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Building on the success of positron emission tomography (PET)/CT scanners, a hybrid imaging technology, combining PET with the versatile attributes of magnetic resonance imaging (MRI), has been in use for over ten years. At the Forschungszentrum Julich, one of the four prototypes available worldwide, combining a commercial 3T MRI with a newly developed BrainPET insert, was installed in 2008, allowing simultaneous data acquisition with PET and MRI. Here, we report on our ten-year-long experience of the methods and technology developed through this paper with numerous patients and volunteers. In the course of our research, a number of different radiotracers, such as [18F]Fluoroethyltyrosine, [11C]Flumazenil, [11C]Raclopride, and [15O]H2O, have been applied while simultaneously measuring several different MR sequences. The PET data obtained with the BrainPET has demonstrated higher image quality and superior resolution capability compared to the stand-alone PET scanner, ECAT Exact HR+. Furthermore, the simultaneous acquisition of MR images alongside BrainPET scans, using different sequences, showed an excellent quality without any relevant artifacts and provided useful complementary information. Our experiences with the hybrid 3T MR-BrainPET indicate a promising basis for further developments of this unique technique, allowing simultaneous brain imaging in a clinical setting.

Purpose Today, attenuation correction (AC) of positron emission tomography/magnetic resonance (PET/MR) hardware components is performed by using an established method from PET/CT hybrid imaging. As shown in previous studies, the established mathematical conversion from computed tomography (CT) to PET attenuation coefficients may, however, lead to incorrect results in PET quantification when applied to AC of hardware components in PET/MR. The purpose of this study is to systematically investigate the attenuating properties of various materials and electronic components frequently used in the context of PET/MR hybrid imaging. The study, thus, aims at improving hardware component attenuation correction in PET/MR. Materials and methods Overall, 38 different material samples were collected; a modular phantom was used to for CT, PET, and PET/MR scanning of all samples. Computed tomography-scans were acquired with a tube voltage of 140 kVp to determine Hounsfield Units (HU). PET transmission scans were performed with 511 keV to determine linear attenuation coefficients (LAC) of all materials. The attenuation coefficients were plotted to obtain a HU to LAC correlation graph, which was then compared to two established conversions from literature. Hardware attenuation maps of the different materials were created and applied to PET data reconstruction following a phantom validation experiment. From these measurements, PET difference maps were calculated to validate and compare all three conversion methods. Results For each material, the HU and corresponding LAC could be determined and a bi-linear HU to LAC conversion graph was derived. The corresponding equation was y = 1.64 ∗ 10 - 5 × HU + 1000 + 8.3 ∗ 10 - 2 . While the two established conversions lead to a mean quantification PET bias of 4.69% ± 0.27% and -2.84% ± 0.72% in a phantom experiment, PET difference measurements revealed only 0.5 % bias in PET quantification when applying the new conversion resulting from this study. Conclusions An optimized method for the conversion of CT to PET attenuation coefficients has been derived by systematic measurement of 38 different materials. In contrast to established methods, the new conversion also considers highly attenuating materials, thus improving attenuation correction of hardware components in PET/MR hybrid imaging.

We aimed to evaluate the diagnostic potential of dual-time-point imaging with positron emission tomography (PET) using O-(2-[(18)F]fluoroethyl)-L-tyrosine ((18)F-FET) for non-invasive grading of cerebral gliomas compared with a dynamic approach.Thirty-six patients with histologically confirmed cerebral gliomas (21 primary, 15 recurrent; 24 high-grade, 12 low-grade) underwent dynamic PET from 0 to 50 min post-injection (p.i.) of (18)F-FET, and additionally from 70 to 90 min p.i. Mean tumour-to-brain ratios (TBRmean) of (18)F-FET uptake were determined in early (20-40 min p.i.) and late (70-90 min p.i.) examinations. Time-activity curves (TAC) of the tumours from 0 to 50 min after injection were assigned to different patterns. The diagnostic accuracy of changes of (18)F-FET uptake between early and late examinations for tumour grading was compared to that of curve pattern analysis from 0 to 50 min p.i. of (18)F-FET.The diagnostic accuracy of changes of the TBRmean of (18)F-FET PET uptake between early and late examinations for the identification of HGG was 81% (sensitivity 83%; specificity 75%; cutoff - 8%; p 0.001), and 83% for curve pattern analysis (sensitivity 88%; specificity 75%; p 0.001).Dual-time-point imaging of (18)F-FET uptake in gliomas achieves diagnostic accuracy for tumour grading that is similar to the more time-consuming dynamic data acquisition protocol.• Dual-time-point imaging is equivalent to dynamic FET PET for grading of gliomas. • Dual-time-point imaging is less time consuming than dynamic FET PET. • Costs can be reduced due to higher patient throughput. • Reduced imaging time increases patient comfort and sedation might be avoided. • Quicker image interpretation is possible, as no curve evaluation is necessary.

