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 Magnetic Resonance Materials in Physics, Biology and Medicine   [SJR: 0.928]   [H-I: 40]   [2 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 0968-5243 - ISSN (Online) 1352-8661    Published by Springer-Verlag  [2335 journals]
• Theory of MRI contrast in the annulus fibrosus of the intervertebral disc
• Abstract: Objective Here we develop a three-dimensional analytic model for MR image contrast of collagen lamellae in the annulus fibrosus of the intervertebral disc of the spine, based on the dependence of the MRI signal on collagen fiber orientation. Materials and methods High-resolution MRI scans were performed at 1.5 and 7 T on intact whole disc specimens from ovine, bovine, and human spines. An analytic model that approximates the three-dimensional curvature of the disc lamellae was developed to explain inter-lamellar contrast and intensity variations in the annulus. The model is based on the known anisotropic dipolar relaxation of water in tissues with ordered collagen. Results Simulated MRI data were generated that reproduced many features of the actual MRI data. The calculated inter-lamellar image contrast demonstrated a strong dependence on the collagen fiber angle and on the circumferential location within the annulus. Conclusion This analytic model may be useful for interpreting MR images of the disc and for predicting experimental conditions that will optimize MR image contrast in the annulus fibrosus.
PubDate: 2016-08-01

• Minimizing the effects of magnetization transfer asymmetry on
inhomogeneous magnetization transfer (ihMT) at ultra-high magnetic field
(11.75 T)
• Abstract: Objectives The recently reported inhomogeneous magnetization transfer technique (ihMT) has been proposed for specific imaging of inhomogeneously broadened lines, and has shown great promise for characterizing myelinated tissues. The ihMT contrast is obtained by subtracting magnetization transfer images obtained with simultaneous saturation at positive and negative frequency offsets (dual frequency saturation experiment, MT +/−) from those obtained with single frequency saturation (MT +) at the same total power. Hence, ihMT may be biased by MT-asymmetry, especially at ultra-high magnetic field. Use of the average of single positive and negative frequency offset saturation MT images, i.e., (MT ++MT −) has been proposed to correct the ihMT signal from MT-asymmetry signal. Materials and methods The efficiency of this correction method was experimentally assessed in this study, performed at 11.75 T on mice. Quantitative corrected ihMT and MT-asymmetry ratios (ihMTR and MTRasym) were measured in mouse brain structures for several MT-asymmetry magnitudes and different saturation parameter sets. Results Our results indicated a “safe” range of magnitudes (/MTRasym/<4 %) for which MT-asymmetry signal did not bias the corrected ihMT signal. Moreover, experimental evidence of the different natures of both MT-asymmetry and inhomogeneous MT contrasts were provided. In particular, non-zero ihMT ratios were obtained at zero MTRasym values. Conclusion MTRasym is not a confounding factor for ihMT quantification, even at ultra-high field, as long as MTRasym is restricted to ±4 %.
PubDate: 2016-08-01

• Endoluminal high-resolution MR imaging protocol for colon walls analysis
in a mouse model of colitis
• Abstract: Objective An endoluminal magnetic resonance (MR) imaging protocol including the design of an endoluminal coil (EC) was defined for high-spatial-resolution MR imaging of mice gastrointestinal walls at 4.7 T. Materials and methods A receive-only radiofrequency single-loop coil was developed for mice colon wall imaging. Combined with a specific protocol, the prototype was first characterized in vitro on phantoms and on vegetables. Signal-to-noise ratio (SNR) profiles were compared with a quadrature volume birdcage coil (QVBC). Endoluminal MR imaging protocol combined with the EC was assessed in vivo on mice. Results The SNR measured close to the coil is significantly higher (10 times and up to 3 mm of the EC center) than the SNR measured with the QVBC. The gain in SNR can be used to reduce the in-plane pixel size up to 39 × 39 µm2 (234 µm slice thickness) without time penalty. The different colon wall layers can only be distinguished on images acquired with the EC. Conclusion Dedicated EC provides suitable images for the assessment of mice colon wall layers. This proof of concept provides gains in spatial resolution and leads to adequate protocols for the assessment of human colorectal cancer, and can now be used as a new imaging tool for a better understanding of the pathology.
PubDate: 2016-08-01

