3D localized lactate detection in muscle tissue using double-quantum filtered 1 H MRS with adiabatic refocusing pulses at 7 T.

PURPOSE: Lactate is a key metabolite in skeletal muscle and whole-body physiology. Its MR visibility in muscle is affected by overlapping lipid signals and fiber orientation. Double-quantum filtered (DQF) 1 H MRS selectively detects lactate at 1.3 ppm, but at ultra-high field the efficiency of slice-selective 3D-localization with conventional RF pulses is limited by bandwidth. This novel 3D-localized 1 H DQF MRS sequence uses adiabatic refocusing pulses to unambiguously detect lactate in skeletal muscle at 7 T. METHODS: Lactate double-quantum coherences were 3D-localized using slice-selective Shinnar-Le Roux optimized excitation and adiabatic refocusing pulses (similar to semi-LASER). DQF MR spectra were acquired at 7 T from lactate phantoms, meat specimens with injected lactate (exploring multiple TEs and fiber orientations), and human gastrocnemius in vivo during and after exercise (without cuff ischemia). RESULTS: Lactate was readily detected, achieving the full potential of 50% signal with a DQF, in solution. The effects of fiber orientation and TE on the lactate doublet (peak splitting, amplitude, and phase) were in good agreement with theory and literature. Exercise-induced lactate accumulation was detected with 30 s time resolution. CONCLUSION: This novel 3D-localized 1 H DQF MRS sequence can dynamically detect glycolytically generated lactate in muscle during exercise and recovery at 7 T.

Interleaved 31 P MRS/1 H ASL for analysis of metabolic and functional heterogeneity along human lower leg muscles at 7T.

PURPOSE: MR offers the unique possibility to noninvasively investigate cellular energy metabolism via 31P MRS, while blood perfusion, which provides oxygen and substrates to the tissue, is accessible by arterial spin labeling (ASL) 1H MRI. Because metabolic and hemodynamic parameters are linked, it would be desirable to study them simultaneously. A 3D-resolved method is presented that allows such measurements with high spatiotemporal resolution and has the potential to discern differences along an exercising muscle. METHODS: Multi-voxel localized 31 P MRS was temporally interleaved with multi-slice pASL 1H MRI. Phosphorus spectra were collected from two adjacent positions in gastrocnemius medialis (GM) during rest, submaximal plantar flexion exercise and recovery, while perfusion and T2* -weighted axial images were acquired at the same time. Seventeen healthy volunteers (9 f / 8 m) were studied at 7 T. RESULTS: An increase of postexercise perfusion and T2* -weighted signal in GM positively correlated with end-exercise PCr depletion and pH drop. At proximal positions functional and metabolic activity was higher than distally, that is, perfusion increase and peak T2* -weighted signal, end-exercise PCr depletion, end-exercise pH, and PCr recovery time constant were significantly different. An NOE-induced SNR increase of approximately 20 % (P < .001), at rest, was found in interleaved 31 P spectra, when comparing to 31 P-only acquisitions. CONCLUSIONS: A technique for fast, simultaneous imaging of muscle functional heterogeneity in ASL, T2* and acquisition of time-resolved 31 P  MRS data is presented. These single exercise recovery experiments can be used to investigate local variations during disease progression in patients suffering from vascular or muscular diseases.

Design, Implementation, and Evaluation of a Head and Neck MRI RF Array Integrated with a 511 keV Transmission Source for Attenuation Correction in PET/MR.

The goal of this work is to further improve positron emission tomography (PET) attenuation correction and magnetic resonance (MR) sensitivity for head and neck applications of PET/MR. A dedicated 24-channel receive-only array, fully-integrated with a hydraulic system to move a transmission source helically around the patient and radiofrequency (RF) coil array, is designed, implemented, and evaluated. The device enables the calculation of attenuation coefficients from PET measurements at 511 keV including the RF coil and the particular patient. The RF coil design is PET-optimized by minimizing photon attenuation from coil components and housing. The functionality of the presented device is successfully demonstrated by calculating the attenuation map of a water bottle based on PET transmission measurements; results are in excellent agreement with reference values. It is shown that the device itself has marginal influence on the static magnetic field B0 and the radiofrequency transmit field B1 of the 3T PET/MR system. Furthermore, the developed RF array is shown to outperform a standard commercial 16-channel head and neck coil in terms of signal-to-noise ratio (SNR) and parallel imaging performance. In conclusion, the presented hardware enables accurate calculation of attenuation maps for PET/MR systems while improving the SNR of corresponding MR images in a single device without degrading the B0 and B1 homogeneity of the scanner.

