A prototype high-resolution animal positron tomograph with avalanche photodiode arrays and LSO crystals

To fully utilize positron emission tomography (PET) as a non-invasive tool for tissue characterization, dedicated instrumentation is being developed which is specially suited for imaging mice and rats. Semiconductor detectors, such as avalanche photodiodes (APDs), may offer an alternative to photomultiplier tubes for the readout of scintillation crystals. Since the scintillation characteristics of lutetium oxyorthosilicate (LSO) are well matched to APDs, the combination of LSO and APDs seems favourable, and the goal of this study was to build a positron tomograph with LSO-APD modules to prove the feasibility of such an approach. A prototype PET scanner based on APD readout of small, individual LSO crystals was developed for tracer studies in mice and rats. The tomograph consists of two sectors (86 mm distance), each comprising three LSO-APD modules, which can be rotated for the acquisition of complete projections. In each module, small LSO crystals (3.7 x 3.7 x 12 mm3) are individually coupled to one channel within matrices containing 2x8 square APDs (2.6 x 2.6 mm2 sensitive area per channel). The list-mode data are reconstructed with a penalized weighted least squares algorithm which includes the spatially dependent line spread function of the tomograph. Basic performance parameters were measured with phantoms and first experiments with rats and mice were conducted to introduce this methodology for biomedical imaging. The reconstructed field of view covers 68 mm, which is 80% of the total detector diameter. Image resolution was shown to be 2.4 mm within the whole reconstructed field of view. Using a lower energy threshold of 450 keV, the system sensitivity was 350 Hz/MBq for a line source in air in the centre of the field of view. In a water-filled cylinder of 4.6 cm diameter, the scatter fraction at the centre of the field of view was 16% (450 keV threshold). The count rate was linear up to 700 coincidence counts per second. In vivo studies of anaesthetized rats and mice showed the feasibility of in vivo imaging using this PET scanner. The first LSO-APD prototype tomograph has been successfully introduced for in vivo animal imaging. APD arrays in combination with LSO crystals offer new design possibilities for positron tomographs with finely granulated detector channels.     

Simultaneous MR/PET imaging of the human brain: feasibility study

The purpose of this study was to apply a magnetic resonance (MR) imaging-compatible positron emission tomographic (PET) detector technology for simultaneous MR/PET imaging of the human brain and skull base. The PET detector ring consists of lutetium oxyorthosilicate (LSO) scintillation crystals in combination with avalanche photodiodes (APDs) mounted in a clinical 3-T MR imager with use of the birdcage transmit/receive head coil. Following phantom studies, two patients were simultaneously examined by using fluorine 18 fluorodeoxyglucose (FDG) PET and MR imaging and spectroscopy. MR/PET data enabled accurate coregistration of morphologic and multifunctional information. Simultaneous MR/PET imaging is feasible in humans, opening up new possibilities for the emerging field of molecular imaging.     

Performance evaluation of microPET: a high-resolution lutetium oxyorthosilicate PET scanner for animal imaging.

A new dedicated PET scanner, microPET, was designed and developed at the University of California, Los Angeles, for imaging small laboratory animals. The goal was to provide a compact system with superior spatial resolution at a fraction of the cost of a clinical PET scanner. METHODS: The system uses fiberoptic readout of individually cut lutetium oxyorthosilicate (LSO) crystals to achieve high spatial resolution. Each microPET detector consists of an 8 x 8 array of 2 x 2 x 10-mm LSO scintillation crystals that are coupled to a 64-channel photomultiplier tube by optical fibers. The tomograph consists of 30 detectors in a continuous ring with a 17.2-cm diameter and fields of view (FOVs) of 11.25 cm in the transaxial direction and 1.8 cm in the axial direction. The system has eight crystal rings and no interplane septa. It operates exclusively in the three-dimensional mode and has an electronically controlled bed that is capable of wobbling with a radius of 300 microm. We describe the performance of the tomograph in terms of its spatial, energy and timing resolution, as well as its sensitivity and counting-rate performance. We also illustrate its overall imaging performance with phantom and animal studies that demonstrate the potential applications of this device to biomedical research. RESULTS: Images reconstructed with three-dimensional filtered backprojection show a spatial resolution of 1.8 mm at the center of the FOV (CFOV), which remains <2.5 mm for the central 5 cm of the transaxial FOV. The resulting volumetric resolution of the system is <8 microL. The absolute system sensitivity measured with a 0.74 MBq (20 microCi) 68Ge point source at the CFOV is 5.62 Hz/kBq. The maximum noise equivalent counting rate obtained with a 6.4-cm diameter cylinder spanning the central 56% of the FOV is 10 kcps, whereas the scatter fraction is 37% at the CFOV for an energy window of 250-650 keV and the same diameter cylinder. CONCLUSION: This is the first PET scanner to use the new scintillator LSO and uses a novel detector design to achieve high volumetric spatial resolution. The combination of imaging characteristics of this prototype system (resolution, sensitivity, counting-rate performance and scatter fraction) opens up new possibilities in the study of animal models with PET.     

