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 Magnetic Resonance Materials in Physics, Biology and Medicine   [SJR: 0.928]   [H-I: 40]   [1 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 0968-5243 - ISSN (Online) 1352-8661    Published by Springer-Verlag  [2276 journals]
• Effect of $$T_{2}^{*}$$ T 2 ∗ correction on contrast kinetic model
analysis using a reference tissue arterial input function at 7 T
• Abstract: Objectives We aimed to investigate the effect of $$T_{2}^{*}$$ correction on estimation of kinetic parameters from T 1-weighted dynamic contrast enhanced (DCE) MRI data when a reference-tissue arterial input function (AIF) is used. Materials and methods DCE-MRI data were acquired from seven mice with 4T1 mouse mammary tumors using a double gradient echo sequence at 7 T. The AIF was estimated from a region of interest in the muscle. The extended Tofts model was used to estimate pharmacokinetic parameters in the enhancing part of the tumor, with and without $$T_{2}^{*}$$ correction of the lesion and AIF. The parameters estimated with $$T_{2}^{*}$$ correction of both the AIF and lesion time-intensity curve were assumed to be the reference standard. Results For the whole population, there was significant difference (p < 0.05) in transfer constant (K trans) between $$T_{2}^{*}$$ corrected and not corrected methods, but not in interstitial volume fraction (v e). Individually, no significant differences were found in K trans and v e of four and six tumors, respectively, between the $$T_{2}^{*}$$ corrected and not corrected methods. In contrast, K trans was significantly underestimated, if the $$T_{2}^{*}$$ correction was not used, in other tumors for which the median K trans was larger than 0.4 min−1. Conclusion $$T_{2}^{*}$$ effect on tumors with high K trans may not be negligible in kinetic model analysis, even if AIF is estimated from reference tissue where the concentration of contrast agent is relatively low.
PubDate: 2015-12-01

• The repeatability of T2 relaxation time measurement of human knee
articular cartilage
• Abstract: Objectives To assess short- and long-term repeatability of T2 relaxation time measurements of the knee articular cartilage. Materials and methods The right knees of nine asymptomatic volunteers (age 30–38 years, five male, four female) were imaged at 1.5 T in three sessions 1 and 2 weeks apart. To observe short-term repeatability, the measurements were repeated three times within one of the three imaging sessions for each volunteer. T2 relaxation time was mapped using a multi-slice multi-echo spin echo sequence in axial and sagittal planes. Cartilage was manually segmented and repeatability, as measured by root-mean-square coefficient of variation (CVRMS) was evaluated both for the entire bulk cartilage of each joint surface in the slice and separately for each region of interest (ROI) at different topographical locations and separately for the superficial and deep half of each ROI. Results For bulk T2, the long-term repeatability was 3.2, 5.4, and 3.7 %, and the short-term reproducibility was 3.9, 3.9, and 3.4 % for bulk femoral, tibial, and patellar cartilage, respectively. There were no significant differences between long-term and short-term repeatability in superficial or deep cartilage when comparing CVRMS values (p = 0.338 and 0.700, respectively). For individual ROIs, the repeatability varied between 2.5 and 22.2 % depending on the topographical location. Conclusion The current results show mostly good repeatability. However, there were remarkable variations of T2 between bulk cartilage and different ROIs, bulk cartilage showing better repeatability. With careful patient positioning T2 can be accurately determined for different cartilage surfaces.
PubDate: 2015-12-01

• Optical tracking with two markers for robust prospective motion correction
for brain imaging
• Abstract: Objective Prospective motion correction (PMC) during brain imaging using camera-based tracking of a skin-attached marker may suffer from problems including loss of marker visibility due to the coil and false correction due to non-rigid-body facial motion, such as frowning or squinting. A modified PMC system is introduced to mitigate these problems and increase the robustness of motion correction. Materials and methods The method relies on simultaneously tracking two markers, each providing six degrees of freedom, that are placed on the forehead. This allows us to track head motion when one marker is obscured and detect skin movements to prevent false corrections. Experiments were performed to compare the performance of the two-marker motion correction technique to the previous single-marker approach. Results Experiments validate the theory developed for adaptive marker tracking and skin movement detection, and demonstrate improved image quality during obstruction of the line-of-sight of one marker when subjects squint or when subjects squint and move simultaneously. Conclusion The proposed methods eliminate two common failure modes of PMC and substantially improve the robustness of PMC, and they can be applied to other optical tracking systems capable of tracking multiple markers. The methods presented can be adapted to the use of more than two markers.
PubDate: 2015-12-01

