Correction: Assessment of single-vessel cerebral blood velocity by phase contrast fMRI.

[This corrects the article DOI: 10.1371/journal.pbio.3000923.].

Signal-to-noise ratio versus field strength for small surface coils.

The increasing signal-to-noise ratio (SNR) is the main reason to use ultrahigh field MRI. Here, we investigate the dependence of the SNR on the magnetic field strength, especially for small animal applications, where small surface coils are used and coil noise cannot be ignored. Measurements were performed at five field strengths from 3 to 14.1 T, using 2.2-cm surface coils with an identical coil design for transmit and receive on two water samples with and without salt. SNR was measured in a series of spoiled gradient echo images with varying flip angle and corrected for saturation based on a series of flip angle and T1 measurements. Furthermore, the noise figure of the receive chain was determined and eliminated to remove instrument dependence. Finally, the coil sensitivity was determined based on the principle of reciprocity to obtain a measure for ultimate SNR. Before coil sensitivity correction, the SNR increase in nonconductive samples is highly supralinear with B0 1.6-2.7, depending on distance to the coil, while in the conductive sample, the growth is smaller, being around linear close to the surface coil and increasing up to a B0 2.0 dependence when moving away from the coil. After sensitivity correction, the SNR increase is independent of loading with B0 2.1. This study confirms the supralinear increase of SNR with increasing field strengths. Compared with most human measurements with larger coil sizes, smaller surface coils, as mainly used in animal studies, have a higher contribution of coil noise and thus a different behavior of SNR at high fields.

Assessment of single-vessel cerebral blood velocity by phase contrast fMRI.

Current approaches to high-field functional MRI (fMRI) provide 2 means to map hemodynamics at the level of single vessels in the brain. One is through changes in deoxyhemoglobin in venules, i.e., blood oxygenation level-dependent (BOLD) fMRI, while the second is through changes in arteriole diameter, i.e., cerebral blood volume (CBV) fMRI. Here, we introduce cerebral blood flow-related velocity-based fMRI, denoted CBFv-fMRI, which uses high-resolution phase contrast (PC) MRI to form velocity measurements of flow. We use CBFv-fMRI in measure changes in blood velocity in single penetrating microvessels across rat parietal cortex. In contrast to the venule-dominated BOLD and arteriole-dominated CBV fMRI signals, CBFv-fMRI is comparable from both arterioles and venules. A single fMRI platform is used to map changes in blood pO2 (BOLD), volume (CBV), and velocity (CBFv). This combined high-resolution single-vessel fMRI mapping scheme enables vessel-specific hemodynamic mapping in animal models of normal and diseased states and further has translational potential to map vascular dementia in diseased or injured human brains with ultra-high-field fMRI.

Evaluation of the contribution of individual arteries to the cerebral blood supply in patients with Moyamoya angiopathy: comparison of vessel-encoded arterial spin labeling and digital subtraction angiography.

PURPOSE: Vessel-encoded arterial spin labeling (VE-ASL) is able to provide noninvasive information about the contribution of individual arteries to the cerebral perfusion. The aim of this study was to compare VE-ASL to the diagnostic standard digital subtraction angiography (DSA) with respect to its ability to visualize vascular territories. METHODS: In total, 20 VE-ASL and DSA data sets of 17 patients with Moyamoya angiopathy with and without revascularization surgery were retrospectively analyzed. Two neuroradiologists independently assessed the agreement between VE-ASL and DSA using a 4-point Likert scale (no- very high agreement). Additionally, grading of the vascular supply of subterritories (A1-A2, M1-M6) on the VE-ASL images and angiograms was performed. The intermodal agreement was calculated for all subterritories in total and for the subdivision into without and after revascularization (direct or indirect bypass). RESULTS: There was a very high agreement between the VE-ASL and the DSA data sets (median = 1, modus = 1) with a substantial inter-rater agreement (kw = 0.762 (95% CI 0.561-0.963)). The inter-modality agreement between VE-ASL and DSA in vascular subterritories was almost perfect for all subterritories (k = 0.899 (0.865-0.945)), in the subgroup of direct revascularized subterritories (k = 0.827 (0.738-0.915)), in the subgroup of indirect revascularized subterritories (k = 0.843 (0.683-1.003)), and in the subgroup of never revascularized subterritories (k = 0.958 (0.899-1.017)). CONCLUSION: Vessel-encoded ASL seems to be a promising non-invasive method to depict the contributions of individual arteries to the cerebral perfusion before and after revascularization surgery.