A BGO detector unit for a stationary high resolution positron emission tomograph

A new encoding scheme was developed that is applicable to bismuth germanate (BGO) detector units for a high resolution positron emission tomograph. The detector unit is composed of eight equal-size, 3 mm wide BGO crystals coupled to a dual-module square photomultiplier tube (PMT) through a pair of light guides. The light guides divide the scintillation light between the two modules of the PMT, with a ratio dependent on the crystal of origin. Mean detector pair spatial resolution as measured by the point spread function was 3.9 mm full width at half maximum (FWHM) and reconstructed image spatial resolution was 4.8 mm FWHM at the center of the field of view. Timing resolution between two detector units was 6.0 ns FWHM. Energy resolution was 24% FWHM for 511 keV gamma rays. The finest spots of the Derenzo phantom were clearly resolved.     

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.     

Preliminary study on potential of the jPET-D4 human brain scanner for small animal imaging

Objective  One trend in positron emission tomography (PET) instrumentation over the last decade has been the development of scanners dedicated to small animals such as rats and mice. Thicker crystals, which are necessary to obtain higher sensitivity, result in degraded spatial resolution in the peripheral field-of-view (FOV) owing to the parallax error. On the other hand, we are developing the jPET-D4, which is a dedicated human brain PET scanner that has a capability for depth-of-interaction (DOI) measurement. Although its crystal width is about twice that of commercially available small animal PET scanners, we expect the jPET-D4 to have a potential for small animal imaging by making full use of the DOI information. In this article, we investigate the jPET-D4’s potential for small animal imaging by comparing it with the microPET Focus220, a state-of-the-art PET scanner dedicated to small animals. Methods  The jPET-D4 uses four-layered GSO crystals measuring 2.9 mm × 2.9 mm × 7.5 mm, whereas the microPET Focus220 uses a single layer of LSO crystals measuring 1.5 mm × 1.5 mm × 10.0 mm. First, the absolute sensitivity, counting rate performance and spatial resolution of both scanners were measured. Next a small hot-rod phantom was used to compare their imaging performance. Finally, a rat model with breast tumors was imaged using the jPET-D4. Results  Thanks to the thicker crystals and the longer axial FOV, the jPET-D4 had more than four times higher sensitivity than the microPET Focus220. The noise equivalent counting-rate performance of the jPETD4 reached 1,024 kcps for a rat-size phantom, whereas that of the microPET Focus220 reached only 165 kcps. At the center of the FOV, the resolution was 1.7 mm for the microPET Focus220, whereas it was 3.2 mm for the jPET-D4. On the other hand, the difference of resolution became smaller at the off-center position because the radial resolution degraded faster for the microPET Focus220. The results of phantom imaging showed that the jPET-D4 was comparable to the microPET Focus220 at the off-center position even as the microPET Focus220 outperformed the jPET-D4 except for the peripheral FOV. Conclusions  The jPET-D4 human brain PET scanner, which was designed to achieve not only high resolution but also high sensitivity by measuring DOI information, was proven to have a potential for small animal imaging.     

Technical performance evaluation of a human brain PET/MRI system

Objectives

Technical performance evaluation of a human brain PET/MRI system.

Methods

The magnetic field compatible positron emission tomography (PET) insert is based on avalanche photodiode (APD) arrays coupled with lutetium oxyorthosilicate (LSO) crystals and slip-fits into a slightly modified clinical 3-T MRI system. The mutual interference between the two imaging techniques was minimised by the careful design of the hardware to maintain the quality of the B ₀ and B ₁ field homogeneity.

