Room: Track 1
Purpose: By collecting the photons Compton-scattered out of the megavoltage therapy beam, a scatter image of the treated volume may be captured to identify the tumor position. Due to the low number of scattered photons compared to direct imaging, implementation is challenging. Here, a prototype scatter imaging camera is characterized and improvements are explored.
Methods: The spatial resolution and relative sensitivity of a pinhole and parallel-hole camera were experimentally characterized using a 0.5 cm diameter scattering sphere irradiated with a 6 MV FFF therapy beam. For the pinhole camera, pinholes of various diameters (3, 4, 5 mm) were characterized. Three different parallel-hole collimator configurations (Siemens LEHR, MELP, 2 stacked MELPs) were also characterized. To increase sensitivity, a detector was constructed of 2 mm pitch x 20 mm length BGO crystal scintillator arrays read-out by an amorphous silicon (a-Si) detector. The operation of the BGO and 700 µm CsI detectors were compared through contrast-to-noise (CNR) measurements.
Results: For the prototype pinhole camera, point spread functions (PSFs) were measured with full-width half-maximum (FWHM) widths of 4 - 8 mm (3 - 5 mm diameter pinholes). As the diameter is decreased (5 to 3 mm), the sensitivity decreased by 50%. At a distance of 45 cm, LEHR, MELP, and MELPx2 parallel-hole collimators display scatter PSFs with FWHM of 29, 42, and 20 mm, and respective sensitivities 22x, 26x, and 4x that of the 5 mm pinhole. The BGO detector displayed a 1.8x increase in CNR over the CsI detector.
Conclusion: Methods for characterizing scatter cameras were developed. For the pinhole collimator, there is a trade-off between sensitivity and spatial resolution. Available parallel-hole collimators maintain high sensitivity at large 50 cm distances. Our data suggest that larger scintillator crystals coupled to modified a-Si detectors improve sensitivity and may facilitate clinical implementation of the technique.