ORCID Profile
0000-0002-2626-7622
Current Organisations
Liverpool Hospital
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Macquarie University
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Publisher: Elsevier BV
Date: 03-1994
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012069
Abstract: Many different chemical radiation dosimeters have been fabricated over the last 20 years. In the search for new dosimeters, next to being sensitive to clinical radiation doses, several other physicochemical characteristics need to be satisfied, such as stability of the dose response, spatial integrity, temperature independence, dose rate independence and tissue equivalence. The development of new dosimeters is often hindered by a limited access to radiation facilities to irradiate hundreds of test tubes or cuvettes to study these physicochemical properties. To facilitate this basic experimental research, we propose the use of an inexpensive UVC irradiator. While care is required in extrapolating the results obtained with UV radiation to high energetic X-rays, for several studies, a UV irradiator is a handy tool for first line investigation of new dosimeters. In this study, we calculated the dose distribution in a cylindrical test tube when being rotated during UV exposure. A quantitative analysis allows the optimization of the set-up to obtain dose rates in the s le in similar order of magnitude that are delivered at a clinical Linac. Regardless the usefulness of a UVC irradiator in the laboratory for preliminary testing, it should not be a complete replacement for measurements with high energetic X-rays.
Publisher: IOP Publishing
Date: 12-2006
Publisher: IOP Publishing
Date: 26-06-2013
Publisher: IOP Publishing
Date: 16-12-2008
DOI: 10.1088/0031-9155/54/2/007
Abstract: A gelatin phantom containing an optically scattering funnel-shaped region of elevated optical density (OD) was used to examine light-scattering-induced artefacts in a cone-beam optical CT scanner used for gel dosimetry. To simulate polymer gel dosimeters, the opacity was introduced by adding a colloidal scatterer to the gelatin. Scatter results in an underestimate of OD (hence dose). In line profiles of OD taken from 3D reconstructions of the funnel, those profiles with a long pathlength through high OD regions exhibited a 'dishing' (or 'cupping') artefact, while those of short pathlength exhibited the opposite effect-'doming'. These phenomena are accounted for by a model that includes the effect of stray, scattered light.
Publisher: IOP Publishing
Date: 05-2009
Publisher: Springer Science and Business Media LLC
Date: 08-02-2011
Publisher: IOP Publishing
Date: 05-2009
Publisher: Elsevier BV
Date: 04-2022
Publisher: IOP Publishing
Date: 12-01-2015
Publisher: IOP Publishing
Date: 2022
DOI: 10.1088/1742-6596/2167/1/012029
Abstract: 4D radiation dosimetry using a highly radiation-sensitive polymer gel dosimeter with real-time quantitative MRI readout is presented as a technique to acquire the accumulated radiation dose distribution during image guided radiotherapy (IGRT) on an MRI-Linac. Optimized T 2 weighted TSE scans are converted into quantitative ΔR 2 maps and subsequently to radiation dose maps. The potential of real-time 4D radiation dosimetry in a theragnostic MRI-Linac is demonstrated in test tubes, for a square beam in a cylindrical gel phantom, for a simple step-and-shoot irradiation in a head phantom and a dynamic arc treatment on a cylindrical gel phantom using a rotating couch. The optimal sequence parameters for maximal dose resolution in the dynamic MRI acquisition will be presented and the trade off between MRI scanning speed and dose resolution will be discussed. A further improvement in temporal resolution using a keyhole imaging approach is the focus of future research.
Publisher: Radiological Society of North America (RSNA)
Date: 09-1994
Publisher: Elsevier BV
Date: 12-1996
Publisher: IOP Publishing
Date: 22-08-2013
DOI: 10.1088/0031-9155/58/18/6241
Abstract: A quantitative comparison of two full three-dimensional (3D) gel dosimetry techniques was assessed in a clinical setting: radiochromic gel dosimetry with an in-house developed optical laser CT scanner and polymer gel dosimetry with magnetic resonance imaging (MRI). To benchmark both gel dosimeters, they were exposed to a 6 MV photon beam and the depth dose was compared against a diamond detector measurement that served as golden standard. Both gel dosimeters were found accurate within 4% accuracy. In the 3D dose matrix of the radiochromic gel, hotspot dose deviations up to 8% were observed which are attributed to the fabrication procedure. The polymer gel readout was shown to be sensitive to B0 field and B1 field non-uniformities as well as temperature variations during scanning. The performance of the two gel dosimeters was also evaluated for a brain tumour IMRT treatment. Both gel measured dose distributions were compared against treatment planning system predicted dose maps which were validated independently with ion chamber measurements and portal dosimetry. In the radiochromic gel measurement, two sources of deviations could be identified. Firstly, the dose in a cluster of voxels near the edge of the phantom deviated from the planned dose. Secondly, the presence of dose hotspots in the order of 10% related to inhomogeneities in the gel limit the clinical acceptance of this dosimetry technique. Based on the results of the micelle gel dosimeter prototype presented here, chemical optimization will be subject of future work. Polymer gel dosimetry is capable of measuring the absolute dose in the whole 3D volume within 5% accuracy. A temperature stabilization technique is incorporated to increase the accuracy during short measurements, however keeping the temperature stable during long measurement times in both calibration phantoms and the volumetric phantom is more challenging. The sensitivity of MRI readout to minimal temperature fluctuations is demonstrated which proves the need for adequate compensation strategies.
Publisher: Elsevier BV
Date: 03-1999
Publisher: Elsevier BV
Date: 09-2013
Publisher: IOP Publishing
Date: 16-09-2002
DOI: 10.1088/0031-9155/47/19/301
Abstract: Polymer gel dosimeters offer a wide range of potential applications in the three-dimensional verification of complex dose distribution such as in intensity-modulated radiotherapy (IMRT). Until now, however, polymer gel dosimeters have not been widely used in the clinic. One of the reasons is that they are difficult to manufacture. As the polymerization in polymer gels is inhibited by oxygen, all free oxygen has to be removed from the gels. For several years this was achieved by bubbling nitrogen through the gel solutions and by filling the phantoms in a glove box that is perfused with nitrogen. Recently another gel formulation was proposed in which oxygen is bound in a metallo-organic complex thus removing the problem of oxygen inhibition. The proposed gel consists of methacrylic acid, gelatin, ascorbic acid, hydroquinone and copper(II)sulphate and is given the acronym MAGIC gel dosimeter. These gels are fabricated under normal atmospheric conditions and are therefore called 'normoxic' gel dosimeters. In this study, a chemical analysis on the MAGIC gel was performed. The composition of the gel was varied and its radiation response was evaluated. The role of different chemicals and the reaction kinetics are discussed. It was found that ascorbic acid alone was able to bind the oxygen and can thus be used as an anti-oxidant in a polymer gel dosimeter. It was also found that the anti-oxidants N-acetyl-cysteine and tetrakis(hydroxymethyl)phosphonium were effective in scavenging the oxygen. However, the rate of oxygen scavenging is dependent on the anti-oxidant and its concentration with tetrakis(hydroxymethyl)phosphonium being the most reactive anti-oxidants. Potentiometric oxygen measurements in solution provide an easy way to get a first impression on the rate of oxygen scavenging. It is shown that cupper(II)sulphate operates as a catalyst in the oxidation of ascorbic acid. We, therefore, propose some new normoxic gel formulations that have a less complicated chemical formulation than the MAGIC gel.
Publisher: The Royal Society of Chemistry
Date: 10-07-2020
Publisher: CRC Press
Date: 31-10-2017
Publisher: IOP Publishing
Date: 05-2009
Publisher: IOP Publishing
Date: 2022
DOI: 10.1088/1742-6596/2167/1/012010
Abstract: Patient-specific dosimeters are ideal for quality assurance of radiotherapy treatments. Such dosimeters would ideally need to be manufactured in a short amount of time so that the time between initially imaging the patient, treating the patient-specific dosimeter, reading out the dosimeter, and then finally treating the patient, is minimised. 3D printing allows for rapid manufacture of complex 3D objects. The FlexyDos3D dosimeter is a silicone-based material that has enough strength for the dosimeter to hold its shape without a mould or other structure supporting it. A custom 3D printer has been built for making dosimeters from the FlexyDos3D material. Testing of the 3D printer is ongoing.
Publisher: IOP Publishing
Date: 05-2009
Publisher: IOP Publishing
Date: 09-01-2019
Publisher: Elsevier BV
Date: 04-2022
DOI: 10.1016/J.EJMP.2022.02.010
Abstract: A novel anthropomorphic head and neck phantom which features the emulation of blood flow and perfusion is proposed. The phantom is helpful in both education and research and contains major blood vessels and a porous silicone elastomer brain compartment with microvascular capillary flow. The porous brain compartment is fabricated by use of a novel cast-moulding-dissolution technique. The skull and vertebra are fabricated by a combination of 3D printing and cast-moulding and are tissue equivalent with CT numbers ranging from 1000 HU to 1200 HU. The elastic structure of the phantom allows ultrasound imaging in the neck region. MRI compatible pressure sensors measure the pressure in the carotic arteries and the jugular veins and pulsatile flow is created by use of a peristaltic pump. The pressure-flow dynamics are physiologically realistic and also matches well with computational simulations of porous Darcy flow. The phantom can be used to optimize and validate MRI pulse sequences and protocols for flow imaging, MR angiography, Arterial Spin Labeling (ASL) and dynamic contrast enhanced (DCE) MRI.