Free-breathing Arterial Spin Labeling MRI for the Detection of Pulmonary Embolism.

Background Arterial spin labeling (ASL) MRI can be used to assess organ perfusion but has yet to be implemented for perfusion evaluation of the lung. Purpose To evaluate pseudo-continuous ASL (PCASL) MRI for the detection of acute pulmonary embolism (PE) and its potential as an alternative to CT pulmonary angiography (CTPA). Materials and Methods Between November 2020 and November 2021, 97 patients (median age, 61 years; 48 women) with suspected PE were enrolled in this prospective study. PCASL MRI was performed within a 72-hour period following CTPA under free-breathing conditions and included three orthogonal planes. The pulmonary trunk was labeled during systole, and the image was acquired during diastole of the subsequent cardiac cycle. Additionally, multisection, coronal, balanced, steady-state free-precession imaging was carried out. Two radiologists blindly assessed overall image quality, artifacts, and diagnostic confidence (five-point Likert scale, 5 = best). Patients were categorized as positive or negative for PE, and a lobe-wise assessment in PCASL MRI and CTPA was conducted. Sensitivity and specificity were calculated on a patient level with the final clinical diagnosis serving as the reference standard. Interchangeability between MRI and CTPA was also tested with use of an individual equivalence index (IEI). Results PCASL MRI was performed successfully in all patients with high scores for image quality, artifact, and diagnostic confidence (κ ≥ .74). Of the 97 patients, 38 were positive for PE. PCASL MRI depicted PE correctly in 35 of 38 patients with three false-positive and three false-negative findings, resulting in a sensitivity of 35 of 38 patients (92% [95% CI: 79, 98]) and a specificity of 56 of 59 patients (95% [95% CI: 86, 99]). Interchangeability analysis revealed an IEI of 2.6% (95% CI: 1.2, 3.8). Conclusion Free-breathing pseudo-continuous arterial spin labeling MRI depicted abnormal lung perfusion caused by acute pulmonary embolism and may be useful as a contrast material-free alternative to CT pulmonary angiography for selected patients. German Clinical Trials Register no. DRKS00023599 © RSNA, 2023.

A CMOS NMR needle for probing brain physiology with high spatial and temporal resolution.

Magnetic resonance imaging and spectroscopy are versatile methods for probing brain physiology, but their intrinsically low sensitivity limits the achievable spatial and temporal resolution. Here, we introduce a monolithically integrated NMR-on-a-chip needle that combines an ultra-sensitive 300 µm NMR coil with a complete NMR transceiver, enabling in vivo measurements of blood oxygenation and flow in nanoliter volumes at a sampling rate of 200 Hz.

Concurrent intrinsic optical imaging and fMRI at ultra-high field using magnetic field proof optical components.

Intrinsic optical imaging (IOI) is a well established technique to quantify activation-related hemodynamical changes at the surface of the brain, which can be used to investigate the underlying processes of BOLD signal formation. To directly and quantitatively relate IOI and fMRI, simultaneous measurements with the two modalities are necessary. Here, a novel technical solution for a completely in-bore setup is presented, which uses only magnetic field proof components and thus allows concurrent recordings with a quality similar to that obtained in separate experiments. Measurements of the somatosensory cortex of rats with electrical forepaw stimulation were used to verify this approach. The high spatial and temporal resolution of the fMRI data, which is possible due to the high magnetic field of 14.1 T, the use of a point-spread function-based distortion correction and optimized additional anatomical images, allowed accurate colocalization of the images of the two modalities. Accordingly, detailed investigations of the temporal and spatial relationships between the hemodynamic parameters and the fMRI signal, which demonstrate the linear dependence of the BOLD effect on changes in the concentrations of oxygenated and deoxygenated hemoglobin, are possible. Comparisons between the signals emerging from arterial, venous and parenchymal areas are possible and show clearly distinct characteristics. The presented setup allows combining MRI measurements and optical recordings without serious losses in the data quality of either modality. While the proposed combination of fMRI and IOI can help to gain valuable insight into the generation of the BOLD effect, the setup can be easily modified to include different types of optical or MRI measurements.

