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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  [2281 journals]
• From ultrahigh to extreme field magnetic resonance: where physics, biology
and medicine meet
• PubDate: 2016-05-24

• Toward 20 T magnetic resonance for human brain studies: opportunities
for discovery and neuroscience rationale
• Abstract: An initiative to design and build magnetic resonance imaging (MRI) and spectroscopy (MRS) instruments at 14 T and beyond to 20 T has been underway since 2012. This initiative has been supported by 22 interested participants from the USA and Europe, of which 15 are authors of this review. Advances in high temperature superconductor materials, advances in cryocooling engineering, prospects for non-persistent mode stable magnets, and experiences gained from large-bore, high-field magnet engineering for the nuclear fusion endeavors support the feasibility of a human brain MRI and MRS system with 1 ppm homogeneity over at least a 16-cm diameter volume and a bore size of 68 cm. Twelve neuroscience opportunities are presented as well as an analysis of the biophysical and physiological effects to be investigated before exposing human subjects to the high fields of 14 T and beyond.
PubDate: 2016-05-18

• A novel alternative to classify tissues from T 1 and T 2 relaxation times
for prostate MRI
• Abstract: Objective To segment and classify the different attenuation regions from MRI at the pelvis level using the T 1 and T 2 relaxation times and anatomical knowledge as a first step towards the creation of PET/MR attenuation maps. Materials and methods Relaxation times were calculated by fitting the pixel-wise intensities of acquired T 1- and T 2-weighted images from eight men with inversion-recovery and multi-echo multi-slice spin-echo sequences. A decision binary tree based on relaxation times was implemented to segment and classify fat, muscle, prostate, and air (within the body). Connected component analysis and an anatomical knowledge-based procedure were implemented to localize the background and bone. Results Relaxation times at 3 T are reported for fat (T 1 = 385 ms, T 2 = 121 ms), muscle (T 1 = 1295 ms, T 2 = 40 ms), and prostate (T 1 = 1700 ms, T 2 = 80 ms). The relaxation times allowed the segmentation–classification of fat, prostate, muscle, and air, and combined with anatomical knowledge, they allowed classification of bone. The good segmentation–classification of prostate [mean Dice similarity score (mDSC) = 0.70] suggests a viable implementation in oncology and that of fat (mDSC = 0.99), muscle (mDSC = 0.99), and bone (mDSCs = 0.78) advocates for its implementation in PET/MR attenuation correction. Conclusion Our method allows the segmentation and classification of the attenuation-relevant structures required for the generation of the attenuation map of PET/MR systems in prostate imaging: air, background, bone, fat, muscle, and prostate.
PubDate: 2016-05-09

• MRI-based assessment of liver perfusion and hepatocyte injury in the
murine model of acute hepatitis
• Abstract: Objective To assess alterations in perfusion and liver function in the concanavalin A (ConA)-induced mouse model of acute liver failure (ALF) using two magnetic resonance imaging (MRI)-based methods: dynamic contrast-enhanced MRI (DCE-MRI) with Gd-EOB-DTPA contrast agent and arterial spin labelling (ASL). Materials and methods BALB/c mice were studied using a 9.4 T MRI system. The IntraGateFLASHTM and FAIR-EPI pulse sequences were used for optimum mouse abdomen imaging. Results The average perfusion values for the liver of the control and ConA group were equal to 245 ± 20 and 200 ± 32 ml/min/100 g (p = 0.008, respectively). DCE-MRI showed that the time to the peak of the image enhancement was 6.14 ± 1.07 min and 9.72 ± 1.69 min in the control and ConA group (p < 0.001, respectively), while the rate of the contrast wash-out in the control and ConA group was 0.037 ± 0.008 and 0.021 ± 0.008 min−1 (p = 0.004, respectively). These results were consistent with hepatocyte injury in the ConA-treated mice as confirmed by histopathological staining. Conclusions Both the ASL and DCE-MRI techniques represent a reliable methodology to assess alterations in liver perfusion and hepatocyte integrity in murine hepatitis.
PubDate: 2016-05-09

