Performance evaluation of microPET: a high-resolution lutetium oxyorthosilicate PET scanner for animal imaging

Abstract

A new dedicated PET scanner, microPET, was designed and developed at the University of California, Los Angeles, for imaging small laboratory animals. The goal was to provide a compact system with superior spatial resolution at a fraction of the cost of a clinical PET scanner.

Methods: The system uses fiberoptic readout of individually cut lutetium oxyorthosilicate (LSO) crystals to achieve high spatial resolution. Each microPET detector consists of an 8 x 8 array of 2 x 2 x 10-mm LSO scintillation crystals that are coupled to a 64-channel photomultiplier tube by optical fibers. The tomograph consists of 30 detectors in a continuous ring with a 17.2-cm diameter and fields of view (FOVs) of 11.25 cm in the transaxial direction and 1.8 cm in the axial direction. The system has eight crystal rings and no interplane septa. It operates exclusively in the three-dimensional mode and has an electronically controlled bed that is capable of wobbling with a radius of 300 microm. We describe the performance of the tomograph in terms of its spatial, energy and timing resolution, as well as its sensitivity and counting-rate performance. We also illustrate its overall imaging performance with phantom and animal studies that demonstrate the potential applications of this device to biomedical research.

Results: Images reconstructed with three-dimensional filtered backprojection show a spatial resolution of 1.8 mm at the center of the FOV (CFOV), which remains <2.5 mm for the central 5 cm of the transaxial FOV. The resulting volumetric resolution of the system is <8 microL. The absolute system sensitivity measured with a 0.74 MBq (20 microCi) 68Ge point source at the CFOV is 5.62 Hz/kBq. The maximum noise equivalent counting rate obtained with a 6.4-cm diameter cylinder spanning the central 56% of the FOV is 10 kcps, whereas the scatter fraction is 37% at the CFOV for an energy window of 250-650 keV and the same diameter cylinder.

Conclusion: This is the first PET scanner to use the new scintillator LSO and uses a novel detector design to achieve high volumetric spatial resolution. The combination of imaging characteristics of this prototype system (resolution, sensitivity, counting-rate performance and scatter fraction) opens up new possibilities in the study of animal models with PET.

FIGURE 1

(A) Photograph of microPET tomograph gantry with bed and laser positioning system. (B) Position histogram from typical microPET block detector, illustrating clear separation of 64 crystals. (C) MicroPET block detector with crystal array on left, coupled through optical fibers to MC-PMT on right.

FIGURE 2

Intrinsic spatial resolution for typical microPET block detectors in coincidence, measured by stepping ²²Na point source across FOV.

FIGURE 3

Linear spatial resolution components (radial, tangential and axial) of reconstructed image as function of radial offset from CFOV.

FIGURE 4

Volumetric spatial resolution of reconstructed image, expressed in microliters, as function of radial offset from CFOV.

FIGURE 5

Map of absolute system sensitivity in transverse and axial directions across FOV of microPET for 250- to 650-keV energy window.

FIGURE 6

NEC rates for three phantoms, representing rat body (small), cat head (medium) and monkey head (large), for 250- to 650-keV energy window.

FIGURE 7

Plot of quantitative evaluation of microPET shows true activity measured with well counter versus activity measured with ROIs in reconstructed images of compartment phantom.

FIGURE 8

Reconstructed images of two miniature versions of Derenzo phantom imaged with microPET (A and B) and EXACT HR+ (C and D). Images show hot-rod versions (A and C) and cold-rod versions (B and D). Diameters of rods in each of five segments were 2.5, 2.0, 1.5, 1.25 and 1.0 mm, respectively, and center-to-center spacing was four times rod diameter.

FIGURE 9

Images from 300-g rat injected with 74 MBq (2 mCi) ¹⁸FDG and scanned for 16 bed positions and 8-min acquisitions per bed position. Coronal slices from tomographic reconstructed images (A–D). Projection through same dataset (E).

FIGURE 10

Coronal slices from brain of 300-g rat injected with 74 MBq (2 mCi) ¹⁸FDG and imaged for 40 min. Cortex is clearly resolved, and major subcortical structures such as thalamus and striatum also can be discerned.

FIGURE 11

Short- and long-axes slices through heart of 352-g rat injected with 70 MBq (1.9 mCi) ¹⁸FDG and imaged for 32 min.

FIGURE 12

Coronal slices showing bone scan of 30-g mouse injected with 37 MBq (1 mCi) ¹⁸F⁻ and imaged for 8 bed positions and 8 min per bed position.

FIGURE 13

Transverse (A) and coronal (B) slices of adult vervet monkey injected with 185 MBq (5 mCi) ¹¹C-WIN 35,428 and imaged at 2 bed positions for 50 min. Images show ¹¹C-WIN 35,428 binding to dopamine transporter in striatum.

FIGURE 14

Transverse slices and time-activity curves for 300-g rat injected with 37 MBq (1 mCi) ¹¹C-WIN 35,428 (A) and 30-g mouse injected with 6.7 MBq (180 µCi) ¹¹C-WIN 35,428 (B). Images show ¹¹C-WIN 35,428 binding to dopamine transporter in striatum.