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.     

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.     

Fabrication and characterization of a 0.5-mm lutetium oxyorthosilicate detector array for high-resolution PET applications.

With the increasing use of in vivo imaging in mouse models of disease, there are many interesting applications that demand imaging of organs and tissues with submillimeter resolution. Though there are other contributing factors, the spatial resolution in small-animal PET is still largely determined by the detector pixel dimensions. METHODS: In this work, a pair of lutetium oxyorthosilicate (LSO) arrays with 0.5-mm pixels was coupled to multichannel photomultiplier tubes and evaluated for use as high-resolution PET detectors. RESULTS: Flood histograms demonstrated that most crystals were clearly identifiable. Energy resolution varied from 22% to 38%. The coincidence timing resolution was 1.42-ns full width at half maximum (FWHM). The intrinsic spatial resolution was 0.68-mm FWHM as measured with a 30-gauge needle filled with (18)F. The improvement in spatial resolution in a tomographic setting is demonstrated using images of a line source phantom reconstructed with filtered backprojection and compared with images obtained from 2 dedicated small-animal PET scanners. Finally, a projection image of the mouse foot is shown to demonstrate the application of these 0.5-mm LSO detectors to a biologic task. CONCLUSION: A pair of highly pixelated LSO detections has been constructed and characterized for use as high-spatial-resolution PET detectors. It appears that small-animal PET systems capable of a FWHM spatial resolution of 600 microm or less are feasible and should be pursued.     

Imaging characteristics of a 3-dimensional GSO whole-body PET camera.

A whole-body 3-dimensional PET scanner using gadolinium oxyorthosilicate (GSO) crystals has been designed to achieve high sensitivity and reduced patient scanning time. This scanner has a diameter of 82.0 cm and an axial field of view of 18 cm without interplane septa. The detector comprises of 4 x 6 x 20 mm(3) GSO crystals coupled via an optically continuous light guide to an array of 420 photomultiplier tubes (39-mm diameter) in a hexagonal arrangement. The patient port diameter is 56 cm, and 2.86-cm (1.125 in.) thick lead shielding is used to fill in the region up to the detector ring. METHODS: Performance measurements on the scanner were made using the National Electrical Manufactures Association (NEMA) NU 2-2001 procedures. Additional counting rate measurements with a large phantom were performed to evaluate imaging characteristics for heavier patients. The image-quality torso phantom with hot or cold spheres was also measured as a function of counting rate to evaluate different techniques for randoms and scatter subtraction as well as to determine an optimum imaging time. RESULTS: The transverse and axial resolutions near the center are 5.5 and 5.6 mm, respectively. The absolute sensitivity of this scanner measured with a 70-cm-long line source is 4.36 cps/kBq, whereas the scatter fraction is 40% with a 20 x 70 cm line source cylinder. For the same cylinder, the peak noise equivalent count (NEC) rate of 30 kcps at an activity concentration of 9.25 kBq/mL (0.25 micro Ci/mL) leads to a 7% increase in the peak NEC value. A significant reduction in the peak NEC is observed with a larger 35 x 70 cm line source cylinder. Image-quality measurements show that the small 10-mm sphere in the NEMA NU 2-2001 image-quality phantom is clearly visible in a scan time of 3 min, and there is no noticeable degradation in image contrast at high activity levels. CONCLUSION: This whole-body scanner represents a new generation of 3D, high-sensitivity, and high-performance PET cameras capable of producing high-quality images in <30 min for a full patient scan. The use of a pixelated GSO Anger-logic detector leads to a high-sensitivity scanner design with good counting rate capability due to the reduced light spread in the detector and fast decay time of GSO. The light collection over the detector is fairly uniform, leading to a good energy resolution and, thus, reduced scatter in the collected data due to a tight energy gate.     

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).     

Performance of a whole-body counter with five high-purity germanium detectors.

A germanium (Ge) detector system has been installed in a whole-body counting facility at NIRS (Japan). The system employs five detectors that have relative efficiencies of more than 80%. A basic performance of the Ge detectors was tested and compared with that of the former NaI(Tl) detector system. Although the Ge detector system has advantages of improved radionuclide identification capability and MDA values, it also has disadvantages of high operation costs and demanding maintenance such as supply of liquid nitrogen.     

Performance evaluation of a modular gamma camera using a detectability index.