• Robust imaging of hippocampal inner structure at 7T: in vivo acquisition
protocol and methodological choices
• Abstract: Objective Motion-robust multi-slab imaging of hippocampal inner structure in vivo at 7T. Materials and methods Motion is a crucial issue for ultra-high resolution imaging, such as can be achieved with 7T MRI. An acquisition protocol was designed for imaging hippocampal inner structure at 7T. It relies on a compromise between anatomical details visibility and robustness to motion. In order to reduce acquisition time and motion artifacts, the full slab covering the hippocampus was split into separate slabs with lower acquisition time. A robust registration approach was implemented to combine the acquired slabs within a final 3D-consistent high-resolution slab covering the whole hippocampus. Evaluation was performed on 50 subjects overall, made of three groups of subjects acquired using three acquisition settings; it focused on three issues: visibility of hippocampal inner structure, robustness to motion artifacts and registration procedure performance. Results Overall, T2-weighted acquisitions with interleaved slabs proved robust. Multi-slab registration yielded high quality datasets in 96 % of the subjects, thus compatible with further analyses of hippocampal inner structure. Conclusion Multi-slab acquisition and registration setting is efficient for reducing acquisition time and consequently motion artifacts for ultra-high resolution imaging of the inner structure of the hippocampus.
PubDate: 2016-05-02

• In vivo functional connectome of human brainstem nuclei of the ascending
arousal, autonomic, and motor systems by high spatial resolution 7-Tesla
fMRI
• Abstract: Objective Our aim was to map the in vivo human functional connectivity of several brainstem nuclei with the rest of the brain by using seed-based correlation of ultra-high magnetic field functional magnetic resonance imaging (fMRI) data. Materials and methods We used the recently developed template of 11 brainstem nuclei derived from multi-contrast structural MRI at 7 Tesla as seed regions to determine their connectivity to the rest of the brain. To achieve this, we used the increased contrast-to-noise ratio of 7-Tesla fMRI compared with 3 Tesla and time-efficient simultaneous multi-slice imaging to cover the brain with high spatial resolution (1.1-mm isotropic nominal resolution) while maintaining a short repetition time (2.5 s). Results The delineated Pearson’s correlation-based functional connectivity diagrams (connectomes) of 11 brainstem nuclei of the ascending arousal, motor, and autonomic systems from 12 controls are presented and discussed in the context of existing histology and animal work. Conclusion Considering that the investigated brainstem nuclei play a crucial role in several vital functions, the delineated preliminary connectomes might prove useful for future in vivo research and clinical studies of human brainstem function and pathology, including disorders of consciousness, sleep disorders, autonomic disorders, Parkinson’s disease, and other motor disorders.
PubDate: 2016-04-28

• A comparison of phase imaging and quantitative susceptibility mapping in
the imaging of multiple sclerosis lesions at ultrahigh field
• Abstract: Objective The aim of this study was to compare the use of high-resolution phase and QSM images acquired at ultra-high field in the investigation of multiple sclerosis (MS) lesions with peripheral rings, and to discuss their usefulness for drawing inferences about underlying tissue composition. Materials and methods Thirty-nine Subjects were scanned at 7 T, using 3D T 2*-weighted and T 1-weighted sequences. Phase images were then unwrapped and filtered, and quantitative susceptibility maps were generated using a thresholded k-space division method. Lesions were compared visually and using a 1D profiling algorithm. Results Lesions displaying peripheral rings in the phase images were identified in 10 of the 39 subjects. Dipolar projections were apparent in the phase images outside of the extent of several of these lesions; however, QSM images showed peripheral rings without such projections. These projections appeared ring-like in a small number of phase images where no ring was observed in QSM. 1D profiles of six well-isolated example lesions showed that QSM contrast corresponds more closely to the magnitude images than phase contrast. Conclusions Phase images contain dipolar projections, which confounds their use in the investigation of tissue composition in MS lesions. Quantitative susceptibility maps correct these projections, providing insight into the composition of MS lesions showing peripheral rings.
PubDate: 2016-04-25

