Development of a CMOS-based optical computed tomography system (CMOS-OCT) for 3D radiotherapy dosimetry

Optical Computed Tomography (OCT) was developed as a 3D imaging system for measuring radiotherapy dose delivered to radiochromic dosimeters. The imaging system reconstructs dose distributions delivered to a radiochromic dosimeter such as PRESAGE that changes its colour when irradiated. Current optical CT systems in the market are based on CCD image sensor and have slow imaging speed. The study exploited recent advances in CMOS image sensor (CIS) technology to improve the imaging speed of the OCT system. The study characterised the components in the in-house developed CMOS-based Optical CT (CMOS-OCT) imaging system and investigates the feasibility of the system for imaging 3D radiochromic dosimeters. A green dye solution of various concentration was used to mimic colour variation in a PRESAGE radiochromic dosimeter from its response to radiation dose. The solution was filled in a plastic cylinder to study the linearity and uniformity of the system. A rotary stage was constructed using a stepper motor to hold and rotate the dosimeter between the CIS and a large area LED. The components of the imaging system were integrated and controlled using LabVIEW (National Instrument, Austin, TX). A graphical user interface (GUI) was also developed to acquire projection images of the dosimeter. The measured field of view (FOV) of the CIS is 125 mm by 90 mm that can cover the whole PRESAGE dosimeter. A projection image is captured at every 1.8 degree rotation of the dosimeter, at every second, that amounted to 200 projection images for a 360 degree rotation. Current limitation of the imaging speed is the rotation speed of the motor of 1.8 degree per second which can be improved with upgrading the stepper motor. At an imaging speed only limited by the maximum sensor frame rate of 28.5 fps, the scanning time will be reduced to 7 s compared to 200 s. The results show that the system is capable of capturing the projection images of a 3D translucent object with good linearity and uniformity. Further experiments will be carried out to optimise the development of the system to reconstruct dose distributions in 3D from a PRESAGE dosimeter.