Performance evaluation of the Philips MOSAIC small animal PET scanner

Purpose In this study an evaluation of the performance of the Philips MOSAIC small animal PET scanner is presented, with special emphasis on the ability of the system to provide quantitatively accurate PET images. Methods The performance evaluation was structured according to NEMA-like procedures. Results The transaxial spatial resolution of the system (radial component) ranged between 2.7 mm FWHM at the centre and 3.2 mm FWHM at a radial offset of 45 mm from the centre. The axial spatial resolution of the system ranged between 3.4 mm FWHM at the centre and 5.8 mm FWHM at a radial offset of 45 mm from the centre. The scatter fraction was determined for a mouse- as well as for a rat-sized phantom, and the values obtained were 9.6% and 16.8%, respectively. For the mouse phantom, the maximum count rate measured was 560 kcps at 93 MBq; the maximum NEC rate equalled 308 kcps at 1.7 MBq/ml. For the rat phantom, these values were 400 kcps at 100 MBq and 129 kcps at 0.24 MBq/ml, respectively. The sensitivity of the system was derived to be 0.65%. An energy window between 410 and 665 keV was used in all experiments. Conclusion The MOSAIC system exhibits moderate spatial resolution and sensitivity values, but good NEC performance. In combination with its relatively large field of view, the system allows for high-throughput whole-body imaging of mice and rats. The accurate measurement of relative changes in radiotracer distributions is feasible.     

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 feasibility study of a prototype PET insert device to convert a general-purpose animal PET scanner to higher resolution.

We developed a prototype system to evaluate the feasibility of using a PET insert device to achieve higher resolution from a general-purpose animal PET scanner. METHODS: The system consists of a high-resolution PET detector, a computer-controlled rotation stage, and a custom mounting plate. The detector consists of a cerium-doped lutetium oxyorthosilicate array (12 x 12 crystals, 0.8 x 1.66 x 3.75 mm(3) each) directly coupled to a position-sensitive photomultiplier tube (PS-PMT). The detector signals were fed into the scanner electronics to establish coincidences between the 2 systems. The detector was mounted to a rotation stage that is attached to the scanner via the custom mounting plate after removing the transmission source holder. The rotation stage was concentric with the center of the scanner. The angular offset of the insert detector was calibrated via optimizing point-source images. In all imaging experiments, coincidence data were collected from 9 angles to provide 180 degrees sampling. A (22)Na point source was imaged at different offsets from the center to characterize the in-plane resolution of the insert system. A (68)Ge point source was stepped across the axial field of view to measure the sensitivity of the system. A 23.2-g mouse was injected with 38.5 MBq of (18)F-fluoride and imaged at 3 h after injection for 2 h. RESULTS: The transverse image resolution of the PET insert device ranges from 1.1- to 1.4-mm full width at half maximum (FWHM) without correction for the point-source dimension. This corresponds to approximately 33% improvement over the resolution of the original scanner (1.7- to 1.8-mm FWHM) in 2 of the 3 directions. The sensitivity of the device is 0.064% at the center of the field, 46-fold lower than the sensitivity of an existing animal PET scanner. The mouse bone scan had improved image resolution using the PET insert device over that of the existing animal PET scanner alone. CONCLUSION: We have demonstrated the feasibility of using a high-resolution insert device in an existing PET scanner to provide high-resolution PET. A PET insert device with more detector modules will improve sensitivity and may become an alternative to special-purpose PET systems for high-resolution PET.     

Clinical evaluation of 2D versus 3D whole-body PET image quality using a dedicated BGO PET scanner

Purpose Three-dimensional positron emission tomography (3D PET) results in higher system sensitivity, with an associated increase in the detection of scatter and random coincidences. The objective of this work was to compare, from a clinical perspective, 3D and two-dimensional (2D) acquisitions in terms of whole-body (WB) PET image quality with a dedicated BGO PET system.Methods 2D and 3D WB emission acquisitions were carried out in 70 patients. Variable acquisition parameters in terms of time of emission acquisition per axial field of view (aFOV) and slice overlap between sequential aFOVs were used during the 3D acquisitions. 3D and 2D images were reconstructed using FORE+WLS and OSEM respectively. Scatter correction was performed by convolution subtraction and a model-based scatter correction in 2D and 3D respectively. All WB images were attenuation corrected using segmented transmission scans. Images were blindly assessed by three observers for the presence of artefacts, confidence in lesion detection and overall image quality using a scoring system.Results Statistically significant differences between 2D and 3D image quality were only obtained for 3D emission acquisitions of 3 min. No statistically significant differences were observed for image artefacts or lesion detectability scores. Image quality correlated significantly with patient weight for both modes of operation. Finally, no differences were seen in image artefact scores for the different axial slice overlaps considered, suggesting the use of five slice overlaps in 3D WB acquisitions.Conclusion 3D WB imaging using a dedicated BGO-based PET scanner offers similar image quality to that obtained in 2D considering similar overall times of acquisitions.     