The performance of a modular gamma camera for the task of detecting signals in random noisy backgrounds was evaluated experimentally. The results were compared with a theoretical computer simulation. METHODS: The camera uses a 10 x 10 cm thallium-doped sodium iodide crystal, a 2 x 2 array of 53 x 53 mm photomultiplier tubes, and a parallel-hole collimator (1.5-mm bore width, 23.6-mm bore length). The camera was positioned to look down into a 10-cm-deep water bath that filled its field of view (FOV). The top surface of the water was 5 cm from the front face of the camera. The camera has 3-mm intrinsic spatial resolution (SR) in the center of its FOV and 9-mm system SR for objects 5 cm below the top surface of the water. Uniform and nonuniform random background data were collected by imaging the bath containing 740 MBq (20 mCi) (99m)Tc. Nonuniformities were created by placing water-filled objects in the bath. Each signal dataset was collected by imaging a water-filled plastic sphere, injected with (99m)Tc and set at a specific depth (Z) in the bath. Data were collected for many signal diameters (D) (4, 7, 10, 13, 16, 28 mm) at 1 depth (5 cm) and for 1 signal diameter (10 mm) at several depths (1, 3, 5, 7, 9 cm). Sets of signal-present/signal-absent image pairs (380 pairs, 10(5) events per image) for known contrasts (C) were generated for use in ideal-observer studies in which the detectability (d') was calculated. Contrast-detail (log C vs. log D) plots were created. The theoretical simulation, developed for uniform backgrounds, provided data for comparison. RESULTS: The detectability increased linearly with C and decreased nonlinearly with decreasing D or increasing Z. The C required to achieve a specific d' increased sharply for D < SR. For C = 5, D = 10 mm, and d' = 1.2, the camera consistently detected signals for Z < 6 cm. Similar results were found for nonuniform backgrounds. The theoretical simulation verified the results for uniform backgrounds. CONCLUSION: The methodology presented here provides a way of evaluating gamma cameras on the basis of signal-detection performance for specified lesions, with particular application to scintimammography.     

Evaluation of efficiency of a multi-crystal scintillation camera Digirad 2020tc Imager using a solid-state detectors

Digirad 2020tc Imager is the movable scintillation camera, consisting of combining multi-crystal scintillators (CsI(Tl)) and photo-diodes. Total numbers of element are 4096, which are further divided into 16 x 16 modules. Each module contains 4 x 4 elements. We have examined Digirad 2020tc according to NEMA (National Electrical Manufactures Association), and the following results are obtained; the maximum count rate; 221 kcps, total system uniformity; 1.3% (integral uniformity), 0.9% (differential uniformity), system spatial resolution; 6.97 +/- 0.72 mm (the LEHR collimator to 99mTc source at 10 cm), intrinsic energy resolution; 12.8%, total system sensitivity; 3270.8 cpm/MBq (with LEHR collimator using 99mTc source at 10 cm). Further more, we determined the contrast of an imaging using the pin-hole (100 microns phi) 99mTc source in order to know the signal per noise (S/N) ratio among the pixels (S/N; 93.4 +/- 46.2 (first pixels)). Although the physical dimension of the camera has a smaller field of view, comparing with the standard camera, Digirad 2020tc has the equivalent characteristics as well as that of the standard camera and its field view is enough to measure the adult lung perfusion using a diverging collimator. We will further examine Digirad 2020tc with its movable portability and expect applications in nuclear medicine.     

Performance characteristics of a new 3-dimensional continuous-emission and spiral-transmission high-sensitivity and high-resolution PET camera evaluated with the NEMA NU 2-2001 standard.

