High resolution polymer gel dosimetry by parameter selective MR‐microimaging on a whole body scanner at 3 T

Berg, Andreas; Ertl, Adolf; Moser, Ewald

doi:10.1118/1.1358304

Cited by 44 publications

(34 citation statements)

References 22 publications

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“…This equipment offers high sensitivity, in-plane resolution, and RF homogeneity, as well as a thin slice thickness, and has been previously used for highresolution polymer gel dosimetry and diffusivity imaging for detection of arthritis-associated degenerative changes. 26,27 We chose a T2-weighted multislice multiecho ͑MSME͒ MR sequence with the following acquisition parameters as our reference standard: TR= 4000 ms, TE= 25 ms, NA= 3, SBW= 13 kHz, flip angle= 90°, matrix size ͑MTX͒ = 256 ϫ 256, FOV= 30ϫ 30 mm 2 , slice thickness= 1 mm, no interslice gap, and 20 consecutive axial slices per scan. For each of the four acquisition parameters ͑NA, TR, TE, and SBW͒, whose effects on texture features of different categories were investigated, we used five different values for image acquisition ͑see below͒, while all other parameters were kept constant.…”

Section: Iib Mr Imagingmentioning

confidence: 99%

Effects of MRI acquisition parameter variations and protocol heterogeneity on the results of texture analysis and pattern discrimination: An application‐oriented study

et al. 2009

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MRI texture features are generally considered to be sensitive to variations in signal-to-noise ratio and spatial resolution, which represents an obstacle for the widespread clinical application of texture-based pattern discrimination with MRI. This study investigates the sensitivity of texture features of different categories (co-occurrence matrix, run-length matrix, absolute gradient, autoregressive model, and wavelet transform) to variations in the number of acquisitions (NAs), repetition time (TR), echo time (TE), and sampling bandwidth (SBW) at different spatial resolutions. Special emphasis was placed on the influence of MRI protocol heterogeneity and implications for the results of pattern discrimination. Experiments were performed using two polystyrene spheres and agar gel phantoms with different nodular patterns. T2-weighted multislice multiecho images were obtained using a 3.0 T scanner equipped with a microimaging gradient insert coil. Linear discriminant analysis and k nearest neighbor classification were used for texture-based pattern discrimination. Results show that texture features of all categories are increasingly sensitive to acquisition parameter variations with increasing spatial resolution. Nevertheless, as long as the spatial resolution is sufficiently high, variations in NA, TR, TE, and SBW have little effect on the results of pattern discrimination. Texture features derived from the co-occurrence matrix are superior to features of other categories because they enable discrimination of different patterns close to the resolution limits for the smallest structures of physical texture even for datasets that are heterogeneous with regard to different acquisition parameters, including spatial resolution.

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Section: Iib Mr Imagingmentioning

confidence: 99%

Effects of MRI acquisition parameter variations and protocol heterogeneity on the results of texture analysis and pattern discrimination: An application‐oriented study

et al. 2009

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“…12 Polymer gel dosimetry seems to be able to address the problems related to small field dosimetry 26 since the method is dose integrating and the gel substance ensures tissue equivalence, 27 absence of field perturbation since the gel comprises both the phantom and detector material, excellent ͑submillimeter͒ spatial resolution, and very small sensitive volumes. [28][29][30][31][32][33] Their performance for beam profile and depth dose measurements of narrow stereotactic beams has been evaluated and discussed in a previous work 26 without however performing a detailed investigation for relative OF measurements.…”

Section: Introductionmentioning

confidence: 99%

Relative output factor measurements of a 5 mm diameter radiosurgical photon beam using polymer gel dosimetry

et al. 2005

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Besides the fine spatial resolution inherent in polymer gel-magnetic resonance imaging (MRI) dosimetry, the method also features the potential for multiple measurements of varying sensitive volume in a single experiment by integrating results in MRI voxels of finite dimensions (i.e., in plane resolution by slice thickness). This work exploits this feature of polymer gel dosimetry to propose an experimental technique for relative output factor (OF) measurements of small radiosurgical beams. Two gel vials were irradiated with a 5 and 30 mm diameter 6 MV radiosurgery beam and MR scanned with the same slice thickness and three different in plane resolutions. Using this experimental data set, 5 mm OF measurements with the PinPoint ion chamber are simulated by integrating results over a sensitive volume equal to that of the chamber. Results are found in agreement within experimental uncertainties with actual PinPoint measurements verifying the validity of the proposed experimental procedure. The polymer gel data set is subsequently utilized for OF measurements of the 5 mm beam with varying sensitive volume to discuss the magnitude of detector volume averaging effects. Seeking to correct for volume averaging, results are extrapolated to zero sensitive volume yielding a 5 mm OF measurement of (0.66+/-5%). This result compares reasonably with corresponding ionometric and radiographic film measurements of this work and corresponding, limited, data in the literature. Overall, results suggest that polymer gel dosimetry coupled with the proposed experimental procedure helps overcome not only tissue-equivalence and beam perturbation implications but also volume averaging and positioning uncertainties which constitute the main drawback in small radiosurgical beam dosimetry.