• Topographical variations in zonal properties of canine tibial articular
cartilage due to early osteoarthritis: a study using 7-T magnetic
resonance imaging at microscopic resolution
• Abstract: Objective Our aim was to determine topographical variations in zonal properties of articular cartilage over the medial tibia in an experimental osteoarthritis (OA) model using 7-T magnetic resonance imaging (MRI). Materials and methods An anterior cruciate ligament (ACL)-transection canine model was subjected to study at 8 (six) and 12 (seven) weeks after the surgery. Each medial tibia was divided into five topographical locations. For each specimen, T2 relaxation (at 0° and 55°) was quantified at microscopic resolution. The imaging data grouped the five locations into two topographical areas (meniscus-covered and -uncovered). Results The T2 (55°) bulk values from the meniscus-covered area were significantly lower than those from the uncovered area. The total cartilage thicknesses on the meniscus-covered area were significantly thinner than those on the meniscus-uncovered area. Significant differences in the T2 (0°) values were observed in most thicknesses of the four subtissue zones and whole-tissue from the uncovered area, while the same significant changes were detected in the superficial zone from the meniscus-covered area. Conclusion By quantifying high-resolution imaging data both topographically and depth-dependently (zonal-wise), this study demonstrates that the rate of disease progression varies topographically over the medial tibia. Future correlation with OA pathology could lead to better detection of early OA.
PubDate: 2016-08-01

• Design of a mobile, homogeneous, and efficient electromagnet with a large
field of view for neonatal low-field MRI
• Abstract: Objective In this work, a prototype of an effective electromagnet with a field-of-view (FoV) of 140 mm for neonatal head imaging is presented. The efficient implementation succeeded by exploiting the use of steel plates as a housing system. We achieved a compromise between large sample volumes, high homogeneity, high B0 field, low power consumption, light weight, simple fabrication, and conserved mobility without the necessity of a dedicated water cooling system. Materials and methods The entire magnetic resonance imaging (MRI) system (electromagnet, gradient system, transmit/receive coil, control system) is introduced and its unique features discussed. Furthermore, simulations using a numerical optimization algorithm for magnet and gradient system are presented. Results Functionality and quality of this low-field scanner operating at 23 mT (generated with 500 W) is illustrated using spin-echo imaging (in-plane resolution 1.6 mm × 1.6 mm, slice thickness 5 mm, and signal-to-noise ratio (SNR) of 23 with a acquisition time of 29 min). B0 field-mapping measurements are presented to characterize the homogeneity of the magnet, and the B0 field limitations of 80 mT of the system are fully discussed. Conclusion The cryogen-free system presented here demonstrates that this electromagnet with a ferromagnetic housing can be optimized for MRI with an enhanced and homogeneous magnetic field. It offers an alternative to prepolarized MRI designs in both readout field strength and power use. There are multiple indications for the clinical medical application of such low-field devices.
PubDate: 2016-08-01

• Amide proton transfer (APT) magnetic resonance imaging of prostate cancer:
comparison with Gleason scores
• Abstract: Objective To evaluate the utility of amide proton transfer (APT) imaging in estimating the Gleason score (GS) of prostate cancer (Pca). Materials and methods Sixty-six biopsy-proven cancers were categorized into four groups according to the GS: GS-6 (3 + 3); GS-7 (3 + 4/4 + 3); GS-8 (4 + 4) and GS-9 (4 + 5/5 + 4). APT signal intensities (APT SIs) and apparent diffusion coefficient (ADC) values of each GS group were compared by one-way analysis of variance with Tukey’s HSD post hoc test. Results The mean and standard deviation of the APT SIs (%) and ADC values (×10−3 mm2/s) were as follows: GS-6, 2.48 ± 0.59 and 1.16 ± 0.26; GS-7, 5.17 ± 0.66 and 0.92 ± 0.18; GS-8, 2.56 ± 0.85 and 0.86 ± 0.17; GS-9, 1.96 ± 0.75 and 0.85 ± 0.18, respectively. The APT SI of the GS-7 group was highest, and there were significant differences between the GS-6 and GS-7 groups and the GS-7 and GS-9 groups (p < 0.05). The ADC value of the GS-6 group was significantly higher than each value of the GS-7, GS-8, and GS-9 groups (p < 0.05), but no significant differences were obtained among the GS-7, GS-8, and GS-9 groups. Conclusion The mean APT SI in Pca with a GS of 7 was higher than that for the other GS groups.
PubDate: 2016-08-01

