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Simultaneous MR-PET is an increasingly popular multimodal imaging technique that is able to combine metabolic information obtained from PET with anatomical/functional information from MRI. One of the key technological challenges of the technique is the integration of a PET-transparent MR coil system, a solution to which is demonstrated here for a double-tuned 1 H/ 31 P head coil at 3 T. Two single-resonant birdcage coils tuned to the 1 H and 31 P resonances were arranged in an interleaved fashion and electrically decoupled with the use of trap circuits. All high 511 keV quanta absorbing components were arranged outside the PET field-of-view in order to minimize count rate reduction. The materials inside the PET field-of-view were carefully evaluated and chosen for minimum impact on the PET image quality. As far as possible, the coil case was geometrically optimized to avoid sharp transitions in attenuation, which may potentially result in streaking artefacts during PET image reconstruction. The coil caused a count rate loss of just above 5% when inserted into the PET detector ring. Except for the anterior region, which was designed to maintain free openings for increased patient comfort, an almost uniform distribution of 511 keV attenuation was maintained around the circumference of the coil. MR-related performance for both nuclei was similar or slightly better than that of a commercial double-tuned coil, despite the MR-PET coil having a close-fitting RF screen to shield the PET and MR electronics from possible electromagnetic interferences.

ObjectiveThe positron range is a fundamental, detector-independent physical limitation to special resolution in positron emission tomography (PET) as it causes a significant blurring of the reconstructed PET images. A major challenge for positron range correction methods is to provide accurate range kernels that inherently incorporate the generally inhomogeneous stopping power, especially at tissue boundaries. In this work, we propose a novel approach to generate accurate three-dimensional (3-D) blurring kernels both in homogenous and heterogeneous media to improve PET spatial resolution.ApproachIn the proposed approach, positron energy deposition was approximately tracked along straight paths, depending on the positron stopping power of the underlying material. The positron stopping power was derived from the attenuation coefficient of 511keV gamma photons according to the available PET attenuation maps. Thus, the history of energy deposition is taken into account within the range of kernels. Special emphasis was placed on facilitating the very fast computation of the positron annihilation probability in each voxel.ResultsPositron path distributions of 18F in low-density polyurethane were in high agreement with Geant4 simulation at an annihilation probability larger than 10−2∼10−3 of the maximum annihilation probability. The Geant4 simulation was further validated with measured 18F depth profiles in these polyurethane phantoms. The tissue boundary of water with cortical bone and lung was correctly modeled. Residual artifacts from the numerical computations were in the range of 1%. The calculated annihilation probability in voxels shows an overall difference of less than 20% compared to the Geant4 simulation.SignificanceThe proposed method significantly improves spatial resolution for non-standard isotopes by providing accurate range kernels, even in the case of significant tissue inhomogeneities.

Physics in medicine and biology 67(23), 235004 (2022). doi:10.1088/1361-6560/aca1f3, Published by IOP Publ., Bristol

Objective. The positron range is a fundamental, detector-independent physical limitation to spatial resolution in positron emission tomography (PET) as it causes a significant blurring of underlying activity distribution in the reconstructed images. A major challenge for positron range correction methods is to provide accurate range kernels that inherently incorporate the generally inhomogeneous stopping power, especially at tissue boundaries. In this work, we propose a novel approach to generate accurate three-dimensional (3D) blurring kernels both in homogenous and heterogeneous media to improve PET spatial resolution. Approach. In the proposed approach, positron energy deposition was approximately tracked along straight paths, depending on the positron stopping power of the underlying material. The positron stopping power was derived from the attenuation coefficient of 511 keV gamma photons according to the available PET attenuation maps. Thus, the history of energy deposition is taken into account within the range of kernels. Special emphasis was placed on facilitating the very fast computation of the positron annihilation probability in each voxel. Results. Positron path distributions of 18F in low-density polyurethane were in high agreement with Geant4 simulation at an annihilation probability larger than 10−2 ∼ 10−3 of the maximum annihilation probability. The Geant4 simulation was further validated with measured 18F depth profiles in these polyurethane phantoms. The tissue boundary of water with cortical bone and lung was correctly modeled. Residual artifacts from the numerical computations were in the range of 1%. The calculated annihilation probability in voxels shows an overall difference of less than 20% compared to the Geant4 simulation. Significance. The proposed method is expected to significantly improve spatial resolution for non-standard isotopes by providing sufficiently accurate range kernels, even in the case of significant tissue inhomogeneities.

