Noninvasive imaging at the molecular level is an emerging field in biomedical research. This paper introduces a new technology synergizing two leading imaging methodologies: positron emission tomography (PET) and magnetic resonance imaging (MRI). Although the value of PET lies in its high-sensitivity tracking of biomarkers in vivo, it lacks resolving morphology. MRI has lower sensitivity, but produces high soft-tissue contrast and provides spectroscopic information and functional MRI (fMRI). We have developed a three-dimensional animal PET scanner that is built into a 7-T MRI. Our evaluations show that both modalities preserve their functionality, even when operated isochronously. With this combined imaging system, we simultaneously acquired functional and morphological PET-MRI data from living mice. PET-MRI provides a powerful tool for studying biology and pathology in preclinical research and has great potential for clinical applications. Combining fMRI and spectroscopy with PET paves the way for a new perspective in molecular imaging.

This paper presents two new rebinning algorithms for the reconstruction of three-dimensional (3-D) positron emission tomography (PET) data. A rebinning algorithm is one that first sorts the 3-D data into an ordinary two-dimensional (2-D) data set containing one sinogram for each transaxial slice to be reconstructed; the 3-D image is then recovered by applying to each slice a 2-D reconstruction method such as filtered-backprojection. This approach allows a significant speedup of 3-D reconstruction, which is particularly useful for applications involving dynamic acquisitions or whole-body imaging. The first new algorithm is obtained by discretizing an exact analytical inversion formula. The second algorithm, called the Fourier rebinning algorithm (FORE), is approximate but allows an efficient implementation based on taking 2-D Fourier transforms of the data. This second algorithm was implemented and applied to data acquired with the new generation of PET systems and also to simulated data for a scanner with an 18 degrees axial aperture. The reconstructed images were compared to those obtained with the 3-D reprojection algorithm (3DRP) which is the standard "exact" 3-D filtered-backprojection method. Results demonstrate that FORE provides a reliable alternative to 3DRP, while at the same time achieving an order of magnitude reduction in processing time.

The microPET Primate 4-ring system (P4) is an animal PET tomograph with a 7.8 cm axial extent, a 19 cm diameter transaxial field of view (FOV) and a 22 cm animal port. The system is composed of 168 detector modules, each with an 8 x 8 array of 2.2 x 2.2 x 10 mm3 lutetium oxyorthosilicate crystals, arranged as 32 crystal rings 26 cm in diameter. The detector crystals are coupled to a Hamamatsu R5900-C8 PS-PMT via a 10 cm long optical fibre bundle. The detectors have a timing resolution of 3.2 ns, an average energy resolution of 26%, and an average intrinsic spatial resolution of 1.75 mm. The system operates in 3D mode without inter-plane septa, acquiring data in list mode. The reconstructed image spatial resolution ranges from 1.8 mm at the centre to 3 mm at 4 cm radial offset. The tomograph has a peak system sensitivity of 2.25% at the centre of the FOV with a 250-750 keV energy window. The noise equivalent count rate peaks at 100-290 kcps for representative object sizes. Images from two phantoms and three different types of laboratory animal demonstrate the advantage of the P4 system over the original prototype microPET. including its threefold improvement in sensitivity and a large axial FOV sufficient to image an entire mouse in a single bed position.