• A USPIO doped gel phantom for R 2* relaxometry
• Abstract: Objective This work describes a phantom containing regions of controlled R2* (1/T2*) values to provide a stable reference object for testing implementations of R2* relaxometry commonly used for liver and heart iron assessments. Materials and methods A carrageenan-strengthened gadolinium DTPA doped agarose gel was used to enclose nine gels additionally doped with ultra-small superparamagnetic iron oxide. R2* values were determined at 1.5 T using multi-echo GRE sequences and exponential regression of pixel values from a region of interest against echo time using non-linear regression algorithms. We measured R2*, R2 and R1 values and the inter-scan and inter-operator reproducibility. Results The phantom reliably demonstrated R2* values in seven steps between 22.4 s−1 (SE 1.98) and 441.9 s−1 (SE 6.76), with an R2* relaxivity (r2*) of 792 (SE 5.6) mM−1 s−1. The doped gels displayed a concentration-dependent R2ʹ component of R2* phantom, indicating superparamagnetic enhancement effects. We observed no significant change in relaxivity (r2*) over 12 months, and estimate a useful life of 3 years. Detailed descriptions of the production process and calculators are been provided as Online Resources. Conclusion The phantom provides a durable test object with controlled R2* relaxation behaviour, useful for a range of R2* relaxometry reference work.
PubDate: 2016-07-19

• Assessing the accuracy of using oscillating gradient spin echo sequences
with AxCaliber to infer micron-sized axon diameters
• Abstract: Objective Current magnetic resonance imaging (MRI) axon diameter measurements rely on the pulsed gradient spin-echo sequence, which is unable to provide diffusion times short enough to measure small axon diameters. This study combines the AxCaliber axon diameter fitting method with data generated from Monte Carlo simulations of oscillating gradient spin-echo sequences (OGSE) to infer micron-sized axon diameters, in order to determine the feasibility of using MRI to infer smaller axon diameters in brain tissue. Materials and methods Monte Carlo computer simulation data were synthesized from tissue geometries of cylinders of different diameters using a range of gradient frequencies in the cosine OGSE sequence . Data were fitted to the AxCaliber method modified to allow the new pulse sequence. Intra- and extra-axonal water were studied separately and together. Results The simulations revealed the extra-axonal model to be problematic. Rather than change the model, we found that restricting the range of gradient frequencies such that the measured apparent diffusion coefficient was constant over that range resulted in more accurate fitted diameters. Thus a careful selection of frequency ranges is needed for the AxCaliber method to correctly model extra-axonal water, or adaptations to the method are needed. This restriction helped reduce the necessary gradient strengths for measurements that could be performed with parameters feasible for a Bruker BG6 gradient set. For these experiments, the simulations inferred diameters as small as 0.5 μm on square-packed and randomly packed cylinders. The accuracy of the inferred diameters was found to be dependent on the signal-to-noise ratio (SNR), with smaller diameters more affected by noise, although all diameter distributions were distinguishable from one another for all SNRs tested. Conclusion The results of this study indicate the feasibility of using MRI with OGSE on preclinical scanners to infer small axon diameters.
PubDate: 2016-07-13