Publisher: IOP Publishing
Date: 11-2010
Publisher: The Royal Society of Chemistry
Date: 10-07-2020
Publisher: IOP Publishing
Date: 06-11-2007
DOI: 10.1088/0031-9155/52/23/001
Abstract: Carnosine has been shown to be present in the skeletal muscle and in the brain of a variety of animals and humans. Despite the various physiological functions assigned to this metabolite, its exact role remains unclear. It has been suggested that carnosine plays a role in buffering in the intracellular physiological pHi range in skeletal muscle as a result of accepting hydrogen ions released in the development of fatigue during intensive exercise. It is thus postulated that the concentration of carnosine is an indicator for the extent of the buffering capacity. However, the determination of the concentration of this metabolite has only been performed by means of muscle biopsy, which is an invasive procedure. In this paper, we utilized proton magnetic resonance spectroscopy (1H MRS) in order to perform absolute quantification of carnosine in vivo non-invasively. The method was verified by phantom experiments and in vivo measurements in the calf muscles of athletes and untrained volunteers. The measured mean concentrations in the soleus and the gastrocnemius muscles were found to be 2.81 +/- 0.57/4.8 +/- 1.59 mM (mean +/- SD) for athletes and 2.58 +/- 0.65/3.3 +/- 0.32 mM for untrained volunteers, respectively. These values are in agreement with previously reported biopsy-based results. Our results suggest that 1H MRS can provide an alternative method for non-invasively determining carnosine concentration in human calf muscle in vivo.
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012043
Abstract: Dual-wavelength scanning is a technique eliminating the need of phantom repositioning for optical-CT gel readout. To further diminish artifacts caused by phantom impurities, we hereby propose a novel dual-wavelength imaging method based on phantom impurity recognition and correction. In this method, impurities in motion trajectories during phantom rotation are recognized at the reference wavelength via motion-tracking as a binary mask, which is then applied to correct impurity-contaminated pixels at the data wavelength. Compared with the existent dual-wavelength imaging method, the proposed method is demonstrated to be capable of reducing impurity-induced artifacts and improving imaging SNR and CNR in the same process.
Publisher: IOP Publishing
Date: 12-01-2015
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012044
Abstract: Chloroform in the FlexyDos3D dosimeter acts as a radical initiator which brings about the colour change of the dosimeter when the radicals react with the leucomalachite green. However, the volatility of the chloroform likely results in a rapid loss of chloroform from the dosimeter. Gravimetric analysis and NMR diffusion-ordered coherence spectroscopy were used to measure the diffusion and evaporation rates of chloroform from the dosimeter and found that both rates were both significantly large resulting in a rapid loss of chloroform. Dose maps of irradiated phantoms aged for different times found a significant difference in the dose measurements of the dosimeter, likely a result of the different chloroform concentrations within the dosimeters.
Publisher: Elsevier BV
Date: 12-2001
DOI: 10.1016/S1278-3218(01)00137-8
Abstract: The physical and dosimetrical characteristics of a device, designed to visualize the isocenter position on electronic portal images, were examined. The device, to be mounted on the gantry head of the accelerator, containing five spheric lead markers, was designed in order to visualize the isocenter position on portal images. A quality control device was designed to check the reliability of this technique. The disturbance of the dose distribution by the markers was studied with gel dosimetry. The use of markers resulted in a precise and accurate method to visualize the isocenter on portal images. A maximum underdosage of 11%, due to attenuation by the markers, was observed. The use of markers to visualize the isocenter position on portal images, is a fast and reliable method when analyzing patient setup errors with online electronic portal imaging.
Publisher: IOP Publishing
Date: 06-12-2013
DOI: 10.1088/0031-9155/58/1/43
Abstract: This study quantifies some major physico-chemical factors that influence the validity of MRI (PAGAT) polymer gel dosimetry: temperature history (pre-, during and post-irradiation), oxygen exposure (post-irradiation) and volumetric effects (experiment with phantom in which a small test tube is inserted). Present results confirm the effects of thermal history prior to irradiation. By exposing a polymer gel s le to a linear temperature gradient of ∼2.8 °C cm⁻¹ and following the dose deviation as a function of post-irradiation time new insights into temporal variations were added. A clear influence of the temperature treatment on the measured dose distribution is seen during the first hours post-irradiation (resulting in dose deviations up to 12%). This effect diminishes to 5% after 54 h post-irradiation. Imposing a temperature offset (maximum 6 °C for 3 h) during and following irradiation on a series of calibration phantoms results in only a small dose deviation of maximum 4%. Surprisingly, oxygen diffusing in a gel dosimeter up to 48 h post-irradiation was shown to have no effect. Volumetric effects were studied by comparing the dose distribution in a homogeneous phantom compared to the dose distribution in a phantom in which a small test tube was inserted. This study showed that the dose measured inside the test tube was closer to the ion chamber measurement in comparison to the reference phantom without test tube by almost 7%. It is demonstrated that physico-chemical effects are not the major causes for the dose discrepancies encountered in the reproducibility study discussed in the concurrent paper (Vandecasteele and De Deene 2013a Phys. Med. Biol. 58 19-42). However, it is concluded that these physico-chemical effects are important factors that should be addressed to further improve the dosimetric accuracy of 3D MRI polymer gel dosimetry.
Publisher: IOP Publishing
Date: 26-06-2013
Publisher: Elsevier BV
Date: 12-2008
DOI: 10.1016/J.APRADISO.2008.06.005
Abstract: The radiological properties of the PRESAGE and PAGAT polymer dosimeters have been investigated and their water equivalence determined for use in radiotherapy dosimetry. The radiological water equivalence of each of the polymer dosimeters was determined by comparing the photon and electron interaction cross-sections over the 10 keV-20 MeV energy range and by Monte Carlo modelling the depth dose from a linear accelerator using the BEAMnrc software package. PRESAGE was found to have an effective Z-value and mass density (kgm(-3)) approximately 17% and 10% higher than water, respectively. A maximum difference of 85% was discovered in the photoelectric interaction probability curve of PRESAGE when compared to water over the energy range 10-100 keV, partially due to the Z(3) dependence of the photoelectric effect. The mass radiative stopping power ratios and mass scattering power ratios were both found to have less than 9% difference from water. The depth dose for PRESAGE from a 6MV photon beam had an absolute percentage difference to water of less than 2% and a relative percentage difference of less than 8%. The mass density of PAGAT was found to be 2.6% higher than water due to its high gelatine and monomer concentration. The cross-sectional attenuation and absorption coefficient ratios were found to be within 5% for energies between 10 and 100 keV and within 1% for energies between 100 keV and 20 MeV. The mass collisional stopping power, mass radiative stopping power and mass scattering power ratios were all less than 1% over the energy range studied. The depth dose had an absolute percentage difference to water of less than 1% and a relative percentage difference of less than 2.5%. These results indicate that the PAGAT polymer gel formulation is more radiological water equivalent than the PRESAGE formulation. However, the PRESAGE dosimeter offers some advantages in terms of ease of use and its lack of water equivalence may be overcome with dosimetric correction factors.
Publisher: Elsevier BV
Date: 10-2015
Publisher: IOP Publishing
Date: 03-12-2002
DOI: 10.1088/0031-9155/47/24/307
Abstract: Ultrasonic speed of propagation and attenuation were investigated as a function of absorbed radiation dose in PAG and MAGIC polymer gel dosimeters. Both PAG and MAGIC gel dosimeters displayed a dependence of ultrasonic parameters on absorbed dose with attenuation displaying significant changes in the dose range investigated. The ultrasonic attenuation dose sensitivity at 4 MHz in MAGIC gels was determined to be 4.7 +/- 0.3 dB m(-1) Gy(-1) and for PAG 3.9 +/- 0.3 dB m(-1) Gy(-1). Ultrasonic speed dose sensitivities were 0.178 +/- 0.006 m s(-1) Gy(-1) for MAGIC gel and -0.44 +/- 0.02 m s(-1) Gy(-1) for PAG. Density and compressional elastic modulus were investigated to explain the different sensitivities of ultrasonic speed to radiation for PAG and MAGIC gels. The different sensitivities were found to be due to differences in the compressional elastic modulus as a function of dose for the two formulations. To understand the physical phenomena underlying the increase in ultrasonic attenuation with dose, the viscoelastic properties of the gels were studied. Results suggest that at ultrasonic frequencies, attenuation in polymer gel dosimeters is primarily due to volume viscosity. It is concluded that ultrasonic attenuation significantly increases with absorbed dose. Also, the ultrasonic speed in polymer gel dosimeters is affected by changes in dosimeter elastic modulus that are likely to be a result of polymerization. It is suggested that ultrasound is a sufficiently sensitive technique for polymer gel dosimetry.
Publisher: SAGE Publications
Date: 1995
Publisher: IOP Publishing
Date: 03-07-2002
DOI: 10.1088/0031-9155/47/14/307
Abstract: In this study the stability of different polymer gel dosimeters is investigated. Further to a previous chemical stability study on a (6%T, 50%C) PAG gel, the change in slope and intercept of the linear part of the R2-dose plot is recorded with time for different gel formulations. In addition to this R2-dose-response stability study, the dose edge of a half-blocked field was recorded with time. Three different PAG type polymer gels, a hydroxyethyl acrylate (HEA) gel and two different normoxic polymer gels were investigated. In the PAG type polymer gels, the relative concentration of gelatin and comonomers was varied in order to study the influence of the different components, that constitute the dosimeter, on the stability. It is shown that the R2-dose-response stability is largely determined by the chemical composition of the gel dosimeters. All the PAG gel dosimeters and the normoxic gel dosimeters are found to preserve the integrity of the dose distribution up to 22 days after irradiation. The half-life of the change in dose sensitivity of a MAGIC gel is found to be 18 h compared to 5.7 h for a (6%T, 50%C) PAG gel. A maximum relative decrease in dose sensitivity of 21% was noted for the MAGIC gel compared to an increase of 50% for a (6%T, 50%C) PAG gel. A loss of integrity of the dose distribution was found in the HEA gel.
Publisher: SAGE Publications
Date: 1995
DOI: 10.1177/028418519503600105
Abstract: Forty-five patients with known or suspected musculoskeletal tumors were examined with static and dynamic MR imaging to evaluate the safety, tolerability and diagnostic utility of gadodiamide injection and to assess the diagnostic value of dynamic MR imaging and parametric “first-pass” (FP) images. The proportion of patients presenting more diagnostic information on the contrast-enhanced compared to the precontrast spin-echo examinations was determined. The dynamic enhancement characteristics were evaluated with time-intensity curves and parametric images of the FP enhancement rate. The tolerance of gadodiamide injection was good. Contrast enhancement was useful for delineating tumor from muscle, and differentiating viable from necrotic tissue and cystic from solid lesions. Malignant tumors showed a significantly higher slope value, earlier onset of enhancement, and higher maximum enhancement than benign lesions. However, slope values could not be used to predict the malignant potential of a lesion, due to overlap between highly vascular benign and low vascular malignant lesions. By displaying highly vascular areas, parametric FP images provided useful information on the most active part in a tumor before biopsy and for assessing the incorporation of bone-chip allografts. Static, dynamic and FP MR imaging using gadodiamide injection appears safe and provides useful information for diagnosis, biopsy and follow-up of musculoskeletal lesions.