• Repeatability of in vivo quantification of atherosclerotic carotid artery
plaque components by supervised multispectral classification
• Abstract: Objective To evaluate the agreement and scan–rescan repeatability of automated and manual plaque segmentation for the quantification of in vivo carotid artery plaque components from multi-contrast MRI. Materials and methods Twenty-three patients with 30–70 % stenosis underwent two 3T MR carotid vessel wall exams within a 1 month interval. T1w, T2w, PDw and TOF images were acquired around the region of maximum vessel narrowing. Manual delineation of the vessel wall and plaque components (lipid, calcification, loose matrix) by an experienced observer provided the reference standard for training and evaluation of an automated plaque classifier. Areas of different plaque components and fibrous tissue were quantified and compared between segmentation methods and scan sessions. Results In total, 304 slices from 23 patients were included in the segmentation experiment, in which 144 aligned slice pairs were available for repeatability analysis. The correlation between manual and automated segmented areas was 0.35 for lipid, 0.66 for calcification, 0.50 for loose matrix and 0.82 for fibrous tissue. For the comparison between scan sessions, the coefficient of repeatability of area measurement obtained by automated segmentation was lower than by manual delineation for lipid (9.9 vs. 17.1 mm2), loose matrix (13.8 vs. 21.2 mm2) and fibrous tissue (24.6 vs. 35.0 mm2), and was similar for calcification (20.0 vs. 17.6 mm2). Conclusion Application of an automated classifier for segmentation of carotid vessel wall plaque components from in vivo MRI results in improved scan–rescan repeatability compared to manual analysis.
PubDate: 2015-12-01

• Acoustic-noise-optimized diffusion-weighted imaging
• Abstract: Objective This work was aimed at reducing acoustic noise in diffusion-weighted MR imaging (DWI) that might reach acoustic noise levels of over 100 dB(A) in clinical practice. Materials and methods A diffusion-weighted readout-segmented echo-planar imaging (EPI) sequence was optimized for acoustic noise by utilizing small readout segment widths to obtain low gradient slew rates and amplitudes instead of faster k-space coverage. In addition, all other gradients were optimized for low slew rates. Volunteer and patient imaging experiments were conducted to demonstrate the feasibility of the method. Acoustic noise measurements were performed and analyzed for four different DWI measurement protocols at 1.5T and 3T. Results An acoustic noise reduction of up to 20 dB(A) was achieved, which corresponds to a fourfold reduction in acoustic perception. The image quality was preserved at the level of a standard single-shot (ss)-EPI sequence, with a 27–54 % increase in scan time. Conclusions The diffusion-weighted imaging technique proposed in this study allowed a substantial reduction in the level of acoustic noise compared to standard single-shot diffusion-weighted EPI. This is expected to afford considerably more patient comfort, but a larger study would be necessary to fully characterize the subjective changes in patient experience.
PubDate: 2015-12-01

• Active decoupling of RF coils using a transmit array system
• Abstract: Objective Implementation of a decoupling method for isolation of transmit and receive radio frequency (RF) coils for concurrent excitation and acquisition (CEA) MRI in samples with ultra-short T2*. Materials and methods The new phase and amplitude (PA) decoupling method is implemented in a clinical 3T-MRI system equipped with a parallel transmit array system. For RF excitation, two transmit coils are used in combination with a single receive coil. The transmit coil is geometrically decoupled from the receive coil, and the remaining B 1-induced voltages in the receive coil during CEA are minimized by the second transmit coil using a careful adjustment of the phase and amplitude settings in this coil. Isolation of the decoupling scheme and transmit noise behavior are analyzed for different loading conditions, and a CEA MRI experiment is performed in a rubber phantom with sub-millisecond T2* and in an ex vivo animal. Results Geometrical (20 dB) and PA decoupling (50 dB) provided a total isolation of 70 dB between the transmit and receive coils. Integration of a low-noise RF amplifier was necessary to minimize RF transmit noise. CEA MR images could be reconstructed from a rubber phantom and an ex vivo animal. Conclusion CEA MRI can be implemented in clinical MRI systems using active decoupling with parallel transmit array capabilities with minor hardware modifications.
PubDate: 2015-12-01

