A comparative study of NaI(Tl), CeBr3, and CZT for use in a real-time simultaneous nuclear and fluoroscopic dual-layer detector.

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.

Performance of a dual-layer scanner for hybrid SPECT/CBCT.

Fluoroscopic procedures involving radionuclides would benefit from interventional nuclear imaging by obtaining real-time feedback on the activity distribution. We have previously proposed a dual-layer detector that offers such procedural guidance by simultaneous fluoroscopic and nuclear planar imaging. Acquisition of single photon computed tomography (SPECT) and cone beam computed tomography (CBCT) could provide additional information on the activity distribution. This study investigates the feasibility and the image quality of simultaneous SPECT/CBCT, by means of phantom experiments and simulations. Simulations were performed to study the obtained reconstruction quality for (i) clinical SPECT/CT, (ii) a dual-layer scanner configured with optimized hardware, and (iii) our (non-optimized) dual-layer prototype. Experiments on an image quality phantom and an anthropomorphic phantom (including extrahepatic depositions with volumes and activities close to the median values encountered in hepatic radioembolization) were performed with a clinical SPECT/CT scanner and with our dual-layer prototype. Nuclear images were visually and quantitatively evaluated by measuring the tumor/non-tumor (T/N) ratio and contrast-to-noise ratio (CNR). The simulations showed that the maximum obtained CNR was 38.8 ± 0.8 for the clinical scanner, 30.2 ± 0.9 for the optimized dual-layer scanner, and 20.8 ± 0.4 for the prototype scanner. T/N ratio showed a similar decline. The phantom experiments showed that performing simultaneous SPECT/CBCT is feasible. The CNR obtained from the SPECT reconstruction of largest sphere in the image quality phantom was 43.1 for the clinical scanner and 28.6 for the developed prototype scanner. The anthropomorphic phantom showed that the extrahepatic depositions were detected with both scanners. A dual-layer detector is able to simultaneously acquire SPECT and CBCT. Both CNR and T/N ratio are worse than that of a clinical system, but the phantom experiments showed that extrahepatic depositions with volumes and activities close to the median values encountered in hepatic radioembolization could be distinguished.