Intravascular BOLD signal characterization of balanced SSFP experiments in human blood at high to ultrahigh fields.

PURPOSE: To investigate the intravascular contribution to the overall balanced SSFP (bSSFP) BOLD effect in human blood at high to ultrahigh field strengths (3 T, 9.4 T, and 14.1 T). METHODS: Venous blood prepared at two different oxygenation levels (deoxygenated: Y ≈ 71%, oxygenated: Y ≈ 94%) was measured with phase-cycled bSSFP for varying TRs/flip angles at 3 T, 9.4 T, and 14.1 T. The oxygen sensitivity was analyzed by intrinsic MIRACLE (motion-insensitive rapid configuration relaxometry)-R2 estimation and passband signal differences. The intravascular BOLD-related signal change was extracted from the measured data for microvasculature and macrovasculature, and compared with the extravascular contribution obtained by Monte Carlo simulations. RESULTS: The MIRACLE-R2 values showed a characteristic increase with longer TRs in deoxygenated blood, corroborating that SE-R2 data cannot be used to assess the intravascular bSSFP BOLD effect. Passband bSSFP signal differences measured at optimal flip angles of 30° at 3 T and 20° at 9.4 T/14.1 T revealed considerable relative intravascular contributions of 95%/70% at 3 T, 74%/43% at 9.4 T, 66%/46% at 14.1 T for TR = 5 ms, and 90%/65% at 3 T, 36%/27% at 9.4 T, 13%/15% at 14.1 T for TR = 10 ms in macrovascular/microvascular regimes. CONCLUSION: The results indicate that intravascular effects have to be considered to better understand the origin of bSSFP BOLD contrast in functional MRI experiments, especially at short TRs. The MIRACLE-R2 method demonstrated the ability to quantify the apparent decrease in R2 due to rapid RF refocusing.

Quantitative and simultaneous measurement of oxygen consumption rates in rat brain and skeletal muscle using 17 O MRS imaging at 16.4T.

PURPOSE: Oxygen-17 (17 O) MRS imaging, successfully used in the brain, is extended by imaging the oxygen metabolic rate in the resting skeletal muscle and used to determine the total whole-body oxygen metabolic rate in the rat. METHODS: During and after inhalations of 17 O2 gas, dynamic 17 O MRSI was performed in rats (n = 8) ventilated with N2 O or N2 at 16.4 T. Time courses of the H217 O concentration from regions of interest located in brain and muscle tissue were examined and used to fit an animal-adapted 3-phase metabolic model of oxygen consumption. CBF was determined with an independent washout method. Finally, body oxygen metabolic rate was calculated using a global steady-state approach. RESULTS: Cerebral metabolic rate of oxygen consumption was 1.97 ± 0.19 µmol/g/min on average. The resting metabolic rate of oxygen consumption in skeletal muscle was 0.32 ± 0.12 µmol/g/min and >6 times lower than cerebral metabolic rate of oxygen consumption. Global oxygen consumed by the body was 24.2 ± 3.6 mL O2 /kg body weight/min. CBF was estimated to be 0.28 ± 0.02 mL/g/min and 0.34 ± 0.06 mL/g/min for the N2 and N2 O ventilation condition, respectively. CONCLUSION: We have evaluated the feasibility of 17 O MRSI for imaging and quantifying the oxygen consumption rate in low metabolizing organs such as the skeletal muscle at rest. Additionally, we have shown that CBF is slightly increased in the case of ventilation with N2 O. We expect this study to be beneficial to the application of 17 O MRSI to a wider range of organs, although further validation is advised.

Impact of prospective motion correction, distortion correction methods and large vein bias on the spatial accuracy of cortical laminar fMRI at 9.4 Tesla.