The SET-3000 G/X (clinical tomograph with high resolution and a large axial field of view) is a 3-dimensional (3D) (only) dedicated PET camera with germanium oxyorthosilicate (GSO) and bismuth germanate (BGO) scintillators. The main characteristic of the SET-3000 G/X PET scanner is 3D continuous-emission and spiral-transmission (CEST) scanning, yielding a reduction in whole-body scan time. We evaluated the physical performance of the SET-3000 G/X PET scanner with the National Electrical Manufacturers Association (NEMA) NU 2-2001 standard. METHODS: A GSO 3D emission scanner is combined with a BGO transmission scanner separated axially by a lead shield. In the GSO scanner, small and thick scintillators (2.45 x 5.1 x 30 mm(3)) are arranged in small blocks (23.1 x 52 mm) to achieve high resolution and a high counting rate. The detector ring has a large solid angle with a diameter of 664 mm and an axial coverage of 260 mm (50 rings). The transmission scanner consists of BGO block detectors with a diameter of 798 mm and an axial width of 23.1 mm and is equipped with a rotating (137)Cs point source of 740 MBq and a tungsten collimator. The low- and high-energy thresholds are set to 400 and 700 keV, respectively, in the emission system. The coincidence time window is set to 6 ns. In CEST acquisition, the patient couch moves continuously through the emission and transmission scanners in a 1-way motion. Emission coincidence data are acquired in the histogram mode with on-the-fly Fourier rebinning, and transmission single data are acquired with emission contamination correction. RESULTS: With the NEMA NU 2-2001 standard, the main performance results were as follows: the average (radial and tangential) transverse and axial spatial resolutions (full width at half maximum) at 1 cm and at 10 cm off axis were 3.49 and 5.04 mm and 4.48 and 5.40 mm, respectively; the average sensitivity for the 2 radial positions (0 and 10 cm) was 20.71 cps/kBq; the scatter fraction was 50%; the peak noise equivalent count rate was 62.3 kcps at 9.8 kBq/mL; and the peak random rate was 542.1 kcps at 37.6 kBq/mL. CONCLUSION: The new integrated SET-3000 G/X PET scanner has good overall performance, including high resolution and sensitivity, and has the potential of reducing whole-body acquisition time to less than 10 min while improving small-lesion detectability with a low radiation dose.     

PET with a dual-head coincidence camera: spatial resolution, scatter fraction, and sensitivity.

Scintillation cameras with options for detecting positron annihilation quanta in the coincidence acquisition mode may be the most innovative diagnostic devices introduced in nuclear medicine during the last few years. Besides conventional low-energy imaging in the collimated single-photon mode, these options offer a relatively inexpensive opportunity to perform uncollimated PET by switching into the coincidence acquisition mode. Instead of collimators, scatter frames (with 2 optional configurations: axial or open scatter frame) can be mounted to reduce the amount of quanta reaching the detectors from parts of the patient's body outside the field of view. This study investigates the coincidence imaging properties of the scintillation camera by measuring spatial resolution, scatter fraction, sensitivity, and count-rate response for 18F. METHODS: A needle in air and a plastic tube in water, each filled with 18F, were oriented axially and transversally to measure the transverse and axial spatial resolutions, respectively. Using either the axial or the open scatter frame, a standard cylinder filled homogeneously with activity was studied over several half-life periods to deduce the respective scatter and random fractions of the system by means of a sinogram technique. The activity of the cylinder was kept low to determine the sensitivity to coincidence events for both scatter frames. RESULTS: Depending on the distance between the line source and the axis of rotation and on the choice of the axial acceptance angle used to reconstruct the coincidence events (single-slice rebinning algorithm), the axial resolution was found to be between 6 and 10 mm (full width at half maximum) with the axial scatter frame mounted. The transversal resolution was 6-6.5 mm on the axis of rotation, independent of the scatter frame used. The scatter fraction amounted to roughly 25% for the axial and 38% for the open scatter frames. The sensitivity when measuring true coincidence pairs ran to nearly 650 Hz/kBq/mL, when acquisition was performed with the axial scatter frame using a 30%-wide photopeak energy window. When acquiring with the open scatter frame, the sensitivity increased to nearly 3000 Hz/kBq/mL. Using the axial scatter frame, the homogeneously filled cylinder could be scanned with a maximum true coincidence rate of 2000 Hz for an activity of 55-60 MBq. Although this maximum true coincidence counting rate did not change significantly when the acquisition was performed with the open scatter frame, the respective activity in the standard cylinder was decreased to 10-15 MBq. CONCLUSION: The spatial resolution of the scintillation camera is sufficient for high-resolution coincidence imaging. Compared with a dedicated PET scanner, the scatter fraction is relatively high and should therefore be corrected adequately. The relatively low sensitivity and the rather low maximum true coincidence counting rate are considerably inferior compared with a conventional PET scanner. However, these drawbacks can be partially compensated for, facilitating its clinical use.     

Clinical applications of PET in brain tumors.