• The effect of water suppression on the hepatic lipid quantification, as
assessed by the LCModel, in a preclinical and clinical scenario
• Abstract: Objective To investigate the effect of water suppression on the hepatic lipid quantification, using the LCModel. Materials and methods MR spectra with and without water suppression were acquired in the liver of mice at 4.7 T and patients at 3 T, and processed with the LCModel. The Cramér–Rao Lower Bound (CRLB) values of the seven lipid resonances were determined to assess the impact of water suppression on hepatic lipid quantification. A paired t test was used for comparison between the CRLBs obtained with and without water suppression. Results For the preclinical data, in the high (low) fat fraction subset an overall impairment in hepatic lipid quantification, i.e. an increase of CRLBs (no significant change of CRLBs) was observed in spectra acquired with water suppression. For the clinical data, there were no substantial changes in the CRLB with water suppression. Because (1) the water suppression does not overall improve the quantification of the lipid resonances and (2) the MR spectrum without water suppression is always acquired for fat fraction calculation, the optimal data-acquisition strategy for liver MRS is to acquire only the MR spectrum without water suppression. Conclusion For quantification of hepatic lipid resonances, it is advantageous to perform MR spectroscopy without water suppression in a clinical and preclinical scenario (at moderate fields).
PubDate: 2015-11-21

• Contrast-optimized composite image derived from multigradient echo cardiac
magnetic resonance imaging improves reproducibility of myocardial contours
and T2* measurement
• Abstract: Objectives Reproducibility of myocardial contour determination in cardiac magnetic resonance imaging is important, especially when determining T2* values per myocardial segment as a prognostic factor of heart failure or thalassemia. A method creating a composite image with contrasts optimized for drawing myocardial contours is introduced and compared with the standard method on a single image. Materials and methods A total of 36 short-axis slices from bright-blood multigradient echo (MGE) T2* scans of 21 patients were acquired at eight echo times. Four observers drew free-hand myocardial contours on one manually selected T2* image (method 1) and on one image composed by blending three images acquired at TEs providing optimum contrast-to-noise ratio between the myocardium and its surrounding regions (method 2). Results Myocardial contouring by method 2 met higher interobserver reproducibility than method 1 (P < 0.001) with smaller Coefficient of variance (CoV) of T2* values in the presence of myocardial iron accumulation (9.79 vs. 15.91 %) and in both global myocardial and mid-ventricular septum regions (12.29 vs. 16.88 and 5.76 vs. 8.16 %, respectively). Conclusion The use of contrast-optimized composite images in MGE data analysis improves reproducibility of myocardial contour determination, leading to increased consistency in the calculated T2* values enhancing the diagnostic impact of this measure of iron overload.
PubDate: 2015-11-03

• A semi-automated “blanket” method for renal segmentation from
non-contrast T1-weighted MR images
• Abstract: Objective To investigate the precision and accuracy of a new semi-automated method for kidney segmentation from single-breath-hold non-contrast MRI. Materials and methods The user draws approximate kidney contours on every tenth slice, focusing on separating adjacent organs from the kidney. The program then performs a sequence of fully automatic steps: contour filling, interpolation, non-uniformity correction, sampling of representative parenchyma signal, and 3D binary morphology. Three independent observers applied the method to images of 40 kidneys ranging in volume from 94.6 to 254.5 cm3. Manually constructed reference masks were used to assess accuracy. Results The volume errors for the three readers were: 4.4 % ± 3.0 %, 2.9 % ± 2.3 %, and 3.1 % ± 2.7 %. The relative discrepancy across readers was 2.5 % ± 2.1 %. The interactive processing time on average was 1.5 min per kidney. Conclusions Pending further validation, the semi-automated method could be applied for monitoring of renal status using non-contrast MRI.
PubDate: 2015-10-29