Until recently, no direct comparison between [15O]water positron emission tomography (PET) and arterial spin labeling (ASL) for measuring cerebral blood flow (CBF) was possible. With the introduction of integrated, hybrid magnetic resonance (MR)-PET scanners, such a comparison becomes feasible. This study presents results of CBF measurements recorded simultaneously with [15O]water and ASL. A 3T MR-BrainPET scanner was used for the simultaneous acquisition of pseudo-continuous ASL (pCASL) magnetic resonance imaging (MRI) and [15O]water PET. Quantitative CBF values were compared in 10 young healthy male volunteers at baseline conditions. A statistically significant ( P15O]water PET became possible for the first time so that temporal, physiologic, and functional variations were avoided. Regional and individual differences were found despite the overall similarity between ASL and PET, requiring further detailed investigations.

In hybrid magnetic resonance-positron emission tomography (MR-PET) studies with the Siemens 3T MR-BrainPET scanner an instantaneous reduction of the PET sensitivity was observed during execution of certain MR sequences. This interference was investigated in detail with custom-made as well as standard clinical MR sequences. The radio-frequency pulses, the switched gradient fields and the constant magnetic field were examined as the relevant parameters of the magnetic resonance imaging (MRI) system as well as the air temperature within the PET detectors. Our investigation comprised the analysis of the analog PET signals, the total count rates, the geometric distribution of the count rate reduction within the BrainPET detector as well as reconstructed images. The fast switching magnetic field gradients were identified to distort the analog PET detector signals. The measured count rate reduction was found to be less than 3%, but only up to 2% in the case of echo planar imaging sequences, as applied in functional MRI. For clinical sequences routinely used in hybrid MR-BrainPET measurements, a correction method has been designed, implemented, and evaluated .

Purpose The use of magnetic resonance (MR) radiofrequency (RF) surface coils is a prerequisite for high-quality positron emission tomography (PET)/MR imaging. In lack of in-gantry transmission (TX) sources, the exact position of the RF coils is unknown in PET/MR, and may, therefore, lead to false attenuation correction (AC) of the emission (EM) data. The authors assess lesion and background quantification in AC-PET by mimicking different PET/MR imaging situations using a whole-body (WB) PET-only tomograph. Methods Phantom experiments were performed on a PET tomograph with 68 Ge-rod TX sources. First, a 15-cm plastic cylinder was filled uniformly with [18F]-FDG to simulate a head study. Second, a NEMA NU-2001 image quality phantom (35 x 25 x 25 cm3) was filled uniformly with [18F]-FDG to simulate torso imaging. The phantom contained six lesions (10-38 mm diameter, lesion-to-background ratio 6:1) centred around a 5 cm diameter lung insert. EM and TX measurements were acquired with and without MR head (cylinder) and surface (NU-2001 phantom) RF coils in place. The following imaging situations were mimicked in both head and torso phantom studies: (1) PET scan without MR coils in EM and TX for reference, (2) PET scan with coils in both EM and TX, and (3) PET scan with coils in EM but without coils in TX. Two more set-ups were performed for the torso phantom: (4) PET scan with coils in EM only and phantom shifted slightly compared to (3), and (5) PET scan with coils in EM and TX following local displacement of the surface coils. PET EM data (1)-(4) were corrected for attenuation and scatter using cold TX data. Imaging situations (1)-(3) were repeated with the cylinder phantom and head coil in a combined PET/MR prototype system employing template-based AC. Results Head phantom: In case the MR head coils were not accounted for during AC (3), central and peripheral background activity concentration was underestimated by 13%-19% when compared to the reference setup (1). The effects of MR coil omission during AC was replicated in the repeat study with the combined PET/MR prototype. Torso phantom: All lesions were equally visible on all AC-PET images. The effects of disregarding MR surface RF coils during AC [(3) vs (1)] were 4%, or less. A slightly higher bias was observed when accounting for the RF surface coils that were shifted between EM and TX (5). The effect of coil misalignment and neglect during AC on the quantification of the simulated lungs was insignificant compared to the noise levels in AC-PET. Conclusions Unaccounted attenuation from MR surface coils causes a regional bias of AC-PET data in body regions near the MR coils. Bias of central regions was more noticeable in smaller-size objects. In torso studies with body surface coils, the visibility of central lesions on PET was unaffected by MR coils following incomplete AC. Coil misalignment of several cm between emission and attenuation images causes an error that was comparable to that arising from unaccounted MR coil attenuation but small compared to the average standard deviation of the activity concentration levels.