• 7 Tesla quantitative hip MRI: a comparison between TESS and CPMG for T2
mapping
• Abstract: Objectives We aimed to evaluate the feasibility of triple-echo steady state (TESS) T2 mapping as an alternative to conventional multi-echo-spin-echo (CPMG) T2 mapping for the quantitative assessment of hip joint cartilage at 7 T. Materials and methods A total of eight healthy volunteers and three patients were included. Reproducibility of both techniques was evaluated in five volunteers in five scans each. T2 relaxation times were measured by manually drawing regions of interest in multiple regions of the hip joint. Data from both methods were compared using Pearson correlation coefficient, intra-class correlation coefficient, and coefficient of repeatability. The overall image quality and presence of artifacts was assessed. Results Cartilage transplant and surrounding fluid were well depicted by both methods. Compared to CPMG, TESS provided systematically reduced T2 values (43.3 ± 7.3 vs. 19.2 ± 5.5 ms for acetabular cartilage, and 41.4 ± 5.6 vs. 21.7 ± 5.2 ms for femoral cartilage), in line with previously reported values. No correlation between both methods was found. TESS yielded a slightly better reproducibility than CPMG, while CPMG showed pronounced sensitivity to B1 inhomogeneities. Conclusion TESS seems to be an attractive alternative to CPMG for improvements in quantitative hip joint imaging at 7 T, allowing shortening of the total acquisition time paired with insensitivity to B1, while rendering comparable image quality with good repeatability.
PubDate: 2016-04-25

• Electrodynamics and radiofrequency antenna concepts for human magnetic
resonance at 23.5 T (1 GHz) and beyond
• Abstract: Objective This work investigates electrodynamic constraints, explores RF antenna concepts and examines the transmission fields (B 1 + ) and RF power deposition of dipole antenna arrays for 1H magnetic resonance of the human brain at 1 GHz (23.5 T). Materials and methods Electromagnetic field (EMF) simulations are performed in phantoms with average tissue simulants for dipole antennae using discrete frequencies [300 MHz (7.0 T) to 3 GHz (70.0 T)]. To advance to a human setup EMF simulations are conducted in anatomical human voxel models of the human head using a 20-element dipole array operating at 1 GHz. Results Our results demonstrate that transmission fields suitable for 1H MR of the human brain can be achieved at 1 GHz. An increase in transmit channel density around the human head helps to enhance B 1 + in the center of the brain. The calculated relative increase in specific absorption rate at 23.5 versus 7.0 T was below 1.4 (in-phase phase setting) and 2.7 (circular polarized phase setting) for the dipole antennae array. Conclusion The benefits of multi-channel dipole antennae at higher frequencies render MR at 23.5 T feasible from an electrodynamic standpoint. This very preliminary finding opens the door on further explorations that might be catalyzed into a 20-T class human MR system.
PubDate: 2016-04-20

• The traveling heads: multicenter brain imaging at 7 Tesla
• Abstract: Objective This study evaluates the inter-site and intra-site reproducibility of 7 Tesla brain imaging and compares it to literature values for other field strengths. Materials and methods The same two subjects were imaged at eight different 7 T sites. MP2RAGE, TSE, TOF, SWI, EPI as well as B1 and B0 field maps were analyzed quantitatively to assess inter-site reproducibility. Intra-site reproducibility was measured with rescans at three sites. Results Quantitative measures of MP2RAGE scans showed high agreement. Inter-site and intra-site reproducibility errors were comparable to 1.5 and 3 T. Other sequences also showed high reproducibility between the sites, but differences were also revealed. The different RF coils used were the main source for systematic differences between the sites. Conclusion Our results show for the first time that multi-center brain imaging studies of the supratentorial brain can be performed at 7 T with high reproducibility and similar reliability as at 3T. This study develops the basis for future large-scale 7 T multi-site studies.
PubDate: 2016-04-20

