Technical Note Performance evaluation of a small-animal PETCT system based on NEMA NU 4– 2008 standa

1.Abstract

Purpose: he Metis^TM PET/CT is a self-developed, silicon photomultiplier (SiPM) detector-based, rodent PET/CT system. The

objective of this study was to evaluate the performance of the system using the National Electrical Manufacturers Association

(NEMA) NU 4–2008 standard protocol.

Methods:Energy resolution, spatial resolution, sensitivity, scatter fraction (SF), noise-equivalent count rate (NECR),and image

quality (IQ) characteristics were measured. A micro Derenzo phantom experiment was performed to evaluate the spatial resolution using three-dimensional ordered-subsets expectation maximization (3D-OSEM) and maximum likelihood expectation maximization (MLEM)

reconstructed images. In addition, the CT imaging agent Ioverol 350 was mixed with fluorine-18 (¹⁸F)-fluorodeoxyglucose (FDG) and then injected into the micro Derenzo phantom to evaluate the PET/CT imaging. In vivo PET/CT imaging studies were also conducted in a healthy mouse and rat using ¹⁸F-FDG.

Results:The mean energy resolution of the system was 15.3%. The tangential resolution was 0.82 mm full-width half-maximum (FWHM) at the center of the field of the view (FOV), and the radial and axial resolution were generally lower than 2.0 mm FWHM. The spatial resolution was significantly improved when using 3D-OSEM,especially the axial FWHM could be improved by

up to about 57%. The system absolute sensitivity was 7.7% and 6.8% for an energy window of 200–750 and 350–750 keV respectively. The scatter fraction was 8.2% and 12.1% for the mouse and rat like phantom respectively. The peak NECR

was 1343.72 kcps at 69 MBq and 640.32 kcps at 53 MBq for the mouse- and rat-like phantom respectively. The 1-mm fillable rod in the IQ phantom can be clearly observed.We can identify the 0.6-mm aperture of the micro Derenzo phantom

image clearly using 3D-OSEM (10 subsets, 5 iterations). We also performed the fusion of the PET and CT images of the mouse

and the brain imaging of the rat.

Conclusions: The results show that the system has the characteristics of high-resolution, high-sensitivity, and excellent IQ

and is suitable for rodent imaging-based research.

2.INTRODUCTION

Experimental equipment：

FIGURE 1 The appearance of the MetisTM PET/CT system coincidence time window of 2 ns. Table 1 lists the structural parameters of PET/CT system.

Energy resolution：

The mean energy resolution of the whole PET/CT system is 15.3%, with the best and worst energy resolutions being 7.4% and 49.8% respectively.

Spatial resolution：

The measured FWHM of 22Na point source using FBP in three directions are shown in Figure 3a. The FWHM resolutions are between 0.82 and 1.94 mm. The FWHM volumetric resolutions for the axial FOV and 1/4 axial FOV from center are 1.16 and 1.56 mm3 respectively. Figure 3b shows the FWHM of the point source after 3D-OSEM reconstruction with scatter and random corrections.

Figure 4 shows the PET images of the micro Derenzo phantom, and the CT image after injecting Ioverol 350 developer. The tangential resolution is the best when using 3D-OSEM (10 subsets, 5 iterations), between 0.6 and 0.5 mm. The radial resolution

is 0.7 mm, and the axial resolution is the worst, between 1.0 and 0.9 mm.

The absolute peak sensitivity of the system is 6.8% and maximum sensitivity is 0.062 cps/Bq with the energy window of 350–750 keV at the center of FOV, while the absolute peak sensitivity value is 7.7% and maximum sensitivity is 0.070 cps/Bq with the energy window of 200–750 keV. Figure 5 shows the sensitivity profiles measured along the system axially using the 22Na

point source.

The SFs for the mouse- and rat-like phantoms are 8.2% and 12.1% respectively. The total, true, random,and scatter

coincidence count rates performance for two phantoms are shown in Figure 6. The NECR is 1343.72 kcps at 69 MBq for the mouse-like phantom, and 640.32 kcps for the rat-like phantom at 53 MBq, whereas the peak true count rate is 1666.94 kcps at 69

MBq and 1038.48 kcps at 113 MBq for mouse- and rat-like phantoms, respectively.

Imaging studies：

The PET images of the IQ phantom reconstructed using MLEM are shown in Figure 7a, with even the smallest (i.e., the 1-mm) rod visualized. The average, maximum, minimum activity concentration and the %STD in the uniform area are shown in Table 2. RCs and %STD for five rods of different size are shown in Figure 7b. The SOR is 0.20 (19.47%) for the water-filled chamber and 0.14 (17.75%) for the air-filled chamber.

Figure 8 shows coronal-section PET and CT images of a healthy mouse using PMOD software. Figure 9 shows a clear PET images of the healthy rat brain.

2.DISCUSSION

Different PET system designs and implementations will lead to differences in the sensitivity of systems. Compared with previous preclinical systems, MetisTM PET/CT system has a smaller scintillation crystal size, smaller detection ring diameter, and larger axial extent, which increases the solid angle coverage and therefore increases the system sensitivity.Owing to the increased sensitivity, the NECRs have also been significantly improved. The system peak NECR values are 1343.72 kcps for the mouse-like phantom and 640.32 kcps for the rat-like phantom, which are better than most commercial small animal PET/CT systems.