mGluR5 receptor availability is associated with lower levels of negative symptoms and better cognition in male patients with chronic schizophrenia

Consistent findings postulate disturbed glutamatergic function (more specifically a hypofunction of the ionotropic NMDA receptors) as an important pathophysiologic mechanism in schizophrenia. However, the role of the metabotropic glutamatergic receptors type 5 (mGluR5) in this disease remains unclear. In this study, we investigated their significance (using [11 C]ABP688) for psychopathology and cognition in male patients with chronic schizophrenia and healthy controls. In the patient group, lower mGluR5 binding potential (BPND ) values in the left temporal cortex and caudate were associated with higher general symptom levels (negative and depressive symptoms), lower levels of global functioning and worse cognitive performance. At the same time, in both groups, mGluR5 BPND were significantly lower in smokers (F[27,1] = 15.500; p = .001), but without significant differences between the groups. Our findings provide support for the concept that the impaired function of mGluR5 underlies the symptoms of schizophrenia. They further supply a new perspective on the complex relationship between tobacco addiction and schizophrenia by identifying glutamatergic neurotransmission-in particularly mGluR5-as a possible connection to a shared vulnerability. Keywords: chronic schizophrenia; cognition; mGluR5 receptor; negative symptoms; positron emission tomography

Alternative headphones for patient noise protection and communication in PET-MR studies of the brain

IntroductionDue to the high noise emission generated by the gradients in magnetic resonance imaging (MRI), an efficient method of noise protection is mandatory. In addition to providing hearing protection, appropriate headphone systems also serve to facilitate communication between the operator and the patient. However, in combined PET-MR devices, use of common pneumatic headphones, as delivered by the manufacturer, is problematic due to the potential generation of attenuation artefacts in the PET measurement. Furthermore, modern multichannel head coils rarely provide space for conventional headphones. This work presents an alternative system, which aims to address these limitations while still being appropriate for both patient noise protection and communication in PET-MR.Material and methodsAs an alternative to the standard headphones supplied with the PET-MR (3T MR-BrainPET, Siemens), the possibility of using earphones built out of commercially available earplugs has been investigated. The air channel (E-A-RLink) of the earplug is connected to the tubes of the original headphones. The attenuation characteristics of the conventional headphones and of the modified earphones were measured using a dedicated PET system with a 68Ge transmission source. For this purpose, the headphones, and then the earphones, were attached to a non-radioactive head phantom. To investigate the influence of the different phones on PET emission images, measurements of the head phantom, filled with 18F solution, were performed in the PET-MR. A measurement of the head phantom without headphones or earphones was used as a reference.ResultsThe linear attenuation coefficient of the headphones was 0.11 cm-1 and that of the head phantom 0.10 cm-1. The earphones were not identifiable in the transmission image. The emission image showed an activity underestimation of 10% near the headphones, compared to the reference image, whereas the earphones did not affect the image. Communication with the patient via the earphones was successful, and the noise protection—as confirmed by investigated subjects—was satisfying.ConclusionThe presented earphones, which can be connected to the existing patient communication system, are a preferable alternative to the conventional headphones, as, in contrast to the use of headphones, qualitative and quantitative errors in the PET images can be avoided. Patient acceptance of the earphones was high, despite the increase in preparation time before the PET-MR study

Improving the CT (140 kVp) to PET (511 keV) conversion in PET/MR Hardware Component Attenuation Correction

PurposeToday, 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 methodsOverall, 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.ResultsFor 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 urn:x-wiley:00942405:media:mp14091:mp14091-math-0001 . 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.ConclusionsAn 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

MR-Based PET Motion Correction Procedure for Simultaneous MR-PET Neuroimaging of Human Brain

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Effects of Magnetic Fields of up to 9.4 T on Resolution and Contrast of PET Images as Measured with an MR-BrainPET

Simultaneous, hybrid MR-PET is expected to improve PET image resolution in the plane perpendicular to the static magnetic field of the scanner. Previous papers have reported this either by simulation or experiment with simple sources and detector arrangements. Here, we extend those studies using a realistic brain phantom in a recently installed MR-PET system comprising a 9.4 T MRI-scanner and an APD-based BrainPET insert in the magnet bore. Point and line sources and a 3D brain phantom were filled with 18F (low-energy positron emitter), 68Ga (medium energy positron emitter) or 120I, a non-standard positron emitter (high positron energies of up to 4.6 MeV). Using the BrainPET insert, emission scans of the phantoms were recorded at different positions inside and outside the magnet bore such that the magnetic field was 0 T, 3 T, 7 T or 9.4 T. Brain phantom images, with the 'grey matter' compartment filled with 18F, showed no obvious resolution improvement with increasing field. This is confirmed by practically unchanged transaxial FWHM and 'grey/white matter' ratio values between at 0T and 9.4T. Field-dependent improvements in the resolution and contrast of transaxial PET images were clearly evident when the brain phantom was filled with 68Ga or 120I. The grey/white matter ratio increased by 7.3% and 16.3%, respectively. The greater reduction of the FWTM compared to FWHM in 68Ga or 120I line-spread images was in agreement with the improved contrast of 68Ga or 120I images. Notwithstanding elongations seen in the z-direction of 68Ga or 120I point source images acquired in foam, brain phantom images show no comparable extension. Our experimental study confirms that integrated MR-PET delivers improved PET image resolution and contrast for medium- and high-energy positron emitters even though the positron range is reduced only in directions perpendicular to the magnetic field

Fast 3D kernel computation method for positron range correction in PET

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

Design and Construction of a PET-Compatible Double-Tuned 1H/31P MR Head Coil

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