One of the challenges in PET/MRI is the derivation of an attenuation map to correct the PET image for attenuation. Different methods have been suggested for deriving the attenuation map from an MR image. Because the low signal intensity of cortical bone on images acquired with conventional MRI sequences makes it difficult to detect this tissue type, these methods rely on some sort of anatomic precondition to predict the attenuation map, raising the question of whether these methods will be usable in the clinic when patients may exhibit anatomic abnormalities. Methods: We propose the use of the transverse relaxation rate, derived from images acquired with an ultrashort echo time sequence to classify the voxels into 1 of 3 tissue classes (bone, soft tissue, or air), without making any assumptions on patient anatomy. Each voxel is assigned a linear attenuation coefficient corresponding to its tissue class. A reference CT scan is used to determine the voxel-by-voxel accuracy of the proposed method. The overall accuracy of the MRI-based attenuation correction is evaluated using a method that takes into account the nonlocal effects of attenuation correction. Results: As a proof of concept, the head of a pig was used as a phantom for imaging. The new method yielded a correct tissue classification in 90% of the voxels. Five human brain PET/CT and MRI datasets were also processed, yielding slightly worse voxel-by-voxel performance, compared to a CT-derived attenuation map. The PET datasets were reconstructed using the segmented MRI attenuation map derived with the new method, and the resulting images were compared with segmented CT-based attenuation correction. An average error of around 5% was found in the brain. Conclusion: The feasibility of using the transverse relaxation rate map derived from ultrashort echo time MR images for the estimation of the attenuation map was shown on phantom and clinical brain data. The results indicate that the new method, compared with CTbased attenuation correction, yields clinically acceptable errors. The proposed method does not make any assumptions about patient anatomy and could therefore also be used in cases in which anatomic abnormalities are present. A new generation of medical imaging scanners, which will combine the high sensitivity of functional imaging by PET with the wide range of applications of MRI scanners, is currently being developed (1-4). Combined PET/MRI offers several advantages over PET/CT, of which the following relate to the particular strength of the MRI method. First, it is possible to acquire anatomic images with better soft-tissue contrast than CT. Second, the radiation dose to the patient is significantly smaller. Third, other applications such as spectroscopy or molecular imaging are also possible with MRI. A PET/MRI scanner will also be a useful research tool, for example, enabling researchers to validate molecular MRI protocols against PET, which can be considered the current gold standard in molecular imaging.One of the difficulties in the design of a m...

Decreased cerebral blood flow (CBF) may contribute to the pathology of multiple sclerosis (MS), but the underlying mechanism is unknown. We investigated whether the potent vasoconstrictor endothelin-1 (ET-1) is involved. We found that, compared with controls, plasma ET-1 levels in patients with MS were significantly elevated in blood drawn from the internal jugular vein and a peripheral vein. The jugular vein/peripheral vein ratio was 1.4 in patients with MS vs. 1.1 in control subjects, suggesting that, in MS, ET-1 is released from the brain to the cerebral circulation. Next, we performed ET-1 immunohistochemistry on postmortem white matter brain samples and found that the likely source of ET-1 release are reactive astrocytes in MS plaques. We then used arterial spin-labeling MRI to noninvasively measure CBF and assess the effect of the administration of the ET-1 antagonist bosentan. CBF was significantly lower in patients with MS than in control subjects and increased to control values after bosentan administration. These data demonstrate that reduced CBF in MS is mediated by ET-1, which is likely released in the cerebral circulation from reactive astrocytes in plaques. Restoring CBF by interfering with the ET-1 system warrants further investigation as a potential new therapeutic target for MS.M ultiple sclerosis (MS) is a poorly understood chronic disorder of the CNS, characterized by focal inflammatory demyelinating lesions and degenerative processes (1). Immune responses play a crucial role in the pathogenesis of focal lesions that constitute the pathological substrate for relapses. However, the underlying mechanism of the progressive degeneration of axons, which is the primary determinant of long-term disability in MS, is not clear (2), and treatment is lacking.A number of studies found that cerebral blood flow (CBF) is globally impaired in early diagnosed relapsing-remitting MS and primary progressive MS, indicating that it is an integral part of the disease that is already present at the time of diagnosis (3-5). Animal studies have shown that chronic hypoperfusion of the brain can lead to neurodegenerative changes, including axonal degeneration (6).The underlying mechanism of reduced CBF in MS is unknown. Plasma levels of the potent vasoconstrictor endothelin-1 (ET-1) (7) were found to be elevated in patients with MS (8), and this was associated with alterations of extraocular blood flow (9). The reason for the increase in ET-1 levels in MS was unclear, and the findings have not received much attention. We hypothesized that ET-1 might play a role in reducing CBF in MS. ResultsInternal Jugular Vein ET-1 Levels. Individuals with critical illness and systemic conditions known to be associated with increased levels of ET-1 were excluded (10). Ten subjects with MS according to the revised McDonald criteria (11) and 10 matched control subjects were included. Patients with MS were autonomously seeking treatment at the department of cardiovascular and thoracic surgery of Onze-Lieve-Vrouw Ziekenhuis Aalst for so-c...

Diffusion weighted imaging sequences are now widely available on Magnetic Resonance Imaging (MRI) scanners. Diffusion Tensor Imaging (DTI) of the brain is able to show white matter tracts and is now commonly used in human medicine to study brain anatomy, tumors, structural pathways,. . . The purpose of this study was to show the interest of DTI to reveal the white matter fibers in the dogs' brain. DTI MR Images for this study were obtained with a 3 T system of 4 dogs euthanized for other reasons than neurological disorders. Combined fractional anisotropic (FA) and directional maps were obtained in the first 2 hours after death. The heads were amputated immediately after scanning and stored in 10% formalin until preparation for dissection. An experienced anatomist tracked white matter tracts with clinical relevance using the scanner software. The selected tracts were volume rendered and correlated with gross dissection. Using DTI we were able to track relevant neurological connections, such as the corticospinal tract, the optic and the cerebellar tract. The three dimensional anatomy is better presented using modern visualization techniques. DTI seems to be a valuable tool in order to present clinically relevant white matter tracts to neurological clinicians and researchers. Anat Rec, 296:340-349, 2013. V C 2013 Wiley Periodicals, Inc.Key words: brain; diffusion tensor imaging; dog; anatomy Since a few years, Magnetic Resonance Imaging is a reference technique for imaging the brain in different planes (sagittal, transversal, coronal) and the use of 1,5T to 7T MRI allows more and more accurate and detailed visualization of white matter localization than conventional CT-Scan and X-Ray (Van Thielen et al., 2010). However, an atlas of all the white matter tracts would be particularly useful for providing detailed anatomical data that is not available in studies based on conventional MRI data (Lawes et al., 2008). So we

Objectives To compare the diagnostic usefulness of highfield with low-field magnetic resonance imaging (MRI)and stereomicroscopic autopsy for examination of the heart in fetuses at or under 20 weeks' gestation. (95% CI,) and specificity 100. 0% (95% CI,. Eight fetuses out of 10 with congenital heart disease (CHD) were classified as having major CHD. Methods High-field MRI at 9.4 T was able to identify seven out of the eight cases of major CHD.Conclusion High-field MRI at 9.4 T seems to be an acceptable alternative approach to invasive stereomicroscopic autopsy for fetuses with CHD at or below 20 weeks' gestation.

Our results suggest that in MS patients there is no relationship between reduced CBF in NAWM and impaired axonal mitochondrial metabolism or astrocytic PCr metabolism.