Publisher: IOP Publishing
Date: 26-06-2000
DOI: 10.1088/0031-9155/45/7/308
Abstract: In BANG gel dosimetry, the spin-spin relaxation rate, R2 = 1/T2, is related to radiation dose that has been delivered to a gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. The accuracy that is aimed for in this quantitative MR application is about 5% relative to the maximum dose. In a conventional imaging MR scanner, however, several imaging artefacts may perturb the final dose map. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose maps are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B1-field inhomogeneities as a source of dose inaccuracy. First, the influence of B1-field inhomogeneities on slice profiles is investigated using a thin-slice phantom. Secondly, a FLASH sequence is used to map the B1-field by assessing the effective flip angle in each voxel of a homogeneous phantom. In addition, both experiments and computer simulations revealed the effects of B1 field inhomogeneities on the measured R2. This work offers a method to correct R2 maps for B1 -field inhomogeneities.
Publisher: IOP Publishing
Date: 26-06-2000
DOI: 10.1088/0031-9155/45/7/307
Abstract: In BANG gel dosimetry, the spin-spin relaxation rate, R2 = I/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower, is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.
Publisher: The Royal Society of Chemistry
Date: 10-07-2020
Publisher: Springer Science and Business Media LLC
Date: 25-06-2010
DOI: 10.1007/S10334-010-0219-6
Abstract: This study presents a reproducible phantom which mimics oxygen-consuming tissue and can be used for the validation of (19)F MRI oximetry. The phantom consists of a haemodialysis filter of which the outer compartment is filled with a gelatin matrix containing viable yeast cells. Perfluorocarbon emulsions can be added to the gelatin matrix to simulate sequestered perfluorocarbons. A blood-substituting perfluorocarbon fluid is pumped through the lumen of the fibres in the filter. (19)F relaxometry MRI is performed with a fast 2D Look-Locker imaging sequence on a clinical 3T scanner. Acute and perfusion-related hypoxia were simulated and imaged spatially and temporally using the phantom. The presented experimental setup can be used to simulate oxygen consumption by somatic cells in vivo and for validating computational biophysical models of hypoxia, as measured with (19)F MRI oximetry.
Publisher: IOP Publishing
Date: 15-06-2009
Publisher: IOP Publishing
Date: 10-09-2004
DOI: 10.1088/0031-9155/49/19/005
Abstract: When irradiating a polymer gel dosimeter to relatively high doses, edge enhancing effects (overshoots) may be noticed near dose gradients, resulting in a loss of spatial dose integrity. These overshoots are believed to be a consequence of monomers diffusing into the high-dose region, where they react with long-living macroradicals. Macroradicals may also be responsible for the temporal chemical instability of post-irradiation polymerization that occurs in the polymer gel dosimeter. In this study, a mathematical model is proposed that simulates the edge enhancing effect. The model is based on the hypothesis that the macroradicals are responsible for both the temporal instability and loss of spatial dose integrity. All input parameters for the model are obtained from independent experiments. The edge enhancing effect is studied both experimentally and theoretically for polymer gel dosimeters with various gelatin concentrations. The change in the edge enhancement is also investigated over post-irradiation time. Comparisons between polymer gel measurements and simulations confirm the hypothesis that there is a strong relation between the spatial and temporal instabilities.
Publisher: IOP Publishing
Date: 11-02-2010
Publisher: IEEE
Date: 04-2007
Publisher: IEEE
Date: 2007
Publisher: Elsevier BV
Date: 12-2022
DOI: 10.1016/J.EJMP.2022.10.017
Abstract: A novel MR-safe anthropomorphic torso phantom with an MR-conditional pneumatic respiration system and inflatable lungs for tissue deformation studies is proposed. The phantom consists of a pair of lungs made from sponges encased in flexible polyurethane. The lung phantom also contains a set of silicone tubes of various diameters to mimic the larger vasculature and airways of the lungs. The lungs are surrounded by a plastic ribcage and immersed in a gelatine hydrogel within a flexible polyurethane skin. A plastic pneumatic pump system was constructed to inflate and deflate the lungs. A fibre optic rotary encoder was constructed to determine the volume of displaced air in the lungs. The pneumatic pump and rotary encoder were constructed of plastic materials to allow placement within the bore of the MR scanner with minimal interaction with the magnetic field. Breath-gated scans and rapid imaging scans (2.5 s per image) were taken of the phantom in the stationary state and during inflation/deflation, and with Cartesian and BLADE k-space s ling. It was found that BLADE shows the least motion artifacts during breathing. This phantom and respiration system shows potential for quality assurance of MRI incorporating breathing corrections and for radiotherapy applications in tracking a moving target.
Publisher: IOP Publishing
Date: 18-09-2001
DOI: 10.1088/0031-9155/46/10/312
Abstract: In high-precision 3D gel dosimetry, long MR measurement times together with a high amount of RF energy being absorbed by the phantom are very common, and result in a spatially dependent temperature rise in the gel. As T2 of the dosimeter gel is temperature dependent, dose estimation will be affected. In this study we assess the temperature rise in the dosimeter gel by use of MR temperature mapping and computer modelling. It is shown that in conventional MR sequences. where linear k-space s ling is used, a temperature rise of 3 C results in a dose underestimation of 10% over the whole dose map. To correct for these dose errors, a compensation method involving centric k-space ordering is suggested. Computer simulations have been performed to analyse the robustness of the proposed method. Applying the compensated sequence, a temperature rise of 3 C leads to a narrow dose artefact of the order of 3% for a 'worst case' situation in which a single pixel dose gradient is assumed. Negligible deviations are found in the rest of the dose map.
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012058
Abstract: Teaching demonstrations of computerized tomography (CT) and Single-Photon Emission Tomography (SPECT) to biomedical engineering and medical physics students is h ered by a limited accessibility to clinical scanners, especially during day time. The use of ionizing radiation and radioactive sources in X-ray CT and SPECT further complicates the design of a teaching laboratory session. We here propose an inexpensive and safe educational demonstration of CT and SPECT on an anthropomorphic phantom whereby a visible light source serves as source and a CCD camera serves as detector. The equivalent of a SPECT radionuclide in optical CT scanning is a chemiluminescent material which can be obtained relatively inexpensive in the form of party glow sticks. The proposed teaching tool comprises several learning outcomes such as hands-on construction of the scanner, the acquisition of images and image reconstruction. Also, different imaging artefacts can be simulated and investigated.
Publisher: Wiley
Date: 18-07-2011
DOI: 10.1002/JMRI.22681
Abstract: To investigate the accuracy of a method neglecting T(2)*-relaxation, for the conversion of spoiled gradient echo pulse sequence signal intensity to contrast agent (CA) concentration, in dynamic contrast enhanced MRI studies. In addition a new closed form conversion expression is proposed that accounts for a first order approximation of T(2)*-relaxation. The accuracy of both conversion methods is compared theoretically by means of simulations for four pulse sequences from literature. Both methods are tested in vivo against the numerical conversion method for measuring the arterial input function in mice. Simulations show that the T(2)*-neglecting method underestimates typical tissue CA concentrations (0 mM to 2 mM) up to 6%, while the errors for arterial concentrations (0 mM to 10 mM) range up to 43%. The results from our first order method are numerically indistinguishable from the simulation input values in tumor tissue, while for arterial concentrations the error is reduced up to a factor 10. In vivo, peak Gd-DOTA concentration is underestimated up to 14% with the T(2)*-neglecting method and up to 0.9% with our first order method. Our conversion method reduces the underestimation of CA concentration severely in a broad physiological concentration range and is easy to perform in any clinical setting.
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012059
Abstract: Several kinds of chemical 3D radiation dosimeters have been fabricated to acquire the dose distribution in clinical radiotherapy. A legitimate concern with every new dosimeter is its dose rate dependent response because, the dose rate in each point of the phantom during a clinical treatment is unknown. Unfortunately, many 3D dosimeters have shown some degree of dose rate dependence. In practice, radiation dosimeters are mostly calibrated using a calibration curve that is obtained by irradiating calibration vials with different doses but using a fixed dose rate. Therefore, when applying a dose calibration to an experimentally obtained dose distribution, a deviation between the measured dose distribution and the actual dose distribution can be expected. In this computational study, the effect of a dose rate dependent dosimeter response on a theoretical dose distribution has been investigated. In order to compare the effect of dose rate, a gamma evaluation is performed between the predefined dose distribution and the dose distribution that is affected by a dose rate dependent radiation response. It is found that a dose rate difference of -10%, results in a gamma pass rate of 100% in the 3D dose distribution.
