Abstract
1153
Introduction: Single photon emission computed tomography (SPECT) using cadmium zinc telluride (CZT) detectors offers an improved quantitative imaging capability over conventional NaI(Tl)-based SPECT. However, when optimizing CZT-based SPECT geometries using simulation, the low-energy tailing effect due to incomplete charge collection and charge sharing in CZT can lead to overestimated performances1,2. In this work, we assessed a variable-aperture fulling-ring CZT-SPECT geometry for whole-body imaging using combination of a Monte Carlo (MC) simulation and a finite element method (FEM) and evaluated imaging performance metrices of the proposed system geometry.
Methods: The proposed CZT-SPECT system geometry uses eight detector heads having 112 × 80 pixelated CZT (1.6 mm × 1.6 mm × 5 mm) paired with tungsten-alloy hole-matching parallel hole collimator3. Each detector head can rotate around the common axes of rotation individually without interference. Two projections from adjacent detector heads are combined to extend the trans-axial FOV with an interpolation method for gap and edge correction. The scanner was modeled by a MC simulation tool (GATE) including photoelectric effect, Compton scattering, and Rayleigh scattering4. To construct the detailed and realistic 3D model of the CZT detector, we calculated charge induction efficiency (CIE) of CZT detector and converted it to a 3D map using an FEM by COMSOL MULTIPHYSICS software5. Charge transport, trapping, diffusion, and bias potential were implemented in the FEM model. To implement the tailing effect to the energy spectrum acquired from MC simulation, we used deposited energies and CIE values at certain points inside detector layer. We simulated a Tc-99m point source (1.5 MBq) for confirmation of the low-energy tailing effect, and also simulated a NEMA IQ phantom with Tc-99m (Spheres to Background ratios = 8 to 1, 10 kBq/mL)6 for evaluation of contrast-to-recovery ratio (CRR) with and without dual-energy scatter correction. The phantom images were reconstructed by an open-source package (STIR)7 with collimator-detector response model and attenuation correction.
Results: The low-energy tailing effect was shown in the point-source energy spectrum when the CIE 3D map was applied. In the NEMA phantom images, there was no visible truncation artifact observed although some parts of the phantom were outside of the FOV of each detector because of the extended FOV combining two adjacent views with gap and edge correction. CRRs with scatter correction is 3.1 ~ 8.1 % higher than those without scatter correction. Conclusion: We demonstrated the feasibility of our proposed full-ring pixelated CZT SPECT system geometry for whole-body imaging such as fully 3D bone SPECT. Furthermore, the results showed that our simulation method combining MC simulation and FEM properly modeled the low-energy tailing effect of CZT detectors, so that our method could potentially be used for optimizing CZT-SPECT system geometries. Acknowledgments: The study was supported in part by National Institute of Biomedical Imaging and Bioengineering grants R01EB026331 and R01EB012965, and National Heart, Lung, and Blood Institute grant R01HL135490.