• Acoustic noise reduction in T 1 - and proton-density-weighted turbo
spin-echo imaging
• Abstract: Objective To reduce acoustic noise levels in T 1-weighted and proton-density-weighted turbo spin-echo (TSE) sequences, which typically reach acoustic noise levels up to 100 dB(A) in clinical practice. Materials and methods Five acoustic noise reduction strategies were combined: (1) gradient ramps and shapes were changed from trapezoidal to triangular, (2) variable-encoding-time imaging was implemented to relax the phase-encoding gradient timing, (3) RF pulses were adapted to avoid the need for reversing the polarity of the slice-rewinding gradient, (4) readout bandwidth was increased to provide more time for gradient activity on other axes, (5) the number of slices per TR was reduced to limit the total gradient activity per unit time. We evaluated the influence of each measure on the acoustic noise level, and conducted in vivo measurements on a healthy volunteer. Sound recordings were taken for comparison. Results An overall acoustic noise reduction of up to 16.8 dB(A) was obtained by the proposed strategies (1–4) and the acquisition of half the number of slices per TR only. Image quality in terms of SNR and CNR was found to be preserved. Conclusions The proposed measures in this study allowed a threefold reduction in the acoustic perception of T 1-weighted and proton-density-weighted TSE sequences compared to a standard TSE-acquisition. This could be achieved without visible degradation of image quality, showing the potential to improve patient comfort and scan acceptability.
PubDate: 2015-10-22

• Parameterization of hyperpolarized 13 C-bicarbonate-dissolution dynamic
nuclear polarization
• Abstract: Objective 13C metabolic MRI using hyperpolarized 13C-bicarbonate enables preclinical detection of pH. To improve signal-to-noise ratio, experimental procedures were refined, and the influence of pH, buffer capacity, temperature, and field strength were investigated. Materials and methods Bicarbonate preparation was investigated. Bicarbonate was prepared and applied in spectroscopy at 1, 3, 14 T using pure dissolution, culture medium, and MCF-7 cell spheroids. Healthy rats were imaged by spectral–spatial spiral acquisition for spatial and temporal bicarbonate distribution, pH mapping, and signal decay analysis. Results An optimized preparation technique for maximum solubility of 6 mol/L and polarization levels of 19–21 % is presented; T1 and SNR dependency on field strength, buffer capacity, and pH was investigated. pH mapping in vivo is demonstrated. Conclusion An optimized bicarbonate preparation and experimental procedure provided improved T1 and SNR values, allowing in vitro and in vivo applications.
PubDate: 2015-10-08

• Surface coil with reduced specific absorption rate for rat MRI at 7 T
• Abstract: Objective A scaled-down slotted surface radio frequency (RF) coil was built, and the specific absorbance rate (SAR) in 100 mg of tissue (SAR100 mg) produced in a rat brain phantom was computed at 7 T. Materials and methods A slotted coil 2-cm in diameter with six circular slots was developed. Its theoretical and experimental performance was computed and compared using the signal-to-noise ratio (SNR) expression and phantom images obtained with a spin echo sequence. Electromagnetic simulations were performed using the finite integral method with saline sphere and rat brain phantoms. SAR100 mg was computed for the circular coil, by varying its radius, and was also computed for the slotted coil. Results The slotted coil quality factor gave a twofold increment over the circular coil, and noise was reduced by 17 %. The experimental SNR of the slotted coil produced a 30 % improvement for points near the coil plane. The theoretical and experimental results showed substantial agreement. Axial map histograms and profiles showed greater SAR100 mg values for the circular coil than for the slotted coil. Conclusions The slotted surface coil offers improved performance and low SAR100 mg for rat brain imaging at 7 T. This approach may be used with new RF coils to investigate SAR in humans.
PubDate: 2015-10-08

• Highly undersampled peripheral Time-of-Flight magnetic resonance
angiography: optimized data acquisition and iterative image reconstruction