At present, a new hybrid imaging technology combining PET and MRI is emerging. The construction of hybrid scanners that allow simultaneous MRI and PET required the development of a new type of MR-compatible PET detector, which was first realized for studying small animals. The first industrial prototypes of such integrated MR-PET scanners for human brain studies have been installed recently. In this article, the feasibility of this new approach is demonstrated by first studies in brain tumor patients. Beyond time saving in patient management and optimal spatial overlay of anatomic and functional information, this technique offers new perspectives with regard to the simultaneous observation of physiological and biochemical processes. Functional brain activation and interaction of receptor ligands during special activation tasks or pharmacological intervention may be explored in humans. Thus, MR-PET opens new horizons in multiparametric neuroimaging.

Head motion can significantly degrade image quality of static and dynamic Positron Emission Tomography (PET) of the human brain. One method to regain acceptable image quality in the presence of motion is to include the correction for motion in the reconstruction process. When applying motion correction, the PET data can be segmented into discrete parts of similar head position, referred to as frames. This framing of the data can reduce the computational overhead necessary for motion correction during the reconstruction process by reducing the number of discrete head positions which have to be accounted for. Here a framing algorithm is presented which minimises residual motion in the framed data, while taking full advantage of the additional information provided by Magnetic Resonance Imaging (MRI) in a simultaneous MR–PET acquisition. In the work presented here information on motion is derived from EPI sequences acquired simultaneously with the PET data. A comparison to images reconstructed with regular framing show a more clearly delineated cortex due to increased contrast between grey matter and white matter. This improvement in image quality is achieved as well as a reduction in the number of frames, thereby reducing the reconstruction time. Preliminary data indicates an efficient reduction of residual intra-frame motion compared to regular framing.

PLoS one 9(4), e95250 (2014). doi:10.1371/journal.pone.0095250, Published by PLoS [u.a.], Lawrence, Kan.

To determine task-specific activations of the human brain in individual subjects, we applied pixel-by-pixel t-map statistics to the regional cerebral perfusion data obtained sequentially by dynamic scanning of [15O]-butanol with positron emission tomography (PET). The listmode data were binned into frames of 2 sec, and multiple corresponding pixel-by-pixel activation-minus-control subtractions were used for t-map calculation. The subtraction frames covering 10–40 sec after tracer arrival in the brain showed the activation-related increase of regional cerebral perfusion. A mismatch of the activation and control data by 2 sec resulted in a mean error of

Measurements of cerebral blood flow (CBF) using 15O-water-PET and ASL-MRI have been compared previously. Until now the two procedures were not performed simultaneously. Using a 3TMR-BrainPET, which integrates a BrainPET developed by Siemens as an insert in a Siemens 3T MAGNETOM MRI, we studied 15O-water-PET and ASL-MRI simultaneously in 10 healthy young men. In this way physiological and functional side conditions could be avoided which are possible in the separate PET/ASL comparisons published until now. Our initial results showed a similar global CBF of 50.1±7.2 ml/100 g/min and 51.9±7.1 ml/100 g/min with PET and ASL, respectively. While the CBF measured with ASL in white matter was lower than that of PET, the opposite result was obtained in gray matter. Direct comparisons in the single subjects showed local differences between ASL and PET as well as obvious distortions in ASL images. Although the global CBF between the two methods is similar, further investigations to improve the measurement of white matter perfusion in ASL are needed. © 2013 Elsevier B.V. All rights reserved.

