SU‐F‐T‐434: Development of a Fan‐Beam Optical Scanner Using CMOS Array for Small Field Dosimetry

Abstract

Purpose: To design and construct a second generation optical computed tomography (OCT) system using a fan‐beam with a CMOS array detector for the 3D dosimetry with polymer gel and radiochromic solid dosimeters. The system was specifically designed for the small field dosimetry. Methods: The optical scanner used a fan‐beam laser, which was produced from a collimated red laser beam (λ=620 nm) with a 15‐degree laser‐line generating lens. The fan‐beam was sent through an index‐matching bath which holds the sample stage and a sample. The emerging laser light was detected with a 2.54 cm‐long CMOS array detector (512 elements). The sample stage rotated through the full 360 degree projection angles at 0.9‐degree increments. Each projection was normalized to the unirradiated sample at the projection angle to correct for imperfections in the dosimeter. A larger sample could be scanned by using a motorized mirror and linearly translating the CMOS detector. The height of the sample stage was varied for a full 3D scanning. The image acquisition and motor motion was controlled by a computer. The 3D image reconstruction was accomplished by a fan‐beam reconstruction algorithm. All the software was developed inhouse with MATLAB. Results: The scanner was used on both PRESAGE and PAGAT gel dosimeters. Irreconcilable refraction errors were seen with PAGAT because the fan beam laser line refracted away from the detector when the field was highly varying in 3D. With PRESAGE, this type of error was not seen. Conclusion: We could acquire tomographic images of dose distributions by the new OCT system with both polymer gel and radiochromic solid dosimeters. Preliminary results showed that the system was more suited for radiochromic solid dosimeters since the radiochromic dosimeters exhibited minimal refraction and scattering errors. We are currently working on improving the image quality by thorough characterization of the OCT system.

Three-dimensional radiation dosimetry using polymer gel and solid radiochromic polymer: From basics to clinical applications

“…A mixture of Clear Poly A and B (Smooth-On Inc., Easton, PA) can cast in any shape. By adding LMG, this material can be quickly turned into SPD[14]. Figure 2 shows the SPD manufactured in-house.…”

“…For an even quicker scan, the third generation OCT uses a broad cone beam of laser light, either parallel beam or a divergent beam in the object, and a charge coupled detector camera as the sensor. Some researchers proposed a fan-beam type scanning system by generating a horizontal fan beam[14,18]. OCT measures the attenuation of photons, and this is represented by the optical density (OD).…”