Performance of a whole-body PET scanner using curve-plate NaI(Tl) detectors.

A whole-body PET scanner, without interplane septa, has been designed to achieve high performance in clinical applications. The C-PET scanner, an advancement of the PENN PET scanners, is unique in the use of 6 curved NaI(Tl) detectors (2.54 cm thick). The scanner has a ring diameter of 90 cm, a patient port diameter of 56 cm, and an axial field of view of 25.6 cm. A (137)Cs point source is used for transmission scans. METHODS: Following the protocols of the International Electrotechnical Commission ([IEC] 61675-1) and the National Electrical Manufacturers Association ([NEMA] NU-2-1994 and an updated version, NU2-2001), point and line sources, as well as uniform cylinders, were used to determine the performance characteristics of the C-PET scanner. An image-quality phantom and patient data were used to evaluate image quality under clinical scanning conditions. Data were rebinned with Fourier rebinning into 2-dimensional (slice-oriented) datasets and reconstructed with an iterative reconstruction algorithm. RESULTS: The spatial resolution for a point source in the transaxial direction was 4.6 mm (full width at half maximum) at the center, and the axial resolution was 5.7 mm. For the NU2-1994 analysis, the sensitivity was 12.7 cps/Bq/mL (444 kcps/microCi/mL), the scatter fraction was 25%, and the peak noise equivalent count rate (NEC) for a uniform cylinder (diameter = 20 cm, length = 19 cm) was 49 kcps at an activity concentration of 11.2 kBq/mL. For the IEC protocol, the peak NEC was 41 kcps at 12.3 kBq/mL, and for the NU2-2001 protocol, the peak NEC was 14 kcps at 3.8 kBq/mL. The NU2-2001 NEC value differed significantly because of differences in the data analysis and the use of a 70-cm-long phantom. CONCLUSION: Compared with previous PENN PET scanners, the C-PET, with its curved detectors and improvements in pulse shaping, integration dead time, and triggering, has an improved count-rate capability and spatial resolution. With the refinements in the singles transmission technique and iterative reconstruction, image quality is improved and scan time is shortened. With single-event transmission scans interleaved between sequential emission scans, a whole-body study can be completed in <1 h. Overall, C-PET is a cost-effective PET scanner that performs well in a broad variety of clinical applications.     

A prototype PET scanner with DOI-encoding detectors.

Detectors with depth-encoding allow a PET scanner to simultaneously achieve high sensitivity and high spatial resolution. METHODS: A prototype PET scanner, consisting of depth-encoding detectors constructed by dual-ended readout of lutetium oxyorthosilicate (LSO) arrays with 2 position-sensitive avalanche photodiodes (PSAPDs), was developed. The scanner comprised 2 detector plates, each with 4 detector modules, and the LSO arrays consisted of 7 x 7 elements, with a crystal size of 0.9225 x 0.9225 x 20 mm and a pitch of 1.0 mm. The active area of the PSAPDs was 8 x 8 mm. The performance of individual detector modules was characterized. A line-source phantom and a hot-rod phantom were imaged on the prototype scanner in 2 different scanner configurations. The images were reconstructed using 20, 10, 5, 2, and 1 depth-of-interaction (DOI) bins to demonstrate the effects of DOI resolution on reconstructed image resolution and visual image quality. RESULTS: The flood histograms measured from the sum of both PSAPD signals were only weakly depth-dependent, and excellent crystal identification was obtained at all depths. The flood histograms improved as the detector temperature decreased. DOI resolution and energy resolution improved significantly as the temperature decreased from 20 degrees C to 10 degrees C but improved only slightly with a subsequent temperature decrease to 0 degrees C. A full width at half maximum (FWHM) DOI resolution of 2 mm and an FWHM energy resolution of 15% were obtained at a temperature of 10 degrees C. Phantom studies showed that DOI measurements significantly improved the reconstructed image resolution. In the first scanner configuration (parallel detector planes), the image resolution at the center of the field of view was 0.9-mm FWHM with 20 DOI bins and 1.6-mm FWHM with 1 DOI bin. In the second scanner configuration (detector planes at a 40 degrees angle), the image resolution at the center of the field of view was 1.0-mm FWHM with 20 DOI bins and was not measurable when using only 1 bin. CONCLUSION: PET scanners based on this detector design offer the prospect of high and uniform spatial resolution (crystal size, approximately 1 mm; DOI resolution, approximately 2 mm), high sensitivity (20-mm-thick detectors), and compact size (DOI encoding permits detectors to be tightly packed around the subject and minimizes number of detectors needed).     