Malignant gliomas and metastatic tumors are the most common brain tumors. Neuroimaging plays a significant role clinically. In low-grade tumors, neuroimaging is needed to evaluate recurrent disease and to monitor anaplastic transformation into high-grade tumors. In high-grade and metastatic tumors, the imaging challenge is to distinguish between recurrent tumor and treatment-induced changes such as radiation necrosis. The current clinical gold standard, MRI, provides superior structural detail but poor specificity in identifying viable tumors in brain treated with surgery, radiation, or chemotherapy. (18)F-FDG PET identifies anaplastic transformation and has prognostic value. The sensitivity and specificity of (18)F-FDG in evaluating recurrent tumor and treatment-induced changes can be improved significantly by co-registration with MRI and potentially by delayed imaging 3-8 h after injection. Amino acid PET tracers are more sensitive than (18)F-FDG in imaging recurrent tumors and in particular recurrent low-grade tumors. They are also promising in differentiating between recurrent tumors and treatment-induced changes.     

Performance profile of FDG-PET and PET/CT for cancer screening based on a Japanese nationwide survey

A total of 50,558 healthy subjects underwent an FDG-PET (including PET/CT) scan with or without combination of other tests for cancer screening in 46 PET centers during fiscal year of 2005 in Japan. Thorough examination was indicated for 9.8% of the cases due to positive findings suggesting possible cancer. On analyzing 43,996 cases from 38 PET centers, where detailed information was obtained, 500 cases of cancers (1.14%) were found, of which 0.90% was PET positive and 0.24% was PET negative, resulting in the relative sensitivity of PET being 79.0%. Cancers of thyroid, colon/rectum, lung and breast were most frequently found (107, 102, 79, 35 cases, respectively) with high PET sensitivity (88%, 90%, 80%, 92%). PET showed an overall positive predictive value of 29.0%. PET/CT had better detection rate, sensitivity, and positive predictive value than dedicated PET (p<0.01).     

Performance characteristics of a gamma camera over a wide range of energies

The problem of choosing which collimator to use for imaging a new isotope has been approached by collecting resolution and sensitivity data for a selected group of isotopes. These have been chosen to be readily available and to have generally a single -ray only. Resolution and sensitivity plots for a low energy collimator and a high energy collimator are presented and their use with several isotopes of interest is discussed. The interpretation of recommendations in the literature on the choice of collimators for newly introduced isotopes would be considerably simpler if data in this format were commonly available.     

ECAT ART — a continuously rotating PET camera: Performance characteristics, initial clinical studies, and installation considerations in a nuclear medicine department

Advances in fully three-dimensional (3D) image reconstruction techniques have permitted the development of a commercial, rotating, partial ring, fully 3D positron emission tomographic (PET) scanner, the ECAT ART. The system has less than one-half the number of bismuth germanate detectors compared with a full ring scanner with the equivalent field of view, resulting in reduced capital cost. The performance characteristics, implications for installation in a nuclear medicine department, and clinical utility of the scanner are presented in this report. The sensitivity (20 cm diameter×20 cm long cylindrical phantom, no scatter correction) is 11400 cps·kBq^–1·ml^–1. This compares with 5800 and 40500 cps·kBq^–1·ml^–1 in 2D and 3D respectively for the equivalent full ring scanner (ECAT EXACT). With an energy window of 350–650 keV the maximum noise equivalent count (NEC) rate was 27 kcps at a radioactivity concentration of ~15 kBq·ml^–1 in the cylinder. Spatial resolution is ~6 mm full width at half maximum on axis degrading to just under 8 mm at a distance of 20 cm off axis. Installation and use within the nuclear medicine department does not appreciably increase background levels of radiation on gamma cameras in adjacent rooms and the dose rate to an operator in the same room is 2 µSv·h^–1 for a typical fluorine-18 fluorodeoxyglucose (¹⁸F-FDG) study with an initial injected activity of 370 MBq. The scanner has been used for clinical imaging with¹⁸F-FDG for neurological and oncological applications. Its novel use for imaging iron-52 transferrin for localising erythropoietic activity demonstrates its sensitivity and resolution advantages over a conventional dual-headed gamma camera. The ECAT ART provides a viable alternative to conventional full ring PET scanners without compromising the performance required for clinical PET imaging.     

Experience with high-dose gadolinium MR imaging in the evaluation of brain metastases.