• Glioma vessel abnormality quantification using time-of-flight MR
angiography
• Abstract: Objectives To differentiate between abnormal tumor vessels and regular brain vasculature using new quantitative measures in time-of-flight (TOF) MR angiography (MRA) data. Materials and methods In this work time-of-flight (TOF) MR angiography data are acquired in 11 glioma patients to quantify vessel abnormality. Brain vessels are first segmented with a new algorithm, efficient monte-carlo image-analysis for the location of vascular entity (EMILOVE), and are then characterized in three brain regions: tumor, normal-appearing contralateral brain, and the total brain volume without the tumor. For characterization local vessel orientation angles and the dot product between local orientation vectors are calculated and averaged in the 3 regions. Additionally, correlation with histological and genetic markers is performed. Results Both the local vessel orientation angles and the dot product show a statistically significant difference (p < 0.005) between tumor vessels and normal brain vasculature. Furthermore, the connection to both histology and the gene expression of the tumor can be found—here, the measures were compared to the proliferation marker Ki-67 [MIB] and genome-wide expression analysis. The results in a subgroup indicate that the dot product measure may be correlated with activated genetic pathways. Conclusion It is possible to define a measure of vessel abnormality based on local vessel orientation angles which can differentiate between normal brain vasculature and glioblastoma vessels.
PubDate: 2016-04-20

• Intravoxel incoherent motion analysis of abdominal organs: computation of
reference parameters in a large cohort of C57Bl/6 mice and correlation to
microvessel density
• Abstract: Objective Diffusion-weighted magnetic resonance imaging (DW-MRI) combined with intravoxel incoherent motion (IVIM) analysis may be applied for assessment of organ lesions, diffuse parenchymal pathologies, and therapy monitoring. The aim of this study was to determine IVIM reference parameters of abdominal organs for translational research in a large cohort of C57Bl/6 laboratory mice. Materials and methods Anesthetized mice (n = 29) were measured in a 4.7 T small-animal MR scanner with a diffusion-weighted echo-planar imaging sequence at the $$b$$ -values 0, 13, 24, 55, 107, 260, 514, 767, 1020 s/mm2. IVIM analysis was conducted on the liver, spleen, renal medulla and cortex, pancreas, and small bowel with computation of the true tissue diffusion coefficient $$D_{\text{t}}$$ , the perfusion fraction $$f_{\text{p}}$$ , and the pseudodiffusion coefficient $$D_{\text{p}}$$ . Microvessel density (MVD) was assessed by immunohistochemistry (IHC) against panendothelial cell antigen CD31. Results Mean values of the different organs [ $$D_{\text{t}}$$ (10−3 mm2/s); $$f_{\text{p}}$$ (%); $$D_{\text{p}}$$ (10−3 mm2/s); MVD (MV/mm2)]: liver 1.15 ± 0.14; 14.77 ± 6.15; 50.28 ± 33.21, 2008.48 ± 419.43, spleen 0.55 ± 0.12; 9.89 ± 5.69; 24.46 ± 17.31; n.d., renal medulla 1.50 ± 0.20; 14.63 ± 4.07; 35.50 ± 18.01; 1231.88 ± 290.61, renal cortex 1.34 ± 0.18; 10.83 ± 3.70; 16.74 ± 6.74; 810.09 ± 193.50, pancreas 1.23 ± 0.22; 20.12 ± 7.46; 29.35 ± 17.82, 591.15 ± 86.25 and small bowel 1.06 ± 0.13; 16.48 ± 3.63; 15.31 ± 7.00; 420.50 ± 168.42. Unlike $$D_{\text{t}}$$ and $$f_{\text{p}}$$ , $$D_{\text{p}}$$ correlates significantly with MVD (r = 0.90, p = 0.037). Conclusion This systematic evaluation of murine abdominal organs with IVIM and MVD analysis allowed to establish reference parameters for future DW-MRI translational research studies on small-animal disease models.
PubDate: 2016-04-19