In vivo MRI of the human finger at 7 T.

PURPOSE: To demonstrate a dedicated setup for ultrahigh resolution MR imaging of the human finger in vivo. METHODS: A radiofrequency coil was designed for optimized signal homogeneity and sensitivity in the finger at ultrahigh magnetic field strength (7 T), providing high measurement sensitivity. Imaging sequences (2D turbo-spin echo (TSE) and 3D magnetization-prepared rapid acquisition gradient echo (MPRAGE)) were adapted for high spatial resolution and good contrast of different tissues in the finger, while keeping acquisition time below 10 minutes. Data was postprocessed to display finger structures in three dimensions. RESULTS: 3D MPRAGE data with isotropic resolution of 200 µm, along with 2D TSE images with in-plane resolutions of 58 × 78 µm2 and 100 × 97 µm2 , allowed clear identification of various anatomical features such as bone and bone marrow, tendons and annular ligaments, cartilage, arteries and veins, nerves, and Pacinian corpuscles. CONCLUSION: Using this dedicated finger coil at 7 T, together with adapted acquisition sequences, it is possible to depict the internal structures of the human finger in vivo within patient-compatible measurement time. It may serve as a tool for diagnosis and treatment monitoring in pathologies ranging from inflammatory or erosive joint diseases to injuries of tendons and ligaments to nervous or vascular disorders in the finger. Magn Reson Med 79:588-592, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Dynamic multivoxel-localized 31 P MRS during plantar flexion exercise with variable knee angle.

Exercise studies investigating the metabolic response of calf muscles using 31 P MRS are usually performed with a single knee angle. However, during natural movement, the distribution of workload between the main contributors to force, gastrocnemius and soleus is influenced by the knee angle. Hence, it is of interest to measure the respective metabolic response of these muscles to exercise as a function of knee angle using localized spectroscopy. Time-resolved multivoxel 31 P MRS at 7 T was performed simultaneously in gastrocnemius medialis and soleus during rest, plantar flexion exercise and recovery in 12 healthy volunteers. This experiment was conducted with four different knee angles. PCr depletions correlated negatively with knee angle in gastrocnemius medialis, decreasing from 79±14 % (extended leg) to 35±23 %(∼40°), and positively in soleus, increasing from 20±21 % to 36±25 %; differences were significant. Linear correlations were found between knee angle and end-exercise PCr depletions in gastrocnemius medialis (R2 =0.8) and soleus (R2 =0.53). PCr recovery times and end-exercise pH changes that correlated with PCr depletion were consistent with the literature in gastrocnemius medialis and differences between knee angles were significant. These effects were less pronounced in soleus and not significant for comparable PCr depletions. Maximum oxidative capacity calculated for all knee angles was in excellent agreement with the literature and showed no significant changes between different knee angles. In conclusion, these findings confirm that plantar flexion exercise with a straight leg is a suitable paradigm, when data are acquired from gastrocnemius only (using either localized MRS or small surface coils), and that activation of soleus requires the knee to be flexed. The present study comprises a systematic investigation of the effects of the knee angle on metabolic parameters, measured with dynamic multivoxel 31 P MRS during muscle exercise and recovery, and the findings should be used in future study design.

Magnetic resonance imaging T1- and T2-mapping to assess renal structure and function: a systematic review and statement paper.

