A High-Performance Self-Collimation SPECT for Small Animal Imaging

Abstract

Stemmed from our novel single-photon imaging concept of detector self-collimation-which leverages detectors themselves as collimators to overcome the inherent resolution-sensitivity trade-off in conventional SPECT-this study presents the design and evaluation of the first full-ring self-collimation SPECT (SC-SPECT) scanner for small animal imaging. The system features four concentric detector rings and two interchangeable high-aperture-ratio tungsten collimator rings optimized for high-resolution (HR) and general-purpose (GP) imaging applications. Detector rings contain 480, 720, 960, and 1,200 evenly distributed GAGG(Ce) scintillators, each measuring 0.84 mm (tangential) × 6 mm (radial) × 20 mm (axial) and separated by 0.84-mm gaps to enable effective photon collimation. Inner detector rings and the collimator ring collectively provide collimation for photons reaching subsequent outer rings. Dual-end SiPM readouts facilitate axial depth-of-interaction measurements. Phantom and mouse studies are performed to assess the system's resolution, sensitivity, and field-of-view volume, and SC-SPECT demonstrates generally superior performance compared with state-of-the-art small-animal SPECT systems. Mouse bone images using ^99mTc-MDP show CT-like resolution, clearly delineating detailed tracer uptake distributions within small structures such as mouse paws and skulls, indicating a significant technological advancement in small-animal SPECT imaging.

Fig. 1.

Comparison of conventional mechanical collimation and novel self-collimation. (a) Mechanical collimation: a mechanical collimator restricts photons to specific paths by absorbing those traveling in other directions. The choice of the aperture size on the collimator determines the trade-off of sensitivity and resolution. (b) Self-collimation: active detectors replace part of the mechanical collimators to guide the photons’ trajectories toward the subsequent layers. Resolution is not solely determined by the aperture size.

Fig. 2.

(a) Photographs of the small animal SC-SPECT system. (b) A schematic diagram and a cross-sectional view of the four detector rings.

Fig. 3.

(a) A schematic diagram and a photograph of the detector cassette. The coordinate system follows the same convention as in Fig. 2B. (b) A schematic diagram of the detector cell. (C) The CovB and DPB used for signal readout.

Fig. 4.

(a) Detector cells’ flood histogram in a cassette. (b) PSD factor histogram of a detector cell. The threshold used to separate GAGG-90 from GAGG-200 is set to 10.4, corresponding to the minimum point between the two peaks. (c) DOI ratio histogram of a scintillator.

Fig. 5.

Sensitivity map of the small animal SC-SPECT with the HR and GP metal rings in the three orthogonal planes, transverse, coronal, and sagittal, passing the origin.

Fig. 6.

(a) Reconstructed images of the hot rod phantom acquired using the HR and GP rings. For each scan, the phantom contains a total activity of 0.18 mCi. Under the T1 scanning protocol, the total scan time is 48 minutes. For the T4 protocol, reconstructions are performed using total scan times of both 48 minutes and 12 minutes. (b) Line profiles of the smallest distinguishable hot rods, evaluated using a criteria of PVR >2. The corresponding PVR values are annotated above each line profile.

Fig. 7.

(a) DOI ratio histograms of a scintillator for collimated planar photon beam incidences at −8 mm, −4 mm, 0 mm, 4 mm, and 8 mm. (b) DOI resolutions at the corresponding five DOI positions.

Fig. 8.

(a) MIPs of the mouse paw acquired with SC-SPECT using the GP metal ring. (b) MIPs of the mouse skull acquired with SC-SPECT using the GP metal ring. (c) MIPs of the mouse skull acquired with micro-CT. (d) Transverse images of the mouse skull acquired with SC-SPECT (6-hour scan) and micro-CT.