• Assessment of the myelin water fraction in rodent spinal cord using
T2-prepared ultrashort echo time MRI
• Abstract: Objective Multi-component T2 relaxation allows for assessing the myelin water fraction in nervous tissue, providing a surrogate marker for demyelination. The assessment of the number and distribution of different T2 components for devising exact models of tissue relaxation has been limited by T2 sampling with conventional MR methods. Materials and methods A T2-prepared UTE sequence was used to assess multicomponent T2 relaxation at 9.4 T of fixed mouse and rat spinal cord samples and of mouse spinal cord in vivo. For in vivo scans, a cryogenically cooled probe allowed for 78-µm resolution in 1-mm slices. Voxel-wise non-negative least square analysis was used to assess the number of myelin water-associated T2 components. Results More than one myelin water-associated T2 component was detected in only 12 % of analyzed voxels in rat spinal cords and 6 % in mouse spinal cords, both in vivo and in vitro. However, myelin water-associated T2 values of individual voxels varied between 0.1 and 20 ms. While in fixed samples almost no components below 1 ms were identified, in vivo, these contributed 14 % of the T2 spectrum. No significant differences in MWF were observed in mouse spinal cord in vivo versus ex vivo measurements. Conclusion Voxel-wise analysis methods using relaxation models with one myelin water-associated T2 component are appropriate for assessing myelin content of nervous tissue.
PubDate: 2016-07-09

• Reproducibility of macromolecule suppressed GABA measurement using motion
and shim navigated MEGA-SPECIAL with LCModel, jMRUI and GANNET
• Abstract: Objective Measuring the pure form of GABA has become increasingly important because of its association with behaviour and certain pathologies. The aim of this study was to assess the reproducibility of GABA measurements using a shim and motion navigated MEGA-SPECIAL sequence with LCModel, jMRUI and GANNET software. Materials and methods Motion and shim navigated MEGA-SPECIAL scans were acquired in 20 healthy subjects. Two acquisitions were performed for each of two regions: the anterior cingulate (ACC) and medial-parietal (PAR) cortices. Absolute GABA concentration ( $${\text{GABA}}_{{{\text{H}}_{2} {\text{O}}}}$$ ) and GABA-to-Creatine ratio (GABA/Cr) were quantified using the three software packages. Results Using the within-subject coefficient of variation (CVws) as an index, reproducibility for both GABAH20 and GABA/Cr ranged from 13 to 22 % in the ACC and 13 to 18 % in PAR using the three software packages. Conclusion Based on CVws, GABA concentrations in both the ACC and PAR are reproducible using a shim and motion navigated MEGA-SPECIAL sequence with any of the three software packages, thus demonstrating the ability to quantify the pure form of GABA using these software in studies relating GABA to pathology and healthy behaviour.
PubDate: 2016-07-08

• Magnetization transfer imaging of cortical bone in vivo using a zero echo
time sequence in mice at 4.7 T: a feasibility study
• Abstract: Objective To investigate the feasibility of magnetization transfer (MT) imaging in mice in vivo for the assessment of cortical bone. Materials and methods MT-zero echo time data were acquired at 4.7 T in six mice using MT preparation pulses with two different flip angles (FAs) and a series of ten different off-resonance frequencies (500–15000 Hz). Regions of interest were drawn at multiple levels of the femoral cortical bone. The MT ratio (MTR) was computed for each combination of FAs and off-resonance frequencies. T1 measurements were used to estimate the direct saturation (DS) using a Bloch equation simulation. Estimation of the absorption line width of cortical bone from T2* measurements was also performed. Results MTR values were higher using 3000° FA than 1000° FA. MTR values decreased toward higher off-resonance frequencies. Maximum mean MTR ± standard deviation (SD) of 58.57 ± 5.22 (range 50.44–70.61) was measured with a preparation pulse of 3000° and off-resonance frequency of 500 Hz. Maximum “true” MT effect was estimated at around 2–3 and 5 kHz, respectively, for 1000° and 3000° FA. Mean full width at half maximum ± SD of 577 ± 91 Hz was calculated for the absorption spectral line of the cortical bone. Conclusion MT imaging can be used for the assessment of cortical bone in mice in vivo. DS effects are negligible using preparation pulses with off-resonance frequencies greater than 3 kHz.
PubDate: 2016-07-06