This systematic review, initiated by the European Cooperation in Science and Technology Action Magnetic Resonance Imaging Biomarkers for Chronic Kidney Disease (PARENCHIMA), focuses on potential clinical applications of magnetic resonance imaging in renal non-tumour disease using magnetic resonance relaxometry (MRR), specifically, the measurement of the independent quantitative magnetic resonance relaxation times T1 and T2 at 1.5 and 3Tesla (T), respectively. Healthy subjects show a distinguishable cortico-medullary differentiation (CMD) in T1 and a slight CMD in T2. Increased cortical T1 values, that is, reduced T1 CMD, were reported in acute allograft rejection (AAR) and diminished T1 CMD in chronic allograft rejection. However, ambiguous findings were reported and AAR could not be sufficiently differentiated from acute tubular necrosis and cyclosporine nephrotoxicity. Despite this, one recent quantitative study showed in renal transplants a direct correlation between fibrosis and T1 CMD. Additionally, various renal diseases, including renal transplants, showed a moderate to strong correlation between T1 CMD and renal function. Recent T2 studies observed increased values in renal transplants compared with healthy subjects and in early-stage autosomal dominant polycystic kidney disease (ADPKD), which could improve diagnosis and progression assessment compared with total kidney volume alone in early-stage ADPKD. Renal MRR is suggested to be sensitive to renal perfusion, ischaemia/oxygenation, oedema, fibrosis, hydration and comorbidities, which reduce specificity. Due to the lack of standardization in patient preparation, acquisition protocols and adequate patient selection, no widely accepted reference values are currently available. Therefore this review encourages efforts to optimize and standardize (multi-parametric) protocols to increase specificity and to tap the full potential of renal MRR in future research.

High-sensitivity TMS/fMRI of the Human Motor Cortex Using a Dedicated Multichannel MR Coil.

PURPOSE: To validate a novel setup for concurrent TMS/fMRI in the human motor cortex based on a dedicated, ultra-thin, multichannel receive MR coil positioned between scalp and TMS system providing greatly enhanced sensitivity compared to the standard birdcage coil setting. METHODS: A combined TMS/fMRI design was applied over the primary motor cortex based on 1Hz stimulation with stimulation levels of 80%, 90%, 100%, and 110% of the individual active motor threshold, respectively. Due to the use of a multichannel receive coil we were able to use multiband-accelerated (MB=2) EPI sequences for the acquisition of functional images. Data were analysed with SPM12 and BOLD-weighted signal intensity time courses were extracted in each subject from two local maxima (individual functional finger tapping localiser, fixed MNI coordinate of the hand knob) next to the hand area of the primary motor cortex (M1) and from the global maximum. RESULTS: We report excellent image quality without noticeable signal dropouts or image distortions. Parameter estimates in the three peak voxels showed monotonically ascending activation levels over increasing stimulation intensities. Across all subjects, mean BOLD signal changes for 80%, 90%, 100%, 110% of the individual active motor threshold were 0.43%, 0.63%, 1.01%, 2.01% next to the individual functional finger tapping maximum, 0.73%, 0.91%, 1.34%, 2.21% next to the MNI-defined hand knob and 0.88%, 1.09%, 1.65%, 2.77% for the global maximum, respectively. CONCLUSION: Our results show that the new setup for concurrent TMS/fMRI experiments using a dedicated MR coil array allows for high-sensitivity fMRI particularly at the site of stimulation. Contrary to the standard birdcage approach, the results also demonstrate that the new coil can be successfully used for multiband-accelerated EPI acquisition. The gain in flexibility due to the new coil can be easily combined with neuronavigation within the MR scanner to allow for accurate targeting in TMS/fMRI experiments.

Interleaved multivoxel 31 P MR spectroscopy.