• Pulsed arterial spin labelling at ultra-high field with a B 1 + -optimised
• Abstract: Objective Arterial spin labelling (ASL) techniques benefit from the increased signal-to-noise ratio and the longer T 1 relaxation times available at ultra-high field. Previous pulsed ASL studies at 7 T concentrated on the superior regions of the brain because of the larger transmit radiofrequency inhomogeneity experienced at ultra-high field that hinders an adequate inversion of the blood bolus when labelling in the neck. Recently, researchers have proposed to overcome this problem with either the use of dielectric pads, through dedicated transmit labelling coils, or special adiabatic inversion pulses. Materials and methods We investigate the performance of an optimised time-resampled frequency-offset corrected inversion (TR-FOCI) pulse designed to cause inversion at much lower peak B 1 + . In combination with a PICORE labelling, the perfusion signal obtained with this pulse is compared against that obtained with a FOCI pulse, with and without dielectric pads. Results Mean grey matter perfusion with the TR-FOCI was 52.5 ± 10.3 mL/100 g/min, being significantly higher than the 34.6 ± 2.6 mL/100 g/min obtained with the FOCI pulse. No significant effect of the dielectric pads was observed. Conclusion The usage of the B 1 + -optimised TR-FOCI pulse results in a significantly higher perfusion signal. PICORE–ASL is feasible at ultra-high field with no changes to operating conditions.
PubDate: 2016-04-15

• 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

• 3D T 2 -weighted imaging at 7T using dynamic k T -points on
single-transmit MRI systems
• Abstract: Objectives For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B 1 + inhomogeneity. Previously, it was shown that a single kT-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static kT-points). This work demonstrates that the B 1 + dependence of T 2-weighted imaging can be further mitigated by designing a specific kT-point pulse for each pulse of a 3D TSE sequence (dynamic kT-points) even on single-channel transmit systems Materials and methods By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic kT-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic kT-points were inserted into the TSE sequence to acquire in vivo images at 7T. Results The improvement provided by the dynamic kT-points over the static kT-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic kT-points showed systematic improvement of signal and contrast at 7T over regular TSE—especially in cerebellar and temporal lobe regions without the need of parallel transmission. Conclusion Designing dynamic kT-points for a 3D TSE sequence allows the acquisition of T 2-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B 1 + inhomogeneity.
PubDate: 2016-04-08

• 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: 2016-04-08

• Scan time minimization in hepatic diffusion-weighted imaging: evaluation
of the simultaneous multislice acceleration technique with different
acceleration factors and gradient preparation schemes
• Abstract: Objective To evaluate simultaneous multislice (sms) accelerated diffusion-weighted imaging (DWI) of the liver in comparison to conventional sequences. Materials and methods Ten volunteers underwent DWI of the liver at 1.5 T. Four different sms-accelerated sequences with monopolar and bipolar gradient preparation (MP, BP) and acceleration factors 2 and 3 (sms2-DWI, sms3-DWI) were compared to conventional DWI (c-DWI). Image quality criteria rated on a 5-point Likert scale (5 = excellent), image quality sum scores (maximum 120), and ADC were compared using Friedman test and Dunn-Bonferroni post hoc test. Bland–Altman plots were calculated for ADC comparison. p values <0.05 were considered significant. Results Sms2-DWI offered scan time minimization of 67 % without significant difference in image quality (sum score: sms2-DWI MP/BP: 97 ± 8/92 ± 9; c-DWI MP/BP: 99 ± 8/97 ± 8). Sms3-DWI offered slight additional scan time minimization with significantly inferior image quality (sum score: sms3-DWI MP/BP: 75 ± 14/69 ± 14; p < 0.001). MP preparation provided slightly higher image quality in sms-DWI without statistical significance. ADC in sms-DWI were significantly lower (sms2-DWI MP 1.01 × 10−3 mm2/s; c-DWI MP 1.20 × 10−3 mm2/s; p < 0.001). Conclusion Sms2-DWI provides considerable scan time minimization without significant shortcomings in image quality. Sms3-DWI provides significantly inferior image quality without further scan time minimization. Potentially lower ADC in sms-DWI should be considered in clinical routine.
PubDate: 2016-04-02