The Inveon dedicated PET (DPET) scanner is the latest generation of preclinical PET systems devoted to high-resolution and high-sensitivity murine model imaging. In this study, we report on its performance based on the National Electrical Manufacturers Association (NEMA) NU-4 standards. Methods: The Inveon DPET consists of 64 lutetium oxyorthosilicate block detectors arranged in 4 contiguous rings, with a 16.1-cm ring diameter and a 12.7-cm axial length. Each detector block consists of a 20 · 20 lutetium oxyorthosilicate crystal array of 1.51 · 1.51 · 10.0 mm elements. The scintillation light is transmitted to position-sensitive photomultiplier tubes via optical light guides. Energy resolution, spatial resolution, sensitivity, scatter fraction, and counting-rate performance were evaluated. The NEMA NU-4 image-quality phantom and a healthy mouse injected with 18 F-FDG and 18 F 2 were scanned to evaluate the imaging capability of the Inveon DPET. Results: The energy resolution at 511 keV was 14.6% on average for the entire system. In-plane radial and tangential resolutions reconstructed with Fourier rebinning and filtered backprojection algorithms were below 1.8-mm full width at half maximum (FWHM) at the center of the field of view. The radial and tangential resolution remained under 2.0 mm, and the axial resolution remained under 2.5-mm FWHM within the central 4-cm diameter of the field of view. The absolute sensitivity of the system was 9.3% for an energy window of 250-625 keV and a timing window of 3.432 ns. At a 350-to 625-keV energy window and a 3.432-ns timing window, the peak noise equivalent counting rate was 1,670 kcps at 130 MBq for the mouse-sized phantom and 590 kcps at 110 MBq for the rat-sized phantom. The scatter fractions at the same acquisition settings were 7.8% and 17.2% for the mouse-and rat-sized phantoms, respectively. The mouse image-quality phantom results demonstrate that for typical mouse acquisitions, the image quality correlates well with the measured performance parameters in terms of image uniformity, recovery coefficients, attenuation, and scatter corrections. Conclusion: The Inveon system, compared with previous generations of preclinical PET systems from the same manufacturer, shows significantly improved energy resolution, sensitivity, axial coverage, and counting-rate capabilities. Tomographi c systems dedicated to noninvasive, in vivo imaging of preclinical animal models have been widely used at research institutes in recent years (1,2). With the ability to longitudinally image the same subject, each individual animal can serve as its own control. Therefore, intersubject variability can be minimized. Because of the dramatic difference in size between humans and rodents, small-animal PET imposes higher performance requirements than does clinical PET, particularly on image resolution and sensitivity. The resolution and sensitivity improvements are mainly achieved by using smaller crystal sizes, smaller detector ring diameters, and longer axial coverage. With the goal of improving...

The microPET R4 scanner is a dedicated positron emission tomograph (PET) for studies of rodents. A number of scanner parameters such as spatial resolution, sensitivity, scatter, and count rate performance were determined in this work, which showed that the microPET R4 is a suitable PET scanner for small animals like mice and rats. In the center of the field of view (FOV) a maximal sensitivity of 43.66 cps/kBq for a centered point source was calculated from a measurement with a germanium-68 line source within an energy widow of 250-750 keV. A spatial resolution of 1.85 mm full-width at half-maximum (FWHM) in the axial direction and 1.66 mm FWHM in the transaxial direction was measured in the center with a 1-mm-diameter sodium-22 point source. Within the inner 20 mm of the FOV the volumetric resolution is better than 15.6 micro l, corresponding to a linear resolution of less than 2.5 mm in all three dimensions. Images of a high-resolution phantom and from mice and rat studies illustrate the good performance of the scanner. A maximal noise equivalent count rate (NECR) was reached at 174 kcps for a mouse phantom and at 93 kcps for a rat phantom (energy window 250-750 keV). Scatter fractions were measured between 0.30 and 0.42 for an energy window of 250-750 keV and phantom diameters similar to mice and rats. A comparison with the microPET P4 model for primates illustrates the gain in sensitivity due to a smaller detector ring diameter but also the changes in NECR.

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MicroPET II is a newly developed PET (positron emission tomography) scanner designed for high-resolution imaging of small animals. It consists of 17 640 LSO crystals each measuring 0.975 × 0.975 × 12.5 mm 3 , which are arranged in 42 contiguous rings, with 420 crystals per ring. The scanner has an axial field of view (FOV) of 4.9 cm and a transaxial FOV of 8.5 cm. The purpose of this study was to carefully evaluate the performance of the system and to optimize settings for in vivo mouse and rat imaging studies. The volumetric image resolution was found to depend strongly on the reconstruction algorithm employed and averaged 1.1 mm (1.4 µl) across the central 3 cm of the transaxial FOV when using a statistical reconstruction algorithm with accurate system modelling. The sensitivity, scatter fraction and noise-equivalent count (NEC) rate for mouse-and rat-sized phantoms were measured for different energy and timing windows. Mouse imaging was optimized with a wide open energy window (150-750 keV) and a 10 ns timing window, leading to a sensitivity of 3.3% at the centre of the FOV and a peak NEC rate of 235 000 cps for a total activity of 80 MBq (2.2 mCi) in the phantom. Rat imaging, due to the higher scatter fraction, and the activity that lies outside of the field of view, achieved a maximum NEC rate of 24 600 cps for a total activity of 80 MBq (2.2 mCi) in the phantom, with an energy window of 250-750 keV and a 6 ns timing window. The sensitivity at the centre of the FOV for these settings is 2.1%. This work demonstrates that different scanner settings are necessary to optimize the NEC count rate for different-sized animals and different injected doses. Finally, phantom and in vivo animal studies are presented to demonstrate the capabilities of microPET II for small-animal imaging studies.

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