An eight-channel detector for an /sup 192/Ir brachytherapy source-based computed tomography scanner

et al. 2006

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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.

Section: Iia1 Detector Arraymentioning

confidence: 99%

Section: Iia2 Detector Electronics and Multiplexing Unitmentioning

confidence: 99%

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

Rathee

et al. 2006

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“…As seen in this figure, the receptor consists of ten separate blocks of bonded eightelement CdWO 4 arrays. The components of these blocks have been described earlier [20][21][22] and the bonding process of the eight-element CdWO 4 crystal array discussed in detail. 23 Briefly, each block, consisting of eight 0.275ϫ 0.8 ϫ 1.0 cm 3 CdWO 4 crystals ͑Rexon Components Inc., Beachwood, OH͒, is constructed by bonding the crystals in white gelcoat ͑Ashland chemical type 1 polyester gelcoat͒ commonly used as the outer coat for fiberglass boats.…”

Section: Image Receptormentioning

confidence: 99%

A bench‐top megavoltage fan‐beam CT using ‐photodiode detectors. II. Image performance evaluation

Fallone

et al. 2006

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Megavoltage computed tomography (MVCT) is a potential imaging tool for positioning and dose delivery verification during image guided radiotherapy. The problem with many MVCT detectors, however, is their low detective quantum efficiency (DQE) which leads to poor low contrast resolution (LCR) and high image noise. This makes separating the tumors from the soft tissue background difficult. This manuscript describes the imaging performance of our bench-top MVCT scanner that uses an 80-element detector array consisting of CdWO4-photodiode elements with a DQE of 19% in 6 MV and 26% in Co60 beams [T. T. Monajemi, S. Steciw, B. G. Fallone, and S. Rathee, "Modelling scintillator-photodiodes as detectors for megavoltage CT," Med. Phys. 31, 1225-1234 (2004)] at zero frequency. The imaging experiments presented were carried out mainly in a Co60 teletherapy unit, while the beam hardening characteristics of the system were also presented for a 6 MV beam. During image evaluation, persistent ring artifacts, caused by air gaps at the ends of the eight-element detector blocks, were removed by using a calibration procedure. The measured contrast of a low contrast target with a 20 mm diameter was determined to be independent of dose, between 2.1 and 17 cGy. The measured LCR of a target with a nominal contrast of 2.8% was reduced from 2.3% to 1.2% when the contrast target diameter was reduced from 15 to 5 mm, using 17 cGy for imaging. The signal to noise ratio of this system is shown to be proportional to the square root of dose. Most importantly, a low contrast target with a diameter of 6 mm and a nominal contrast level of 1.5% is resolved with a radiation dose of 2.1 cGy in the Co60 beam. The spatial resolution in the Co60 beam is limited to one line pair per centimeter mainly due to the size of the Co60 source.

“…The prototype hardware used to read the signal from the scintillator-photodiode system in continuous radiation is similar to that described in an earlier work. 31 Briefly, for the Co 60 beam, the photo-current (I p ) is integrated on a 1 nano-Farad capacitor for a specified period of time (⌬T), amplified (gainϭ47) and digitized using 16-bit analog-to-digital ͑A-to-D͒ converter ͑PCI6034E, National Instruments, Austin, TX͒. Timing control signals are derived from an independent clock circuit ͑in-house͒ and by using Labview ͑National Instruments, Austin, TX͒ software running on a PC.…”

Section: B Measurementsmentioning

confidence: 99%

Modeling scintillator‐photodiodes as detectors for megavoltage CT

Steciw²,

Fallone

et al. 2004

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The use of cadmium tungstate (CdWO4) and cesium iodide [CsI(Tl)] scintillation detectors is studied in megavoltage computed tomography (MVCT). A model describing the signal acquired from a scintillation detector has been developed which contains two steps: (1) the calculation of the energy deposited in the crystal due to MeV photons using the EGSnrc Monte Carlo code; and (2) the transport of the optical photons generated in the crystal voxels to photodiodes using the optical Monte Carlo code DETECT2000. The measured detector signals in single CdWO4 and CsI(Tl) scintillation crystals of base 0.275 x 0.8 cm2 and heights 0.4, 1, 1.2, 1.6 and 2 cm were, generally, in good agreement with the signals calculated with the model. A prototype detector array which contains 8 CdWO4 crystals, each 0.275 x 0.8 x 1 cm3, in contact with a 16-element array of photodiodes was built. The measured attenuation of a Cobalt-60 beam as a function of solid water thickness behaves linearly. The frequency dependent modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)] were measured for 1.25 MeV photons (in a Cobalt-60 beam). For 6 MV photons, only the MTF(f) was measured from a linear accelerator, where large pulse-to-pulse fluctuations in the output of the linear accelerator did not allow the measurement of the NPS(f). A two-step Monte Carlo simulation was used to model the detector's MTF(f), NPS(f) and DQE(f). The DQE(0) of the detector array was found to be 26% and 19% for 1.25 MeV and 6 MV photons, respectively. For 1.25 MeV photons, the maximum discrepancies between the measured and modeled MTF(f), relative NPS(f) and the DQE(f) were found to be 1.5%, 1.2%, and 1.9%, respectively. For the 6 MV beam, the maximum discrepancy between the modeled and the measured MTF(f) was found to be 2.5%. The modeling is sufficiently accurate for designing appropriate detectors for MVCT.