Enhanced Megavoltage Imaging for Radiotherapy by Light-Field Imaging of Scintillators

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. In addition, conventional EPIDs do not provide any form of spectral decomposition, and the megavoltage detectors that do are expensive. Light-field imaging offers a way to image a volumetric light source like a scintillator by recording the emitted 4D light field and computationally refocusing from a single exposure. Computational reconstruction allows for out-of-focus light to be removed, allowing for an order of magnitude thicker scintillators to be imaged while preserving resolution and allowing for spectral extraction. A forward model was developed to simulate light-field cameras and scintillators to explore the potential of light-field cameras in scintillator imaging. With the forward model, DQEs and spectral performance were evaluated. A prosumer light-field camera was used experimentally and evaluated in terms of DQE and spectral performance to evaluate the simulation model. In addition, a conventional scientific camera was used experimentally. The simulations revealed that light-field cameras provide an advantage over conventional cameras for especially thick scintillators; however, the noise from reconstruction is the limiting factor preventing the light-field camera from being competitive with multilayered EPIDs for thinner scintillators. In addition, the spectral performance of light-field cameras for X-ray-based imaging was marginal at 225 kV and negligible in megavoltage images. The scientific camera results were validated by simulation, and the prosumer lightfield camera was shown to be unsuitable for scientific studies