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“…Several dose verification experiments using gel dosimetry in Gamma Knife based radiosurgery have been published. 7,29,30 The voxel size in these studies was between 0.1ϫ 0.1ϫ 1.0 mm 3 and 0.5ϫ 0.5ϫ 2.0 mm 3 , which compares to the 1 ϫ 1 ϫ 1 mm 3 imaging resolution obtained by optical CT imaging. 13 However, these dose verifications are based on single isocenter irradiations only, which are less representative from a clinical point of view.…”

Section: Customization For Imrtmentioning

confidence: 55%

Introducing gel dosimetry in a clinical environment: Customization of polymer gel composition and magnetic resonance imaging parameters used for 3D dose verifications in radiosurgery and intensity modulated radiotherapy

et al. 2007

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Radiation sensitive gels have been used as dosimeters for clinical dose verification of different radiation therapy modalities. However, the use of gels is not widespread, because careful techniques are required to achieve the dose precision and accuracy aimed for in clinical dose verification. Here, the introduction of gel dosimetry in a clinical environment is described, including the whole chain of customizations and preparations required to introduce magnetic resonance (MR) based gel dosimetry into clinical routine. In order to standardize gel dosimetry in dose verifications for radiosurgery and intensity modulated radiotherapy (IMRT), we focused on both the customization of the gel composition and of the MR imaging parameters to increase its precision. The relative amount of the components of the normoxic, methacrylic acid based gel (MAGIC) was changed to obtain linear and steep dose response relationships. MR imaging parameters were customized for the different dose ranges used in order to lower the relative standard deviation of the measured transversal relaxation rate (R2). An optimization parameter was introduced to quantify the change in the relative standard deviation of R2 (sigma(R2,rel)) taking the increase in MR time into account. A 9% methacrylic acid gel customized for radiosurgery was found to give a linear dose response up to 40 Gy with a slope of 0.94 Gy(-1) s(-1), while a 6% methacrylic acid gel customized for IMRT had a linear range up to 3 Gy with a slope of 1.86 Gy(-1) s(-1). With the help of an introduced optimization parameter, the mean sigma(R2,rel) was improved by 13% for high doses and by 55% for low doses, without increasing MR time to unacceptable values. A mean dose resolution of less than 0.13 Gy has been achieved with the gel and imaging parameters customized for IMRT and a dose resolution from 0.97 Gy (at 5 Gy) to 2.15 Gy (at 40 Gy) for the radiosurgery dose range. The comparisons of calculated and measured relative 3D dose distributions performed for radiosurgery and IMRT showed an acceptable overall correlation. The gamma criterion for the radiosurgery verification with a voxel size of 1.5 x 1.5 x 1.5 mm3 was passed by 96.8% of the voxels (1.5 mm distance, 8% in dose). For the IMRT verification using a voxel size of 1.25 x 1.25 x 5 mm3 the gamma criterion was passed by 50.3% of the voxels (3 mm distance, 3% dose uncertainty). Using dedicated data analysis and visualization software, MR based normoxic gel dosimetry was found to be a valuable tool for clinically based dose verification, provided that customized gel compositions and MR imaging parameters are used. While high dose precision was achieved, further work is required to achieve clinically acceptable dose accuracy.

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High resolution polymer gel dosimetry by parameter selective MR‐microimaging on a whole body scanner at 3 T

Cited by 44 publications

References 22 publications

Effects of MRI acquisition parameter variations and protocol heterogeneity on the results of texture analysis and pattern discrimination: An application‐oriented study

Effects of MRI acquisition parameter variations and protocol heterogeneity on the results of texture analysis and pattern discrimination: An application‐oriented study

Relative output factor measurements of a 5 mm diameter radiosurgical photon beam using polymer gel dosimetry

Introducing gel dosimetry in a clinical environment: Customization of polymer gel composition and magnetic resonance imaging parameters used for 3D dose verifications in radiosurgery and intensity modulated radiotherapy

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