Technical note
Spatial resolution of the Inveon small-animal PET scanner for the entire field of view

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Abstract

In high throughput small-animal positron emission tomography (PET) studies, it can be advantageous to scan more than one animal at the same time. To optimally position these animals in the scanner, both sensitivity and spatial resolution have to be known across the entire field of view (FOV). The Siemens Inveon small-animal PET scanner has been characterized according to NEMA NU 4-2008 standards, but the spatial resolution has only been determined in a small portion of the FOV and only for filtered backprojection (FBP). In this study, the spatial resolution in the entire FOV was determined for the Inveon using FBP, ordered subset expectation maximization in 2 dimensions (OSEM2D) and maximum a posteriori (MAP) reconstructions. OSEM2D produced the best spatial resolution in the axial center of the FOV, but MAP outperformed the two other reconstruction algorithms in terms of uniformity of the resolution.

Introduction

The Inveon (Siemens Medical Solutions, Knoxville TN, USA) small-animal Positron Emission Tomograph (PET) has been characterized in several studies [1], [2], [3], [4]. In some of these studies the NEMA (NEMA NU-4 2008, [5]) characterization standards were used. Visser et al. [1] and Constantinescu et al. [2] determined transaxial spatial resolution for the axial center of the scanner. Bao et al. [3] and Kemp et al. [4] extended these measurements to profiles taken at an offset one quarter of the axial length from the center. Visser et al. used maximum a posteriori (MAP) reconstructions next to the NEMA-specified filtered backprojection (FBP).

In high throughput small-animal PET studies where several mice are scanned simultaneously, information about the spatial resolution and sensitivity across the entire field of view (FOV) is helpful to optimally position the animals in the scanner. With its relatively large axial length, the Inveon is especially well equipped for multi-animal studies. Typically, up to four mice can be scanned simultaneously in different configurations.

For FBP and other 2 dimensional (2D) reconstruction algorithms, dept-of-interaction (DOI) effects for photons with oblique incidence angles relative to the detector face, seriously deteriorate the spatial resolution in off-axis positions. It is therefore desirable to use 3 dimensional (3D) statistical reconstruction algorithms that should provide a more uniform spatial resolution. Both axial and transaxial blurring are suppressed during reconstruction because the point spread functions (PSFs) throughout the FOV are being used in the projection matrix during reconstruction by the ordered subset expectation maximization in 3 dimensions (OSEM3D) and MAP algorithms.

In this study we extended the standard NEMA protocol and determined spatial resolution in tangential and radial directions for the full FOV, for three different reconstruction algorithms.

Section snippets

Data acquisition and image reconstruction

Spatial resolution of the Inveon was measured using a 22Na point source with an active diameter of less than 0.25 mm embedded in a Lucite disk (Isotope Products Laboratories, Berlin, Germany). The activity of the point source was 0.92 MBq, as measured in a dose calibrator (VDC-404; Veenstra, Joure, The Netherlands).

Spatial resolution measurements were not corrected for source dimension, positron range, or non-colinearity of the 2 photons involved in positron annihilation. The point source was

Results

Table 1 shows the minimum and maximum values of the FWHM and FWTM for the three reconstruction algorithms.

MAP has the most uniform spatial resolution with FWHM in tangential direction between 1.29 and 1.47 mm, and a FWHM in radial direction between 1.30 and 1.88 mm. OSEM2D has an overall smaller FWHM tangentially. In the radial direction, the FWHM for OSEM2D is only smaller than MAP in the transaxial center.

Fig. 1A and B shows the FWHM in the radial and tangential directions for the three

Discussion and conclusion

The present study shows that with MAP reconstruction, the most uniform distribution of spatial resolution can be obtained. OSEM2D performs slightly better in the center of the FOV, but causes a large increase at the edges of the FOV. FBP has no advantages with respect to spatial resolution.

Since MAP results in relatively small variations in spatial resolution across the FOV, the positions with the highest sensitivity should be used for imaging and mice may be placed besides each other, or on

Conflicts of interest

None

References (6)

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There are more references available in the full text version of this article.

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