A bench-top megavoltage fan-beam CT using CdWO4-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 CdWO4 scintillators coupled to photodiodes. Each CdWO4 crystal is 2.75 x 8 x 10 mm3. 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 (R2) 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.
Rathee
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Department of Medical Physics, Cross Cancer Institute; Department of Oncology, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G IZ2, Canada.
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Monajemi
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Rickey
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Fallone
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D004867:Equipment Design; D019544:Equipment Failure Analysis; D008904:Miniaturization; D010777:Photochemistry / Q000295:instrumentation; D012588:Scintillation Counting / Q000295:instrumentation*; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*; D014057:Tomography, X-Ray Computed / Q000295:instrumentation* / Q000379:methods; D014159:Transducers