Abstract
Simultaneous acquisition of nuclear and fluoroscopic projections could be of benefit for image-guided radionuclide administration. A gamma camera positioned behind an x-ray flat panel detector can accomplish such simultaneous acquisition, but the gamma camera performance suffers from the intense x-ray dose. A regular NaI(Tl)-based camera has nominal performance up to 0.02 nGy dose per pulse, whereas 10 nGy dose is expected for our foreseen applications. We evaluated the performance of CeBr3- and CZT-based detectors and investigated a cost-effective improvement of a regular NaI(Tl)-based camera by the introduction of a high-pass filter and shorting circuit.A CeBr3-based detector was exposed to 5 mGy x-ray dose and the resulting light emission was measured over time to quantify the crystal afterglow, allowing comparison with a previously measured NaI(Tl)-based detector. The NaI(Tl)-, CeBr3- and CZT-based detectors were exposed to x-ray pulse sequences with dose from 0.06 to 60 nGy, while being irradiated with a gamma source. The mean gamma energy and energy resolution in between the x-ray pulses were measured as a reference of the detector performance.The afterglow signal after 3 ms was 14.1% for the NaI(Tl)-based detector, whereas for the CeBr3-based detector it was only 0.1%. The limits for a proper functioning detectors are 0.32 nGy for the NaI(Tl)-based detector with high-pass filter and shorting circuit and 18.94 nGy for the one with CeBr3. No energy degradation was observed for the CZT module in the studied dose range.The performance of regular NaI(Tl)-based gamma cameras deteriorates when exposed to high x-ray doses. CeBr3 and CZT are much better suited for introduction into a dual-layer detector but have high associated costs. Addition of a high-pass filter and shorting circuit into the PMT of a NaI(Tl)-based detector is a cost-effective solution that works well for low dose levels.
Highlights
Single-photon emission computed tomography (SPECT) has been combined with computed tomography (CT) in hybrid SPECT/CT scanners to provide attenuation correction and anatomical reference for nuclear images
We envision two situations in which simultaneous acquisition of nuclear and fluoroscopic images could be of benefit: (i) for guidance of radioisotope administration in an interventional setting, inasmuch as direct feedback on the activity distribution with anatomical reference is obtained; and (ii) for improvement of SPECT image quality, by exploiting the anatomical information from the fluoroscopic images to correct for respiratory motion artifacts in the SPECT reconstruction
The dual-layer detector consists of an x-ray flat panel detector, which is placed in front of a gamma camera mounted with cone beam collimator
Summary
Single-photon emission computed tomography (SPECT) has been combined with computed tomography (CT) in hybrid SPECT/CT scanners to provide attenuation correction and anatomical reference for nuclear images. There exist scanners on the market that could technically perform such simultaneous acquisition (e.g. the Brightview XCT developed by Philips Healthcare or systems in research (Kalki et al 1997)).,these systems are too large to enable smooth integration in the intervention room (required for the guidance of radioisotope administration) and the nuclear and fluoroscopic projections are captured under different angles, making simultaneous image interpretation difficult. By positioning the x-ray tube in the focal point of the cone beam collimator, fluoroscopic and nuclear projections capture the same field of view. This design has two advantages over the above mentioned systems: (i) the acquired fluoroscopic and nuclear projections intrinsically overlap, and (ii) the system can be manufactured considerably more compact and mobile by placing the detector on a custom c-arm (see figure 1 for an illustration)
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