NEMA NU 4-2008 performance evaluation and MR compatibility tests of an APD-based small animal PET-insert for simultaneous PET/MR imaging.

An avalanche photodiode (APD)-based small animal positron emission tomography (PET)-insert was fully evaluated for its PET performance, as well as potential influences on magnetic resonance imaging (MRI) performance. This PET-insert has an extended axial field of view (FOV) compared with the previous design to increase system sensitivity, as well as an updated cooling and temperature regulation to enable stable and reproducible PET acquisitions. The PET performance was evaluated according to the National Electrical Manufacturers Association NU4-2008 protocol. The energy and timing resolution's full width at half maximum were 16.1% and 4.7 ns, respectively. The reconstructed radial spatial resolution of the PET-insert was 1.8 mm full width at half maximum at the center FOV using filtered back projection for reconstruction and sensitivity was 3.68%. The peak noise equivalent count rates were 70 kcps for a rat-like and 350 kcps for a mouse-like phantom, respectively. Image quality phantom values and contrast recovery were comparable to state-of-the art PET-inserts and standalone systems. Regarding MR compatibility, changes in the mean signal-to-noise ratio for turbo spin echo and echo-planar imaging sequences were below 8.6%, for gradient echo sequences below 1%. Degradation of the mean homogeneity was below 2.3% for all tested sequences. The influence of the PET-insert on theB0maps was negligible and no influence on functional MRI sequences was detected. A mouse and rat imaging study demonstrated the feasibility ofin vivosimultaneous PET/MRI.

Performance evaluation of the inveon dedicated PET preclinical tomograph based on the NEMA NU-4 standards.

UNLABELLED: 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 x 20 lutetium oxyorthosilicate crystal array of 1.51 x 1.51 x 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(-) 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. The performance of the Inveon is suitable for successful murine model imaging experiments.

Evaluation of transmission methodology and attenuation correction for the microPET Focus 220 animal scanner.

An accurate, low noise estimate of photon attenuation in the subject is required for quantitative microPET studies of molecular tracer distributions in vivo. In this work, several transmission-based measurement techniques were compared, including coincidence mode with and without rod windowing, singles mode with two different energy sources ((68)Ge and (57)Co), and postinjection transmission scanning. In addition, the effectiveness of transmission segmentation and the propagation of transmission bias and noise into the emission images were examined. The (57)Co singles measurements provided the most accurate attenuation coefficients and superior signal-to-noise ratio, while (68)Ge singles measurements were degraded due to scattering from the object. Scatter correction of (68)Ge transmission data improved the accuracy for a 10 cm phantom but over-corrected for a mouse phantom. (57)Co scanning also resulted in low bias and noise in postinjection transmission scans for emission activities up to 20 MBq. Segmentation worked most reliably for transmission data acquired with (57)Co but the minor improvement in accuracy of attenuation coefficients and signal-to-noise may not justify its use, particularly for small subjects. We conclude that (57)Co singles transmission scanning is the most suitable method for measured attenuation correction on the microPET Focus 220 animal scanner.

Performance evaluation of the microPET focus: a third-generation microPET scanner dedicated to animal imaging.

UNLABELLED: The microPET Focus is the latest generation microPET system dedicated to high-resolution animal imaging and incorporates several changes to enhance its performance. This study evaluated the basic performance of the scanner and compared it with the Primate (P4) and Rodent (R4) models. METHODS: The system consists of 168 lutetium oxyorthosilicate (LSO) detectors arranged in 4 contiguous rings, with a 25.8-cm diameter and a 7.6-cm axial length. Each detector consists of a 12 x 12 LSO crystal array of 1.51 x 1.51 x 10.00 mm3 elements. The scintillation light is transmitted to position-sensitive photomultiplier tubes via optical fiber bundles. The system was evaluated for its energy and spatial resolutions, sensitivity, and noise equivalent counting rate. Phantoms and animals of varying sizes were scanned to evaluate its imaging capability. RESULTS: The energy resolution averages 18.5% for the entire system. Reconstructed image resolution is 1.3-mm full width at half maximum (FWHM) at the center of field of view (CFOV) and remains under 2 mm FWHM within the central 5-cm-diameter FOV in all 3 dimensions. The absolute sensitivity of the system is 3.4% at the CFOV for an energy window of 250-750 keV and a timing window of 10 ns. The noise equivalent counting-rate performance reaches 645 kcps for a mouse-size phantom using 250- to 750-keV and 6-ns settings. Emission images of a micro-Derenzo phantom demonstrate the improvement in image resolution compared with previous models. Animal studies exhibit the capability of the system in studying disease models using mouse, rat, and nonhuman primates. CONCLUSION: The Focus has significantly improved performance over the previous models in all areas evaluated. This system represents the state-of-the-art scintillator-based animal PET scanner currently available and is expected to advance the potential of small animal PET.

Cardiac PET imaging in mice with simultaneous cardiac and respiratory gating.

