MicroPET II: design, development and initial performance of an improved microPET scanner for small-animal imaging

Abstract

MicroPET II is a newly developed PET (positron emission tomography) scanner designed for high-resolution imaging of small animals. It consists of 17 640 LSO crystals each measuring 0.975 × 0.975 × 12.5 mm 3 , which are arranged in 42 contiguous rings, with 420 crystals per ring. The scanner has an axial field of view (FOV) of 4.9 cm and a transaxial FOV of 8.5 cm. The purpose of this study was to carefully evaluate the performance of the system and to optimize settings for in vivo mouse and rat imaging studies. The volumetric image resolution was found to depend strongly on the reconstruction algorithm employed and averaged 1.1 mm (1.4 µl) across the central 3 cm of the transaxial FOV when using a statistical reconstruction algorithm with accurate system modelling. The sensitivity, scatter fraction and noise-equivalent count (NEC) rate for mouse-and rat-sized phantoms were measured for different energy and timing windows. Mouse imaging was optimized with a wide open energy window (150-750 keV) and a 10 ns timing window, leading to a sensitivity of 3.3% at the centre of the FOV and a peak NEC rate of 235 000 cps for a total activity of 80 MBq (2.2 mCi) in the phantom. Rat imaging, due to the higher scatter fraction, and the activity that lies outside of the field of view, achieved a maximum NEC rate of 24 600 cps for a total activity of 80 MBq (2.2 mCi) in the phantom, with an energy window of 250-750 keV and a 6 ns timing window. The sensitivity at the centre of the FOV for these settings is 2.1%. This work demonstrates that different scanner settings are necessary to optimize the NEC count rate for different-sized animals and different injected doses. Finally, phantom and in vivo animal studies are presented to demonstrate the capabilities of microPET II for small-animal imaging studies.

NEMA NU4-2008 Image Quality Performance Report for the microPET Focus 120 and for Various Transmission and Reconstruction Methods

“…Recently, the National Electrical Manufacturers Association (NEMA) released the NU4-2008 (NU4) standards, which provide a unified full protocol for performance measurements of small-animal PET (15). The performance in terms of spatial resolution, counting rate, and sensitivity has been evaluated for these 5 scanners, in particular for the F-120, using procedures similar to those given in NU4 (6)(7)(8)(9)(10)(11)(12)(13)(14)16). However, for image quality (IQ) evaluation, several phantoms that differed from the NU4 IQ phantom were used.…”

“…From the analysis of these results, the most suitable method for transmission measurement was selected. Finally, these transmission data were used to reconstruct the phantom emission images with all available reconstruction methods provided with the scanner: FORE and 2D-FBP or 2D ordered-subset expectation maximization (OSEM) (18) for various numbers of iterations, 3-dimensional (3D) filtered backprojection (3DRP) (19), 3D-OSEM (16,18,20) for various numbers of iterations, and a 3D maximum a posteriori (MAP) algorithm dedicated to the microPET and Inveon scanners family (21)(22)(23).…”

Primer on molecular imaging technology

“…Furthermore, this light collection efficiency varies as a function of interaction location within the crystal, and so energy and time resolutions suffer as a result. Typically, in order to achieve acceptable light collection with 1-mm crystal pixels, their length is limited to ∼10 mm [3][4][5], but this significantly compromises the probability of absorbing 511-keV photons and hence limits the overall photon sensitivity performance.…”

Resolution properties of a prototype continuous miniature crystal element (cMiCE) scanner

“…While the HIDAC small animal PET system was able to achieve <1.0 mm image resolution using multi-wire proportional chamber technology combined with lead converter plates [3], it had a low intrinsic detection efficiency. Prototype research scintillator based PET systems have been able to achieve <1.0 mm FWHM image resolution [4][5][6][7][8]; however, due in part to high costs for fabricating ultra-high resolution discrete crystal detectors none of these systems were commercialized. Thus limitations in adoption are the fabrication complexity and costs for PET detector systems utilizing discrete crystals with cross-sections of less than 1.0 mm by 1.0 mm.…”