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

Abstract

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.

Simulation studies of a full‐ring, CZT SPECT system for whole‐body imaging of 99mTc and 177Lu

“…The capability of our system to image those isotopes needs to be evaluated in dedicated studies, although the imaging performance of higher energy gamma-rays will likely be degraded.A way to improve spatial resolution and IQ could be to combine our system with a dual-layer collimator concept, as presented in Ref 31. Given the larger subject size, a scatter correction becomes much more relevant in the current work than in our previous organ-specific studies. 7 The mDEW scatter correction shows a good performance for 99m Tc and 177 Lu,but more sophisticated methods will be necessary to account for the low-energy tail when more than one gamma-ray is present,like for the low energy 177 Lu emission or in a multi-tracer imaging scenario. MC-based models could solve this issue.…”

“…Each head has the ability to swivel as shown in Figure 2. The detector heads are located at 240 mm radius around the center of the imaging region, which is further than in our previous brain imaging simulation study 7 in order to accommodate a human torso. Given the further distance, the heads are oriented such that their shorter size are parallel to the Z axis, as opposed to our previous study.…”

“…The Derenzo phantom is a 108-mm radius plastic cylinder that contains six groups of rods of 88 mm length of different diameters (19.1, 12.7, 11.1, 9.5, 7.9, 6.4 mm), which are greater than in our previous brain study. 7 Two different Derenzo phantoms are simulated: a hot-rod phantom and a cold-rod phantom. The inactive material (rods for the cold-rod phantom and main body of the phantom for the hot-rod one) was simulated as polyethylene while the aqueous radioactive solution (contained in the rods for the hot-rod and in the main cavity for the cold-rod) was simulated as water.…”

“…In Ref. 7 we proposed a full‐ring CZT SPECT system with variable aperture, that is, with detectors that can move in the radial direction in order to accommodate the detectors as close as possible to the subject. This design featured the addition of parallel‐hole tungsten collimators designed in‐house for a good performance in the 100–250 keV energy range 8 .…”

“…Our goal is to prove that our design can provide a multi‐purpose solution for a number of clinical cases such as whole‐body 99m Tc imaging for metastatic cancer as well as 177 Lu imaging during radiotherapy treatment of gastroenteropancreatic neuroendocrine tumors 11 and metastatic‐castration‐resistant prostate cancer 12 . Our detector model is improved with respect to 7 by including a realistic CZT response based on a finite element method (FEM 13 ). To adapt our approach to whole‐body imaging, we add an energy‐window‐based scatter correction 14 .…”