Investigation of dosimetric characteristic of NIPAM polymer gel using x-ray CT

Introduction: Polymer gel dosimeters contain chemical materials sensitive to the radiation which are polymerized by the radiation as a function of absorbed dose. So information of spatial dose distribution can be extracted by imaging from irradiated gel. Among imaging techniques, computed tomography (CT) poses as an attractive method because of practical advantages such as accessibility, easy and speed of image acquisition and dose-response is stable and relatively insensitive to environmental factors. The most major limitation that prevents this method from clinical application is high noise in images that obtained from this system. In this study, imaging parameters in purpose of noise reduction of CT image and then concentration impact of each component of NIPAM gel on dose-response have been investigated. Finally, imaging appropriate parameter for read-out gel and NIPAM gel composition with maximum dose sensitivity for X-ray CT imaging of gel are introduced which would be an attempt to facilitate the application of this method in clinical applications. Materials and Methods: NIPAM gel was made according to the method described by Chain et al.; after the preparation, they were poured inside the vials and retained in refrigerator at 4° C for 24 hours. After this stage, gel-contained vials were subject to irradiation with a cobalt- 60 machine in dose ranging 0-50 Gy. After this, the irradiated gels were imaged with the CT scan. Ultimately, resultant DICOM images were processed in MATLAB and CT mean value of the vials as well as corresponding standard deviation were taken. Results: In this study, the imaging protocol for the Siemens SOMATOS PLUSE 4 CT scanner was investigated to reduce image noise. Parameters of 140 kV, 206 mAs, 5 mm slice thickness and 10AH kernel-based reconstruction are suitable options to evaluate polymer gel using aforementioned device. A number of 25 images for averaging could reduce the noise to an acceptable level. NIPAM polymer gel with 5% gelatin, 10 mM THPC, 17% NIPAM monomer and 3% BIS indicates the biggest changes in the CT value at 0-50 Gy ranging dose for CT imaging method. Conclusion: high-percentage monomer NIPAM polymer gel shows high sensitivity to CT imaging method and has good reproducibility and linear response range; as well as since appropriately choosing imaging protocols and thus achieved image processing reduce the noise to an acceptable level, standing these factors along with applicable advantages of CT method such as accessibility in radiotherapy centers, ease and speed of the image provision, as well as being low-cost exhibits that NIPAM polymer gel dosimeter with CT imaging method has a good potential to become a three-dimensional dosimeter in radiation therapy clinical applications.