Results

The signal-to-noise ratio (SNR) and the homogeneity of the MR images were minimally influenced by the presence of the PET. Measurements according to the Function Biomedical Informatics Research Network (FBIRN) protocol proved the combined system’s ability to perform functional MRI (fMRI). The performance of the PET insert was evaluated according to the National Electrical Manufacturers Association (NEMA) standard. The noise equivalent count rate (NEC) peaked at 30.7?×?10³?counts/s at 7.3?kBq/mL. The point source sensitivity was greater than 7?%. The spatial resolution in the centre field of view was less than 3?mm. Patient data sets clearly revealed a noticeably good PET and MR image quality.

Conclusion

PET and MRI phantom tests and first patient data exhibit the device’s potential for simultaneous multiparametric imaging.

Key Points

? Combination of PET and MRI is a new emerging imaging technology. ? Evaluated brain PET/MRI enables uncompromised imaging performance. ? PET/MRI aims to provide multiparametric imaging allowing acquisition of morphology and metabolism.     

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

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.     

High resolution BrainPET combined with simultaneous MRI

After the successful clinical introduction of PET/CT, a novel hybrid imaging technology combining PET with the versatile attributes of MRI is emerging. At the Forschungszentrum Jülich, one of four prototypes available worldwide combining a commercial 3T MRI with a newly developed BrainPET insert has been installed, allowing simultaneous data acquisition with PET and MRI. The BrainPET is equipped with LSO crystals of 2.5 mm width and Avalanche photodiodes (APD) as readout electronics. Here we report on some performance characteristics obtained by phantom studies and also on the initial BrainPET studies on various patients as compared with a conventional HR+ PET-only scanner. MATERIAL, METHODS: The radiotracers [18F]-fluoro-ethyl-tyrosine (FET), [11C]-flumazenil and [18F]-FP-CIT were applied. RESULTS: Comparing the PET data obtained with the BrainPET to those of the HR+ scanner demonstrated the high image quality and the superior resolution capability of the BrainPET. Furthermore, it is shown that various MR images of excellent quality could be acquired simultaneously with BrainPET scans without any relevant artefacts. DISCUSSION, CONCLUSION: Initial experiences with the hybrid MRI/BrainPET indicate a promising basis for further developments of this unique technique allowing simultaneous PET imaging combined with both anatomical and functional MRI.     

Performance characteristics of a whole-body positron tomograph

This paper describes an investigation of some of the important physical characteristics of a whole-body positron tomograph consisting of two rings of bismuth germanate detectors of dimensions 5.6 mm X 30 mm X 30 mm (512/ring). The resolution applicable to in vivo imaging is six mm or more, depending on radionuclide and reconstruction filter and is very uniform over the field of view normally used. Axial resolution can be varied by moving side collimation (maximum approximately 16 mm FWHM with interplane septa). A simple scheme has been devised to correct for loss of true coincidence events with varying count rate based on the total random and multiple coincidence rates. It is concluded that correction for scattered radiation should be implemented for reliable quantitation.     

Performance characteristics obtained for a new 3-dimensional lutetium oxyorthosilicate-based whole-body PET/CT scanner with the National Electrical Manufacturers Association NU 2-2001 standard.

This article reports the results of performance measurements obtained for the lutetium oxyorthosilicate (LSO)-based whole-body PET/CT scanner Biograph 16 HI-REZ with the National Electrical Manufacturers Association (NEMA) NU 2-2001 standard. The Biograph 16 HI-REZ combines a multislice (16-slice) spiral CT scanner with a PET scanner composed of 24.336 LSO crystals arranged in 39 rings. The crystal dimensions are 4.0x4.0x20 mm3, and the crystals are organized in 13x13 blocks coupled to 4 photomultiplier tubes each. The 39 rings allow the acquisition of 81 images 2.0 mm thick, covering an axial field of view of 162 mm. The low- and high-energy thresholds are set to 425 and 650 keV, respectively, acquiring data within a 4.5-ns-wide coincidence window. METHODS: Performance measurements for the LSO-based PET/CT scanner were obtained with the NEMA NU 2-2001 standard, taking into account issues deriving from the presence of intrinsic radiation. RESULTS: The results obtained with the NEMA NU 2-2001 standard measurements were as follows: average transverse and axial spatial resolutions (full width at half maximum) at 1 cm and at 10 cm off axis of 4.61 (5.10) mm and 5.34 (5.91) mm, respectively; average sensitivity of 4.92 counts per second per kilobecquerel for the 2 radial positions (0 and 10 cm); 34.1% system scatter fraction; and peak noise equivalent count (NEC) rates of 84.77 kilocounts per second (kcps) at 28.73 kBq/mL (k=1 in the NEC formula; noiseless random correction) and 58.71 kcps at 21.62 kBq/mL (k=2; noisy random correction). CONCLUSION: The new integrated PET/CT system Biograph 16 HI-REZ has good overall performance, with, in particular, a high resolution, a low scatter fraction, and a very good NEC response.     

