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Scanning rats on the high resolution research tomograph (HRRT): a comparison study with a dedicated micro-PET.

PURPOSE
The Siemens ECAT high resolution research tomograph (HRRT) is a dedicated human brain PET camera with a 6% absolute sensitivity and a (2.3 mm)(3) spatial resolution, improving to (1.8 mm)(3) when point spread function (PSF) modeling algorithms are used. These values are very close to those of dedicated small animal PET cameras such as the Siemens microPET FOCUS 120 (F120). The larger axial and transaxial field of view of the HRRT compared to the F120 allows, in principle, for simultaneous imaging of several rodents thus potentially reducing scanning costs and time. This study investigates the feasibility of using the HRRT for quantitative small animal brain studies.


METHODS
We compare, in terms of magnitude, reproducibility, and asymmetry, the nondisplaceable tissue input binding potentials (BP(ND)) in the striata obtained from [(11)C]methylphenidate scans of the same rats imaged on both the F120 and the HRRT. The animal studies are complemented by a phantom study aimed at investigating noise properties relevant to the size of typical regions of interest used in rat brain image analysis.


RESULTS
(i) The BP(ND) values obtained from HRRT data are lower than those obtained on the F120 by 38% when PSF modeling is not used, while they are 7% higher with PSF modeling. (ii) The within animal reproducibility on the HRRT is 18% without PSF modeling, worse than the 6% reproducibility on the F120, and is even further degraded to a value of 27% with the use of PSF modeling. (iii) The asymmetry between the left and right striatum in healthy rats worsens from 4.7% in the F120 images to 7.8% in the HRRT images reconstructed without PSF modeling, and is even worse with a value of 14.8% when PSF modeling is used. (iv) Overshooting artifacts and clumpiness in the noise structure of the HRRT images reconstructed with PSF modeling are clearly visible.


CONCLUSIONS
The spatial resolution achieved on the HRRT without the use of resolution recovery techniques is not sufficient to allow for reliable quantitative small animal brain imaging. While PSF modeling in the reconstruction of the HRRT images in principle improves the resolution close to the level of the F120, it also introduces small scale nonuniformity artifacts and overshooting artifacts which preclude reliable quantitative small animal brain imaging on the HRRT.