• Accelerated visualization of selected intracranial arteries by cycled
super-selective arterial spin labeling
• Abstract: Objective To accelerate super-selective arterial spin labeling (ASL) angiography by using a single control condition denoted as cycled super-selective arterial spin labeling. Materials and methods A single non-selective control image is acquired that is shared by selective label images. Artery-selective imaging is possible by geometrically changing the position of the labeling focus to more than one artery of interest during measurement. The presented approach is compared to conventional super-selective imaging in terms of its labeling efficiency inside and outside the labeling focus using numerical simulations and in vivo measurements. Additionally, the signal-to-noise ratios of the images are compared to non-selective ASL angiography and analyzed using a two-way ANOVA test and calculating the Pearson’s correlation coefficients. Results The results indicate that the labeling efficiency is not reduced within the labeled artery, but can increase as a function of distance to the artery of interest when compared to conventional super-selective ASL. In the final images, no statistically significant difference of image quality can be observed while the acquisition duration could be reduced when the major brain feeding arteries are being tagged. Conclusion Using super-selective arterial spin labeling, a single non-selective control acquisition suffices for reconstructing selective angiograms of the cerebral vasculature, thereby accelerating image acquisition of the major intracranial arteries without notable loss of information.
PubDate: 2016-06-29

• Quantitative T 1 and T 2 MRI signal characteristics in the human brain:
different patterns of MR contrasts in normal ageing
• Abstract: Objective The objective of this study was to examine age-dependent changes in both T1-weighted and T2-weighted image contrasts and spin-echo T2 relaxation time in the human brain during healthy ageing. Methods A total of 37 participants between the ages of 49 and 87 years old were scanned with a 3 Tesla system, using T1-weighted, T2 weighted and quantitative spin-echo T2 imaging. Contrast between image intensities and T2 values was calculated for various regions, including between individual hippocampal subfields. Results The T1 contrast-to-noise (CNR) and gray:white signal intensity ratio (GWR) did not change in the hippocampus, but it declined in the cingulate cortex with age. In contrast, T2 CNR and GWR declined in both brain regions. T2 relaxation time was almost constant in gray matter and most (but not all) hippocampal subfields, but increased substantially in white matter, pointing to an age effect on water relaxation in white matter. Conclusions Changes in T1 and T2 MR characteristics influence the appearance of brain images in later life and should be considered in image analyses of aged subjects. It is speculated that alterations at the cell biology level, with concomitant alterations to the local magnetic environment, reduce dephasing and subsequently prolong spin-echo T2 through reduced diffusion effects in later life.
PubDate: 2016-06-22

• Dynamic susceptibility contrast perfusion MRI using phase-based venous
output functions: comparison with pseudo-continuous arterial spin
labelling and assessment of contrast agent concentration in large veins
• Abstract: Objectives Contrast agent (CA) relaxivities are generally not well established in vivo, and the relationship between frequency/phase shift and magnetic susceptibility might be a useful alternative for CA quantification. Materials and methods Twenty volunteers (25–84 years old) were investigated using test–retest pre-bolus dynamic susceptibility-contrast (DSC) magnetic resonance imaging (MRI). The pre-bolus phase-based venous output function (VOF) time integral was used for arterial input function (AIF) rescaling. Resulting cerebral blood flow (CBF) data for grey matter (GM) were compared with pseudo-continuous arterial spin labelling (ASL). During the main bolus CA passage, the apparent spatial shift (pixel shift) of the superior sagittal sinus (seen in single-shot echo-planar imaging (EPI)) was converted to CA concentration and compared with conventional ΔR2*-based data and with a predicted phase-based VOF from the pre-bolus experiment. Results The phase-based pre-bolus VOF resulted in a reasonable inter-individual GM CBF variability (coefficient of variation 28 %). Comparison with ASL CBF values implied a tissue R2*-relaxivity of 32 mM−1 s−1. Pixel-shift data at low concentrations (data not available at peak concentrations) were in reasonable agreement with the predicted phase-based VOF. Conclusion Susceptibility-induced phase shifts and pixel shifts are potentially useful for large-vein CA quantification. Previous predictions of a higher R2*-relaxivity in tissue than in blood were supported.
PubDate: 2016-06-13