Investigation of single, random, and true counts from natural radioactivity in LSO-based clinical PET

OBJECTIVE: Lutetium oxyorthosilicate (LSO) contains natural radioactivity that emits beta particles and three gamma photons simultaneously. These beta particles and gamma photons increase the single and random rates in a positron emission tomography (PET) system while a beta particle and gamma photon produced in the same decay of Lu-176 and detected by another detector can be beta-gamma coincidence true events. The purpose of this work is to measure the single, random, and true count rates due to the natural radioactivity in LSO and determine the optimum lower energy threshold level for an energy window in an LSO-based clinical PET. METHODS: First, we measured the energy spectra of these beta particles and gamma photons in LSO using a single crystal to obtain the basic data. Then, we measured single, random, and true count rates of an LSO-based clinical PET from the natural radioactivity as a function of the lower energy threshold. RESULTS: In the PET, single and random count rates due to the natural background activity were gradually decreased as the lower energy threshold level increased. The true count rates due to the beta-gamma coincidence were more than 10 kcps below a lower energy threshold of 250 keV. However, these true count rates due to the natural radioactivity in LSO can be decreased to less than 1 kcps at a lower energy threshold level set at more than 350 keV. CONCLUSION: With these considerations, in an LSO-based clinical PET, a lower energy threshold level set at above 350 keV is recommended.     

Dynamic imaging of striatal D2 receptors in mice using quad-HIDAC PET.

The novel, dedicated small animal PET tomograph, quad-HIDAC, offers submillimeter resolution in instrumental characterization experiments. The aim of this study was to establish the tomograph's utility in a biologic application and to demonstrate the feasibility of rapid dynamic neuroreceptor imaging in mice. METHODS: We used the well-established, high-affinity dopamine D(2) receptor PET ligand (18)F-fallypride for imaging striatal D(2) receptors in NMRI mice. Dynamic PET data were acquired using the quad-HIDAC tomograph and subject to 2 different kinetic modeling approaches. The cerebellum, a brain region devoid of D(2) receptors, was chosen as a reference region for kinetic modeling. RESULTS: The resolution of the quad-HIDAC camera allowed clear visualization of the left and right mouse striatum with high target-to-nontarget signal ratios. The sensitivity of the tomograph permitted the generation of time-activity curves with initial time frames of 120 s. PET experiments acquiring data for 150 min demonstrated that the binding potential of (18)F-fallypride could be fitted robustly with both reference tissue models for scan durations of >or=40 min. Voxel-wise modeling resulted in parametric maps of high quality. The values for the binding potential in the striatum reached approximately 14, consistent with striatum-to-cerebellum ratios extracted from regional time-activity curves. Comparison of in vivo PET imaging results with ex vivo postmortem tissue sampling analyses indicated discrepancies in signal intensity, possibly resulting from scatter and random background in the cerebellum region of interest and leading to an overestimation of cerebellar activity concentrations and degradation of striatum-to-cerebellum ratios in PET experiments. Intraperitoneal injection of the unlabeled D(2) receptor antagonist haloperidol 30 min before intravenous injection of (18)F-fallypride blocked tracer accumulation in the striatum by >95%. CONCLUSION: The quad-HIDAC camera represents a powerful tool for future dynamic neuroreceptor PET studies in mice and rats under numerous pharmacologic or pathophysiologic conditions.     

Imaging in vivo herpes simplex virus thymidine kinase gene transfer to tumour-bearing rodents using positron emission tomography and [18F]FHPG