Publisher: Elsevier BV
Date: 1994
DOI: 10.1016/0730-725X(94)92193-8
Abstract: The purpose of this study was to evaluate the diagnostic value of parametric images which display the first-pass of Gd-DTPA after intravenous bolus injection. Single slice dynamic contrast-enhanced MRI (Turbo-FLASH) with a temporal resolution of 2.41 s was performed in 40 benign and malignant musculoskeletal lesions. Parametric 'first-pass' images were obtained by using a new postprocessing technique, derived from a home written computer program, which calculated the steepest slope of the time-intensity curve pixel by pixel. First-pass images provided qualitative and quantitative information, which was useful for diagnosis and therapy. Demonstration of the areas with the highest vascularization was very helpful in planning the best biopsy site in 6 patients and to delineate tumor from peritumoral muscle edema in 4 patients. In 10 patients the first-pass images provided additional information, which was not visible on the conventional spin-echo images. The most promising application was the ability to evaluate the response to preoperative chemotherapy in a patient with Ewing's sarcoma. Although a highly statistically significant difference (p < 0.001) was found between the first-pass slope values of benign and malignant lesions, the sensitivity (77%) and specificity (71%) of this method were too low to permit a definite differentiation of highly vascular benign from malignant lesions. Pathologic examination of 17 lesions, performed in the plane of the first-pass image, showed a good correlation between the first-pass slope values and the vascularization of the tissues.(ABSTRACT TRUNCATED AT 250 WORDS)
Publisher: IOP Publishing
Date: 26-06-2013
Publisher: IOP Publishing
Date: 12-01-2015
Publisher: Oxford University Press (OUP)
Date: 30-12-2013
DOI: 10.1093/RPD/NCT352
Abstract: In spite of considerable progress in neutron dosimetry, there is no dosemeter that is capable of measuring neutron doses independently of the neutron spectrum with good accuracy. Carbon-doped aluminium oxide (Al2O3:C) is a sensitive material for ionising radiation (beta-ray, X ray and electron) and has been used for applications in personal and medical dosimetry as an optically stimulated luminescence (OSL) dosemeter. Al2O3:C has a low sensitivity to neutron radiation this prevents its application to neutron fields, representing a disadvantage of Al2O3:C-OSL when compared with LiF, which is used as a thermoluminescent detector. Recently an improvement for neutron dosimetry (Passmore and Kirr. Neutron response characterisation of an OSL neutron dosemeter. Radiat. Prot. Dosim. 2011 144: 155-60) uses Al2O3:C coated with (6)Li2CO3 (OSLN),which gives the high-sensitive response as known for Al2O3:C with the advantage of being also sensitive to thermal neutrons. In this article, the authors compare small-size detectors (droplets) of Al2O3:C (OSL) and of Al2O3:C+(6)Li2CO3 (OSLN) and discuss the advantages and drawbacks of both materials, regarding size vs. response.
Publisher: IOP Publishing
Date: 05-2017
Publisher: IOP Publishing
Date: 26-06-2013
Publisher: Wiley
Date: 1995
Abstract: The noninvasive thermometry method is based on the temperature dependence of the proton resonance frequency (PRF). High-quality temperature images can be obtained from phase information of standard gradient-echo sequences with an accuracy of 0.2 degrees C in phantoms. This work was focused on the in vivo capabilities of this method. An experimental setup was designed that allows a qualitative in vivo verification. The lower-leg muscles of a volunteer were cooled and afterwards reheated with an external water bolus. The temperature of the bolus water varied between 17 degrees C and 37 degrees C. The in vivo temperature images can be used to extract the temperature in muscle tissue. The data in the fat tissue are difficult to interpret because of the predominance of susceptibility effects. The results confirm the method's potential for hyperthermia control.
Publisher: IOP Publishing
Date: 12-2006
Publisher: IOP Publishing
Date: 06-12-2013
DOI: 10.1088/0031-9155/58/1/63
Abstract: In MRI (PAGAT) polymer gel dosimetry, there exists some controversy on the validity of 3D dose verifications of clinical treatments. The relative contribution of important sources of uncertainty in MR scanning to the overall accuracy and precision of 3D MRI polymer gel dosimetry is quantified in this study. The performance in terms of signal-to-noise and imaging artefacts was evaluated on three different MR scanners (two 1.5 T and a 3 T scanner). These include: (1) B₀-field inhomogeneity, (2) B₁-field inhomogeneity, (3) dielectric effects (losses and standing waves) and (4) temperature inhomogeneity during scanning. B₀-field inhomogeneities that amount to maximum 5 ppm result in dose deviations of up to 4.3% and deformations of up to 5 pixels. Compensation methods are proposed. B₁-field inhomogeneities were found to induce R₂ variations in large anthropomorphic phantoms both at 1.5 and 3 T. At 1.5 T these effects are mainly caused by the coil geometry resulting in dose deviations of up to 25%. After the correction of the R₂ maps using a heuristic flip angle-R₂ relation, these dose deviations are reduced to 2.4%. At 3 T, the dielectric properties of the gel phantoms are shown to strongly influence B₁-field homogeneity, hence R₂ homogeneity, especially of large anthropomorphic phantoms. The low electrical conductivity of polymer gel dosimeters induces standing wave patterns resulting in dose deviations up to 50%. Increasing the conductivity of the gel by adding NaCl reduces the dose deviation to 25% after which the post-processing is successful in reducing the remaining inhomogeneities caused by the coil geometry to within 2.4%. The measurements are supported by computational modelling of the B₁-field. Finally, temperature fluctuations of 1 °C frequently encountered in clinical MRI scanners result in dose deviations up to 15%. It is illustrated that with adequate temperature stabilization, the dose uncertainty is reduced to within 2.58%.
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012021
Abstract: The pre- and post-irradiation scan strategy for optical-CT gel readout often turns out to be corrupted by angular mismatch between these two scans. In this study, we used computational simulations to investigate the influence of angular mismatch. Two phantoms are constructed: one cylindrical phantom with synthetic impurities and one elliptical phantom. The reconstructed results of angular mismatched pre- and post-data show that the dual-scan method is very sensitive to repositioning error, and positive-negative pair errors can be easily identified around impurities and phantom edges. From the simulation results, we believe that the angular mismatch should be less than 0.1 degree.
Publisher: IOP Publishing
Date: 25-04-2007
DOI: 10.1088/0031-9155/52/10/006
Abstract: Polymer gel dosimeters offer a wide range of applications in the three-dimensional verification of complex radiation dose distributions such as in intensity-modulated radiotherapy (IMRT). With the release of polymer gel dosimeters that can be fabricated in normal atmospheric ('normoxic') conditions, the gel manufacturing process has been significantly simplified. Gel dosimeters are calibrated by use of a series of calibration vials irradiated with known doses or by use of a calibration phantom with a known dose distribution. The overall accuracy of the polymer gel dosimeters is determined by different dosimetric properties. In this study, we show the influence of the temperature history during storage of the gel dosimeter on the dose response curve for two gel dosimeters using the monomers acrylamide/N,N'-methylene-bis-acrylamide (nPAG) and methacrylic acid (nMAG) respectively and bis[tetrakis(hydroxymethyl)phosphonium]sulphate (THP) as antioxidant in both gel dosimeters. This study reveals that differences in temperature history after fabrication of normoxic polymer gel dosimeters may compromise the dosimetric accuracy. It was found that the acrylamide based gel dosimeter (nPAG) is less dependent on the post-manufacture temperature history than the methacrylic acid based gel dosimeter (nMAG). The importance of an equal temperature history for the gel dosimeter and calibration vials is emphasized by this study. A reproducibility study has also been performed on the nPAG gel dosimeter when additional efforts are made to control the temperature changes upon cooling.
Publisher: Elsevier BV
Date: 2002
Publisher: IOP Publishing
Date: 28-03-2011
DOI: 10.1088/0031-9155/56/8/013
Abstract: In (19)F MRI oximetry, a method used to image tumour hypoxia, perfluorocarbons serve as oxygenation markers. The goal of this study is to evaluate the impact of perfluorocarbon distribution and concentration in (19)F MRI oximetry through a computer simulation. The simulation studies the correspondence between (19)F measured (pO(FNMR)(2)) and actual tissue oxygen tension (pO(2)) for several tissue perfluorocarbon distributions. For this, a Krogh tissue model is implemented which incorporates the presence of perfluorocarbons in blood and tissue. That is, in tissue the perfluorocarbons are distributed homogeneously according to Gaussian diffusion profiles, or the perfluorocarbons are concentrated in the capillary wall. Using these distributions, the oxygen tension in the simulation volume is calculated. The simulated mean oxygen tension is then compared with pO(FNMR)(2), the (19)F MRI-based measure of pO(2) and with pO(0)(2), pO(2) in the absence of perfluorocarbons. The agreement between pO(FNMR)(2) and actual pO(2) is influenced by vascular density and perfluorocarbon distribution. The presence of perfluorocarbons generally gives rise to a pO(2) increase in tissue. This effect is enhanced when perfluorocarbons are also present in blood. Only the homogeneous perfluorocarbon distribution in tissue with no perfluorocarbons in blood guarantees small deviations of pO(FNMR)(2) from pO(2). Hence, perfluorocarbon distribution in tissue and blood has a serious impact on the reliability of (19)F MRI-based measures of oxygen tension. In addition, the presence of perfluorocarbons influences the actual oxygen tension. This finding may be of great importance for further development of (19)F MRI oximetry.
Publisher: IOP Publishing
Date: 23-01-2015
DOI: 10.1088/0031-9155/60/4/1543
Abstract: Three dimensional radiation dosimetry has received growing interest with the implementation of highly conformal radiotherapy treatments. The radiotherapy community faces new challenges with the commissioning of image guided and image gated radiotherapy treatments (IGRT) and deformable image registration software.A new three dimensional anthropomorphically shaped flexible dosimeter, further called 'FlexyDos3D', has been constructed and a new fast optical scanning method has been implemented that enables scanning of irregular shaped dosimeters. The FlexyDos3D phantom can be actuated and deformed during the actual treatment. FlexyDos3D offers the additional advantage that it is easy to fabricate, is non-toxic and can be molded in an arbitrary shape with high geometrical precision.The dosimeter formulation has been optimized in terms of dose sensitivity. The influence of the casting material and oxygen concentration has also been investigated. The radiophysical properties of this new dosimeter are discussed including stability, spatial integrity, temperature dependence of the dosimeter during radiation, readout and storage, dose rate dependence and tissue equivalence.
Publisher: IOP Publishing
Date: 09-03-2000
DOI: 10.1088/0031-9155/45/4/304
Abstract: The aim of this work is to investigate the temporal stability of a polyacrylamide gelatin hydrogel used for 3D monomer olymer gel dosimetry techniques involving different methods of analysis. Long-term instabilities for a similar gel have recently been reported, but differ markedly from those described in this work. Two kinds of long-term instabilities are described. One affects the slope of the dose-R2 plot and is related to post-irradiation polymerization of the comonomer olymer aggregates. It is observed that post-irradiation polymerization only lasts 12 hours after irradiation. The other instability affects the intercept of the dose-R2 plot, lasts for up to 30 days and is related to the gelation process of gelatin. Further studies were performed on gelatin gels of varying compositions to obtain a better understanding of the molecular mechanism that causes the instability due to gelation. The studies included observations of the spin-spin and spin-lattice relaxation rates in combination with diffusion measurements and optical measurements. It is shown that the heating history during the manufacture of the gel affects the absolute R2 value of the gel but not its variation. The findings presented in this study may help in producing more stable and reproducible monomer olymer gel dosimeters.