PURPOSE: Separate measurements are required when investigating multiple exercising muscles with singlevoxel-localized dynamic 31 P-MRS. With multivoxel spectroscopy, 31 P-MRS time-series spectra are acquired from multiple independent regions during one exercise-recovery experiment with the same time resolution as for singlevoxel measurements. METHODS: Multiple independently selected volumes were localized using temporally interleaved semi-LASER excitations at 7T. Signal loss caused by mutual saturation from shared excitation or refocusing slices was quantified at partial and full overlap, and potential contamination was investigated in phantom measurements. During an exercise-recovery experiment both gastrocnemius medialis and soleus of two healthy volunteers were measured using multivoxel acquisitions with a total TR of 6 s, while avoiding overlap of excitation slices. RESULTS: Signal reduction by shared adiabatic refocusing slices selected 1 s after the preceding voxel was between 10% (full overlap) and 20% (half overlap), in a phantom measurement. In vivo data were acquired from both muscles within the same exercise experiment, with 13-18% signal reduction. Spectra show phosphocreatine, inorganic phosphate, adenosine-triposphate, phosphomonoesters, and phosphodiesters. CONCLUSION: Signal decrease was relatively low compared to the 2-fold increase in information. The approach could help to improve the understanding in metabolic research and is applicable to other organs and nuclei. Magn Reson Med 77:921-927, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Sex differences in the functional connectivity of the amygdalae in association with cortisol.

Human amygdalae are involved in various behavioral functions such as affective and stress processing. For these behavioral functions, as well as for psychophysiological arousal including cortisol release, sex differences are reported. Here, we assessed cortisol levels and resting-state functional connectivity (rsFC) of left and right amygdalae in 81 healthy participants (42 women) to investigate potential modulation of amygdala rsFC by sex and cortisol concentration. Our analyses revealed that rsFC of the left amygdala significantly differed between women and men: Women showed stronger rsFC than men between the left amygdala and left middle temporal gyrus, inferior frontal gyrus, postcentral gyrus and hippocampus, regions involved in face processing, inner-speech, fear and pain processing. No stronger connections were detected for men and no sex difference emerged for right amygdala rsFC. Also, an interaction of sex and cortisol appeared: In women, cortisol was negatively associated with rsFC of the amygdalae with striatal regions, mid-orbital frontal gyrus, anterior cingulate gyrus, middle and superior frontal gyri, supplementary motor area and the parietal-occipital sulcus. Contrarily in men, positive associations of cortisol with rsFC of the left amygdala and these structures were observed. Functional decoding analyses revealed an association of the amygdalae and these regions with emotion, reward and memory processing, as well as action execution. Our results suggest that functional connectivity of the amygdalae as well as the regulatory effect of cortisol on brain networks differs between women and men. These sex-differences and the mediating and sex-dependent effect of cortisol on brain communication systems should be taken into account in affective and stress-related neuroimaging research. Thus, more studies including both sexes are required.

Simultaneous and interleaved acquisition of NMR signals from different nuclei with a clinical MRI scanner.

PURPOSE: Modification of a clinical MRI scanner to enable simultaneous or rapid interleaved acquisition of signals from two different nuclei. METHODS: A device was developed to modify the local oscillator signal fed to the receive channel(s) of an MRI console. This enables external modification of the frequency at which the receiver is sensitive and rapid switching between different frequencies. Use of the device was demonstrated with interleaved and simultaneous 31 P and 1 H spectroscopic acquisitions, and with interleaved 31 P and 1 H imaging. RESULTS: Signal amplitudes and signal-to-noise ratios were found to be unchanged for the modified system, compared with data acquired with the MRI system in the standard configuration. CONCLUSION: Interleaved and simultaneous 1 H and 31 P signal acquisition was successfully demonstrated with a clinical MRI scanner, with only minor modification of the RF architecture. While demonstrated with 31 P, the modification is applicable to any detectable nucleus without further modification, enabling a wide range of simultaneous and interleaved experiments to be performed within a clinical setting. Magn Reson Med 76:1636-1641, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Dynamic PCr and pH imaging of human calf muscles during exercise and recovery using (31) P gradient-Echo MRI at 7 Tesla.