Functional imaging with sub-millimeter spatial resolution is a basic requirement for assessing functional MRI (fMRI) responses across different cortical depths and is used extensively in the emerging field of laminar fMRI. Such studies seek to investigate the detailed functional organization of the brain and may develop to a new powerful tool for human neuroscience. However, several studies have shown that measurement of laminar fMRI responses can be biased by the image acquisition and data processing strategies. In this work, measurements with three different gradient-echo EPI BOLD fMRI protocols with a voxel size down to 650 â€‹µm isotropic were performed at 9.4 â€‹T. We estimated how prospective motion correction can help to improve spatial accuracy by reducing the number of spatial resampling steps in postprocessing. In addition, we demonstrate key requirements for accurate geometric distortion correction to ensure that distortion correction maps are properly aligned to the functional data and that strong variations of distortions near large veins can lead to signal overlays which cannot be corrected for during postprocessing. Furthermore, this study illustrates the spatial extent of bias induced by pial and other larger veins in laminar BOLD experiments. Since these issues under investigation affect studies performed with more conventional spatial resolutions, the methods applied in this work may also help to improve the understanding of the BOLD signal more broadly.

Imaging Pulmonary Blood Flow Using Pseudocontinuous Arterial Spin Labeling (PCASL) With Balanced Steady-State Free-Precession (bSSFP) Readout at 1.5T.

BACKGROUND: Quantitative assessment of pulmonary blood flow and visualization of its temporal and spatial distribution without contrast media is of clinical significance. PURPOSE: To assess the potential of electrocardiogram (ECG)-triggered pseudocontinuous arterial spin labeling (PCASL) imaging with balanced steady-state free-precession (bSSFP) readout to measure lung perfusion under free-breathing (FB) conditions and to study temporal and spatial characteristics of pulmonary blood flow. STUDY TYPE: Prospective, observational. SUBJECTS: Fourteen volunteers; three patients with pulmonary embolism. FIELD STRENGTH/SEQUENCES: 1.5T, PCASL-bSSFP. ASSESSMENT: The pulmonary trunk was labeled during systole. The following examinations were performed: 1) FB and timed breath-hold (TBH) examinations with a postlabeling delay (PLD) of 1000 msec, and 2) TBH examinations with multiple PLDs (100-1500 msec). Scan-rescan measurements were performed in four volunteers and one patient. Images were registered and the perfusion was evaluated in large vessels, small vessels, and parenchyma. Mean structural similarity indices (MSSIM) was computed and time-to-peak (TTP) of parenchymal perfusion in multiple PLDs was evaluated. Image quality reading was performed with three independent blinded readers. STATISTICAL TESTS: Wilcoxon test to compare MSSIM, perfusion, and Likert scores. Spearman's correlation to correlate TTP and cardiac cycle duration. The repeatability coefficient (RC) and within-subject coefficient of variation (wCV) for scan-rescan measurements. Intraclass correlation coefficient (ICC) for interreader agreement. RESULTS: Image registration resulted in a significant (P < 0.05) increase of MSSIM. FB perfusion values were 6% higher than TBH (3.28 ± 1.09 vs. 3.10 ± 0.99 mL/min/mL). TTP was highly correlated with individuals' cardiac cycle duration (Spearman = 0.89, P < 0.001). RC and wCV were better for TBH than FB (0.13-0.19 vs. 0.47-1.54 mL/min/mL; 6-7 vs. 19-60%). Image quality was rated very good, with ICCs 0.71-0.89. DATA CONCLUSION: ECG-triggered PCASL-bSSFP imaging of the lung at 1.5T can provide very good image quality and quantitative perfusion maps even under FB. The course of labeled blood through the lung shows a strong dependence on the individuals' cardiac cycle duration. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY STAGE: 2 J. MAGN. RESON. IMAGING 2020;52:1767-1782.

Quantification of hydroxyl exchange of D-Glucose at physiological conditions for optimization of glucoCEST MRI at 3, 7 and 9.4 Tesla.