• Abstract: Object The aim of this study was to investigate the acceleration of peripheral Time-of-Flight magnetic resonance angiography using Compressed Sensing and parallel magnetic resonance imaging (MRI) while preserving image quality and vascular contrast. Materials and methods An analytical sampling pattern is proposed that combines aspects of parallel MRI and Compressed Sensing. It is used in combination with a dedicated Split Bregman algorithm. This approach is compared with current state-of-the-art patterns and reconstruction algorithms. Results The acquisition time was reduced from 30 to 2.5 min in a study using ten volunteer data sets, while showing improved sharpness, better contrast and higher accuracy compared to state-of-the-art techniques. Conclusion This study showed the benefits of the proposed dedicated analytical sampling pattern and Split Bregman algorithm for optimizing the Compressed Sensing reconstruction of highly accelerated peripheral Time-of-Flight data.
PubDate: 2015-10-01

• Reproducibility of pharmacological ASL using sequences from different
vendors: implications for multicenter drug studies
• Abstract: Object The current study assesses the multicenter feasibility of pharmacological arterial spin labeling (ASL) by comparing a caffeine-induced relative cerebral blood flow decrease (%CBF↓) measured with two pseudo-continuous ASL sequences as provided by two major vendors. Materials and methods Twenty-two healthy volunteers were scanned twice with both a 3D spiral (GE) and a 2D EPI (Philips) sequence. The inter-session reproducibility was evaluated by comparisons of the mean and within-subject coefficient of variability (wsCV) of the %CBF↓, both for the total cerebral gray matter and on a voxel level. Results The %CBF↓ was larger when measured with the 3D spiral sequence (23.9 ± 5.9 %) than when measured with the 2D EPI sequence (19.2 ± 5.6 %) on a total gray matter level (p = 0.02), and on a voxel level in the posterior watershed area (p < 0.001). There was no difference between the gray matter wsCV of the 3D spiral (57.3 %) and 2D EPI sequence (66.7 %, p = 0.3), whereas on a voxel level, the wsCV was visibly different between the sequences. Conclusion The observed differences between ASL sequences of both vendors can be explained by differences in the employed readout modules. These differences may seriously hamper multicenter pharmacological ASL, which strongly encourages standardization of ASL implementations.
PubDate: 2015-10-01

• Recreational alcohol use induces changes in the concentrations of
choline-containing compounds and total creatine in the brain: a 1 H MRS
study of healthy subjects
• Abstract: Objective It has previously been reported that even social alcohol consumption affects the magnetic resonance spectroscopy (MRS) signals of choline-containing compounds (tCho). The purpose of this study was to investigate whether the consumption of alcohol affects the concentrations of the metabolites tCho, N-acetylaspartate, creatine, or myo-inositol and/or their T 2 relaxation times. Materials and methods 1H MR spectra were obtained at 3 T from a frontal white matter voxel of 25 healthy subjects with social alcohol consumption (between 0 and 25.9 g/day). Absolute brain metabolite concentrations and T 2 relaxation times of metabolites were examined via MRS measurements at different echo times. Metabolite concentrations and their T 2 relaxation times were correlated with subjects’ alcohol consumption, controlling for age. Results We observed positive correlations of absolute tCho and phosphocreatine and creatine (tCr) concentrations with alcohol consumption but no correlation between any metabolite T 2 relaxation time and alcohol consumption. Conclusions This study shows that even social alcohol consumption affects the concentrations of tCho and tCr in cerebral white matter. Future studies assessing brain tCho and tCr levels should control for the confounding factor alcohol consumption.
PubDate: 2015-10-01