PURPOSE: Simultaneous acquisition of spatially resolved (31) P-MRI data for evaluation of muscle specific energy metabolism, i.e., PCr and pH kinetics. METHODS: A three-dimensional (3D) gradient-echo sequence for multiple frequency-selective excitations of the PCr and Pi signals in an interleaved sampling scheme was developed and tested at 7 Tesla (T). The pH values were derived from the chemical shift-induced phase difference between the resonances. The achieved spatial resolution was ∼2 mL with image acquisition time below 6 s. Ten healthy volunteers were studied performing plantar flexions during the delay between (31) P-MRI acquisitions, yielding a temporal resolution of 9-10 s. RESULTS: Signal from anatomically matched regions of interest had sufficient signal-to-noise ratio to allow single-acquisition PCr and pH quantification. The Pi signal was clearly detected in voxels of actively exercising muscles. The PCr depletions were in gastrocnemius 42 ± 14% (medialis), 48 ± 17% (lateralis) and in soleus 20 ± 11%. The end exercise pH values were 6.74 ± 0.18 and 6.65 ± 0.27 for gastrocnemius medialis and lateralis, respectively, and 6.96 ± 0.12 for soleus muscle. CONCLUSION: Simultaneous acquisition of PCr and Pi images with high temporal resolution, suitable for measuring PCr and pH kinetics in exercise-recovery experiments, was demonstrated at 7T. This study presents a fast alternative to MRS for quantifying energy metabolism of posterior muscle groups of the lower leg. Magn Reson Med 75:2324-2331, 2016. © 2015 Wiley Periodicals, Inc.

Disrupted effective connectivity between the amygdala and orbitofrontal cortex in social anxiety disorder during emotion discrimination revealed by dynamic causal modeling for FMRI.

Social anxiety disorder (SAD) is characterized by over-reactivity of fear-related circuits in social or performance situations and associated with marked social impairment. We used dynamic causal modeling (DCM), a method to evaluate effective connectivity, to test our hypothesis that SAD patients would exhibit dysfunctions in the amygdala-prefrontal emotion regulation network. Thirteen unmedicated SAD patients and 13 matched healthy controls performed a series of facial emotion and object discrimination tasks while undergoing fMRI. The emotion-processing network was identified by a task-related contrast and motivated the selection of the right amygdala, OFC, and DLPFC for DCM analysis. Bayesian model averaging for DCM revealed abnormal connectivity between the OFC and the amygdala in SAD patients. In healthy controls, this network represents a negative feedback loop. In patients, however, positive connectivity from OFC to amygdala was observed, indicating an excitatory connection. As we did not observe a group difference of the modulatory influence of the FACE condition on the OFC to amygdala connection, we assume a context-independent reduction of prefrontal control over amygdalar activation in SAD patients. Using DCM, it was possible to highlight not only the neuronal dysfunction of isolated brain regions, but also the dysbalance of a distributed functional network.

Additive gene-environment effects on hippocampal structure in healthy humans.

Hippocampal volume loss has been related to chronic stress as well as genetic factors. Although genetic and environmental variables affecting hippocampal volume have extensively been studied and related to mental illness, limited evidence is available with respect to G × E interactions on hippocampal volume. The present MRI study investigated interaction effects on hippocampal volume between three well-studied functional genetic variants (COMT Val158Met, BDNF Val66Met, 5-HTTLPR) associated with hippocampal volume and a measure of environmental adversity (life events questionnaire) in a large sample of healthy humans (n = 153). All three variants showed significant interactions with environmental adversity with respect to hippocampal volume. Observed effects were additive by nature and driven by both recent as well as early life events. A consecutive analysis of hippocampal subfields revealed a spatially distinct profile for each genetic variant suggesting a specific role of 5-HTTLPR for the subiculum, BDNF Val66Met for CA4/dentate gyrus, and COMT Val158Met for CA2/3 volume changes. The present study underscores the importance of G × E interactions as determinants of hippocampal volume, which is crucial for the neurobiological understanding of stress-related conditions, such as mood disorders or post-traumatic stress disorder (PTSD).

Power balance and loss mechanism analysis in RF transmit coil arrays.

