An x-ray CT polymer gel dosimetry prototype: II. Gel characterization and clinical application (original) (raw)
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Journal of Applied Clinical Medical Physics, 2002
This study presents a 3D dose mapping of complex dose distributions using an x-ray computed tomography ͑CT͒ polymer gel dosimetry technique. Two polyacrylamide gels ͑PAGs͒ of identical composition were irradiated with the same four arc stereotactic treatment to maximum doses of 15 Gy ͑PAG1͒ and 8 Gy ͑PAG2͒. The PAGs were CT imaged using a previously defined protocol that involves image averaging and background subtraction to improve image quality. For comparison with the planned isodose distribution, the PAG images were converted to relative dose maps using a CT number-dose calibration curve or simple division. The PAG images were then co-registered with the planning CT images in the BrainLab ® treatment planning software which automatically provides reconstructed sagittal and coronal images for 3D evaluation of measured and planned dose. The hypointense high dose region in both sets of gel images agreed with the planned 80% isodose contour and was shifted by up to 1.5 and 3.0 mm in the axial and reconstructed planes, respectively. This demonstrates the ability of the CT gel technique to accurately localize the high dose region produced by the stereotactic treatment. The resulting agreement of the measured relative dose volume for PAG1 was within 3.0 mm for the 50% and 80% isodose surfaces. However, the dose contrast was too low in PAG2 to allow for accurate definition of measured relative dose surfaces. Thus, a PAG should be irradiated to higher doses if quantitative relative dose information is required. Unfortunately, this implies use of an additional PAG and its CT number dose response since doses greater than 8 -10 Gy fall outside the linear regions of the response.
Assessment of the effects of CT dose in averaged x-ray CT images of a dose-sensitive polymer gel
Journal of Physics: Conference Series, 2015
The signal-to-noise ratio achievable in x-ray computed tomography (CT) images of polymer gels can be increased by averaging over multiple scans of each sample. However, repeated scanning delivers a small additional dose to the gel which may compromise the accuracy of the dose measurement. In this study, a NIPAM-based polymer gel was irradiated and then CT scanned 25 times, with the resulting data used to derive an averaged image and a "zero-scan" image of the gel. Comparison between these two results and the first scan of the gel showed that the averaged and zero-scan images provided better contrast, higher contrast-tonoise and higher signal-to-noise than the initial scan. The pixel values (Hounsfield units, HU) in the averaged image were not noticeably elevated, compared to the zero-scan result and the gradients used in the linear extrapolation of the zero-scan images were small and symmetrically distributed around zero. These results indicate that the averaged image was not artificially lightened by the small, additional dose delivered during CT scanning. This work demonstrates the broader usefulness of the zero-scan method as a means to verify the dosimetric accuracy of gel images derived from averaged x-ray CT data.
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This work presents an approach to the fast determination of a medical accelerator irradiation isocenter as a quality assurance (QA) procedure in radiotherapy. The isocenter determination tool is the tissue equivalent high-resolution 3D polymer gel dosimeter (PABIGnx) in a dedicated container combined with kilovoltage imaging systems and the polyGeVero-CT software package (v. 1.2, GeVero Co., Poland). Two accelerators were employed: Halcyon and TrueBeam (Varian, USA), both equipped with cone beam computed tomography (CBCT) and iterative reconstruction CBCT (iCBCT) algorithms. The scope of this work includes: (i) the examination of factors influencing image quality (reconstruction algorithms and modes), radiation field parameters (dose and multi-leaf collimator (MLC) gaps), fiducial markers, signal averaging for reconstruction algorithms and the scanning time interval between consecutive scans, (ii) the examination of factors influencing the isocenter determination, image processing (...
Isopropanol-based polymer gel dosimeters for use with x-ray CT imaging
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Considerations for x-ray CT polymer gel dosimetry A Jirasek Characterization of the essential dosimetric properties of cosolvent-free polymer gel dosimeters: Recent progress in x-ray CT based normoxic polymer gel dosimetry H Johnston, M Hilts, J Carrick et al. Fundamentals of gel dosimeters K B McAuley and A T Nasr Effects of glycerol on gel response A Jirasek, M Hilts, A Berman et al. An x-ray CT polymer gel dosimetry prototype: II. Gel characterization and clinical application H Johnston, M Hilts, J Carrick et al.
Physica Medica, 2008
Polymer gel dosimetry undertaken on a multislice CT scanner provides an alternative method to conventional dosimetry measurements. Polymer gel dosimeters were used to measure CT radiation doses and compared to TLD and ionization chamber measurements in different diameter phantoms. CTDI was investigated for each of these phantoms for a range of mAs (100e400 mAs), tube voltage (100e135 kV) and nominal slice width (2e32 mm). Linear fits of the CTDI values for mAs show for the smallest phantom diameter an increase in CTDI of 60% for both TLD and polymer gel dosimeters. A similar increase in CTDI of 50% at 100 kVp and 100% for 135 kVp was also noted. It was also shown that slice width variation measured with either polymer gel or TLD was greatest with the smallest slice widths. In summary, it was found that polymer gels can be used as an alternative dosimeter to TLD for the determination of SWDP and subsequent CTDI calculations. ª 2007 Published