Publisher: SAGE Publications
Date: 02-02-2011
Abstract: Background: Differences in brain energy metabolism have been found between migraine patients and controls in previous phosphorus magnetic resonance spectroscopy ( 31 P-MRS) studies, most of them emphasizing migraine with aura (MwA). The aim of this study was to verify potential changes in resting-state brain energy metabolism in patients with migraine without aura (MwoA) compared to control subjects by 31 P-MRS at 3 tesla. Methods: Quantification was performed using the phantom replacement technique. MRS measurements were performed interictally and in the medial occipital lobe of 19 MwoA patients and 26 age-matched controls. Results: A significantly decreased phosphocreatine concentration ([PCr]) was found as in previous studies. While adenosine triphosphate concentration ([ATP]) was considered to be constant in previously published work, this study found a significant decrease in the measured [ATP] in MwoA patients. The inorganic phosphate ([P i ]) and magnesium ([Mg 2+ ]) concentrations were not significantly different between MwoA patients and controls. Conclusions: The altered metabolic concentrations indicate that the energy metabolism in MwoA patients is impaired, certainly in a subgroup of patients. The actual decrease in [ATP] adds further strength to the theory of the presence of a mitochondrial component in the pathophysiology of migraine.
Publisher: Elsevier BV
Date: 02-2008
DOI: 10.1016/J.JMR.2007.10.014
Abstract: Diffusion weighted magnetic resonance imaging enables the visualization of fibrous tissues such as brain white matter. The validation of this non-invasive technique requires phantoms with a well-known structure and diffusion behavior. This paper presents anisotropic diffusion phantoms consisting of parallel fibers. The diffusion properties of the fiber phantoms are measured using diffusion weighted magnetic resonance imaging and bulk NMR measurements. To enable quantitative evaluation of the measurements, the diffusion in the interstitial space between fibers is modeled using Monte Carlo simulations of random walkers. The time-dependent apparent diffusion coefficient and kurtosis, quantifying the deviation from a Gaussian diffusion profile, are simulated in 3D geometries of parallel fibers with varying packing geometries and packing densities. The simulated diffusion coefficients are compared to the theory of diffusion in porous media, showing a good agreement. Based on the correspondence between simulations and experimental measurements, the fiber phantoms are shown to be useful for the quantitative validation of diffusion imaging on clinical MRI-scanners.
Publisher: IOP Publishing
Date: 21-01-2003
Publisher: IOP Publishing
Date: 21-05-2002
DOI: 10.1088/0031-9155/47/11/304
Abstract: Off-resonance RF pre-saturation was used to obtain contrast in MRI images of polymer gel dosimeters irradiated to doses up to 50 Gy. Two different polymer gel dosimeters composed of 2-hydroxyethyl-acrylate or methacrylic acid monomers mixed with N, N'-methylene-bisacrylamide (BIS), dispersed in an aqueous gelatin matrix were evaluated. Radiation-induced polymerization of the co-monomers generates a fast-relaxing insoluble polymer. Saturation of the polymer using off-resonance Gaussian RF pulses prior to a spin-echo readout with a short echo time leads to contrast that is dependent on the absorbed dose. This contrast is attributed to magnetization transfer (MT) between free water and the polymer, and direct saturation of water was found to be negligible under the prevailing experimental conditions. The usefulness of MT imaging was assessed by computing the dose resolution obtained with this technique. We found a low value of dose resolution over a wide range of doses could be obtained with a single experiment. This is an advantage over multiple spin echo (MSE) experiments using a single echo spacing where an optimal dose resolution is achieved over only very limited ranges of doses. The results suggest MT imaging protocols may be developed into a useful tool for polymer gel dosimetry.
Publisher: IOP Publishing
Date: 05-10-2001
DOI: 10.1088/0031-9155/46/11/304
Abstract: The aim of this work was to investigate the applicability of MR-based polymer gel dosimetry to measure the absorbed dose distribution at short distance from an iridium-192 brachytherapy point source. In this paper, different methodological problems that may result in significant errors in the measured dose distribution are discussed. First of all the extent to which physicochemical mechanisms alter the dose response is discussed. The most important among these are the oxygen permeability of the catheter material and monomer-diffusion-related effects during irradiation. The effect of oxygen on the dose-R2 curve has been determined quantitatively and an oxygen map is performed using a well-defined external irradiation beam. The effect of diffusion of monomer during irradiation has been simulated. Another contribution of errors is related to magnetic susceptibility differences between the catheter and the gel during scanning the irradiated gel. The magnetic field distortion has been mapped by using both an experimental MRI technique and by simulation. Moreover, in constructing a dose-versus-distance curve by polar averaging, the sensitivity to the exact distance between source and point of measurement and to partial volume effects is illustrated. An optimization program is proposed to determine the location of the source on a sub-pixel scale.
Publisher: Elsevier BV
Date: 03-2012
DOI: 10.1016/J.EJRAD.2011.01.106
Abstract: To study age-related metabolic changes in N-acetylaspartate (NAA), total creatine (tCr), choline (Cho) and myo-inositol (Ins). Proton magnetic resonance spectroscopy (1H-MRS) was performed in the posterior cingulate cortex (PCC) and the left hippoc us (HC) of 90 healthy subjects (42 women and 48 men aged 18-76 years, mean±SD, 48.4±16.8 years). Both metabolite ratios and absolute metabolite concentrations were evaluated. Analysis of covariance (ANCOVA) and linear regression were used for statistical analysis. Metabolite ratios Ins/tCr and Ins/H2O were found significantly increased with age in the PCC (P<0.05 and P≤0.001, respectively), and in the HC (P<0.01 for both). An increased tCr/H2O was only observed in the PCC (P<0.01). Following absolute quantification based on the internal water signal, significantly increased concentrations of Ins and tCr in the PCC confirmed the relative findings (P<0.01 for both). Age-related increases of tCr and Ins are found in the PCC, whereas this holds only true for Ins in the HC, indicating possible gliosis in the ageing brain. No age-dependent NAA decreases were observed in the PCC nor the HC. The 1H-MRS results in these specific brain regions can be important to differentiate normal ageing from age-related pathologies such as mild cognitive impairment (MCI) and Alzheimer's disease.
Publisher: IOP Publishing
Date: 02-09-2008
DOI: 10.1088/0031-9155/53/19/009
Abstract: Diffusion weighted magnetic resonance imaging offers a non-invasive tool to explore the three-dimensional structure of brain white matter in clinical practice. Anisotropic diffusion hardware phantoms are useful for the quantitative validation of this technique. This study provides guidelines on how to manufacture anisotropic fibre phantoms in a reproducible way and which fibre material to choose to obtain a good quality of the diffusion weighted images. Several fibre materials are compared regarding their effect on the diffusion MR measurements of the water molecules inside the phantoms. The diffusion anisotropy influencing material properties are the fibre density and diameter, while the fibre surface relaxivity and magnetic susceptibility determine the signal-to-noise ratio. The effect on the T(2)-relaxation time of water in the phantoms has been modelled and the diffusion behaviour inside the fibre phantoms has been quantitatively evaluated using Monte Carlo random walk simulations.
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012018
Abstract: Dry scanners have been proposed as alternatives to scanners in which a refractive index matching fluid is used. Previous dry scanners either have a small field-of-view or employ a thick cast with a higher refractive index than the phantom. This may not always be feasible. A new design is proposed in this study where two aspherical meniscus lenses are employed to direct the laser beams in parallel lines through the phantom.
Publisher: IEEE
Date: 05-2008
Publisher: Springer Berlin Heidelberg
Date: 2008
Publisher: IOP Publishing
Date: 19-01-2006
DOI: 10.1088/0031-9155/51/3/012
Abstract: Polymer gel dosimeters offer a wide range of applications in the three-dimensional verification of complex dose distributions such as in intensity-modulated radiotherapy. One of the major difficulties with polymer gel dosimeters is their sensitivity to oxygen, as oxygen inhibits the radiation-induced polymerization reaction. For several years, oxygen was removed from the gels by bubbling the sol with inert gases for several hours during the gel fabrication. Also, the gel had to be poured in containers with low oxygen permeability and solubility. Recently, it was found that these technical difficulties can easily be solved by adding an antioxidant to the gel. These gels are called 'normoxic' gels as they can be produced under normal atmospheric conditions. In this study several properties of polymer gel dosimeters have been investigated: the dose sensitivity, the temporal and spatial stability of the gel, the sensitivity of the dose response to temperature during irradiation and during MR imaging, the energy dependence and the dose-rate dependence. This study reveals that the normoxic polymer gel dosimeter based on methacrylic acid (nMAG) studied in this work has inferior radiation properties as compared to the polyacrylamide gelatine (PAG) gel dosimeters. It is shown that from the three different gel dosimeters investigated in this study, the nPAG gel dosimeter results in a less sensitive gel dosimeter but with superior radiation properties as compared to the nMAG gel dosimeter. The importance of investigating relevant radiation properties of gel dosimeters apart from the radiation sensitivity-prior to their use for dosimetric validation experiments-is illustrated and emphasized throughout this study. Other combinations of monomer and gelling agent may result in more reliable normoxic polymer gel dosimeters.