PURPOSE: To establish a framework for transmit array power balance calculations based on power correlation matrices to accurately quantify the loss contributions from different mechanisms such as coupling, lumped components, and radiation. THEORY AND METHODS: Starting from Poynting's theorem, power correlation matrices are derived for all terms in the power balance, which is formulated as a matrix equation. Finite-difference time-domain simulations of two 7 T eight-channel head array coils at 297.2 MHz are used to verify the theoretical considerations and demonstrate their application. Care is taken to accurately incorporate all loss mechanisms. The power balance for static B1 phase shims as well as two-dimensional spatially selective transmit SENSE pulses is shown. RESULTS: The simulated power balance shows an excellent agreement with theory, with a maximum power imbalance of less than 0.11%. Power loss contributions from the different loss mechanisms vary significantly between the investigated setups, and depending on the excitation mode imposed on the coil. CONCLUSION: The presented approach enables a straightforward loss evaluation for an arbitrary excitation of transmit coil arrays. Worst-case power imbalance and losses are calculated in a straightforward manner. This allows for deeper insight into transmit array loss mechanisms, incorporation of radiated power components in specific absorption rate calculations and verification of electromagnetic simulations.

Dynamic ASL and T2-weighted MRI in exercising calf muscle at 7 T: a feasibility study.

PURPOSE: The aim of this study was to develop a measurement protocol for noninvasive simultaneous perfusion quantification and T2 *-weighted MRI acquisition in the exercising calf muscle at 7 Tesla. METHODS: Using a nonmagnetic ergometer and a dedicated in-house built calf coil array, dynamic pulsed arterial spin labeling (PASL) measurements with a temporal resolution of 12 s were performed before, during, and after plantar flexion exercise in 16 healthy volunteers. RESULTS: Postexercise peak perfusion in gastrocnemius muscle (GAS) was 27 ± 16 ml/100g/min, whereas in soleus (SOL) and tibialis anterior (TA) muscles it remained at baseline levels. T2 *-weighted and ASL time courses in GAS showed comparable times to peak of 161 ± 72 s and 167 ± 115 s, respectively. The T2 *-weighted signal in the GAS showed a minimum during exercise (88 ± 6 % of the baseline signal) and a peak during the recovery (122 ± 9%), whereas in all other muscles only a signal decrease was observed (minimum 91 ± 6% in SOL; 87 ± 8% in TA). CONCLUSION: We demonstrate the feasibility of dynamic perfusion quantification in skeletal muscle at 7 Tesla using PASL. This may help to better investigate the physiological processes in the skeletal muscle and also in diseases such as diabetes mellitus and peripheral arterial disease.

Novel inductive decoupling technique for flexible transceiver arrays of monolithic transmission line resonators.

PURPOSE: This article presents a novel inductive decoupling technique for form-fitting coil arrays of monolithic transmission line resonators, which target biomedical applications requiring high signal-to-noise ratio over a large field of view to image anatomical structures varying in size and shape from patient to patient. METHODS: Individual transmission line resonator elements are mutually decoupled using magnetic flux sharing by overlapping annexes. This decoupling technique was evaluated by electromagnetic simulations and bench measurements for two- and four-element arrays, comparing single- and double-gap transmission line resonator designs, combined either with a basic capacitive matching scheme or inductive pickup loop matching. The best performing array was used in 7T MRI experiments demonstrating its form-fitting ability and parallel imaging potential. RESULTS: The inductively matched double-gap transmission line resonator array provided the best decoupling efficiency in simulations and bench measurements (<-15 dB). The decoupling and parallel imaging performance proved robust against mechanical deformation of the array. CONCLUSION: The presented decoupling technique combines the robustness of conventional overlap decoupling regarding coil loading and operating frequency with the extended field of view of nonoverlapped coils. While demonstrated on four-element arrays, it can be easily expanded to fabricate readily decoupled form-fitting 2D arrays with an arbitrary number of elements in a single etching process.

A novel coil array for combined TMS/fMRI experiments at 3 T.