AIMS: To determine individual glucose hydroxyl exchange rates at physiological conditions and use this information for numerical optimization of glucoCEST/CESL preparation. To give guidelines for in vivo glucoCEST/CESL measurement parameters at clinical and ultra-high field strengths. METHODS: Five glucose solution samples at different pH values were measured at 14.1 T at various B1 power levels. Multi-B1 -Z-spectra Bloch-McConnell fits at physiological pH were further improved by the fitting of Z-spectra of five pH values simultaneously. The obtained exchange rates were used in a six-pool Bloch-McConnell simulation including a tissue-like water pool and semi-solid MT pool with different CEST and CESL presaturation pulse trains. In vivo glucose injection experiments were performed in a tumor mouse model at 7 T. RESULTS AND DISCUSSION: Glucose Z-spectra could be fitted with four exchanging pools at 0.66, 1.28, 2.08 and 2.88 ppm. Corresponding hydroxyl exchange rates could be determined at pH = 7.2, T = 37°C and 1X PBS. Simulation of saturation transfer for this glucose system in a gray matter-like and a tumor-like system revealed optimal pulses at different field strengths of 9.4, 7 and 3 T. Different existing sequences and approaches are simulated and discussed. The optima found could be experimentally verified in an animal model at 7 T. CONCLUSION: For the determined fast exchange regime, presaturation pulses in the spin-lock regime (long recover time, short yet strong saturation) were found to be optimal. This study gives an estimation for optimization of the glucoCEST signal in vivo on the basis of glucose exchange rate at physiological conditions.

In-vivo quantitative structural imaging of the human midbrain and the superior colliculus at 9.4T.

We explored anatomical details of the superior colliculus (SC) by in vivo magnetic resonance imaging (MRI) at 9.4T. The high signal-to-noise ratio allowed the acquisition of high resolution, multi-modal images with voxel sizes ranging between 176 × 132 × 600 µm and (800)3µm. Quantitative mapping of the longitudinal relaxation rate R1, the effective transverse relaxation rate R2*, and the magnetic susceptibility QSM was performed in 14 healthy volunteers. The images were analyzed in native space as well as after normalization to a common brain space (MNI). The coefficient-of-variation (CoV) across subjects was evaluated in prominent regions of the midbrain, reaching the best reproducibility (CoV of 5%) in the R2* maps of the SC in MNI space, while the CoV in the QSM maps remained high regardless of brain-space. To investigate whether more complex neurobiological architectural features could be detected, depth profiles through the SC layers towards the red nucleus (RN) were evaluated at different levels of the SC along the rostro-caudal axis. This analysis revealed alterations of the quantitative MRI parameters concordant with previous post mortem histology studies of the cyto- and myeloarchitecture of the SC. In general, the R1 maps were hyperintense in areas characterized by the presence of abundant myelinated fibers, and likely enabled detection of the deep white layer VII of the SC adjacent to the periaqueductal gray. While R1 maps failed to reveal finer details, possibly due to the relatively coarse spatial sampling used for this modality, these could be recovered in R2* maps and in QSM. In the central part of the SC along its rostro-caudal axis, increased R2* values and decreased susceptibility values were observed 2 mm below the SC surface, likely reflecting the myelinated fibers in the superficial optic layer (layer III). Towards the deeper layers, a second increase in R2* was paralleled by a paramagnetic shift in QSM suggesting the presence of an iron-rich layer about 3 mm below the surface of the SC, attributed to the intermediate gray layer (IV) composed of multipolar neurons. These results dovetail observations in histological specimens and animal studies and demonstrate that high-resolution multi-modal MRI at 9.4T can reveal several microstructural features of the SC in vivo.

Fast and efficient free induction decay MR spectroscopic imaging of the human brain at 9.4 Tesla.

PURPOSE: The purpose of this work was to develop a fast and efficient MRSI-FID acquisition scheme and test its performance in vivo. The aim was to find a trade-off between the minimal total acquisition time and signal-to-noise ratio of the acquired spectra. METHODS: Measurements were performed on a 9.4 Tesla system. Sequence optimization included redesign of water suppression, optimization of the sequence gradients, and improvement of the sampling efficiency by minimizing the read-out time. This resulted in an acquisition time of 2:47 and 22:13 minutes for 2D (TR = 57 ms; 3-mm in-plane resolution) and 3D MRSI (TR = 57 ms; 16 slices; 3-mm isotropic resolution), respectively. RESULTS: Despite strong T1 weighting and first-order phase problems, it was possible to obtain spectra of an acceptable quality. The average line width calculated for the tCr peak across the entire field of view was 26.9 ± 9.6 Hz for 2D and 30.0 ± 11.3 Hz for 3D MRSI. In 3D measurements, the percent fraction of voxels fitted with Cramer-Rao lower bounds below 10% was 53.3 ± 4.1%, 63.4 ± 8.4%, and 81.0 ± 2.9% for Glu, tCr, and tNAA, respectively. CONCLUSION: Considering the typically long duration of high-resolution MRSI, the proposed technique may be of interest for clinical applications and/or studies that focus on following the biochemistry of dynamic processes. Magn Reson Med 78:1281-1295, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Signal-to-noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 tesla using current receive coil arrays.

