A bench‐top megavoltage fan‐beam CT using Cd W O 4 ‐photodiode detectors. I. System description and detector characterization

We describe the components of a bench‐top megavoltage computed tomography (MVCT) scanner that uses an 80‐element detector array consisting of Cd W O 4scintillators coupled to photodiodes. Each Cd W O 4crystal is 2.75 × 8 × 10mm 3 . The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient ( R 2 ) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by this detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detector's large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h ; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation < 0.5 % ); however, any smaller permanent damage could not be assessed.