A Prototype High-Resolution Small-Animal PET Scanner Dedicated to Mouse Brain Imaging

Abstract

We developed a prototype small-animal PET scanner based on depth-encoding detectors using dual-ended readout of small scintillator elements to produce high and uniform spatial resolution suitable for imaging the mouse brain.

Methods: The scanner consists of 16 tapered dual-ended-readout detectors arranged in a 61-mm-diameter ring. The axial field of view (FOV) is 7 mm, and the transaxial FOV is 30 mm. The scintillator arrays consist of 14 × 14 lutetium oxyorthosilicate elements, with a crystal size of 0.43 × 0.43 mm at the front end and 0.80 × 0.43 mm at the back end, and the crystal elements are 13 mm long. The arrays are read out by 8 × 8 mm and 13 × 8 mm position-sensitive avalanche photodiodes (PSAPDs) placed at opposite ends of the array. Standard nuclear-instrumentation-module electronics and a custom-designed multiplexer are used for signal processing.

Results: The detector performance was measured, and all but the crystals at the very edge could be clearly resolved. The average intrinsic spatial resolution in the axial direction was 0.61 mm. A depth-of-interaction resolution of 1.7 mm was achieved. The sensitivity of the scanner at the center of the FOV was 1.02% for a lower energy threshold of 150 keV and 0.68% for a lower energy threshold of 250 keV. The spatial resolution within a FOV that can accommodate the entire mouse brain was approximately 0.6 mm using a 3-dimensional maximum-likelihood expectation maximization reconstruction. Images of a hot-rod microphantom showed that rods with a diameter of as low as 0.5 mm could be resolved. The first in vivo studies were performed using (18)F-fluoride and confirmed that a 0.6-mm resolution can be achieved in the mouse head in vivo. Brain imaging studies with (18)F-FDG were also performed.

Conclusion: We developed a prototype PET scanner that can achieve a spatial resolution approaching the physical limits of a small-bore PET scanner set by positron range and detector interaction. We plan to add more detector rings to extend the axial FOV of the scanner and increase sensitivity.

Keywords: brain imaging; high resolution; mouse; positron emission tomography; small animal PET.

Figure 1

(A) Tapered LSO array and two different-sized PSAPDs used in the prototype scanner, (B) scale drawing of the prototype 1-ring scanner, (C) photographs of the scanner without the cover showing the readout electronics (left) and, the completed scanner with animal bed (right).

Figure 2

Reconstructed images from point source measurements. The radial offsets of the point source were 0, 5 and 10 mm. Measurements were made at two different axial locations, (A) in the center of the scanner axially, and (B)1.75 mm away from the central slice. The spatial resolution of the scanner at (C) the center of the axial field of view and (D) in a slice at ¼ axial FOV (1.75 mm from center). Two source positions were measured for radial offsets of 5 and 10 mm as shown in Figure 2A and 2B, therefore there are two data points at these locations.

Figure 3

Reconstructed images of a hot rod phantom obtained from the prototype scanner (A) and the Siemens Inveon D-PET scanner (B).

Figure 4

Reconstructed images of ¹⁸F-fluoride uptake in the mouse skull obtained (A) from the prototype scanner and (B) from the Siemens Inveon D-PET scanner. Line profiles through the skull in the first slice of the images are also shown.

Figure 5

FDG brain images of a mouse obtained from the prototype scanner (A) and from the Siemens Inveon D-PET scanner (B).