Evaluation of a variable-aperture full-ring SPECT system using large-area pixelated CZT modules: A simulation study for brain SPECT applications

Abstract

Purpose: Single photon emission computed tomography (SPECT) scanners using cadmium zinc telluride (CZT) offer compact, lightweight, and improved imaging capability over conventional NaI(Tl)-based SPECT scanners. The main purpose in this study is to propose a full-ring SPECT system design with eight large-area CZT detectors that can be used for a broad spectrum of SPECT radiopharmaceuticals and demonstrate the performance of our system in comparison to the reference conventional NaI(Tl)-based two-head Anger cameras.

Methods: A newly designed full-ring SPECT system is composed of eight large-area CZT cameras (128 mm × 179.2 mm effective area) that can be independently swiveled around their own axes of rotation independently and can have radial motion for varying aperture sizes that can be adapted to different sizes of imaging volume. Extended projection data were generated by conjoining projections of two adjacent detectors to overcome the limited field-of-view (FOV) by each CZT camera. Using Monte Carlo simulations, we evaluated this new system design with digital phantoms including a Derenzo hot rod phantom and a Zubal brain phantom. Comparison of performance metrics such as spatial resolution, sensitivity, contrast-to-noise ratio (CNR), and contrast-recovery ratio was made between our design and conventional SPECT scanners having different pixel sizes and radii of rotation (one clinically well-known type and two arbitrary types matched to our proposed CZT-SPECT geometries).

Results: The proposed scanner could result in up to about three times faster in acquisition time over conventional scan time at same acquisition time per step. The spatial resolution improvement, or deterioration, of our proposed scanner compared to the clinical-type scanner was dependent upon the location of the point source. However, there were overall performance improvements over the three different setups of the conventional scanner particularly in volume sensitivity (approximately up to 1.7 times). Overall, we successfully reconstructed the phantom image for both ^99m Tc-based perfusion and ¹²³ I-based dopamine transporter (DaT) brain studies simulated for our new design. In particular, the striatal/background contrast-recovery ratio in 3-to-1 reference ratio was over 0.8 for the ¹²³ I-based DaT study.

Conclusions: We proposed a variable-aperture full-ring SPECT system using combined pixelated CZT and energy-optimized parallel-hole collimator modules and evaluated the performance of this scanner using relevant digital phantoms and MC simulations. Our studies demonstrated the potential of our new full-ring CZT-SPECT design, showing reduced acquisition time and improved sensitivity with acceptable CNR and spatial resolution.

Keywords: CZT detector; Monte Carlo simulation; SPECT; brain SPECT; full ring.

Figure 1.

Example diagrams for Angular Step 3° (AS-3, top) and Angular Step 5° (AS-5, bottom) image acquisition mode for our proposed SPECT scanner. Each detector head is swiveled from −42° to 42° for AS-3 or −42.5° to 42.5° for AS-5 within the rotation angular range. Each detector head has a collimator (blue) and a detector (black).

Figure 2.

Example diagrams for the gap and shifted center corrections (a) and truncated edge (b). The gap correction and edge correction were needed when an extended projection was generated by combining projections of DH #1 at 23^rd step and DH #0 at 8^th step, and projections of DH #1 at 29^th step and DH #0 at 14^th step, respectively. These examples are based on the AS-3 scanning geometry.

Figure 3.

Derenzo hot-rod phantom (a) has acryl material for the background. The yellow dashed line in the hot-rod region indicates the location of profiles of the hot-rods and the yellow dashed circle and blue circle indicates the ROI and background for CNR, respectively. Original Zubal phantom (b) and activity maps for brain perfusion imaging with Tc-99m (c) and DaT imaging with I-123 (d) and attenuation map (e) for both brain phantom studies. The yellow circles indicate the grey matter and striatal ROIs and the blue circles indicate the background.

Figure 4.

Reconstructed images of the Derenzo hot-rod phantom from three different setups for conventional scanners (Clinical_3.18, Arb_1.6RR150 and Arb_1.6RR185) and our proposed system with two acquisition modes (AS-3 and AS-5). All images were reconstructed with 50 iterations and with a post-processed 3D Gaussian filter with sigma equal to 1 pixel. The phantom images were summed axially (28 slices for the Clinical_3.18 configuration and 55 slices for the others).

Figure 5.

The line profiles of reconstructed images for the Derenzo hot-rod phantom from line profile A (11.1 to 7.9 mm diameter, middle) and line profile B (4.8 mm diameter third row, bottom) as shown in Figure 3a. All values were normalized by the maximum value in the reconstructed images.

Figure 6.

CNRs for two different rods of the Derenzo hot-rod phantom corresponding to 50 and 100 iterations.

Figure 7.

The 3D activity map (left) and the 3D reconstructed images for the brain perfusion study using the three configurations of the conventional scanner and our proposed scanner in AS-3 and AS-5 acquisition modes with 100 iterations and post-processed using a 3D Gaussian filter with a sigma of 1 pixel.

Figure 8.

Left: Brain perfusion image from AS-3 with ground-truth activity map (yellow for location of the line profiles). Center: line profiles for the reference activity phantom (orange, dotted line) compared to the reconstructed brain perfusion images for the Clincal_3.18 (green, dash line), Arb_1.6RR150 (gray, dash line), Arb_1.6RR185 (black, dash line), AS-3 (blue line) and AS-5 (red line) configurations. Right: CNRs as a function of the number of iterations.

Figure 9.

Reconstructed images with 150 iterations and CDR correction for I-123 brain DaT imaging from the three configurations of the conventional scanner and our proposed scanner in AS-3 and AS-5 acquisition modes as a function of striatal-to-background activity ratio. All images in each row were normalized by the maximum value in the images for 10 to 1 striatal/background ratio and postprocessed using a 3D Gaussian filter with a sigma of 1 pixel.

Figure 10.

Upper Left: Brain DaT image from AS-3 with the ground-truth activity map (yellow for the location of the line profiles). Upper Right: Line profiles of the reconstructed brain DaT images compared to the reference activity (orange, dotted line). Caudate (lower left) and putamen (lower right) contrast recovery as a function of the number of iterations for the striatal to background uptake ratio of 10:1. The plots correspond to contrast recovery values when reconstruction was performed based on the attenuation correction with CDR correction.