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 To overcome existing challenges in dynamic B0 shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms. Theory and methods Spherical harmonic dynamic B0 shimming is complicated by eddy currents, ill-posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B0 inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements. Results High-order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B0 homogeneity. Whereas conventional dynamic shim updating optimizations improve the B0 homogeneity, on average, by a factor of 2.1 over second-order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14-fold reduction of the average maximum shim current changes. Conclusions The proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B0 homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B0 shimming and help in making its application more readily available.

Currently, the metabotropic glutamate receptor 5 (mGluR5) is the subject of several lines of research in the context of neurology and is of high interest as a target for positron-emission tomography (PET). Here, we assessed the feasibility of using [11C]ABP688, a specific antagonist radiotracer for an allosteric site on the mGluR5, to evaluate changes in glutamatergic neurotransmission through a mismatch-negativity (MMN) task as a part of a simultaneous and synchronized multimodal PET/MR-EEG study. We analyzed the effect of MMN by comparing the changes in nondisplaceable binding potential (BPND) prior to (baseline) and during the task in 17 healthy subjects by applying a bolus/infusion protocol. Anatomical and functional regions were analyzed. A small change in BPND was observed in anatomical regions (posterior cingulate cortex and thalamus) and in a functional network (precuneus) after the start of the task. The effect size was quantified using Kendall’s W value and was 0.3. The motor cortex was used as a control region for the task and did not show any significant BPND changes. There was a significant ΔBPND between acquisition conditions. On average, the reductions in binding across the regions were - 8.6 ± 3.2% in anatomical and - 6.4 ± 0.5% in the functional network (p ≤ 0.001). Correlations between ΔBPND and EEG latency for both anatomical (p = 0.008) and functional (p = 0.022) regions were found. Exploratory analyses suggest that the MMN task played a role in the glutamatergic neurotransmission, and mGluR5 may be indirectly modulated by these changes.

Simultaneous MR-PET is an increasingly popular multimodal imaging technique that is able to combine metabolic information obtained from PET with anatomical/functional information from MRI. One of the key technological challenges of the technique is the integration of a PET-transparent MR coil system, a solution to which is demonstrated here for a double-tuned

The PET data were acquired using a 3T hybrid MR-BrainPET insert system (SIEMENS, Erlangen, Germany) in list mode. The coincidences were corrected for random events using the delayed window technique with VRR, dead time, attenuation and scattered coincidences (single scatter simulation – SSS method), and physical decay. An adapted NEMA phantom, without lung insert and with six sphere inserts, was used. One insert was filled with 11C and the others contained cold (no activity) water contrasting in a warm background cylinder. Coincidence data were acquired during eight isotope half-lives, giving 163 minutes total acquisition time for 11C, which has a half-life of T1/2 = 20.35 minutes. The ratio of the activity concentration between the sphere (Hot with 28 mm of nominal internal diameter) and the background region (BG) was 1.85:1. This value is frequently found for the ratio of activity concentrations in the grey matter cortex (GM) and cerebellar grey matter (reference region) during the steady-state condition in [11C]ABP688 studies. A cold transmission scan of the adapted NEMA phantom using 68Ge sources was acquired in a Siemens ECAT Exact HR+ PET scanner. This acquisition (2 bed positions, 20 minutes of transmission, reconstructed with OSEM 2D – 6 iterations and 16 subsets and a 256 × 256 matrix) was used to create the attenuation map for the phantom used in the 11C decay study.

Background: Iterative image reconstruction algorithms are widely used in positron emission tomography (PET). However, they are known to contribute to quantitation bias, which is particularly pronounced during dynamic PET studies such as neuroreceptor binding studies with 11C-labelled radiotracers where count rates become low towards the end of the examination. This problem is relevant in case simultaneous PET/MR studies which apply a bolus-infusion protocol to allow the multimodal comparison between control or resting state and stimulation effects in a single session of e.g. 60 min, i.e. 3 half-lives of 11C. A quantitation bias may interfere with stimulation related changes. In this study, we evaluated the impact of the 3D ordinary Poisson OSEM (3D OP-OSEM) on quantitation accuracy reconstructions and the subsequent propagation into binding potential values using a decay study of a 11C filled phantom and a human brain data set. To evaluate the reconstruction bias, we tested different reconstruction framing schemes and propose a framing scheme that keeps the counts per time frame constant over the full acquisition time. We also compared the vendor’s 3D OP-OSEM image reconstruction method to an in-house developed reconstruction (PRESTO toolkit). Results: In general, a bias for low and high activity concentration regions was observed in the range of ± 3% and - 3% to 5%, respectively. Using the alternative proposed framing scheme, a lower bias was achieved for regions with low activity concentration using PRESTO, and a stabilization of the mean bias for the binding potential was achieved at around 5% with the vendor’s reconstruction throughout the relevant activity curve time interval. Conclusions: The bias in activity concentration propagated into the binding potential values leading to a mean bias of around 5%, thus allowing the detection of changes in binding values during equilibrium of > 5%.