A positron camera using position-sensitive detectors: PENN-PET

A single-slice positron camera has been developed with good spatial resolution and high count rate capability. The camera uses a hexagonal arrangement of six position-sensitive NaI(Tl) detectors. The count rate capability of NaI(Tl) was extended to 800k cps through the use of pulse shortening. In order to keep the detectors stationary, an iterative reconstruction algorithm was modified which ignores the missing data in the gaps between the six detectors and gives artifact-free images. The spatial resolution, as determined from the image of point sources in air, is 6.5 mm full width at half maximum. We have also imaged a brain phantom and dog hearts.     

CT attenuation correction for myocardial perfusion quantification using a PET/CT hybrid scanner.

In routine PET, a 10- to 20-min transmission scan with a rotating (68)Ge source is commonly obtained for attenuation correction (AC). AC is time-consuming using this procedure and could considerably be shortened by instead using a rapid CT scan. Our aim was to evaluate the feasibility of CT AC in quantitative myocardial perfusion PET using a hybrid PET/CT scanner. METHODS: (13)N-labeled NH(3) and PET were used to measure myocardial blood flow (MBF) (mL/min/g) at rest and during standard adenosine stress. In group 1 (n = 7), CT scans (0.5 s) of the heart area with different tube currents (10, 40, 80, and 120 mA) were compared with a standard (68)Ge transmission (20 min) and with no AC. In group 2 (n = 3), the repeatability of 8 consecutive CT scans at a tube current of 10 mA was assessed. In group 3 (n = 4), emission was preceded and followed by 3 CT scans (10 mA) and 1 (68)Ge scan for each patient. For reconstruction, filtered backprojection (FBP) was compared with iterative reconstruction (IT). RESULTS: For group 1, no significant difference in mean MBF for resting and hyperemic scans was found when emission reconstructed with (68)Ge AC was compared with emission reconstructed with CT AC at any of the different tube currents. Only emission without any correction differed significantly from (68)Ge AC. For group 2, repeated measurements revealed a coefficient of variance ranging from 2% to 5% and from 2% to 6% at rest and at stress, respectively. For group 3, similar reproducibility coefficients (RC) for MBF were obtained when (68)Ge AC(FBP) was compared with (68)Ge AC(IT) (RC = 0.218) and when CT AC(FBP) was compared with CT AC(IT) (RC = 0.227). Even better reproducibility (lower RC) was found when (68)Ge AC(FBP) was compared with CT AC(FBP) (RC = 0.130) and when (68)Ge AC(IT) was compared with CT AC(IT) (RC = 0.146). CONCLUSION: Our study shows that for the assessment of qualitative and quantitative MBF with a hybrid PET/CT scanner, the use of CT AC (with a tube current of 10 mA) instead of (68)Ge AC provides accurate results.     

