Designing and developing an in-house CCD based optical CT scanner for gel dosimetry

Introduction: Measurement of the complex 3D radiation dose distribution created with advanced radiotherapy techniques such as IMRT and SRS became very important. Polymer gel dosimetry (PGD) is proposed method to recording this dose distribution. MRI, X-ray CT, ultrasound, spectrophotometry and optical CT scanners can be used for readout of Polymer gel dosimeters. Although MRI is gold standard readout instrument, but it is costly and time- consuming. Optical CT is faster and has very lower cost than other methods. The aim of this work is designing and developing an in-house fast CCD based optical computed tomography scanner to extract the 3D radiation dose distribution that recorded in irradiated PGDs. Materials and Methods: in this work we used an array of high power LEDs by 650nm wave length as broad beam light source and a CCD camera as large area detector. The image projections of test object that immersed in a water tank and its rotational movement were controlled by a stepper motor was obtained. Image reconstruction carried out in MATLAB environment with the Filtered back projection method.   Results: Reconstructed image quality is very promising. And it is comparable to results of the other works. Contrast to Noise Ratio (CNR) of reconstructed images without any filters in the back projection algorithm was 0.41. Using ramp filter with 0.2 cutoff frequency, gained a 1.1 in CNR and using Butterworth filter with cutoff frequency of 0.3 gained a CNR of 1.3. Increasing cutoff frequency for Butterworth filter will degrade noise properties of reconstructed image and CNR goes down to 0.82, which is comparable with similar works. Conclusion: Optical CT readout system is considered one of the best options for gel readings. If we can overcome the problems of reconstructed images with light rays, this system can be useful for fast and accurate readout of irradiated polymer gel dosimeters.