PURPOSE: To assess the effectiveness and safety of higher doses of gadoteridol in the MR evaluation of patients with brain metastases. MATERIALS AND METHODS: Thirty-one patients with a clinical suspicion of brain metastases were studied prospectively with gadoteridol, a new, nonionic, low-osmolality contrast agent. Each patient received an initial injection of 0.1 mmol/kg and an additional dose of 0.2 mmol/kg 30 minutes later. Images were obtained before, immediately after, and 10 and 20 minutes after the initial dose. Images also were acquired immediately after the additional dose of gadoteridol. RESULTS: No adverse effects were attributed to the injection of gadoteridol. Four patients' examinations were excluded from analysis because of machine malfunction (two patients) and excessive motion artifact (two patients). Four patients had no detectable metastases. After the additional dose of gadoteridol, there was a marked qualitative improvement in lesion conspicuity and detection. The conspicuity of 80 of 81 lesions was increased in the high-dose studies, and 46 new lesions were detected in 19 of 27 patients. Quantitative image analysis demonstrated a significant increase in normalized mean lesion contrast between the initial-dose and high-dose studies (35 lesions identified in 13 patients, P less than .0001). The additional information gained by high-dose examinations contributed to a potential modification of the treatment in 10 of 27 patients. High-dose examinations increased flow-related artifact in the posterior fossa in 12 of 27 patients. CONCLUSION: Based on our preliminary results, high-dose gadolinium-enhanced MR examinations may have advantages over 0.1 mmol/kg examinations in detecting early and/or small metastases. This may be significant in the management of patients with cerebral metastases.     

Dynamic studies of gadolinium uptake in brain tumors using inversion-recovery echo-planar imaging.

Echo-planar imaging has been used to observe the dynamics of Gd-DTPA uptake in brain tumors. It has been possible to examine both vascular uptake and diffusion across the blood-brain barrier in a single experiment, by using the IR-MBEST echo-planar sequence which combines a high temporal resolution (approximately 3 s) with strong T1 weighting. To model the uptake it is necessary to know the arterial concentration of Gd-DTPA; in this study the signal in the sagittal sinus was measured to avoid the need to take repeated blood samples. The time constant for transfer across the blood-brain barrier was measured to be between 20 and 1050 s for different tumors. The results of the modeling correlated with the results of other assessments of tumor vascularity.     

18F-FET PET differentiation of ring-enhancing brain lesions.

The aim of this study was to explore the differential diagnostic value of PET using the amino acid O-(2-(18)F-fluoroethyl)-L-tyrosine ((18)F-FET) in patients with newly diagnosed solitary intracerebral lesions showing ring enhancement on contrast-enhanced MRI. METHODS: (18)F-FET PET analyses were performed on 14 consecutive patients with intracerebral ring-enhancing lesions. Eleven of the patients were additionally studied with (18)F-FDG PET. In all patients, the main differential diagnosis after MRI was a malignant lesion, in particular glioblastoma multiforme, versus a benign lesion, in particular brain abscess. A malignant tumor was suspected for lesions showing increased (18)F-FET uptake on PET images with a mean lesion-to-brain ratio of at least 1.6 ((18)F-FET PET positive). A nonneoplastic lesion was suspected in cases of minimal or absent (18)F-FET uptake, with a mean lesion-to-brain ratio of less than 1.6 ((18)F-FET PET negative). Histologic diagnosis was obtained by serial biopsies in 13 of the 14 patients. One patient refused the biopsy, but follow-up indicated an abscess because his lesion regressed under antibiotic therapy. RESULTS: Histology and clinical follow-up showed high-grade malignant gliomas in 5 patients and nonneoplastic lesions in 9 patients. The findings of (18)F-FET PET were positive in all 5 glioma patients and in 3 of 9 patients with nonneoplastic lesions, including 2 patients with brain abscesses and 1 patient with a demyelinating lesion. The findings of (18)F-FDG PET were positive (mean lesion-to-gray matter ratio > or = 0.7) in 4 of 4 glioma patients and 3 of 7 patients with nonneoplastic lesions. CONCLUSION: Although (18)F-FET PET has been shown to be valuable for the diagnostic evaluation of brain tumors, our data indicate that, like (18)F-FDG PET, (18)F-FET PET has limited specificity in distinguishing between neoplastic and nonneoplastic ring-enhancing intracerebral lesions. Thus, histologic investigation of biopsy specimens remains mandatory to make this important differential diagnosis.