Radiolabelled ganciclovir analogues have shown promise as imaging agents to detect herpes simplex virus thymidine kinase (HSVtk) expression. This study evaluated the use of positron emission tomography (PET) imaging with 9-[(3-[¹⁸F]fluoro-1-hydroxy-2-propoxy)methyl]guanine ([¹⁸F]FHPG) to assess gene transfer into tumours. HSVtk-positive and HSVtk-negative cell lines were first treated in vitro with [¹⁸F]FHPG. To assess the efficacy of PET in detecting HSVtk expression following in vivo gene transfer, mice were injected intravenously with an adenovirus encoding HSVtk (Ad.HSVtk), a control vector (Ad.Bgl2) or saline. Subcutaneous human glioma xenografts were grown in mice and treated by direct injection of Ad.HSVtk or Ad.Bgl2. Imaging was performed 48 h after transduction. Similar experiments were performed using Fischer rats implanted with syngeneic tumours. The presence of the HSVtk protein was confirmed by immunohistochemistry. Biodistribution studies were also obtained in 14 naive mice. In vitro studies showed high and specific uptake of [¹⁸F]FHPG in HSVtk-positive cell lines, with an uptake ratio of up to 27:1. PET imaging and direct counting of major organs demonstrated HSVtk-specific tracer retention. In mice, HSVtk-positive tumours retained 3.4% dose/gram as compared to 0.6% for control tumours (P=0.03). They were clearly seen on the PET images as early as 100 min post injection. Similar results were obtained with syngeneic rat tumours. Biodistribution studies demonstrated the rapid distribution and clearance of the tracer in all major organs. Our results demonstrate that PET imaging of HSVtk gene transfer to tumours is feasible and is highly specific for HSVtk expression.     

Imaging of weak-source distributions in LSO-based small-animal PET scanners.

Lutetium oxyorthosilicate (LSO)- or lutetium-yttrium oxyorthosilicate (LYSO)-based PET scanners have intrinsic radioactivity in the scintillator crystals due to the presence of (176)Lu, which decays by beta-emission followed by one or more prompt gamma-ray emissions. This leads to intrinsic true counts that can influence the image when scanning low levels of activity. An evaluation of the effects of this intrinsic activity for low levels of activity and different energy windows is performed on an LSO-based small-animal PET scanner. METHODS: Intrinsic count rate and sensitivity were measured for a range of lower-level discriminators (LLDs) ranging from 100 to 750 keV. The noise equivalent count rate (NECR) as a function of LLD for activity levels from 100 Bq to 100 kBq was estimated using a combination of measurement and previously published data for this scanner. Phantom imaging was performed using three (68)Ge sources of strength 55, 220, and 940 Bq and LLD levels of 250, 350, and 400 keV. The images were assessed using a contrast-to-noise ratio (CNR) analysis and by comparing the observed ratio of source activities to the true ratio value. RESULTS: The intrinsic true count rate is reduced from 940 counts per second (cps) for a 250- to 750-keV energy window to <2 cps for a 400- to 750-keV window. There is a corresponding 2-fold drop in sensitivity for detected true events for external positron sources for these 2 energy windows. The NECR versus LLD curves showed a highly peaked shape, with the optimum LLD being approximately 425 keV. The phantom image results were dominated by the intrinsic true counts when an energy window of 250-750 keV was used. The intrinsic true counts were almost completely removed by raising the LLD to 400 keV. The CNR for each of the sources was higher for the narrow energy window and the 55 Bq could be easily visualized in images acquired with LLD levels of 350 and 400 keV but not when the 250-keV LLD was used. Images acquired with an LLD of 400 keV and reconstructed with 2-dimensional filtered backprojection were the most quantitatively accurate. CONCLUSION: It is possible to visualize sources of <1 kBq in LSO-based animal PET systems by raising the LLD to 400 keV to exclude the majority of the counts due to the intrinsic activity present in the LSO.     

Evaluation of the performance characteristics of the PC 4600 positron emission tomograph

The sensitivity, resolution, linearity, and count rate capability for the PC 4600 positron emission tomograph (PET), a neurological PET with five rings of 96 bismuth germanate crystals, are reported along with details of the design of this system. Phantom studies and preliminary human images demonstrate the clinical potential of this new instrument.     

功能性影像技术应用概况

本文对主要的功能性影像技术进行了综述。核索断层成像主要有：单光子发射式计算机断层成像(SPECT)和正电子发射式计算机断层成像(PET)。PET是利用F标记的脱氧葡葡糖(F-FAD)进行代谢显像,可用于细胞活性与功能等研究。磁共振波谱(MIlS)则是一种无创伤的研究人体生化代谢的有力工具;磁源成像(MSI)通过研究细胞内外离子运动所形成的生物电流及其所产生的磁场,从而产生了用于检测心脏或脑的心磁图或脑磁图。阻抗成像(EIT)和光学成像(OCT)则分别利用人体组织电阻率和以近红外或红外波段为成像源进行断层成像。还有将解剖成像和功能成像集合一起的集合成像。     