• Physiological noise in human cerebellar fMRI
• Abstract: Objectives To compare physiological noise contributions in cerebellar and cerebral regions of interest in high-resolution functional magnetic resonance imaging (fMRI) data acquired at 7T, to estimate the need for physiological noise removal in cerebellar fMRI. Materials and methods Signal fluctuations in high resolution (1 mm isotropic) 7T fMRI data were attributed to one of the following categories: task-induced BOLD changes, slow drift, signal changes correlated with the cardiac and respiratory cycles, signal changes related to the cardiac rate and respiratory volume per unit of time or other. $$R_{\text{adj}}^{2}$$ values for all categories were compared across regions of interest. Results In this high-resolution data, signal fluctuations related to the phase of the cardiac cycle and cardiac rate were shown to be significant, but comparable between cerebellar and cerebral regions of interest. However, respiratory related signal fluctuations were increased in the cerebellar regions, with explained variances that were up to 80 % higher than for the primary motor cortex region. Conclusion Even at a millimetre spatial resolution, significant correlations with both cardiac and respiratory RETROICOR components were found in all healthy volunteer data. Therefore, physiological noise correction is highly likely to improve the temporal signal-to-noise ratio (SNR) for cerebellar fMRI at 7T, even at high spatial resolution.
PubDate: 2015-10-01

• Localized semi-LASER dynamic 31 P magnetic resonance spectroscopy of the
soleus during and following exercise at 7 T
• Abstract: Objectives This study demonstrates the applicability of semi-LASER localized dynamic 31P MRS to deeper lying areas of the exercising human soleus muscle (SOL). The effect of accurate localization and high temporal resolution on data specificity is investigated. Materials and methods To achieve high signal-to-noise ratio (SNR) at a temporal resolution of 6 s, a custom-built human calf coil array was used at 7T. The kinetics of phosphocreatine (PCr) and intracellular pH were quantified separately in SOL and gastrocnemius medialis (GM) muscle of nine volunteers, during rest, plantar flexion exercise, and recovery. Results The average SNR of PCr at rest was $$64\pm 15$$ in SOL ( $$83\pm 12$$ in GM). End exercise PCr depletion in SOL ( $$19\pm 9$$  %) was far lower than in GM ( $$74\pm 14$$  %). The pH in SOL increased rapidly and, in contrast to GM, remained elevated until the end of exercise. Conclusion 31P MRS in single-shots every 6 s localized in the deeper-lying SOL enabled quantification of PCr recovery times at low depletions and of fast pH changes, like the initial rise. Both high temporal resolution and accurate spatial localization improve specificity of Pi and, thus, pH quantification by avoiding multiple, and potentially indistinguishable sources for changing the Pi peak shape.
PubDate: 2015-10-01

• ESMRMB 2015, 32nd Annual Scientific Meeting, Edinburgh, UK, 1-3 October:
Author Index
• PubDate: 2015-10-01

• ESMRMB 2015, 32nd Annual Scientific Meeting, Edinburgh, UK, 1-3 October:
Abstracts, Thursday
• PubDate: 2015-10-01

• Experience with magnetic resonance imaging of human subjects with passive
implants and tattoos at 7 T: a retrospective study
• Abstract: Object Over the last decade, the number of clinical MRI studies at 7 T has increased dramatically. Since only limited information about the safety of implants/tattoos is available at 7 T, many centers either conservatively exclude all subjects with implants/tattoos or have started to perform dedicated tests for selected implants. This work presents our experience in imaging volunteers with implants/tattoos at 7 T over the last seven and a half years. Materials and methods 1796 questionnaires were analyzed retrospectively to identify subjects with implants/tattoos imaged at 7 T. For a total of 230 subjects, the type of local transmit/receive RF coil used for examination, imaging sequences, acquisition time, and the type of implants/tattoos and their location with respect to the field of view were documented. These subjects had undergone examination after careful consideration by an internal safety panel consisting of three experts in MR safety and physics. Results None of the subjects reported sensations of heat or force before, during, or after the examination. None expressed any discomfort related to implants/tattoos. Artifacts were reported in 52 % of subjects with dental implants; all artifacts were restricted to the mouth area and did not affect image quality in the brain parenchyma. Conclusion Our initial experience at 7 T indicates that a strict rejection of subjects with tattoos and/or implants is not justified. Imaging can be conditionally performed in carefully selected subjects after collection of substantial safety information and evaluation of the detailed exposure scenario (RF coil/type and position of implant). Among the assessed subjects with tattoos, no side effects from the exposure to 7 T MRI were reported.
PubDate: 2015-09-26

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