PURPOSE: To overcome current limitations in combined transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) studies by employing a dedicated coil array design for 3 Tesla. METHODS: The state-of-the-art setup for concurrent TMS/fMRI is to use a large birdcage head coil, with the TMS between the subject's head and the MR coil. This setup has drawbacks in sensitivity, positioning, and available imaging techniques. In this study, an ultraslim 7-channel receive-only coil array for 3 T, which can be placed between the subject's head and the TMS, is presented. Interactions between the devices are investigated and the performance of the new setup is evaluated in comparison to the state-of-the-art setup. RESULTS: MR sensitivity obtained at the depth of the TMS stimulation is increased by a factor of five. Parallel imaging with an acceleration factor of two is feasible with low g-factors. Possible interactions between TMS and the novel hardware were investigated and were found negligible. CONCLUSION: The novel coil array is safe, strongly improves signal-to-noise ratio in concurrent TMS/fMRI experiments, enables parallel imaging, and allows for flexible positioning of the TMS on the head while ensuring efficient TMS stimulation due to its ultraslim design.

A form-fitted three channel (31) P, two channel (1) H transceiver coil array for calf muscle studies at 7 T.

PURPOSE: To enhance sensitivity and coverage for calf muscle studies, a novel, form-fitted, three-channel phosphorus-31 ((31) P), two-channel proton ((1) H) transceiver coil array for 7 T MR imaging and spectroscopy is presented. METHODS: Electromagnetic simulations employing individually generated voxel models were performed to design a coil array for studying nonpathological muscle metabolism. Static phase combinations of the coil elements' transmit fields were optimized based on homogeneity and efficiency for several voxel models. The best-performing design was built and tested both on phantoms and in vivo. RESULTS: Simulations revealed that a shared conductor array for (31) P provides more robust interelement decoupling and better homogeneity than an overlap array in this configuration. A static B1 (+) shim setting that suited various calf anatomies was identified and implemented. Simulations showed that the (31) P array provides signal-to-noise ratio (SNR) benefits over a single loop and a birdcage coil of equal radius by factors of 3.2 and 2.6 in the gastrocnemius and by 2.5 and 2.0 in the soleus muscle. CONCLUSION: The performance of the coil in terms of B1 (+) and achievable SNR allows for spatially localized dynamic (31) P spectroscopy studies in the human calf. The associated higher specificity with respect to nonlocalized measurements permits distinguishing the functional responses of different muscles.

In vivo MR imaging of the human skin at subnanoliter resolution using a superconducting surface coil at 1.5 Tesla.

PURPOSE: To demonstrate the feasibility of a highly sensitive superconducting surface coil for microscopic MRI of the human skin in vivo in a clinical 1.5 Tesla (T) scanner. MATERIALS AND METHODS: A 12.4-mm high-temperature superconducting coil was used at 1.5T for phantom and in vivo skin imaging. Images were inspected to identify fine anatomical skin structures. Signal-to-noise ratio (SNR) improvement by the high-temperature superconducting (HTS) coil, as compared to a commercial MR microscopy coil was quantified from phantom imaging; the gain over a geometrically identical coil made from copper (cooled or not) was theoretically deduced. Noise sources were identified to evaluate the potential of HTS coils for future studies. RESULTS: In vivo skin images with isotropic 80 µm resolution were demonstrated revealing fine anatomical structures. The HTS coil improved SNR by a factor 32 over the reference coil in a nonloading phantom. For calf imaging, SNR gains of 380% and 30% can be expected over an identical copper coil at room temperature and 77 K, respectively. CONCLUSION: The high sensitivity of HTS coils allows for microscopic imaging of the skin at 1.5T and could serve as a tool for dermatology in a clinical setting.

Localized semi-LASER dynamic (31)P magnetic resonance spectroscopy of the soleus during and following exercise at 7 T.

OBJECTIVES: This study demonstrates the applicability of semi-LASER localized dynamic (31)P 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 [Formula: see text] in SOL ([Formula: see text] in GM). End exercise PCr depletion in SOL ([Formula: see text] %) was far lower than in GM ([Formula: see text] %). The pH in SOL increased rapidly and, in contrast to GM, remained elevated until the end of exercise. CONCLUSION: (31)P 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.