Publisher: Wiley
Date: 2001
DOI: 10.1002/NBM.719
Abstract: Following a heterotopic auxiliary liver transplantation, commonly used measurements are either invasive or non-indicative of in idual viability of the coexisting engrafted and native livers. Magnetic resonance imaging (MRI) was therefore tested for its potential to monitor the post-transplant hepatic viability in a rat model. Thirteen Wistar rats were systematically evaluated with MRI and serum biochemical liver parameters. Post-transplant complications and the causes of animal death were identified by autopsy and histo-pathological examinations. The data of the healthy survivors were compared with those of the rats that developed complications. On MRI, the hepatic complications could be depicted in the in idual livers. A specific pattern of signal evolution was found in the livers of the healthy survivors: the mean T1 relaxation times of the engrafted livers increased immediately after transplantation (476 +/- 64 ms, mean +/- standard deviation, pre-operative 730 +/- 48 ms, week 1) and then declined steadily to a 3 month value of 489 +/- 246 ms, while, following a transient first rise (476 +/- 64 ms, pre-operative 589 +/- 28 ms, week 1), the mean T1 value of the native livers increased again 4 weeks after surgery and reached a 3 month value of 859 +/- 43 ms. However, in the rats with various complications, the mean T1 relaxation times of the engrafted livers continued to increase throughout the first post-operative month (760 +/- 48 ms, week 1 922 +/- 76 ms, week 4), while that of the native liver only varied mildly (546 +/- 25 ms, week 1 473 +/- 25 ms, week 4). After the first post-transplant week, the healthy engrafted livers could already be distinguished from those with complications by a significant decrease in T1 relaxation times. These data suggest that, besides demonstrating major complications, MRI may allow one to monitor the viability of each liver by analysing the relative signal intensity and T1 relaxation times after a heterotopic auxiliary liver transplantation.
Publisher: Elsevier BV
Date: 12-2014
Publisher: Springer Berlin Heidelberg
Date: 2008
Publisher: MDPI AG
Date: 19-09-2022
DOI: 10.3390/GELS8090599
Abstract: Gel dosimetry was developed in the 1990s in response to a growing need for methods to validate the radiation dose distribution delivered to cancer patients receiving high-precision radiotherapy. Three different classes of gel dosimeters were developed and extensively studied. The first class of gel dosimeters is the Fricke gel dosimeters, which consist of a hydrogel with dissolved ferrous ions that oxidize upon exposure to ionizing radiation. The oxidation results in a change in the nuclear magnetic resonance (NMR) relaxation, which makes it possible to read out Fricke gel dosimeters by use of quantitative magnetic resonance imaging (MRI). The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding an indicator such as xylenol orange. The second class of gel dosimeters is the radiochromic gel dosimeters, which also exhibit a color change upon irradiation but do not use a metal ion. These radiochromic gel dosimeters do not demonstrate a significant radiation-induced change in NMR properties. The third class is the polymer gel dosimeters, which contain vinyl monomers that polymerize upon irradiation. Polymer gel dosimeters are predominantly read out by quantitative MRI or X-ray CT. The accuracy of the dosimeters depends on both the physico-chemical properties of the gel dosimeters and on the readout technique. Many different gel formulations have been proposed and discussed in the scientific literature in the last three decades, and scanning methods have been optimized to achieve an acceptable accuracy for clinical dosimetry. More recently, with the introduction of the MR-Linac, which combines an MRI-scanner and a clinical linear accelerator in one, it was shown possible to acquire dose maps during radiation, but new challenges arise.
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012037
Abstract: Several 3D dosimeters are commercially available. However, there are many circumstances that require a customized 3D dosimeter. Ex les include feasibility tests of non-standard treatment modalities, inhomogeneous tissue configurations, unique shapes and sizes and teaching. In this session, general approaches for preparing radiochromic dosimeters, Fricke and polymer gel dosimeters, micelle gel and silicone dosimeters were presented. Advise will be given to developers of new 3D dosimeters. For optical readout, light absorption and scatter can limit the practical size of dosimeters. Specifically, increasing from 5 to 15 cm diameter dosimeters is optically challenging. Strategies to maximize initial optical transmission were presented. For MRI readout, the dose resolution is determined by both the dosimeter sensitivity and the pulse sequence parameters and the accuracy is determined by the sensitivity of the dosimeter to temperature and dose rate, next to imaging performance. For X-ray CT imaging, the dose resolution is determined by the sensitivity of the dosimeter which largely depends on the polymer density that can be achieved. The importance of characterizing the dosimeter in terms of dose sensitivity and stability, spatial integrity, dose rate and fractionation dependence, oxygen and ambient light sensitivity, temperature sensitivity and thermal history were emphasized. The dosimeter requirements also dictate the types of vessels and scanners appropriate for readout. For ex le, the preferred dosimeter formulation may include a compound that is incompatible with the preferred vessel.
Publisher: Springer Science and Business Media LLC
Date: 08-01-2011
DOI: 10.1007/S10334-010-0235-6
Abstract: Dynamic contrast enhanced MRI and pharmacokinetic modelling provide a powerful tool for tumour diagnosis and treatment evaluation. However, several studies show low reproducibility of the technique and poor precision of the transendothelial transfer constant K (trans). This work proposes a theoretical framework describing how finite signal-noise-ratio (SNR) in the MR images is propagated throughout the measurement protocol to uncertainty on the kinetic parameter estimates. After deriving a distribution for the contrast agent concentration, a maximum likelihood estimator (MLM) is proposed that exhibits Cramer-Rao lower bounds (CRLB). An analytical expression is derived for the CRLB that can be used to determine confidence intervals for kinetic parameters and to investigate the influence of protocol parameters as scan time and temporal resolution on K (trans)-precision. K (trans)-uncertainty can be reduced up to 30% by using MLM in comparison with least square estimator. K (trans)-precision is proportional to the SNR and depends strongly on the kinetic parameter values themselves. Minimal scan time and temporal resolution were found to be 15 min and 15 s, respectively, for Gd-DTPA. Temporal resolution should be enhanced by decreasing the NEX parameter (NEX ≤ 1). CRLB provide a golden standard to construct 95% confidence intervals, which can be used to perform protocol optimization and to test the statistical significance of K (trans)-changes in treatment evaluation.
Publisher: IOP Publishing
Date: 12-01-2015
Publisher: IOP Publishing
Date: 12-11-2002
DOI: 10.1088/0031-9155/47/23/310
Abstract: Post-irradiation changes in the linear attenuation coefficient, mu, of polymer gel dosimeters give rise to a change which can be measured with x-ray computed tomography. Polymer gel dosimeters were manufactured consisting of 3% (by weight) acrylamide and 3% N,N'-methylene-bis-acrylamide comonomers dissolved in aqueous gelatin (5% gelatin by total weight and 89% de-ionized distilled water). Mu was measured in a collimated radiation beam of photons from an 241Am source. Density, rho, of polymer gel dosimeters was measured using volumetric flasks with capillary stoppers. The measured post-irradiation data of mu was plotted against the data of rho for different batches, and linear least squares fits gave r2 values of 0.99605 and 0.99953, with P values of less than 0.001. This confirms that the post-irradiation change in mu is proportional to that of rho. The change in rho implies a change in volume regardless of the evaluation modality of the polymer gel dosimeter.
Publisher: Wiley
Date: 06-2008
DOI: 10.1118/1.2962036
Publisher: Wiley
Date: 03-1996
Publisher: Wiley
Date: 26-06-2006
DOI: 10.1118/1.2208939
Abstract: A polymer hydrogel foam is proposed as a potential three dimensional experimental dosimeter for radiation treatment verification in low-density tissue such as the lung. A gel foam is created by beating a radiation sensitive polymer gel mixture in an anoxic atmosphere. The mass density of the gel foam is in the order of 0.25-0.35 kg/dm3. Both nuclear magnetic resonance (NMR) spin-spin relaxation rate (R2) and magnetization transfer ratio (MTR) have been used to map the dose distribution from the gel dosimeter. It is found that MTR has significant advantages compared to R2 for mapping the dose distribution in the polymer gel foam dosimeters. The magnetization transfer ratio is found to be less dependent on the density and microstructure of the gel foam dosimeter while spin-spin relaxation dispersion has been observed making the spin-spin relaxation rate dependent on the interecho time interval. Optical microscopy reveals a microstructure that shows great similarity with human lung tissue. It is also shown how NMR hydrogen proton density measurements can be used to map the density distributions in gel dosimeters.
Publisher: IOP Publishing
Date: 12-01-2015
Publisher: Society of Nuclear Medicine
Date: 05-2010
Publisher: IOP Publishing
Date: 08-2019
DOI: 10.1088/1742-6596/1305/1/012030
Abstract: Previous formulations of the FlexyDos3D dosimeter have shown significant changes in the dose-response over time. In this study, various formulations of the dosimeter were created and tested to see if this stability could be improved. A dosimeter that was stable over a three-day period was found. Rapid manufacture of this dosimeter for patient-specific validation of radiotherapy treatments is desirable. The use of 3D printing manufacturing techniques for thermosetting polymers require high temperatures to cure the polymer within a reasonable time. The effect of different curing temperatures and times were investigated for the FlexyDos3D radiation dosimeter for its effect on stability. No significant difference in the dose-response was found for dosimeters cured for different curing times beyond an hour. A significant dose-response offset was found between dosimeters cured at different temperatures, but the dose-response sensitivity was the same.
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D0NA00991A
Abstract: Here, we demonstrate the design of l -dopamine coated manganese oxide nanoparticle based theranostics for MRI imaging and drug delivery. The release of drug molecules can be controlled by altering the size and chemical composition of nanoparticles.
Publisher: IEEE
Date: 10-2008
Publisher: Elsevier BV
Date: 04-2003
DOI: 10.1016/S0167-8140(02)00376-6
Abstract: When planning an intensity-modulated radiation therapy (IMRT) treatment in a heterogeneous region (e.g. the thorax), the dose computation algorithm of a treatment planning system may need to account for these inhomogeneities in order to obtain a reliable prediction of the dose distribution. An accurate dose verification technique such as monomer olymer gel dosimetry is suggested to verify the outcome of the planning system. The effects of low-density structures: (a) on narrow high-energy (18 MV) photon beams and (b) on a class-solution IMRT treatment delivered to a thorax phantom have been examined using gel dosimetry. The used phantom contained air cavities that could be filled with water to simulate a homogeneous or heterogeneous configuration. The IMRT treatment for centrally located lung tumors was delivered on both cases, and gel derived dose maps were compared with computations by both the GRATIS and Helax-TMS planning system. Dose rebuildup due to electronic disequilibrium in a narrow photon beam is demonstrated. The gel measurements showed good agreement with diamond detector measurements. Agreement between measured IMRT dose maps and dose computations was demonstrated by several quantitative techniques. An underdosage of the planning target volume (PTV) was revealed. The homogeneity of the phantom had only a minor influence on the dose distribution in the PTV. An expansion of low-level isodoses in the lung volume was predicted by collapsed cone computations in the heterogeneous case. For the class-solution described, the dose in centrally located mediastinal tumors can be computed with sufficient accuracy, even when neglecting the lower lung density. Polymer gel dosimetry proved to be a valuable technique to verify dose calculation algorithms for IMRT in 3D in heterogeneous configurations.