Accuracy of 3D acquisition mode for myocardial FDG PET studies using a BGO-based scanner

Purpose The aim of the present study was to evaluate the quantitative and qualitative accuracy of 3D PET acquisitions for myocardial FDG studies. Methods Phantom studies were performed with both a homogeneous and an inhomogeneous phantom. Activity profiles were generated along the phantoms using 2D and several 3D reconstructions, varying the 3D scaling value to adjust the scatter correction algorithm. Furthermore, ten patients underwent a dynamic myocardial FDG PET scan, using an interleaved protocol consisting of frames with alternating 2D and 3D acquisition. For each myocardial study, 13 volumes of interest were defined, representing 13 myocardial segments. First, the optimal scaling value for the scatter correction algorithm was determined using data from the phantom and four patient studies. This scaling value was then applied to all ten patients. 2D and 3D acquisitions were compared for both static (i.e. activity concentrations in the last 2D and 3D frames) and dynamic imaging (calculation of the metabolic rate of glucose). Results For both phantom and patient studies, suboptimal results were obtained when the default scaling value for the scatter correction algorithm was used. After adjusting the scaling value, for all ten myocardial FDG studies, a very good correlation (r ² = 0.99) was obtained between 2D and 3D data. With the present protocol no significant differences were observed in qualitative interpretation. Conclusion The 3D FDG acquisition mode is accurate and has clear advantages over the 2D mode for myocardial FDG studies. A prerequisite is, however, optimisation of the 3D scatter correction algorithm.     

Performance characteristics of an eight-ring whole body PET scanner

The technical characteristics of the multislice whole-body positron emission tomographic scanner (model PC4096-15WB Scanditronix) and its performance parameters are described. Spatial resolution at the center of the field of view was found to be 4.9 mm in-plane and 4.6 mm (cross slices) and 6.0 mm (direct slices) in the axial direction. The sensitivity for true and scattered coincidences is approximately 5,000 cps for direct slices and 7,100 cps for cross slices. At an activity concentration of 37 kBq/ml the system deadtime was approximately 5%. By measuring a uniform phantom with a cold cylindrical insert (5.0 cm diameter), the scatter fraction was found to be approximately 5%. The mean global uniformity over all 15 slices was 6.5%, whereas the local uniformity was found to be 4.3%. No systematic nonuniformities were observed. Finally, various methods for attenuation correction (transmission scan, contour finding, ellipse) were utilized to test their effects on the resulting reconstructed images.     

Median polish for quality assurance of a PET scanner

Sensitivity data for the PC 4600, a multiplanar positron emission tomograph system, were obtained over a period of several months during installation and routine clinical operation. These data were analyzed using the exploratory data analysis techniques of median polish (MP), box and whisker plots, and coded residuals. These techniques proved to be useful in spotting trends and identifying problems. Median polish had advantages over traditional percent difference techniques under some conditions because it allows separate study of more than one effect and is particularly resistant to the influence of outliers. The other exploratory data analysis techniques used are of value in interpreting the results of the MP procedure. The methods presented have direct application to any quantitative multiplanar emission tomographic imaging quality assurance program.     

动物PET研究进展

分子医学研究需要在活体实验动物上观察分子水平的生物学过程,因而正电子发射体层(PET)显像作为目前最成熟的分子显像方法,正被越来越多地用于动物实验。新开发的实验动物专用PET扫描仪的各项性能也逐步趋于完善。该技术将在疾病研究、新药开发、基因治疗等领域发挥重要作用。     

Time-course characterization of the computed tomography contrast enhancement of an iodinated blood-pool contrast agent in mice using a volumetric flat-panel equipped computed tomography scanner

OBJECTIVE: The objective of this study was to determine the time-course of computed tomography (CT) contrast enhancement of an iodinated blood-pool contrast agent. METHODS: Five C57BL/6 mice were anesthetized, imaged at baseline, and given an iodinated blood-pool contrast agent. Micro-CT scans were acquired at 0, 0.25, 0.5, 1, 2, 4, 8, and 24 hours after injection. The mean CT number was determined in a region of interest in 7 organs. RESULTS: The CT contrast enhancement was plotted as a function of time for each organ. We identified an imaging window immediately after injection suitable for visualizing the vascular system and a second imaging window at 24 hours for visualizing liver and spleen. CONCLUSIONS: A single injection of the blood-pool contrast agent can be used for dual-phase investigations of the vasculature (t = 0 hours) and liver (t = 24 hours), which can be applied to studies of liver tumors or disease.     

A combined PET/CT scanner for clinical oncology.

