Abstract
The Hyperion IID PET insert is the first scanner using fully digital silicon photomultipliers for simultaneous PET/MR imaging of small animals up to rabbit size. In this work, we evaluate the PET performance based on the National Eletrical Manufacturers Association (NEMA) NU 4-2008 standard, whose standardized measurement protocols allow comparison of different small-animal PET scanners. The Hyperion IID small-animal PET/MR insert comprises three rings of 20 detector stacks with pixelated scintillator arrays with a crystal pitch of 1 mm, read out with digital silicon photomultipliers. The scanner has a large ring diameter of 209.6 mm and an axial field of view of 96.7 mm. We evaluated the spatial resolution, energy resolution, time resolution and sensitivity by scanning a 22Na point source. The count rates and scatter fractions were measured for a wide range of 18F activity inside a mouse-sized scatter phantom. We evaluated the image quality using the mouse-sized image quality phantom specified in the NEMA NU4 standard, filled with 18F. Additionally, we verified the in-vivo imaging capabilities by performing a simultaneous PET/MRI scan of a mouse injected with 18F-FDG. We processed all measurement data with an energy window of 250 keV to 625 keV and a coincidence time window of 2 ns. The filtered-backprojection reconstruction of the point source has a full width at half maximum (FWHM) of 1.7 mm near the isocenter and degrades to 2.5 mm at a radial distance of 50 mm. The scanner’s average energy resolution is 12.7% (ΔE/E FWHM) and the coincidence resolution time is 609 ps. The peak absolute sensitivity is 4.0% and the true and noise-equivalent count rates reach their peak at an activity of 46 MBq with 483 kcps and 407 kcps, respectively, with a scatter fraction of 13%. The iterative reconstruction of the image quality phantom has a uniformity of 3.7%, and recovery coefficients from 0.29, 0.91 and 0.94 for rod diameters of 1 mm, 3 mm and 5 mm, respectively. After application of scatter and attenuation corrections, the air- and water-filled cold regions have spill-over ratios of 6.3% and 5.4%, respectively. The Hyperion IID PET/MR insert provides state-of-the-art PET performance while enabling simultaneous PET/MRI acquisition of small animals up to rabbit size.
Highlights
Positron emission tomography (PET) and magnetic resonance imaging (MRI) are two imaging modalities which provide complementary information: PET provides high-sensitivity for in-vivo molecular imaging while MRI provides morphological information with high soft-tissue contrast (Judenhofer and Cherry 2013)
We evaluated the image quality using the mousesized image quality phantom specified in the National Eletrical Manufacturers Association (NEMA) NU4 standard, filled with 18F
We present the PET performance evaluation of the Hyperion IID scanner based on NEMA NU-4
Summary
Positron emission tomography (PET) and magnetic resonance imaging (MRI) are two imaging modalities which provide complementary information: PET provides high-sensitivity for in-vivo molecular imaging while MRI provides morphological information with high soft-tissue contrast (Judenhofer and Cherry 2013). The integration of simultaneous PET and MRI is challenging: both modalities interfere strongly with each other if not mitigated, and the physical space available for integration of the two modalities is heavily constrained (Vandenberghe and Marsden 2015) To overcome these challenges, we are the first group to develop a simultaneous PET/MR insert with fully digital silicon photomultipliers: the Hyperion IID insert Weissler et al 2015. We are the first group to develop a simultaneous PET/MR insert with fully digital silicon photomultipliers: the Hyperion IID insert Weissler et al 2015 These integrated detectors digitize the signal of each detected photon directly in the photodetector, eliminating any interferenceprone analog signal transmission over long wires (Timms 1992). This unique approach results in a compact, highly integrated and robust data readout, which facilitates the design of the first small-animal PET/ MRI scanner with a wide bore of 200 mm (160 mm with coil) offering imaging of rabbits and other animals of similar size
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