Publisher: Springer Science and Business Media LLC
Date: 10-2010
Publisher: Elsevier BV
Date: 2015
Publisher: AIP
Date: 2008
DOI: 10.1063/1.3058554
Publisher: Wiley
Date: 2000
DOI: 10.1002/(SICI)1522-2594(200001)43:1<116::AID-MRM14>3.0.CO;2-5
Abstract: The aim of this work was to investigate MR-based polymer gel dosimetry as a three-dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water-filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin-spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root-mean-square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized.
Publisher: IOP Publishing
Date: 12-01-2015
Publisher: IOP Publishing
Date: 05-01-2004
DOI: 10.1088/0031-9155/49/2/008
Abstract: Polymer gel dosimetry was used to assess an intensity-modulated arc therapy (IMAT) treatment for whole abdominopelvic radiotherapy. Prior to the actual dosimetry experiment, a uniformity study on an unirradiated anthropomorphic phantom was carried out. A correction was performed to minimize deviations in the R2 maps due to radiofrequency non-uniformities. In addition, compensation strategies were implemented to limit R2 deviations caused by temperature drift during scanning. Inter- and intra-slice R2 deviations in the phantom were thereby significantly reduced. This was verified in an investigative study where the same phantom was irradiated with two rectangular superimposed beams: structural deviations between gel measurements and computational results remained below 3% outside high dose gradient regions the spatial shift in those regions was within 2.5 mm. When comparing gel measurements with computational results for the IMAT treatment, dose deviations were noted in the liver and right kidney, but the dose-volume constraints were met. Root-mean-square differences between both dose distributions were within 5% with spatial deviations not more than 2.5 mm. Dose fluctuations due to gantry angle discretization in the dose computation algorithm were particularly noticeable in the low-dose region.
Publisher: Elsevier BV
Date: 11-2003
DOI: 10.1016/S0360-3016(03)00663-1
Abstract: Whole abdominopelvic radiation therapy (WAPRT) is a treatment option in the palliation of patients with relapsed ovarian cancer. With conventional techniques, kidneys and liver are the dose- and homogeneity-limiting organs. We developed a planning strategy for intensity-modulated arc therapy (IMAT) and report on the treatment plans of the first 5 treated patients. Five consecutive patients with histologically proven relapsed ovarian cancer were sent to our department for WAPRT. The target volumes and organs at risk (OAR) were delineated on 0.5-cm-thick CT slices. The clinical target volume (CTV) was defined as the total peritoneal cavity. CTV and kidneys were expanded with 0.5 cm. In a preset range of 8 degrees interspaced gantry angles, machine states were generated with an anatomy-based segmentation tool. Machine states of the same class were stratified in arcs. The optimization of IMAT was done in several steps, using a biophysical objective function. These steps included weight optimization of machine states, leaf position optimization adapted to meet the maximal leaf speed constraint, and planner-interactive optimization of the start and stop angles. The final control points (machine states plus associated cumulative monitor unit counts) were calculated using a collapsed cone convolution/superposition algorithm. For comparison, two conventional plans (CONV) were made, one with two fields (CONV2), and one with four fields (CONV4). In these CONV plans, dose to the kidneys was limited by cerrobend blocks. The IMAT and the CONV plans were normalized to a median dose of 33 Gy to the planning target volume (PTV). Monomer olymer gel dosimetry was used to assess the dosimetric accuracy of the IMAT planning and delivery method. The median volume of the PTV was 8306 cc. The mean treatment delivery time over 4 patients was 13.8 min. A mean of 444 monitor units was needed for a fraction dose of 150 cGy. The fraction of the PTV volume receiving more than 90% of the prescribed dose (V(90)) was 9% higher for the IMAT plan than for the CONV4 plan (89.9% vs. 82.5%). Outside a build-up region of 0.8 cm and 1 cm away from both kidneys, the inhomogeneity in the PTV was 15.1% for the IMAT plans and 24.9% for the CONV4 plans (for CONV2 plans, this was 34.9%). The median dose to the kidneys in the IMAT plans was lower for all patients. The 95th percentile dose for the kidneys was significantly higher for the IMAT plans than for the CONV4 and CONV2 plans (28.2 Gy vs. 22.2 Gy and 22.6 Gy for left kidney, respectively). No relevant differences were found for liver. The gel-measured dose was within clinical planning constraints. IMAT was shown to be deliverable in an acceptable time slot and to produce dose distributions that are more homogeneous than those obtained with a CONV plan, with at least equal sparing of the OARs.
Publisher: IOP Publishing
Date: 05-2009
Publisher: IOP Publishing
Date: 11-2010
Publisher: Elsevier BV
Date: 07-1999
Publisher: IOP Publishing
Date: 26-06-2013
Publisher: Elsevier BV
Date: 10-1998
Publisher: IOP Publishing
Date: 29-07-2011
DOI: 10.1088/0031-9155/56/16/019
Abstract: Spoiled gradient echo pulse (SPGRE) sequences are commonly used in dynamic contrast-enhanced MRI (DCE-MRI) studies to measure the contrast agent concentration in a tissue of interest over time. However, due to improper tuning of the SPGRE parameters, concentration uncertainty can be very high, even at high signal-to-noise ratio in the MR measurement. In this work, an optimization procedure is proposed for selecting the optimal value of the SPGRE-flip angle FA(opt), given the expected concentration range. The optimization condition ensures that every concentration in the assumed range has the lowest possible uncertainty. By decoupling the R(1)- and R*(2)-effects caused by the presence of the contrast agent, a contour plot has been generated from which FA(opt) can be read off for any study design. Investigation of ten recent DCE-MRI studies showed that improper flip angle selection unnecessarily increases the concentration uncertainty, up to 742% and 72% on average for the typical physiological concentration ranges of 0-2 mM in tumour tissue and 0-10 mM in blood, respectively. Simulations show that the reduced noise levels on the concentration curves, observed at the optimal flip angle, effectively increase the precision of the kinetic parameters estimates (up to 82% for K(trans), 82% for ν(e) and 92% for ν(p) in the case of an in idually measured arterial input function (AIF), up to 53% for K(trans), 59% for ν(e) and 67% for ν(p) in the case of a standard AIF). In vivo experiments confirm the potential of flip angle optimization to increase the reproducibility of the kinetic parameter estimates.
Publisher: The Royal Society of Chemistry
Date: 10-07-2020
Publisher: Elsevier BV
Date: 08-2008
DOI: 10.1016/J.JMR.2008.05.010
Abstract: The spin-spin relaxation rate R2 (=1/T2) in hydrogel foams measured by use of a multiple spin echo sequence is found to be dependent on the echo time spacing. This property, referred to as R2-dispersion, originates to a large extent from molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. Another contribution to the R2 relaxation rate is surface relaxation. Numerical simulations are performed to investigate the relation between the foam microstructure (the mean air bubble radius and standard deviation of the air bubble radius) and foam composition properties (such as magnetic susceptibilities, diffusion coefficient and surface relaxivity) at one hand and the R2-dispersion at the other hand. The simulated R2-dispersions of gel foam are in agreement with the measured R2-dispersions. By correlating the R2-dispersion parameters and simulated microstructure properties a semi-empirical relationship is obtained that enables the mean air bubble size to be derived from measured R2-dispersion curves. The R2-derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the feasibility of using 1H R2-dispersion measurements to determine the size of air bubbles in hydrogel foams and of alveoli in lung tissue.
Publisher: IOP Publishing
Date: 07-11-2018
Abstract: Deformable 3D radiation dosimetry is receiving growing interest for the validation of image-guided radiotherapy treatments (IGRT) of moving and deformable targets. Previously, a proof-of-concept of a flexible anthropomorphic 3D dosimeter called 'FlexyDos3D' has been demonstrated. One of the concerns with respect to the FlexyDos3D dosimeter is its dose-response instability. The effect of different formulations of the dosimeter on its stability were investigated. A stable formulation for the dosimeter was found by optimising the ratios of curing agent and base of the silicone matrix between 3% and 4.5% [w/w] curing agent. The effects of elevated curing temperatures and times upon the dosimetric properties were also investigated and the dose-response was found to be independent of curing times for curing times over an hour at 120 °C.
