Room: AAPM ePoster Library
Purpose: Megavoltage imaging during radiotherapy provides a beam’s eye view of patient anatomy, allowing for portal dosimetry and real-time tumor position tracking without additional dose. However, current electronic portal imaging devices (EPIDs) suffer poor contrast-to-noise ratios because of their limited (1-2%) detective quantum efficiency (DQE). Conventional EPIDs use a thin layer (<1mm) of scintillator to convert x-rays to visible light that is then detected by an array of photodetectors. The scintillator must be thin in order to reduce blurring caused by light spread. We are seeking order-of-magnitude increases in DQE by increasing the x-ray detection layer to 10-30mm using a transparent scintillator, capturing the emitted 4D light field using a prosumer light-field (LF) camera, and performing computational refocusing to maintain resolution.
Methods: A Lytro Illum LF camera was used to image a 22mm-thick LKH-5 scintillator. To simulate megavoltage beam like conditions, the highest energy setting (225kVp) of a small animal irradiator, XRAD225c, was used. Software was used to control the Illum remotely and to process the camera’s raw data into a microlens array (MLA) image. From the MLA images, focal stacks were reconstructed computationally with ray-based optics. Extended depth-of-field images were then constructed from the focal stacks. Images of a bar phantom and objects with different attenuation properties were taken to assess the resolution and the refocusing capabilities.
Results: The Illum was able to perform refocusing across the scintillator’s full thickness. Focal stacks with 8 focal layers containing the scintillator were produced. Refocused images displayed signs of photon production/travel throughout the scintillator at a resolution of ~8.5pixels/mm.
Conclusion: The images captured by the LF camera would be difficult to achieve with a conventional camera in the same acquisition time and setup constraints. This demonstrates the basic feasibility of the approach and prepares us for future testing using LINAC-generated MV beams.
Funding Support, Disclosures, and Conflict of Interest: Grant funding R21EB028103, NIH T32EB002103 Conflicts: Patrick La Riviere receives research funding from Accuray, Inc (unrelated to this research) and is a consultant for and has stock options in MetriTrack, Inc. (unrelated to this research)