We describe the implementation and validation of a combined dynamic–autoradiographic approach for measuring the regional cerebral blood flow (rCBF) with 15O-butanol. From arterial blood data sampled at a rate of 1 s and list mode data of the cerebral radioactivity accumulated over 100 s, the time shift between blood and tissue curves, the dispersion constant DC, the partition coefficient p, and the CBF were estimated by least squares fitting. Using the fit results, a pixel-by-pixel parametrization of rCBF was computed for a single 40-s (autoradiographic) 15O-butanol uptake image. The mean global CBF found in 27 healthy subjects was 49 ± 8 ml 100 g−1 min−1. Gray and white matter rCBF were 83 ± 20 and 16 ± 3 ml 100 g−1 min−1, respectively, with a corresponding partition coefficient p of 0.77 ± 0.18 and 0.77 ± 0.29 ml/g in both compartments. The quantitative images resulted in a significantly higher gray matter rCBF than the autoradiographic images.

PLoS one 7(11), e48149 (2012). doi:10.1371/journal.pone.0048149, Published by PLoS [u.a.], Lawrence, Kan.

Figure 2 shows reconstructed images of a two-chamber phantom (filled with two activity concentrations) which was measured in 7 different positions. The applied motion of the phantom induces a significant image degradation (left). Motion parameters were extracted from EPI volumes to correct the emission data before the reconstruction with PRESTO. The blurring due to motion completely vanishes (right).

Technological challenges have been overcome to realize the combination of MRI and PET imaging systems. The high image quality of the MR compatible BrainPET scanner has been shown and quantification of PET images with a small error is possible. Here we present our experiences with the long term quantitative stability of this system, evaluating data of daily quality control (dailyQC) and calibration measurements acquired every two weeks.

Our method for attenuation correction (AC) in MR-BrainPET with segmented T1-weighted MR images of the pa-tient's head was applied to data from different MR-BrainPET scanners (Julich, Tubingen) and compared to CT-based results.

SummaryAfter 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]-fluoroethyl- tyrosine (FET), [11C]-flumazenil and [18F]-FP-CIT were applied. Results: Comparing komthe 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.

Attenuation correction of PET data acquired in new hybrid MR-PET scanners which do not offer the possibility of a measured attenuation correction can be done in different ways. A previous report of our group described a method which used attenuation templates. The present study utilizes a new knowledge-based segmentation approach applied on T1-weighted MR images. It examines the position and the tissue membership of each voxel and segments the head volume into attenuation-differing regions: brain tissue, extracerebral soft tissue, skull, air-filled nasal and paranasal cavities as well as the mastoid process. To examine this new approach three groups of subjects having MRI and PET were chosen, the selection criterion being the different MR scanners, while the PET scanner was the ECAT HR+ in all cases: 1) four subjects with 1.5T MR images and CPFPX PET scans, 2) four subjects with 3T MR images and Altanserin PET scans, and 3) three brain tumor patients with 3T MR images from the hybrid MR-BrainPET scanner and FET PET scans. Furthermore, a single subject had 3T MR images, a FDG PET scan, and an additional CT scan. All segmented T1-weighted MR images were converted into attenuation maps for 511 KeV photons with coefficients of 0.096 1/cm for brain tissue, 0.146 1/cm for skull, 0.095 1/cm for soft tissue, 0.054 1/cm for the mastoid process, and 0.0 1/cm for nasal and paranasal cavities. The CT volume was also converted from the Hounsfield units into attenuation coefficients valid for 511 keV photons. The 12 segmented-based attenuation (SBA) maps as well as the CT-based attenuation (CBA) map were first filtered by a 3D Gaussian kernel of 10 mm filter width and then used to reconstruct the corresponding PET emission data. These were compared to the PET images attenuation corrected using the conventional PET-based transmission data (PBA). Relative differences (RD) were calculated from ROIs. For the single subject the RD of CBA data exhibit a mean of 1.66%±0.84% with a range from -0.88% to 3.42%, while the RD's mean of SBA data is 1.42%±2.61% (range from -4.12% to 4.66%). Comparing the results obtained with the SBA correction only, the RD for 1) range from -6.10% to 2.56% for cortical regions and from -6.99% to 5.64% for subcortical regions; for 2) they range from -7.33% to 2.33% for the cortical regions, subcortical ones being not drawn due to the not significant tracer uptake; for 3) the mean over the three subjects resulted in 0.89%±1.10% for ROIs at 48% threshold of the image's maximum and in 2.25%±1.50% for ROIs at 72% threshold. ROIs on the healthy contra-lateral grey matter show a mean of -3.24%±0.87%. In conclusion, the first attenuation correction results obtained with the new segmented-based method on a strongly heterogeneous collective are very promising. Further improvements of the method will be focused on the delineation of the skull.