Publisher: IOP Publishing
Date: 10-01-2011
DOI: 10.1088/0031-9155/56/3/007
Abstract: Recently, novel radiochromic leucodye micelle hydrogel dosimeters were introduced in the literature. In these studies, gel measured electron depth dose profiles were compared with ion chamber depth dose data, from which it was concluded that leucocrystal violet-type dosimeters were independent of dose rate. Similar conclusions were drawn for leucomalachite green-type dosimeters, only after pre-irradiating the s les to a homogeneous radiation dose. However, in our extensive study of the radio-physical properties of leucocrystal violet- and leucomalachite green-type dosimeters, a significant dose rate dependence was found. For a dose rate variation between 50 and 400 cGy min(-1), a maximum difference of 75% was found in optical dose sensitivity for the leucomalachite green-type dosimeter. Furthermore, the measured optical dose sensitivity of the leucomalachite green-type dosimeter was four times lower than the value previously reported in the literature. For the leucocrystal violet-type dosimeter, a maximum difference in optical dose sensitivity of 55% was found between 50 and 400 cGy min(-1). A modified composition of the leucomalachite green-type dosimeter is proposed. This dosimeter is composed of gelatin, sodium dodecyl sulfate, chloroform, trichloroacetic acid and leucomalachite green. The optical dose sensitivity amounted to 4.375 × 10(-5) cm(-1) cGy(-1) (dose rate 400 cGy min(-1)). No energy dependence for photon energies between 6 and 18 MV was found. No temperature dependence during readout was found notwithstanding a temperature dependence during irradiation of 1.90 cGy °C(-1) increase on a total dose of 100 cGy. The novel gel dosimeter formulation exhibits an improved spatial stability (2.45 × 10(-7) cm(2) s(-1) (= 0.088 mm(2) h(-1))) and good water/soft tissue equivalence. Nevertheless, the novel formulation was also found to have a significant, albeit reduced, dose rate dependence, as a maximum difference of 33% was found in optical dose sensitivity when the dose rate varied between 50 and 400 cGy min(-1). By pre-irradiating the novel leucomalachite green-type dosimeter to 500 cGy, the apparent difference in dose response between 200 and 400 cGy min(-1) was eliminated, similar to earlier findings. However, a dose response difference of 38% between 50 and 200 cGy min(-1) was still measured. On the basis of these experimental results it is concluded that the leucodye micelle gel dosimeter is not yet optimal for dose verifications of high precision radiation therapy treatments. This study, however, indicates that the dose rate dependence has a potential for improvement. Future research is necessary to further minimize the dose rate dependence through extensive chemical analysis and optimization of the gel formulation. Some insights into the physicochemical mechanisms were obtained and are discussed in this paper.
Publisher: IOP Publishing
Date: 12-2006
Publisher: IOP Publishing
Date: 06-08-2002
DOI: 10.1088/0031-9155/47/17/306
Abstract: The overall performance of polymer gel dosimeters for three-dimensional radiation dosimetry is determined by the temporal and spatial stability of the gels, dose sensitivity and image quality with respect to both systematic and stochastic deviations. The dose resolution (D(p)delta) is determined by the dose sensitivity and the signal-to-noise ratio (SNR) in the dose images. The dose sensitivity can be altered by changing the chemical composition of the polymer gel. The SNR is determined by the scanner and the imaging sequence. In the dose verification of conformal radiotherapy treatments the chosen number of slices may reach a number of 10-20. For these experiments, to obtain a sufficient SNR within a reasonable measurement time using a certain MR scanner, the imaging sequence should be optimized. A few other studies have emphasized the importance of optimizing the imaging sequence with respect to dose resolution (D(p)delta) or SNR but do not give quantitative values for the optimal sequence parameters for scanning a polymer gel dosimeter in three dimensions. In this paper, it is proved that a multiple spin-echo sequence is preferable to a single spin-echo sequence. It is also shown that when using a multiple spin-echo sequence it is not the inter-echo time that should be optimized but the number of echoes. An algebraical expression is derived for the dose resolution in terms of sequence parameters. A mathematical formalism and look-up tables are provided that can be used to optimize both a single and a slice-selective multiple spin-echo sequence to acquire a set of dose images at various locations. The use of the optimization protocol is illustrated by some ex les. The optimization protocol enables the user to derive the optimal sequence parameters to acquire a set of dose maps obtained by quantitative T2 imaging for each polymer gel dosimeter within the shortest time possible.
Publisher: Elsevier BV
Date: 09-1998
DOI: 10.1016/S0167-8140(98)00087-5
Abstract: It was our aim to investigate NMR-based BANG gel dosimetry as a three-dimensional dosimetry technique in conformal radiotherapy. The BANG gel consisting of gelatin, water and co-monomers was first validated in a cylindrical glass flask for a single standard beam. Next, the gel contained in a human neck-shaped cast was used to verify a treatment plan for the conformal irradiation of a concave tumour in the lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions measured using radiographic film. The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, the difference map between gel-measured and film-measured dose distributions revealed a noise component and a more systematic deviation including structural or space-coherent patterns. The mean absolute value of the difference amounted to 8%. A number of possible causes for this deviation are designated. Polymer gel dosimetry in combination with magnetic resonance imaging is a promising method for dosimetric verification of conformal radiotherapy.
Publisher: Springer Science and Business Media LLC
Date: 13-08-2010
DOI: 10.1007/S10334-010-0221-Z
Abstract: Several studies have demonstrated differences in migraine patients when performing (1)H-MRS however, no studies have performed (1)H-MRS in migraine without aura (MwoA), the most common migraine subtype. The aim of this (1)H-MRS study was to elucidate whether any differences could be found between MwoA patients and controls by performing absolute quantification. (1)H-MRS was performed in 22 MwoA patients and 25 control subjects. Absolute quantification was based on the phantom replacement technique. Corrections were made for T (1) and T (2) relaxation effects, CSF content, coil loading and temperature. The method was validated by phantom measurements and in vivo measurements in the occipital visual cortex. After calibration of the quantification procedure and the implementation of the required correction factors, measured absolute concentrations in the visual cortex of MwoA patients showed no significant differences compared to controls, in contrast to relative results obtained in earlier studies. In this study, we demonstrate the implementation of quantitative in vivo (1)H-MRS spectroscopy in migraine patients. Despite rigorous quantification, no spectroscopic abnormalities could be found in patients with migraine without aura.
Publisher: ASME International
Date: 02-1996
DOI: 10.1115/1.2824035
Publisher: IOP Publishing
Date: 26-06-2013
Publisher: IOP Publishing
Date: 06-12-2013
DOI: 10.1088/0031-9155/58/1/19
Abstract: The intra- and inter-batch accuracy and precision of MRI (polyacrylamide gelatin gel fabricated at atmospheric conditions) polymer gel dosimeters are assessed in full 3D. In the intra-batch study, eight spherical flasks were filled with the same polymer gel along with a set of test tubes that served as calibration phantoms. In the inter-batch study, the eight spherical flasks were filled with different batches of gel. For each spherical phantom, a separate set of calibration phantoms was used. The spherical phantoms were irradiated using a three-field coplanar beam configuration in a very reproducible manner. The calibration phantoms were irradiated to known doses to obtain a dose-R₂ calibration plot which was applied on the corresponding R₂ maps of all spherical phantoms on an in idual basis. The intra-batch study showed high dosimetric precision (3.1%) notwithstanding poor accuracy (mean dose discrepancies up to 13.0%). In the inter-batch study, a similar dosimetric precision (4.3%) and accuracy (mean dose discrepancies up to 13.7%) were found. The poor dosimetric accuracy was attributed to a systematic fault that was related to the calibration method. Therefore, the dose maps were renormalized using an independent ion chamber dose measurement. It is illustrated that with this renormalization, excellent agreement between the gel measured and TPS calculated 3D dose maps is achievable: 97% and 99% of the pixels meet the 3%/3 mm criteria for the intra- and inter-batch experiments, respectively. However renormalization will result in significant dose deviations inside a realistically sized anthropomorphic phantom as will be shown in a concurrent paper.
Publisher: IOP Publishing
Date: 05-2017
Publisher: IOP Publishing
Date: 05-2017
Publisher: IOP Publishing
Date: 18-11-2020
Abstract: 4D radiation dosimetry using a highly radiation-sensitive polymer gel dosimeter with real-time quantitative magnetic resonance imaging (MRI) readout is presented as a technique to acquire the accumulated radiation dose distribution during image-guided radiotherapy on an MRI-Linac. Optimized T 2 -weighted Turbo-Spin-Echo (TSE) scans are converted into quantitative Δ R 2 maps and subsequently to radiation dose maps. The concept of temporal uncertainty is introduced as a metric of effective temporal resolution. A mathematical framework is presented to optimize the echo time of the TSE sequence in terms of dose resolution, and the trade-off between temporal resolution and dose resolution is discussed. The current temporal uncertainty achieved with the MAGAT gel dosimeter on a 1 T MRI-Linac is 3.8 s which is an order of magnitude better than what has been achieved until now. The potential of real-time 4D radiation dosimetry in a theragnostic MRI-Linac is demonstrated for two scenarios: an irradiation with three coplanar beams on a head phantom and a dynamic arc treatment on a cylindrical gel phantom using a rotating couch. The dose maps acquired on the MRI-Linac are compared with a treatment plan and with dose maps acquired on a clinical 3 T MRI scanner. 3D gamma map evaluations for the different modalities are provided. While the presented method demonstrates the potential of gel dosimetry for tracking the dose delivery during radiotherapy in 4D, a shortcoming of the MAGAT gel dosimeter is a retarded dose response. The effect of non-ideal radiofrequency pulses resulting from limitations in the specific absorption rate or B 1 -field inhomogeneity on the TSE acquired Δ R 2 values is analysed experimentally and by use of computational modelling with a Bloch simulator.
Publisher: Springer Science and Business Media LLC
Date: 07-09-2010
Publisher: American Physiological Society
Date: 02-2016
Abstract: In people with a history of low back pain (LBP), structural and functional alterations have been observed at several peripheral and central levels of the sensorimotor pathway. These existing alterations might interact with the way the sensorimotor system responds to pain. We examined this assumption by evaluating the lumbar motor responses to experimental nociceptive input of 15 participants during remission of unilateral recurrent LBP. Quantitative T2 images (muscle functional MRI) were taken bilaterally of multifidus, erector spinae, and psoas at several segmental levels (L3 upper and L4 upper and lower endplate) and during several conditions: 1) at rest, 2) upon trunk-extension exercise without pain, and 3) upon trunk-extension exercise with experimental induced pain at the clinical pain-side (1.5-ml intramuscular hypertonic saline injections in erector spinae). Following experimental pain induction, muscle activity levels similarly reduced for all three muscles, on both painful and nonpainful sides, and at multiple segmental levels ( P = 0.038). Pain intensity and localization from experimental LBP were similar as during recalled clinical LBP episodes. In conclusion, unilateral and unisegmental experimental LBP exerts a generalized and widespread decrease in lumbar muscle activity during remission of recurrent LBP. This muscle response is consistent with previous observed patterns in healthy people subjected to the same experimental pain paradigm. It is striking that similar inhibitory patterns in response to pain could be observed, despite the presence of preexisting alterations in the lumbar musculature during remission of recurrent LBP. These results suggest that motor output can modify along the course of recurrent LBP.
No related grants have been discovered for Yves De Deene.