Polymer gel dosimetry using x-ray computed tomography: a feasibility study⁴

Abstract

A new three-dimensional dosimetry technique using x-ray computed tomography (CT) to analyse polymer gels is proposed. The CT imaging is sensitive to radiation-induced density changes that occur within irradiated polyacrylamide gel (PAG). In this preliminary study, a CT imaging protocol is developed to optimize CT images of PAG; the response of PAG CT number to dose (N(CT)-dose response) and the reproducibility of the response are investigated, and the use of CT to analyse PAG is compared with MRI. Experiments were conducted using two 1.5 l cylindrical PAG phantoms (3% acrylamide, 3% bis and 5% gelatin by weight), one irradiated with four intersecting 10 MV photon beams and the other with 10 sets of 6 MV parallel opposed circular radiosurgery fields. The final imaging protocol involves using optimum CT parameters (120 kVp and 200 mAs for our GE HiSpeed CT/i scanner), image averaging and background subtraction. The N(CT)-dose response is reproducible, linear up to 800-1000 cGy and is relatively insensitive to the gel temperature during imaging. The dose resolution is approximately 50 cGy for an image thickness of 10 mm. Despite the low dose resolution, preliminary results indicate that this CT technique provides accurate localization of high dose gradients such as those observed in stereotactic radiosurgery. Thus, given the availability and speed of CT scanners, the technique has the potential to be a valuable and practical 3D dose verification tool in radiation therapy.

CT gel dosimetry technique: Comparison of a planned and measured 3D stereotactic dose volume

“…The $\Delta N_{\mathit{\text{CT}}}$‐dose response curve for a PAG calibration gel, reproduced here from Hilts et al 19 $\Delta N_{\mathit{\text{CT}}}$ increases approximately linearly with dose up to $\sim 10-12$ Gy and then increases more slowly at higher doses.…”

“…An mAs of 200 was chosen to maximize signal while maintaining the smaller filament mode which produced less image blurring. The quality of this high dose region image set was then further improved by using an image averaging and background subtraction protocol, described in detail in a previous paper 19 . In brief, for each PAG, 16 images were obtained at each scan position throughout the high dose region and then averaged to reduce image noise.…”

“…The final image quality and resolution were affected by the chosen slice thickness and spacing 19 . A slice thickness of 3 mm was chosen to produce an acceptable signal to noise while minimizing the imaging time and tube heating associated with acquiring all of the images used for averaging.…”

“…Most recently, x‐ray computed tomography (CT), a prevalent imaging modality in radiotherapy, has been proposed as a technique for extracting dose information from PAG 19 In the paper, the feasibility of the technique, as well as a protocol for CT imaging, was outlined. Although the signal‐to‐noise ratio (SNR) observed in the CT images was relatively low, the imaging protocol maximized the useful dose information obtainable from the images.…”

Measurement of a PAGAT gel dosimeter by ultrasound computed tomography

“…Following irradiation, the physical properties of these radiosensitive gels undergo changes that can be measured using various medical imaging equipment . Traditionally this imaging has utilised either optical computed tomography (CT) [1,2] magnetic resonance imaging [3][4][5] or x-ray CT [6,7]. Previous studies have examined the feasibility of ultrasound based CT to study dose response in PAG dosimeters [8][9][10][11].…”