The purpose of this study was to compare the diagnostic value of positron emission tomography (PET) using [ 18 F]-fluorodeoxyglucose ( 18 F-FDG) and O -(2-[ 18 F]fluoroethyl)-l-tyrosine ( 18 F-FET) in patients with brain lesions suspicious of cerebral gliomas. Methods Fifty-two patients with suspicion of cerebral glioma were included in this study. From 30 to 50 min after injection of 180 MBq 18 F-FET, a first PET scan ( 18 F-FET scan) was performed. Thereafter, 240 MBq 18 F-FDG was injected and a second PET scan was acquired from 30 to 60 min after the second injection ( 18 F-FET/ 18 F-FDG scan). The cerebral accumulation of 18 F-FDG was calculated by decay corrected subtraction of the 18 F-FET scan from the 18 F-FET/ 18 F-FDG scan. Tracer uptake was evaluated by visual scoring and by lesion-to-background (L/B) ratios. The imaging results were compared with the histological results and prognosis. Results Histology revealed 24 low-grade gliomas (LGG) of World Health Organization (WHO) Grade II and 19 high-grade gliomas (HGG) of WHO Grade III or IV, as well as nine others, mainly benign histologies. The gliomas showed increased 18 F-FET uptake (>normal brain) in 86% and increased 18 F-FDG uptake (>white matter) in 35%. 18 F-FET PET provided diagnostically useful delineation of tumor extent while this was impractical with 18 F-FDG due to high tracer uptake in the gray matter. A local maximum in the tumor area for biopsy guidance could be identified with 18 F-FET in 76% and with 18 F-FDG in 28%. The L/B ratios showed significant differences between LGG and HGG for both tracers but considerable overlap so that reliable preoperative grading was not possible. A significant correlation of tracer uptake with overall survival was found with 18 F-FDG only. In some benign lesions like abscesses, increased uptake was observed for both tracers indicating a limited specificity of both techniques. Conclusions 18 F-FET PET is superior to 18 F-FDG for biopsy guidance and treatment planning of cerebral gliomas. The uptake of 18 F-FDG is associated with prognosis, but the predictive value is limited and a histological evaluation of tumor tissue remains necessary. Therefore, amino acids like 18 F-FET are the preferred PET tracers for the clinical management of cerebral gliomas.

Recently a prototype of a new bimodal scanner was installed in our laboratory. This scanner combines magnetic resonance imaging (MRI) and positron emission tomography (PET) for brain studies. As the PET detector is located within the bore of the MRI scanner, simultaneous measurements are possible. The MR-component consists of a commercial 3T MRI scanner MAGNETOM Trio, whereas the PET detector has been newly developed. The readout electronics of the PET is based on Avalanche photodiodes (APDs) which are not affected by the magnetic field. The inner diameter of the PET ring, which consists of 32 cassettes, is 36 cm so that a MRI head coil can be inserted into this ring. First tests were done to analyze the countrate behavior, the scatter fraction and the image resolution. To assess the image quality of simultaneous MR-PET studies a brain-shaped phantom containing 18Fsolution was scanned. The prompt-minus-delayed countrate showed a peak of 680000 cps at 50 kBq/cc. The scatter fraction was 27%. The image resolution in the centre was between 2.1 and 2.5 mm in x,y,z directions. The MR-PET studies of the brain-shaped phantom showed no visible artifacts of the MR image and the PET images showed much better delineation of the cortex compared to equivalent images recorded with the PET-only scanner Siemens HR+. The tests done until now yielded promising results so that a first qualitative human brain study could be performed.

The long acquisition times (up to hours) in PET brain imaging bear a high risk of head motion, which results in artefacts like blurred images and may even lead to misinterpretation and useless data. With the increased resolution of high performance PET scanners, the influence of head movements becomes more and more relevant. Especially in the analysis of small brain structures, e.g. during ROI-analysis, head motion results in inaccuracies of quantified data. This may also influence the kinetic analysis and generate artifacts in parametric images calculat-ed from a motion-affected image sequence. This work presents the feasibility of head motion registration using an external motion tracking system. The implementation of the multi acquisition frame method and an event-by-event method to correct PET data for motion are described. The effects of motion correction are demonstrated on the basis of phantom measurements and patient data. The influence of motion correction on parametric imaging is described in a receptor study.

