Development and Evaluation of mini-EXPLORER: A Long Axial Field-of-View PET Scanner for Nonhuman Primate Imaging

Abstract

We describe a long axial field-of-view (FOV) PET scanner for high-sensitivity and total-body imaging of nonhuman primates and present the physical performance and first phantom and animal imaging results. Methods: The mini-EXPLORER PET scanner was built using the components of a clinical scanner reconfigured with a detector ring diameter of 43.5 cm and an axial length of 45.7 cm. National Electrical Manufacturers Association (NEMA) NU-2 and NU-4 phantoms were used to measure sensitivity and count rate performance. Reconstructed spatial resolution was investigated by imaging a radially stepped point source and a Derenzo phantom. The effect of the wide acceptance angle was investigated by comparing performance with maximum acceptance angles of 14°-46°. Lastly, an initial assessment of the in vivo performance of the mini-EXPLORER was undertaken with a dynamic ¹⁸F-FDG nonhuman primate (rhesus monkey) imaging study. Results: The NU-2 total sensitivity was 5.0%, and the peak noise-equivalent count rate measured with the NU-4 monkey scatter phantom was 1,741 kcps, both obtained using the maximum acceptance angle (46°). The NU-4 scatter fraction was 16.5%, less than 1% higher than with a 14° acceptance angle. The reconstructed spatial resolution was approximately 3.0 mm at the center of the FOV, with a minor loss in axial spatial resolution (0.5 mm) when the acceptance angle increased from 14° to 46°. The rhesus monkey ¹⁸F-FDG study demonstrated the benefit of the high sensitivity of the mini-EXPLORER, including fast imaging (1-s early frames), excellent image quality (30-s and 5-min frames), and late-time-point imaging (18 h after injection), all obtained at a single bed position that captured the major organs of the rhesus monkey. Conclusion: This study demonstrated the physical performance and imaging capabilities of a long axial FOV PET scanner designed for high-sensitivity imaging of nonhuman primates. Further, the results of this study suggest that a wide acceptance angle can be used with a long axial FOV scanner to maximize sensitivity while introducing only minor trade-offs such as a small increase in scatter fraction and slightly degraded axial spatial resolution.

Keywords: high sensitivity; long axial field of view; positron emission tomography; rhesus monkey; total-body imaging.

FIGURE 1.

(A) Side view of scanner with covers removed showing gantry frame, detector rings, and scanner electronics. (B) View inside detector rings showing detectors mounted between tapered aluminum rails. (C) View from front of scanner with covers on and scanning bed installed.

FIGURE 2.

Axial NEMA NU-2 sensitivity profiles (2-mm axial slices) obtained with 46° acceptance angle.

FIGURE 3.

NEMA NU-4 monkey scatter phantom count rate performance. (A) True-, scatter-, and random-coincidence rates measured with 46° acceptance angle. (B) NECR vs. activity for each acceptance angle. (C) Scatter fraction vs. activity for each acceptance angle.

FIGURE 4.

(A) Transaxial (left) and sagittal (right) image slices of reconstructed uniform cylinder. (B) Radial line profile through average of all transaxial image slices (left) and average image pixel value inside uniform cylinder for each axial slice (right).

FIGURE 5.

Reconstructed point-source spatial resolution vs. radial offset.

FIGURE 6.

Transaxial slice of Derenzo phantom reconstructed image.

FIGURE 7.

Maximum-intensity-projection images from ¹⁸F-FDG rhesus monkey study: 1-s frame at 5 s after injection (reconstructed using kernel method (37)) (A), 0–30 s after injection (B), 55–60 min after injection (C), and 18 h after injection (40-min scan) (D). Further images are provided as Supplemental Figure 4 and Supplemental Videos 1–6.