• Coronary MR angiography at 3T: fat suppression versus water-fat separation
• Abstract: Objectives To compare Dixon water-fat suppression with spectral pre-saturation with inversion recovery (SPIR) at 3T for coronary magnetic resonance angiography (MRA) and to demonstrate the feasibility of fat suppressed coronary MRA at 3T without administration of a contrast agent. Materials and methods Coronary MRA with Dixon water-fat separation or with SPIR fat suppression was compared on a 3T scanner equipped with a 32-channel cardiac receiver coil. Eight healthy volunteers were examined. Contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), right coronary artery (RCA), and left anterior descending (LAD) coronary artery sharpness and length were measured and statistically compared. Two experienced cardiologists graded the visual image quality of reformatted Dixon and SPIR images (1: poor quality to 5: excellent quality). Results Coronary MRA images in healthy volunteers showed improved contrast with the Dixon technique compared to SPIR (CNR blood-fat: Dixon = 14.9 ± 2.9 and SPIR = 13.9 ± 2.1; p = 0.08, CNR blood-myocardium: Dixon = 10.2 ± 2.7 and SPIR = 9.11 ± 2.6; p = 0.1). The Dixon method led to similar fat suppression (fat SNR with Dixon: 2.1 ± 0.5 vs. SPIR: 2.4 ± 1.2, p = 0.3), but resulted in significantly increased SNR of blood (blood SNR with Dixon: 19.9 ± 4.5 vs. SPIR: 15.5 ± 3.1, p < 0.05). This means the residual fat signal is slightly lower with the Dixon compared to the SIPR technique (although not significant), while the SNR of blood is significantly higher with the Dixon technique. Vessel sharpness of the RCA was similar for Dixon and SPIR (57 ± 7 % vs. 56 ± 9 %, p = 0.2), while the RCA visualized vessel length was increased compared to SPIR fat suppression (107 ± 21 vs. 101 ± 21 mm, p < 0.001). For the LAD, vessel sharpness (50 ± 13 % vs. 50 ± 7 %, p = 0.4) and vessel length (92 ± 46 vs. 90 ± 47 mm, p = 0.4) were similar with both techniques. Consequently, the Dixon technique resulted in an improved visual score of the coronary arteries in the water fat separated images of healthy subjects (RCA: 4.6 ± 0.5 vs. 4.1 ± 0.7, p = 0.01, LAD: 4.1 ± 0.7 vs. 3.5 ± 0.8, p = 0.007). Conclusions Dixon water-fat separation can significantly improve coronary artery image quality without the use of a contrast agent at 3T.
PubDate: 2016-04-02

• 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

• Potential influence of Gadolinium contrast on image segmentation in
MR-based attenuation correction with Dixon sequences in whole-body 18F-FDG
PET/MR
• Abstract: Objective To evaluate the influence of Gadolinium contrast agent on image segmentation in magnetic resonance (MR)-based attenuation correction (AC) with four-segment dual-echo time Dixon-sequences in whole-body [18F]-fluorodeoxyglucose positron emission tomography (PET)/MR imaging, and to analyze the consecutive effect on standardized uptake value (SUV). Materials and methods Hybrid imaging with an integrated PET/MR system was performed in 30 oncological patients. AC was based on MR imaging with a Dixon sequence with subsequent automated image segmentation. AC maps (µmaps) were acquired and reconstructed prior to (µmap−gd) and after (µmap+gd) Gd-contrast agent application. For quantification purposes, the SUV of organs and tumors based on both µmaps were compared. Results Tissue classification based on µmap−gd was correct in 29/30 patients; based on µmap+gd, the brain was falsely classified as fat in 12/30 patients with significant underestimation of SUV. In all cancerous lesions, tissue segmentation was correct. All concordant µmaps−gd/+gd resulted in no significant difference in SUV. Conclusion In PET/MR, Gd-contrast agent potentially influences fat/water separation in Dixon-sequences of the head with above-average false tissue segmentation and an associated underestimation of SUV. Thus, MR-based AC should be acquired prior to Gd-contrast agent application. Additionally, integrating the MR-based AC maps into the reading-routine in PET/MR is recommended to avoid interpretation errors in cases where tissue segmentation fails.
PubDate: 2016-04-01

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