The authors investigated the influence of variations in global cerebral blood flow (gCBF) on regional flow changes during visual stimulation. Global flow was varied using different end-expiratory CO2 values (Petco2) between 20 and 70 mm Hg. Visual stimulation was performed with a red LED-array flashing at 8 Hz. Blood flow was measured with O-15-butanol, continuous arterial blood sampling, and positron emission tomography (PET). Global flow changes surpassed the published values of O-15-H2O studies, better fitting the results of the inert gas technique (gCBF at 20, 40, and 70 mm Hg Petco2 ± SD was 31 ± 4, 48 ± 13, and 160 ± 50 mL 100 g−1 min−1, respectively). The relation between Petco2 and CBF in the current study was best described by an exponential rather than a linear function. At low Petco2, the activation-induced flow changes are moderately damped, whereas at high Petco2, they are nearly lost (ΔCBF (±SD): 52% ± 25%, 68% ± 22%, 16% ± 25% at Petco2 = 20, 40, 70 mm Hg, respectively).

The calculation of parametric images in PET studies of neuroreceptors is based on dynamic data which have been recorded over many minutes. It is essential that the subject's head remains unmoved during the PET scan, otherwise the data may become useless in the worst case. This work studies the degrading of parametric images caused by head movements and improvements which is achieved by an appropriate motion correction. The head movements present in PET neuroreceptor studies cause artifacts in the calculation of parametric images. Whereas the activity images look blurred, the parametric images contain discontinuities especially at the cortex. It is concluded that the linear regression is sensitive to the specific errors present in the dynamic images because of the movements

The NEMA NU 2-2001 protocol has been especially designed for the performance evaluation of whole-body positron emission tomography (PET) measurements with scanners acquiring data in three-dimensional (3-D) mode without septa or with septa retracted. We report on the comparative NEMA NU 2-2001 performance tests on four different Siemens/CTI ECAT PET scanners, the two bismuth germanate oxide scanners HR+ and EXACT and the two lutetium oxyorthosilicate scanners ACCEL and EMERGE. All scanners were operated in 3-D mode only. Until now, no complete NEMA NU 2-2001 evaluations have been reported for the HR+ and EXACT. For the ACCEL and the EMERGE, these are the first published performance characteristics as far as we know. The HR+ showed a superior spatial resolution, whereas the ACCEL had the best countrate [noise equivalent count (NEC)] performance above a radioactivity concentration of 2 kBq/cc. The sensitivity of the full-ring scanners did not differ much with numbers between 5982 and 7180 cps/MBq. The sensitivity of the partial-ring scanner EMERGE was 2279 cps/MBq. Although the EMERGE yields lower NEC rates than the EXACT for radioactivity concentrations less than 10 kBq/cc, it is favorable beyond this cross over point. The scatter fraction of all four scanners was about 48%. The lesions to be examined by the image quality test could be seen best with the ACCEL. The NEMA NU 2-2001 performance tests proved to be a valuable guideline for this instrumental comparison.

The whole-body distribution of O-(2-[(18)F]fluoroethyl)- l-tyrosine (FET) was studied in seven patients with brain tumours by positron emission tomography (PET). Based on the IMEDOSE and MIRDOSE procedures, radiation absorbed doses were estimated from whole-body PET scans acquired approximately 70 and 200 min after i.v. injection of 400 MBq FET. After injection of FET, the peak of radioactivity in the blood was observed after 1.5 min, and a plateau of nearly constant radioactivity was reached at 20 min. The whole-body distribution of FET showed the highest activities in the urinary tract. All other organs exhibited only moderate FET uptake (SUV/=1.6) which remained constant between early and late PET scans. No increased uptake was seen in the bone, the biliary tract or the pancreas. Twenty-two percent of the injected activity was excreted 5 h p.i. (approx. 5.3% ID/h). The highest absorbed dose was found for the urinary bladder wall. The effective dose according to ICRP 60 was 16.5 micro Sv/MBq for adults, which would lead to an effective dose of 6.1 mSv in a PET study using 370 MBq FET.