Author: Petr Bruza, Jeremy Eric Hallett, Brian W Pogue 👨🔬
Affiliation: Thayer School of Engineering, Dartmouth College, University of Wisconsin - Madison, University of Wisconsin-Madison 🌍
Purpose: Cherenkov imaging is a powerful tool for localizing entrance and exit dose during radiotherapy treatment. The current implementation uses an intensified-CMOS (iCMOS) camera allowing for high-speed time gating to suppress ambient light and boost Cherenkov signal above the read noise. This setup is highly subject to stray x-rays and read noise, while experiencing limited resolution due to the size of the intensifier’s microchannels. To avoid these limitations, a quantum-CMOS (qCMOS) camera was considered. This technology drastically improves the read noise (<0.5 electron rms) by reducing the pixel readout transistor input capacitance. A non-intensified system inherently improves image resolution and potentially reduces manufacturing cost making it an attractive clinical option.
Methods: A qCMOS camera (Orca Quest2, Hamamatsu) was compared against an iCMOS camera with similar settings. A phantom mimicking human skin tissue was placed under the head of a Varian Truebeam linac and irradiated with 6 and 10 MV 15x15 photon beams. Cameras were placed ~1.75m away, triggered with a scintillator circuit. The exposure time per frame was varied and the Cherenkov signal to background noise ratio (SNR) was measured for each exposure time. Lastly, qCMOS images of a water tank were acquired and compared using a 3%/3mm gamma test against the projection view of the Raystation computed dose to test dosimetric capabilities.
Results: SNR analysis suggests the qCMOS camera is read-noise-limited while the iCMOS system is shot-noise-limited, suggesting that longer exposure time per frame results in greater noise improvement for the qCMOS camera. qCMOS SNR is superior beyond an exposure time of ~23ms per frame. Additionally, the gamma pass rate between the computed and imaged water dose was found to be >94%.
Conclusion: The noise reduction with qCMOS technology along with the inherent cost and resolution benefits associated with non-intensified systems may guide the field into the non-intensified regime.