The availability of accurately aligned, whole-body anatomical (CT) and functional (PET) images could have a significant impact on diagnosing and staging malignant disease and on identifying and localizing metastases. Computer algorithms to align CT and PET images acquired on different scanners are generally successful for the brain, whereas image alignment in other regions of the body is more problematic. METHODS: A combined PET/CT tomograph with the unique capability of acquiring accurately aligned functional and anatomical images for any part of the human body has been designed and built. The PET/CT scanner was developed as a combination of a Siemens Somatom AR.SP spiral CT and a partial-ring, rotating ECAT ART PET scanner. All components are mounted on a common rotational support within a single gantry. The PET and CT components can be operated either separately, or in combined mode. In combined mode, the CT images are used to correct the PET data for scatter and attenuation. Fully quantitative whole-body images are obtained for an axial extent of 100 cm in an imaging time of less than 1 h. When operated in PET mode alone, transmission scans are acquired with dual 137Cs sources. RESULTS: The scanner is fully operational and the combined device has been operated successfully in a clinical environment. Over 110 patients have been imaged, covering a range of different cancers, including lung, esophageal, head and neck, melanoma, lymphoma, pancreas, and renal cell. The aligned PET and CT images are used both for diagnosing and staging disease and for evaluating response to therapy. We report the first performance measurements from the scanner and present some illustrative clinical studies acquired in cancer patients. CONCLUSION: A combined PET and CT scanner is a practical and effective approach to acquiring co-registered anatomical and functional images in a single scanning session.     

Quantitation of small-animal (124)I activity distributions using a clinical PET/CT scanner.

Time-dependent PET imaging can be an important tool in the assessment of radiotracer performance in murine models. We have performed a quantitative analysis of PET images of (124)I, acquired on a clinical PET system using a small-animal phantom. We then compared the recovered activity concentrations with the known activity concentration in the phantom spheres. The recovery coefficients found from the phantom data were applied to in vivo (124)I anti-HER2/neu C6.5 diabody PET data and compared with necropsy biodistribution data from the same tumor-bearing immunodeficient mouse. METHODS: The small-animal phantom consisted of a 4 x 8 cm water-filled acrylic cylinder with hollow spheres filled with water ranging in volume from 0.0625 to 1.0 mL and activity concentration of 27 +/- 2 kBq/mL. The background activity concentrations varied from 0 to 0.05 to 0.10 of the spheres. Data were acquired at 0, 5, and 10 cm from the scanner longitudinal axis. Recovery coefficients were theoretically calculated for spheres of different volume, background-to-target concentrations, and distance from the scanner's longitudinal axis. The theoretic recovery coefficients were applied to the maximum sphere activity concentration measured from the PET images, thus obtaining a recovered activity concentration to be compared with the known activity concentration of the spheres. RESULTS: The mean recovered activity concentration for the phantom spheres was 25 +/- 2 kBq/mL. The (124)I diabody PET image of a mouse with a tumor xenograft was then analyzed using the techniques described. The tumor percentage injected dose per gram estimated from the murine PET image (4.8 +/- 0.4) compared well with those obtained from necropsy studies (5.1). CONCLUSION: This study indicates the feasibility of performing quantitative imaging on murine (124)I antibody fragment PET images using a large-bore clinical scanner, which enables high-throughput studies to evaluate the performance of PET tracers in a timely and cost-effective manner by imaging multiple animals simultaneously. Tracers deemed promising by this screening method can then be further evaluated using traditional necropsy studies. Our group is currently conducting time-dependent (124)I diabody PET and necropsy comparative studies with larger numbers of mice.     

PET performance measurements for an LSO-based combined PET/CT scanner using the National Electrical Manufacturers Association NU 2-2001 standard.

Results of performance measurements for a lutetium oxyorthosilicate (LSO)-based PET/CT scanner using new National Electrical Manufacturers Association (NEMA) NU 2-2001 standards are reported. METHODS: Performance measurements following the NU 2-2001 standards were performed on an LSO-based PET/CT scanner. In addition, issues associated with the application of the NEMA standard to LSO-based tomographs in the presence of intrinsic radiation are discussed. RESULTS: We report on some difficulties experienced in following the suggested NEMA measurement techniques and describe alternative approaches. Measurements with the new standard (as compared with NU-1994) incorporate the effects of activity outside the scanner and facilitate measurements of the entire axial field of view. Realistic clinical conditions are also simulated in image quality measurements of a torso phantom. CONCLUSION: We find that, with appropriate modifications, NU 2-2001 can be successfully applied to LSO-based scanners.