AbstractID: 9549 Title: Use of CCD cameras for Megavoltage Cone Beam CT
A high-resolution CCD based EPID capable of matching the high spatial resolution and SNR (signal to
noise ratio) of MVCT imaging devices was developed. The system uses a CCD camera (1300 x 1030),
optical components and a modified scintillator screen to generate high-resolution images.
The evaluation of image quality and resolution was done with a Pips QC3 phantom. Contrast to noise ratio
(CNR) was compared against a-Si detectors (1024x1024). The phantom was placed at the linac isocenter,
with the detector 40cm below. The measured f50 for the Siemens a-Si flat panel and HRV being 0.49
lp/mm, and 0.43 lp/mm respectively. Flat panel images were corrected for Offset, Gain and Defective
pixels. HRV images were not corrected.
Physical and Integration characteristics of the system, small pixel sizes, adjustable camera lens aperture
combined with thicker scintillator screens resulted in an increase SNR without sensor saturation during
treatment imaging. The same concept can be achieved using hardware pixel binning. A new electronic
readout is implemented in the HRV control circuit that will synchronize the image acquisition with the
linear accelerator and thereby increases the SNR.
Advantages for MVCT and standard imaging are (1) faster frame readout, (2) the camera lens aperture can
be fully opened for the low dose imaging (MVCT) and partially opened for therapy imaging to avoid
saturation; (3) HRV EPID has an adjustable FOV versus a fixed FOV of a-Si EPIDs; (4) The detector is
not exposed to direct radiation fields.
Research sponsored by Siemens