• Denoising of MR spectroscopic imaging data using statistical selection of
principal components
• Abstract: Objectives To evaluate a new denoising method for MR spectroscopic imaging (MRSI) data based on selection of signal-related principal components (SSPCs) from principal components analysis (PCA). Materials and methods A PCA-based method was implemented for selection of signal-related PCs and denoising achieved by reconstructing the original data set utilizing only these PCs. Performance was evaluated using simulated MRSI data and two volumetric in vivo MRSIs of human brain, from a normal subject and a patient with a brain tumor, using variable signal-to-noise ratios (SNRs), metabolite peak areas, Cramer-Rao bounds (CRBs) of fitted metabolite peak areas and metabolite linewidth. Results In simulated data, SSPC determined the correct number of signal-related PCs. For in vivo studies, the SSPC denoising resulted in improved SNRs and reduced metabolite quantification uncertainty compared to the original data and two other methods for denoising. The method also performed very well in preserving the spectral linewidth and peak areas. However, this method performs better for regions that have larger numbers of similar spectra. Conclusion The proposed SSPC denoising improved the SNR and metabolite quantification uncertainty in MRSI, with minimal compromise of the spectral information, and can result in increased accuracy.
PubDate: 2016-06-03

• Radiofrequency pulse design with numerical optimization in the Fourier
domain
• Abstract: Objective and methods A radiofrequency (RF) pulse design technique is presented that uses iterative constrained minimization to determine Fourier domain coefficients for an optimal time domain RF pulse. The design of new RF pulses is especially beneficial for field strengths of 7.0 T and above, where challenges pertaining to specific absorption rate (SAR) are exacerbated. Results and conclusion A pair of 90° and 180° spin-echo pulses was designed to lower SAR without the need for a variable slice gradient. The optimized pulses were deployed to a 7.0 T human scanner to demonstrate a reduction in SAR while retaining signal-to-noise (SNR) ratio.
PubDate: 2016-06-01

• Evidencing different neurochemical profiles between thalamic nuclei using
high resolution 2D-PRESS semi-LASER 1 H-MRSI at 7 T
• Abstract: Objective To demonstrate that high resolution 1H semi-LASER MRSI acquired at 7 T permits discrimination of metabolic patterns of different thalamic nuclei. Materials and methods Thirteen right-handed healthy volunteers were explored at 7 T using a high-resolution 2D-semi-LASER 1H-MRSI sequence to determine the relative levels of N-Acetyl Aspartate (NAA), choline (Cho) and creatine-phosphocreatine (Cr) in eight VOIs (volume <0.3 ml) centered on four different thalamic nuclei located on the Oxford thalamic connectivity atlas. Post-processing was done using the CSIAPO software. Chemical shift displacement of metabolites was evaluated on a phantom and correction factors were applied to in vivo data. Results The global assessment (ANOVA p < 0.05) of the neurochemical profiles (NAA, Cho and Cr levels) with thalamic nuclei and hemispheres as factors showed a significant global effect (F = 11.98, p < 0.0001), with significant effect of nucleus type (p < 0.0001) and hemisphere (p < 0.0001). Post hoc analyses showed differences in neurochemical profiles between the left and the right hemisphere (p < 0.05), and differences in neurochemical profiles between nuclei within each hemisphere (p < 0.05). Conclusion For the first time, using high resolution 2D-PRESS semi-LASER 1H-MRSI acquired at 7 T, we demonstrated that the neurochemical profiles were different between thalamic nuclei, and that these profiles were dependent on the brain hemisphere.
PubDate: 2016-06-01