Performance of the new generation of whole-body PET/CT scanners: Discovery STE and Discovery VCT

Purpose The new GE Discovery STE and Discovery VCT respectively combine 16-slice and 64-slice CT with PET. The PET scanner has a new BGO detector block of 8×6 matrix (6.3×4.7×30 mm³). The aim of this study was to test the performance of the new scanner. Methods The PET performance evaluation was done using NEMA methodology. Owing to improved front-end electronics, the system was tested with different energy window and coincidence timing settings. Results Transaxial resolution FWHM for 2D(3D) mode at 1 cm offset from the centre of the field of view (R1) was 4.87 mm (5.12 mm) and at 10 cm off centre (R10) radially 5.70 mm (5.89 mm) and tangentially 5.84 mm (5.47 mm). The axial resolutions were 4.4 mm (5.18 mm) (R1) and 5.99 mm (5.86 mm) (R10). The sensitivities were 2.3 cps/kBq (8.8 cps/kBq) (R0, centre of field of view) and 2.3 cps/kBq (8.9 cps/kBq) (R10). The system scatter fraction was 21.4% in 2D at an energy of 375 keV (33.9% in 3D mode at a higher energy of 425 keV). Peak noise equivalent count rates (k=1) were 84.9 kcps at 43.9 kBq/ml (2D) and 67.6 kcps at 12.1 kBq/ml (3D). In image quality measurement the hot sphere to background contrast with 10- to 22-mm diameter spheres varied from 14% to 68%, being slightly better in 3D than in 2D mode. Cold sphere contrast was 67% in 2D and 59% in 3D mode. Conclusion GE’s new STE and VCT PET/CT systems have improved spatial resolution without loss in sensitivity. When compared with the LYSO crystal-based GE Discovery RX, the resolution and scatter fraction are comparable, the count rate capability is lower but the sensitivity is higher.     

Simultaneous imaging of magnetic resonance imaging and positron emission tomography by means of MRI-compatible optic fiber-based PET: a validation study in ex vivo rat brain

Purpose  We developed a new type of scintillation detector ring for positron emission tomography (PET). In this device the scintillation detectors were connected with 2.6 m length optic fibers to transport scintillation light to a photomultiplier (PMT) located apart from the detector rings. The present study aimed to test the possibility of simultaneous imaging with PET and magnetic resonance imaging (MRI) by means of the present PET device in ex vivo rat brain. Materials and methods  The scintillation detector ring of 4 cm diameter was located in a magnetic field of 0.15 T open MRI. Simultaneous measurements of PET and MRI were performed in ex vivo rat brain after injection of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) 37 MBq and ¹⁸F-NaF 37 MBq. Simultaneous data acquisition was performed for 10 min for PET and 5 min for T1-weighted MRI. Results  Simultaneous imaging of PET and MRI was possible without noticeable image distortion in the PET and MRI scans. In the fusion images, high uptake of ¹⁸F-FDG was identified in the Harderian glands and striatum. High uptake of ¹⁸F-NaF was found in the skull base and vault. Conclusions  The present study indicated the possibility of simultaneous imaging of PET and MRI by means of optic fiber-based scintillation detectors.     

Evaluation of a positron emission tomography (PET)‐compatible field‐cycled MRI (FCMRI) scanner

Field‐cycled MRI (FCMRI) uses two independent, actively controlled resistive magnets to polarize a sample and to provide the magnetic field environment during data acquisition. This separation of tasks allows for novel forms of contrast, reduction of susceptibility artifacts, and a versatility in design that facilitates the integration of a second imaging modality. A 0.3T/4‐MHz FCMRI scanner was constructed with a 9‐cm‐wide opening through the side for the inclusion of a photomultiplier‐tube–based positron emission tomography (PET) system. The performance of the FCMRI scanner was evaluated prior to integrating PET detectors. Quantitative measurements of the system's signal, phase, and temperature were recorded. The polarizing and readout magnets could be operated continuously at 100 A without risk of damage to the system. Transient instabilities in the readout magnet, caused by the pulsing of the polarizing magnet, dissipated in 50 ms; this resulted in a steady‐state homogeneity of 32 Hz over a 7‐cm‐diameter volume. The short‐ and long‐term phase behaviors of the readout field were sufficiently stable to prevent visible readout or phase‐encode artifacts during imaging. Preliminary MR images demonstrated the potential of the FCMRI scanner and the efficacy of integrating a PET system. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.