Prostate cancer is one of the most common cancers among men and its early detection is critical for its successful treatment. The use of multimodal imaging, such as MR-PET, is most advantageous as it is able to provide detailed information about the prostate. However, as the human prostate is flexible and can move into different positions under external conditions, it is important to localise the focused region-of-interest using both MRI and PET under identical circumstances. In this work, we designed five commonly used linear and quadrature radiofrequency surface coils suitable for hybrid MR-PET use in endorectal applications. Due to the endorectal design and the shielded PET insert, the outer face of the coils investigated was curved and the region to be imaged was outside the volume of the coil. The tilting angles of the coils were varied with respect to the main magnetic field direction. This was done to approximate the various positions from which the prostate could be imaged. The transmit efficiencies and safety excitation efficiencies from simulations, together with the signal-to-noise ratios from the MR images were calculated and analysed. Overall, it was found that the overlapped loops driven in quadrature were superior to the other types of coils we tested. In order to determine the effect of the different coil designs on PET, transmission scans were carried out, and it was observed that the differences between attenuation maps with and without the coils were negligible. The findings of this work can provide useful guidance for the integration of such coil designs into MR-PET hybrid systems in the future.

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.

IEEE transactions on radiation and plasma medical sciences : ITRPFI 3(3), 377-385 (2019). doi:10.1109/TRPMS.2018.2886525, Published by IEEE, New York, NY

PURPOSE: To overcome existing challenges in dynamic B(0) shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms. THEORY AND METHODS: Spherical harmonic dynamic B(0) shimming is complicated by eddy currents, ill-posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B(0) inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements. RESULTS: High-order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B(0) homogeneity. Whereas conventional dynamic shim updating optimizations improve the B(0) homogeneity, on average, by a factor of 2.1 over second-order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14-fold reduction of the average maximum shim current changes. CONCLUSIONS: The proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B(0) homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B(0) shimming and help in making its application more readily available.

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.

One challenge for PET-MR hybrid imaging is the correction for attenuation of the 511 keV annihilation radiation by the required RF transmit and/or RF receive coils. Although there are strategies for building PET transparent Tx/Rx coils, such optimised coils still cause significant attenuation of the annihilation radiation leading to artefacts and biases in the reconstructed activity concentrations. We present a straightforward method to measure the attenuation of Tx/Rx coils in simultaneous MR-PET imaging based on the natural 176Lu background contained in the scintillator of the PET detector without the requirement of an external CT scanner or PET scanner with transmission source. The method was evaluated on a prototype 3T MR-BrainPET produced by Siemens Healthcare GmbH, both with phantom studies and with true emission images from patient/volunteer examinations. Furthermore, the count rate stability of the PET scanner and the x-ray properties of the Tx/Rx head coil were investigated. Even without energy extrapolation from the two dominant γ energies of 176Lu to 511 keV, the presented method for attenuation correction, based on the measurement of 176Lu background attenuation, shows slightly better performance than the coil attenuation correction currently used. The coil attenuation correction currently used is based on an external transmission scan with rotating 68Ge sources acquired on a Siemens ECAT HR + PET scanner. However, the main advantage of the presented approach is its straightforwardness and ready availability without the need for additional accessories.

Motion is a frequent problem in magnetic resonance-positron emission tomography (MR-PET) acquisitions, leading to significant degradations of the image quality. This chapter gives an overview of this issue and potential remedies. First, different ways of measuring the intra-scan motion are discussed. This is sub-divided into external device-based PET-based and MR-based motion detection and tracking. Given that MRI-based methods can be relatively fast, they lend themselves to retrospective as well as prospective correction; in retrospective correction the motion information is used to correct flawed k-space data after the scan is completed, i.e. during reconstruction or post-processing, whereas in prospective motion correction the motion information is used to correct the MRI measurement itself in real time while the scan is still running. The goal of prospective correction is to acquire data that are unaffected by any motion that occurs during the measurement. Thereafter, several different motion correction techniques are presented, which are able to counter the negative effects of motion in both MRI and PET.

