Fast Spiral SPECT with Stationary γ-Cameras and Focusing Pinholes

Abstract

Small-animal SPECT systems with stationary detectors and focusing multiple pinholes can achieve excellent resolutionsensitivity trade-offs. These systems are able to perform fast total-body scans by shifting the animal bed through the collimator using an automated xyz stage. However, so far, a large number of highly overlapping central fields of view have been used, at the cost of overhead time needed for animal repositioning and long image reconstruction times due to high numbers of projection views. Methods: To improve temporal resolution and reduce image reconstruction time for such scans, we have developed and tested spiral trajectories (STs) of the animal bed requiring fewer steps. In addition, we tested multiplane trajectories (MPTs) of the animal bed, which is the standard acquisition method of the U-SPECT-II system that is used in this study. Neither MPTs nor STs require rotation of the animal. Computer simulations and physical phantom experiments were performed for a wide range of numbers of bed positions. Furthermore, we tested STs in vivo for fast dynamic mouse scans. Results: We found that STs require less than half the number of bed positions of MPTs to achieve sufficient sampling. The reduced number of bed positions made it possible to perform a dynamic total-body bone scan and a dy- SPECT is used to quantitatively and visually assess the distribution of radioactive biologic markers (tracers) in vivo in order to, for example, study animal models of disease and test new pharmaceuticals. Most SPECT systems use rotating detectors and collimators to scan an animal or patient. Alternatively, SPECT systems have been developed that have 360°coverage and use stationary detectors and multipinhole collimators, a full-ring stationary system being first realized at the University of Arizona (1,2). Combining stationary setups and focusing multiple pinholes with high pinhole magnification has resulted in dedicated small-animal SPECT systems that have overcome the limitation of poor spatial resolution and sensitivity. Sub-half-millimeter resolutions over the entire mouse and very detailed images of tracer uptake in tiny subcompartments of organs and tumors have been achieved (3-16). In such focusing multipinhole (FMP) SPECT systems, all pinholes focus on a central field of view (CFOV) in order to maximize the scan speed and the count yield for imaging tumors or organs.With FMP SPECT, scans of volumes that are larger than the CFOV (such as total-body scans of mice and rats) are performed by gently moving the animal through the collimator in small steps using a high-precision xyz stage (11); this provides adequate sampling (no artifacts in the reconstructed images). Images are reconstructed with a dedicated iterative algorithm that exploits all projections acquired from all positions of the animal inside the collimator simultaneously, rather than stitching together reconstructions of separate subvolumes (obtained from individual focus positions). This method of combined acquisition and reconstruction is called the ...

VECTor: A Preclinical Imaging System for Simultaneous Submillimeter SPECT and PET

“…Previously, we showed that mouse organs and entire bodies can be imaged with half-minute time frames using high-resolution focused multipinhole SPECT (16,17). Because VECTor has a better sensitivity than these SPECT systems (both for SPECT and PET tracers), we expect that similar time resolutions will be feasible, allowing, for example, tracer kinetic modeling in simultaneous SPECT/PET.…”

“…In contrast, high-end commercial smallanimal PET attains image resolutions of approximately 1 mm (10)(11)(12)(13)(14)(15) and cannot distinguish between simultaneously injected tracers, because positron emission from different tracers results in g-photon pairs with equal energy (511 keV). Recently, it was also shown that fast dynamic (total body) imaging is possible with small-animal SPECT based on stationary detectors (16,17). This ability makes SPECT, besides PET, an important tool in, for example, kinetic modeling.…”

“…Similar to the standard multipinhole SPECT collimators of this system, the clustered-pinhole collimator applies a focusing geometry; all pinhole clusters focus on a central scan volume. This design allows the count yield from a specific organ or a tumor to be increased (21) but also allows imaging of larger regionsup to (dynamic) whole-body mouse scans (17,22). We present a quantitative performance evaluation of VECTor using phantom data and show the results of simultaneously acquired multiple functional data obtained with positron and singlephoton emitters in mice.…”

Performance Assessment of a Preclinical PET Scanner with Pinhole Collimation by Comparison to a Coincidence-Based Small-Animal PET Scanner

“…The manufacturer's software was used for acquisition (version 3.4f) and reconstruction (b version, VECTor 1 ). List-mode acquisitions used the scanning focus method (8) with helical bed motions (9) to move the animal through the hourglass-shaped FOV, which encompasses the entire bore (diameter, 4.8 cm) and has an average length of 3.6 cm. For a mouse brain, 19 bed positions were used.…”

Utilizing High-Energy γ-Photons for High-Resolution 213Bi SPECT in Mice

“…All clustered pinholes together observe a field of view that has the shape of an hourglass with a diameter of 44 mm and an average longitudinal length of 33 mm (10). Total-body images are obtained by moving the animal through the scanner along a spiral trajectory (13). Data are collected in list-mode.…”