• Reproducibility and regional variations of an improved gagCEST protocol
for the in vivo evaluation of knee cartilage at 7 T
• Abstract: Objective The objective was to establish a gagCEST protocol that would enable robust and reproducible assessment of the glycosaminoglycan (GAG) content in knee cartilage at 7 T within a clinically feasible measurement time. Materials and methods Ten young healthy volunteers (mean age 26 years, range 24–28, five males, five females) were examined on a 7 T MR system. Informed consent was obtained from all individual participants prior to enrollment into the study. Each volunteer was measured twice for reproducibility assessment. The examined knee was immobilized using a custom-made fixation device. For the gagCEST measurement, a prototype segmented 3-D RF-spoiled gradient-echo sequence with an improved saturation scheme employing adiabatic pulses was used in a scan time of 19 min. The asymmetry of the Z-spectra (MTRasym) in selected regions of interest in knee cartilage was calculated. Differences in MTRasym between different regions were evaluated using ANOVA and the Bonferroni corrected post hoc test. Results The improvement of the saturation scheme reduced the influence of field inhomogeneities, resulted in more uniform saturation, and allowed for good reproducibility in a reasonable measurement time (19 min), as demonstrated by an intraclass correlation coefficient of 0.77. Improved fixation helped to reduce motion artifacts. Whereas similar MTRasym values were found for weight-bearing and non-weight-bearing femoral cartilage, lower values were observed in the trochlear groove (p = 0.028), patellar (p = 0.015) and tibial cartilage (p < 0.001) when compared to non-weight-bearing femoral cartilage. Conclusion Reasonable reproducibility and sensitivity to regional differences in GAG content suggests that the improved gagCEST protocol might be useful for assessing the biochemical changes in articular cartilage that are associated with early stages of cartilage degeneration.
PubDate: 2016-06-01

• Neuromyelitis optica does not impact periventricular venous density versus
healthy controls: a 7.0 Tesla MRI clinical study
• Abstract: Objective To quantify the periventricular venous density in neuromyelitis optica spectrum disease (NMOSD) in comparison to that in patients with multiple sclerosis (MS) and healthy control subjects. Materials and methods Sixteen patients with NMOSD, 16 patients with MS and 16 healthy control subjects underwent 7.0-Tesla (7T) MRI. The imaging protocol included T2*-weighted (T2*w) fast low angle-shot (FLASH) and fluid-attenuated inversion recovery (FLAIR) sequences. The periventricular venous area (PVA) was manually determined by a blinded investigator in order to estimate the periventricular venous density in a region of interest-based approach. Results No significant differences in periventricular venous density indicated by PVA were detectable in NMOSD versus healthy controls (p = 0.226). In contrast, PVA was significantly reduced in MS patients compared to healthy controls (p = 0.013). Conclusion Unlike patients with MS, those suffering from NMOSD did not show reduced venous visibility. This finding may underscore primary and secondary pathophysiological differences between these two distinct diseases of the central nervous system.
PubDate: 2016-04-12

• Measurements of RF power reflected and radiated by multichannel transmit
MR coils at 7T
• Abstract: Objective The power balance of multichannel transmit coils is a central consideration in assessing performance and safety issues. At ultrahigh fields, in addition to absorption and reflection, radiofrequency (RF) radiation into the far field becomes a concern. Materials and methods We engineered a system for in situ measurement of complex-valued scattering parameter (S-parameter) matrices of multichannel transmit coils that allows for the calculation of the reflected and accepted power for arbitrary steering conditions. The radiated power from an RF coil inside a large single-mode waveguide couples to that mode. Finite-difference time-domain simulations were used for the calculations, and E-field probes were used to measure the electric field distribution, and hence the radiated power, in the waveguide. To test this concept, an eight-channel shielded-loop array for 7T imaging was studied inside a 280-cm-long cylindrical waveguide with a 60-cm diameter. Results For a 7T parallel-transmit coil, the S-parameters were measured and the reflected power calculated as a function of steering conditions. Maximum radiated power was observed for the circularly polarized mode. Conclusion A system was developed for in situ S-parameter measurements combined with a method for determining radiated power, allowing a complete assessment of the power balance of multichannel transmit coils at 7T.
PubDate: 2016-04-02

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