The 11CO2 tracer is of high interest to study carbon transport in living plants. However, conventional PET images of plant leaves suffer from the poor positron annihilation probability in the targeted structures, thus giving strong bias in reconstructed images. Here, PET/MR hybrid scanner systems offer a new method to detect and quantify escaping positrons as 2D projections by exploiting the directed travelling of positrons parallel to the present B0 field of the MR magnet system. Due to the Lorentz force all emitted positrons propagate on spiral trajectories parallel to the B0 field with tiny gyroradii. The introduction of a thin absorber screen within the field-of-view of the PET scanner, forces the annihilation of almost all incoming positrons inside the absorber. The annihilation photon pairs can be registered as coincidences and thus directly indicate the positron impact point on the screen by interpolating the Lines-ofResponse onto the screen. In this way, 2D images are obtained and the amount of positrons leaving the source can be quantified dynamically.

OBJECTIVES O-(2-(18)F-fluoroethyl)-L-tyrosine ((18)F-FET) is an established tracer for brain tumour imaging. (18)F-FET kinetics in gliomas appear to have potential for tumour grading, but the mechanisms remain unclear. The aim of this study was to explore the relationship between regional cerebral blood flow (rCBF) as measured by arterial spin labelling MRI and the kinetic behaviour of (18)F-FET PET in cerebral gliomas. MATERIALS AND METHODS Twenty patients with cerebral gliomas were investigated using arterial spin labelling MRI and dynamic (18)F-FET PET. Time-activity curves (TACs) of (18)F-FET uptake were analysed in 33 different tumour regions. The slopes of TAC during the early (0-5 min; slopeup) and late phases of tracer uptake (17-50 min; slopedown) were fitted using linear regression lines. In addition, TACs of each lesion were assigned to different curve patterns. Furthermore, we calculated tumour-to-brain ratios of (18)F-FET uptake. The relationship between (18)F-FET parameters and rCBF was determined. RESULTS (18)F-FET uptake in the early phase (slopeup) showed a significant correlation with rCBF (r=0.4; P=0.02). In contrast, both slopedown and TAC patterns showed no significant correlation with rCBF. Furthermore, a significant correlation was found between rCBF and tumour-to-brain ratio (r=0.53; P=0.002). CONCLUSION There is a relationship between rCBF and (18)F-FET uptake in cerebral gliomas in the initial uptake phase, but the kinetic behaviour of (18)F-FET uptake in the late phase is not significantly influenced by rCBF. Thus, the differential kinetic pattern of (18)F-FET uptake in high-grade and low-grade gliomas appears to be determined by factors other than rCBF.

Aim : The combination of a high-resolution MR-compatible BrainPET insert operated within a 3 T MAGNETOM Trio MR scanner is an excellent tool for obtaining an image derived input function (IDIF), due to simultaneous imaging. In this work, we explore the possibility of obtaining an IDIF from volumes of interest (VOI) defined over the carotid arteries (CAs) using the MR data. Material and methods : FDG data from three patients without brain disorders were included. VOIs were drawn bilaterally over the CAs on a MPRAGE image using a 50% isocontour (MR50VOI). CA PET/MR co-registration was examined based on an individual and combined CA co-registration. After that, to estimate the IDIF, the MR50VOI average (IDIF-A), four hottest pixels per plane (IDIF-4H) and four hottest pixels in VOI (IDIF-4V) were considered. A model-based correction for residual partial volume effects involving venous blood samples was applied, from which partial volume ( PV ) and spillover ( SP ) coefficients were estimated. Additionally, a theoretical PV coefficient ( PVt ) was calculated based on MR50VOI. Results : The results show an excellent co-registration between the MR and PET, with an area under the curve ratio between both co-registration methods of 1.00±0.04. A good agreement between PV and PVt was found for IDIF-A, with PV of 0.39±0.06 and PVt 0.40±0.03, and for IDIF-4H, with PV of 0.47±0.05 and PVt 0.47±0.03. The SP s were 0.20±0.03 and 0.21±0.03 for IDIF-A and IDIF-4H, respectively. Conclusion : The integration of a high resolution BrainPET in an MR scanner allows to obtain an IDIF from an MR-based VOI. This must be corrected for a residual partial volume effect.