Gating firmware and software were developed for the microPET II small animal scanner. The measured cardiac and respiratory signals were collected and converted to TTL gating signals by a Biopac MP150 data acquisition system and sent to microPET II through two BNC connectors on the front panel. During acquisition, the coincidence monitor takes the average of the last eight gate input cycles and inserts this into the list mode data stream on the falling edge of the gating pulse. This value is then used to determine the current time interval of the next gate cycle when the list mode data are sorted into sinograms. The gating firmware and software were validated by an experiment using a rotating point source. Mouse heart (18F-FDG) and bone (18F(-)) imaging was performed with simultaneous cardiac and respiratory gating. It was clearly demonstrated that the contractile function of the mouse heart can be studied by cardiac-gated imaging with microPET II. The left ventricular volumes at different times of the cardiac cycle were measured and the ejection fraction was calculated. In the bone scan, no detectable movement caused by heart contraction was observed. Respiratory motion was more subtle with virtually no motion for more than 75% of the respiratory cycle. The motion of the mouse heart and bones in the thorax caused by respiration was less than 1 mm. It appears with the current resolution of PET, and the small fraction of the respiratory cycle in which motion occurs, that respiratory gating is probably not necessary for most mouse cardiac studies.

The influence of measurement uncertainties on the evaluation of the distribution volume ratio and binding potential in rat studies on a microPET R4: a phantom study.

In small animal positron emission tomography (PET) imaging, the injectable radiotracer dose is often limited by the tracer mass which, together with the tracer kinetics and scanner sensitivity, dictates the statistical quality of the time activity curves (TACs) used to extract biological parameters. We investigated the effect of measurement uncertainty on the determination of the distribution volume ratio (DVR) and binding potential (BP) as estimated using the tissue input Logan (DVR(L), BP(L)) and the ratio (DVRr, BPr) methods for two tracers, with the Concorde microPET R4 camera. Parameters' coefficients of variation (COV) were estimated from a combination of rat and phantom data. For 11C-dihydrotetrabenazine, the COV was 11% for the BP(L) and 13.4% for the BPr when using TACs obtained from individual regions of interest (ROIs) and segmented attenuation correction. The COVs were reduced to 7.5% (BP(L)) and 8.6% (BPr) when the striatal and cerebellar TACs were estimated as averages of 3 and 2 ROIs respectively. Results obtained for 11C-methylphenidate (MP) yielded approximately 30% higher COVs. With measured attenuation correction, the COVs were on average 100% higher. The presented method can be used to examine the contribution of a variety of imaging conditions to the uncertainty of biologically meaningful parameters.

Optimization and performance evaluation of the microPET II scanner for in vivo small-animal imaging.

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 x 0.975 x 12.5 mm3, 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 microl) 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.

MicroPET II: design, development and initial performance of an improved microPET scanner for small-animal imaging.

MicroPET II is a second-generation animal PET scanner designed for high-resolution imaging of small laboratory rodents. The system consists of 90 scintillation detector modules arranged in three contiguous axial rings with a ring diameter of 16.0 cm and an axial length of 4.9 cm. Each detector module consists of a 14 x 14 array of lutetium oxyorthosilicate (LSO) crystals coupled to a multi-channel photomultiplier tube (MC-PMT) through a coherent optical fibre bundle. Each LSO crystal element measures 0.975 mm x 0.975 mm in cross section by 12.5 mm in length. A barium sulphate reflector material was used between LSO elements leading to a detector pitch of 1.15 mm in both axial and transverse directions. Fused optical fibre bundles were made from 90 microm diameter glass fibres with a numerical aperture of 0.56. Interstitial extramural absorber was added between the fibres to reduce optical cross talk. A charge-division readout circuit was implemented on printed circuit boards to decode the 196 crystals in each array from the outputs of the 64 anode signals of the MC-PMT. Electronics from Concorde Microsystems Inc. (Knoxville, TN) were used for signal amplification, digitization, event qualification, coincidence processing and data capture. Coincidence data were passed to a host PC that recorded events in list mode. Following acquisition, data were sorted into sinograms and reconstructed using Fourier rebinning and filtered hackprojection algorithms. Basic evaluation of the system has been completed. The absolute sensitivity of the microPET II scanner was 2.26% at the centre of the field of view (CFOV) for an energy window of 250-750 keV and a timing window of 10 ns. The intrinsic spatial resolution of the detectors in the system averaged 1.21 mm full width at half maximum (FWHM) when measured with a 22Na point source 0.5 mm in diameter. Reconstructed image resolution ranged from 0.83 mm FWHM at the CFOV to 1.47 mm FWHM in the radial direction, 1.17 mm FWHM in the tangential direction and 1.42 mm FWHM in the axial direction at 1 cm offset from the CFOV. These values represent highly significant improvements over our earlier microPET scanner (approximately fourfold sensitivity increase and 25-35% improvement in linear spatial resolution under equivalent operating conditions) and are expected to be further improved when the system is fully optimized.

Simultaneous PET-MRI: a new approach for functional and morphological imaging.

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.

Performance evaluation of the microPET R4 PET scanner for rodents.

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.