PURPOSE: Relaxation times, transmit homogeneity, signal-to-noise ratio (SNR) and parallel imaging g-factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight-fitting coil arrays. METHODS: The same human subjects were scanned at all three field strengths, using identical sequence parameters and similar 31- or 32-channel receive coil arrays. The SNR of three-dimensional (3D) gradient echo images was determined using a multiple replica approach and corrected with measured flip angle and T2 (*) distributions and the T1 of white matter to obtain the intrinsic SNR. The g-factor maps were derived from 3D gradient echo images with several GRAPPA accelerations. RESULTS: As expected, T1 values increased, T2 (*) decreased and the B1 -homogeneity deteriorated with increasing field. The SNR showed a distinctly supralinear increase with field strength by a factor of 3.10 ± 0.20 from 3T to 7T, and 1.76 ± 0.13 from 7T to 9.4T over the entire cerebrum. The g-factors did not show the expected decrease, indicating a dominating role of coil design. CONCLUSION: In standard experimental conditions, SNR increased supralinearly with field strength (SNR ∼ B0 (1.65) ). To take full advantage of this gain, the deteriorating B1 -homogeneity and the decreasing T2 (*) have to be overcome.

Combination of a multimode antenna and TIAMO for traveling-wave imaging at 9.4 Tesla.

PURPOSE: To investigate the performance of a multimode antenna combined with time-interleaved acquisition of modes (TIAMO) for improved (1)H image homogeneity as compared to conventional traveling-wave imaging in the human brain at 9.4 Tesla (T). METHODS: An adjustable three-port antenna was built to stimulate the propagation of three basic waveguide modes within a 9.4 T scanner bore. For TIAMO, two time-interleaved acquisitions using different linear combinations of these modes were optimized to achieve a homogeneous rooted sum-of-squares combination of their B1+ patterns ( B1,RSS+). The antenna's transmit and receive performance, as well as local specific absorption rate, were analyzed using experiments and numerical simulations. RESULTS: The optimized TIAMO B1,RSS+ combination was superior to radiofrequency shimming. Across the entire brain, it improved the homogeneity of the excitation field by a factor of two and its maximum-to-minimum ratio by almost a factor of five as compared to the circularly polarized mode. The two-fold increase in "virtual" receive channels enhanced the parallel imaging performance and enabled the use of higher acceleration factors. CONCLUSION: Despite the limited number of channels, a remote three-port antenna combined with TIAMO represents an easily implementable setup to achieve void-free (1)H images from the entire brain at 9.4 T, which can be used for anatomical localization and B0 shimming.

Quantitative and functional pulsed arterial spin labeling in the human brain at 9.4 T.

PURPOSE: The feasibility of multislice pulsed arterial spin labeling (PASL) of the human brain at 9.4 T was investigated. To demonstrate the potential of arterial spin labeling (ASL) at this field strength, quantitative, functional, and high-resolution (1.05 × 1.05 × 2 mm(3)) ASL experiments were performed. METHODS: PASL was implemented using a numerically optimized adiabatic inversion pulse and presaturation scheme. Quantitative measurements were performed at 3 T and 9.4 T and evaluated on a voxel-by-voxel basis. In a functional experiment, activation maps obtained with a conventional blood-oxygen-level-dependent (BOLD)-weighted sequence were compared with a functional ASL (fASL) measurement. RESULTS: Quantitative measurements revealed a 23% lower perfusion in gray matter and 17% lower perfusion in white matter at 9.4 T compared with 3 T. Furthermore almost identical transit delays and bolus durations were found at both field strengths whereas the calculated voxel volume corrected signal-to-noise ratio was 1.9 times higher at 9.4 T. This result was confirmed by the high-resolution experiment. The functional experiment yielded comparable activation maps for the fASL and BOLD measurements. CONCLUSION: Although PASL at ultrahigh field strengths is limited by high specific absorption rate, functional and quantitative perfusion-weighted images showing a high degree of detail can be obtained.