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.

We investigated the role of the cerebellum in differential aspects of temporal control of rhythmic auditory motor synchronization using positron emission tomography (PET). Subjects tapped with their right index finger to metronome tones at a mean frequency of .8Hz during 5 conditions: (1) an isochronous rhythm condition, (2) random changes in interval durations, and while the duration of rhythmic intervals was continuously time-modulated following a cosine-wave function at (3) 3%, (4) 7%, and (5) 20% of base interval. Anterior lobe cerebellar neuronal populations showed similar motor-associated activity across all conditions regardless of rhythmic time structure in vermal and hemispheric parts ipsilateral to the movements. Neuronal populations in bilateral anterior posterior lobe, especially in the simple lobule, increased their activity stepwise with each increase in tempo modulation from a steady beat. Neuronal populations in other parts of the posterior lobe showed an increase of activity only during the 20% condition, which involved conscious monitoring of rhythmic pattern synchronization, especially on the left side contralateral to the movements. Differential cerebellar activation patterns correspond to those in contralateral primary (primary sensorimotor), ipsilateral secondary (inferior parietal close to the intraparietal sulcus) and bilateral tertiary (dorsolateral prefrontal cortex) sensorimotor areas of the cerebral cortex, suggesting that distinct functional cortico-cerebellar circuits subserve differential aspects of rhythmic synchronization in regard to rhythmic motor control, conscious and subconscious response to temporal structure, and conscious monitoring of rhythmic pattern tracking.

The majority of all modern PET cameras use LSO and LYSO crystals which contain a small proportion of the radioactive isotope of Lutetium. The aim of this study is to obtain attenuation maps for dedicated head PET/MR scanners based on the background activity of the LSO scintillator. In particular, attenuation maps for the Tx/Rx coils in the PET Field of View have to be provided. Before reconstructing such an attenuation map, the scanners stability with respect to detector temperature and background count rate was examined. First image reconstructions of the 8-channel Tx/Rx head coil and a 3-rod phantom obtained with LSO background prove that the method is feasible as long as the overall scan time is sufficiently long.

The purpose of this research was to determine the cortical circuit involved in encoding and controlling kinesthetically guided reaching movements. We used 15O-butanol positron emission tomography in ten blindfolded able-bodied volunteers in a factorial experiment in which arm (left/right) used to encode target location and to reach back to the remembered location and hemispace of target location (left/right side of midsagittal plane) varied systematically. During encoding of a target the experimenter guided the hand to touch the index fingertip to an external target and then returned the hand to the start location. After a short delay the subject voluntarily moved the same hand back to the remembered target location. SPM99 analysis of the PET data contrasting left versus right hand reaching showed increased (P

The non-pure positron-emitting iodine isotope (120)I (T(1/2)=81 min) is a short-lived alternative to (124)I. (120)I has a positron abundance more than twice that of (124)I and a maximum positron energy of 4 MeV. This study was undertaken to evaluate and characterise the qualitative and quantitative PET imaging of (120)I.(120)I was produced via the (120)Te(p,n) reaction on highly enriched (120)Te. The measurements were done with the Siemens scanner HR+ and the 2D PET scanner GE PC4096+. A cylinder containing three cold inserts and a phantom resembling a human brain slice were used to evaluate half-life, positron abundance and background correction. To analyse the image resolution, a -mm tube placed in water was filled with (120)I and (18)F. Comparisons with (18)F, (124)I and (123)I (measured with SPECT) were made using the Hoffman 3D brain phantom.The half-life of 81.1 min was reproduced by the PET measurements. The PET-based positron abundance ranged from 47.9% to 55.0%. The reconstructed image resolution found with the HR+ was 5.4 mm FWHM (12.3 mm FWTM), in contrast to 4.6 mm (8.6 mm) when using (18)F. Erroneous positive and negative numbers of radioactivity found in the cold inserts became nearly zero when the background of gamma-coincidences was corrected for. Images of the Hoffman phantom were inferior to those obtained when (18)F or (124)I was applied but superior to the (123)I-SPECT images.Our data show that (120)I of high radionuclidic purity can be regarded as a suitable nuclide for the PET imaging of radioiodine-labelled pharmaceuticals.