Spatial Resolution and Sensitivity of the Inveon Small-Animal PET Scanner

Abstract

The Inveon small-animal PET scanner is characterized by a large, 127-mm axial length and a 161-mm crystal ring diameter. The associated high sensitivity is obtained by using all lines of response (LORs) up to the maximum ring difference (MRD) of 79, for which the most oblique LORs form acceptance angles of 38.3°with transaxial planes. The result is 2 phenomena that are normally not encountered in PET scanners: a parallax or depth-of-interaction effect in the axial direction and the breakdown of Fourier rebinning (FORE). Both effects cause a deterioration of axial spatial resolution. Limiting the MRD to smaller values reduces this axial blurring at the cost of sensitivity. Alternatively, 3-dimensional (3D) reconstruction techniques can be used in which the rebinning step is absent. The aim of this study was to experimentally determine the spatial resolution and sensitivity of the Inveon for its whole field of view (FOV). Methods: Spatial resolution and sensitivity were measured using filtered backprojection (FBP) with FORE, FBP with LOR angle-weighted adapted FORE (AFORE), and 3D ordered-subset expectation maximization followed by maximum a posteriori reconstruction (OSEM3D/ MAP). Results: Tangential and radial full width at half maximum (FWHM) showed almost no dependence on the MRD using FORE and FBP. Tangential FWHMs were 1.5 mm in the center of the FOV (CFOV) and 1.8 mm at the edge of the FOV (EFOV). Radial FWHMs were 1.5 and 3.0 mm in the CFOV and EFOV, respectively. In contrast, axial FWHMs increased with the MRD and ranged between 1.1 and 2.0 mm in the CFOV and between 1.5 and 2.7 mm in the EFOV for a MRD between 1 and 79. AFORE improved the axial resolution for a large part of the FOV, but image noise increased. OSEM3D/MAP yielded uniform spatial resolution in all directions, with an average FWHM of 1.65 6 0.06 mm. Sensitivity in the CFOV for the default energy and coincidence time window was 0.068; peak sensitivity was 0.111. Conclusion: The Inveon showed high spatial resolution and high sensitivity, both of which can be maintained using OSEM3D/MAP reconstruction instead of rebinning and 2D algorithms. The Inveon (Siemens) small-animal PET scanner differs from its predecessor, the microPET Focus 120 (F120; Siemens), by its larger lutetium orthosilicate (LSO) detector blocks, improved processing of high-speed events (1-4), and shorter, tapered light guides coupling the detector blocks to the photomultiplier tubes (5). The Inveon contains 4 rings of 16 blocks-20 · 20 detectors of 1.5 · 1.5 mm each, resulting in an axial length of 127 mm. The F120 consists of 4 rings of 24 blocks-12 · 12 detectors of the same size, leading to an axial length of 76 mm. Table 1 compares the geometric properties of the Inveon with several other commercial crystal-based small-animal PET scanners.The main advantage of the larger axial field of view (FOV) of the Inveon is the higher detection efficiency (or sensitivity), with a peak value as specified by the manufacturer of greater than 0.1 in the center of the FOV (CFOV) for the ...

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