Three-layered radio frequency coil arrangement for sodium MRI of the human brain at 9.4 Tesla.

PURPOSE: A multinuclei imaging setup with the capability to acquire both sodium ((23) Na) and proton ((1) H) signals at 9.4 Tesla is presented. The main objective was to optimize coil performance at the (23) Na frequency while still having the ability to acquire satisfactory (1) H images. METHODS: The setup consisted of a combination of three radio frequency (RF) coils arranged in three layers: the innermost layer was a 27-channel (23) Na receive helmet which was surrounded by a four-channel (23) Na transceiver array. The outer layer consisted of a four-channel (1) H dipole array for B0 shimming and anatomical localization. Transmit and receive performance of the (23) Na arrays was compared to a single-tuned (23) Na birdcage resonator. RESULTS: While the transmit efficiency of the (23) Na transceiver array was comparable to the birdcage, the (23) Na receive array provided substantial signal-to-noise ratio (SNR) gain near the surface and comparable SNR in the center. The utility of this customized setup was demonstrated by (23) Na images of excellent quality. CONCLUSION: High SNR, efficient transmit excitation and B0 shimming capability can be achieved for (23) Na MRI at 9.4T using novel coil combination. This RF configuration is easily adaptable to other multinuclei applications at ultra high field (≥ 7T).

(17)O relaxation times in the rat brain at 16.4 tesla.

PURPOSE: Measurement of the cerebral metabolic rate of oxygen (CMRO2 ) by means of direct imaging of the (17) O signal can be a valuable tool in neuroscientific research. However, knowledge of the longitudinal and transverse relaxation times of different brain tissue types is required, which is difficult to obtain because of the low sensitivity of natural abundance H2 (17) O measurements. METHODS: Using the improved sensitivity at a field strength of 16.4 Tesla, relaxation time measurements in the rat brain were performed in vivo and postmortem with relatively high spatial resolutions, using a chemical shift imaging sequence. RESULTS: In vivo relaxation times of rat brain were found to be T1 = 6.84 ± 0.67 ms and T2 * = 1.77 ± 0.04 ms. Postmortem H2 (17) O relaxometry at enriched concentrations after inhalation of (17) O2 showed similar T2 * values for gray matter (1.87 ± 0.04 ms) and white matter, significantly longer than muscle (1.27 ± 0.05 ms) and shorter than cerebrospinal fluid (2.30 ± 0.16 ms). CONCLUSION: Relaxation times of brain H2 (17) O were measured for the first time in vivo in different types of tissues with high spatial resolution. Because the relaxation times of H2 (17) O are expected to be independent of field strength, our results should help in optimizing the acquisition parameters for experiments also at other MRI field strengths.

Safety testing and operational procedures for self-developed radiofrequency coils.

The development of novel radiofrequency (RF) coils for human ultrahigh-field (≥7 T), non-proton and body applications is an active field of research in many MR groups. Any RF coil must meet the strict requirements for safe application on humans with respect to mechanical and electrical safety, as well as the specific absorption rate (SAR) limits. For this purpose, regulations such as the International Electrotechnical Commission (IEC) standard for medical electrical equipment, vendor-suggested test specifications for third party coils and custom-developed test procedures exist. However, for higher frequencies and shorter wavelengths in ultrahigh-field MR, the RF fields may become extremely inhomogeneous in biological tissue and the risk of localized areas with elevated power deposition increases, which is usually not considered by existing safety testing and operational procedures. In addition, important aspects, such as risk analysis and comprehensive electrical performance and safety tests, are often neglected. In this article, we describe the guidelines used in our institution for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures, including coil documentation, user training and regular quality assurance testing, which help to recognize and eliminate safety issues during coil design and operation. Although the procedure is generally applicable to all field strengths, specific requirements with regard to SAR-related safety and electrical performance at ultrahigh-field are considered. The protocol describes an internal procedure and does not reflect consensus among a large number of research groups, but rather aims to stimulate further discussion related to minimum coil safety standards. Furthermore, it may help other research groups to establish their own procedures. Copyright © 2015 John Wiley & Sons, Ltd.