Search

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.

PURPOSE Investigation of the feasibility to perform high resolution quantitative sodium imaging at 9.4 Tesla (T). METHODS A proton patch antenna was combined with a sodium birdcage coil to provide a proton signal without compromising the efficiency of the X nucleus coil. Sodium density weighted images with a nominal resolution of 1 × 1 × 5 mm(3) were acquired within 30 min with an ultrashort echo time sequence. The methods used for signal calibration as well as for B0 B1 and off resonance correction were verified on a phantom and five healthy volunteers. RESULTS An actual voxel volume of roughly 40 µL could be achieved at 9.4T while maintaining an acceptable signal to noise ratio (8 for brain tissue and 35 for cerebrospinal fluid). The measured mean sodium concentrations for gray and white matter were 36?±?2 and 31?±?1 mmol/L of wet tissue which are comparable to values previously reported in the literature. CONCLUSION The reduction of partial volume effects is essential for accurate measurement of the sodium concentration in the human brain. Ultrahigh field imaging is a viable tool to achieve this goal due to its increased sensitivity. Magn Reson Med 2014. © 2014 Wiley Periodicals Inc.

Purpose Measurement of the cerebral metabolic rate of oxygen (CMRO2) by means of direct imaging of the 17O 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 H217O 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 H217O relaxometry at enriched concentrations after inhalation of 17O2 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 H217O were measured for the first time in vivo in different types of tissues with high spatial resolution. Because the relaxation times of H217O 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.

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. Magn Reson Med, 2015. (c) 2015 Wiley Periodicals, Inc.

Purpose To evaluate muscle perfusion in patients with peripheral arterial occlusive disease (PAOD) before and after percutaneous transluminal angioplasty (PTA) of the limb by means of MR arterial spin labeling (ASL) perfusion measurements during reactive hyperemia. Materials and Methods Ten patients with symptomatic PAOD affecting the iliac or femoral vessels were investigated before and after PTA. A pseudo-continuous arterial spin labeling (PCASL) MR technique was applied. Perfusion was measured in soleus and tibialis anterior muscle during reactive hyperemia. Key parameters such as mean perfusion value (Phyp), time-to-peak (TTP) and duration of hyperemia (Thyp) describing the perfusion signal curve were examined. Results Between baseline and post-PTA, Phyp increased in both muscle groups. At the same time, TTP and Thyp decreased in both muscle groups. At the same time the clinically assessed ankle brachial index (ABI) increased from 0.56thinsp;plusmn;thinsp;0.10 to 0.83thinsp;plusmn;thinsp;0.15. The impaired pain-free walking distance improved in all patients. Conclusion PCASL MRI can detect changes of the key perfusion parameters Phyp, TTP, and Thyp after successful PTA of the calf muscles during reactive hyperemia and seems to be a promising tool for monitoring of interventional treatments.

PurposeArranging transmit array elements in multiple rows provides an additional degree of freedom to correct B1+ field inhomogeneities and to achieve whole-brain excitation at ultrahigh field strengths. Receive arrays shaped to the contours of the anatomy increase the signal-to-noise ratio of the image. In this work, the advantages offered by the transmit and receive array techniques are combined for human brain imaging at 9.4 T.MethodsA 16-element dual-row transmit array and a 31-element receive array were developed. Based on an accurate numerical model of the transmit array, the deposited power was calculated for different head sizes and positions. The influence of the receive array on the transmit field was characterized. Parallel imaging performance and signal-to-noise ratio of the receive array were evaluated.ResultsOn average, a two fold increase in signal-to-noise ratio was observed in the whole-brain volume when compared with a 16-channel elliptic microstrip transceiver array. The benefits of combining the two arrays, B1+ shimming in three directions and high receive sensitivity, are demonstrated with high-resolution in vivo images.ConclusionThe dual-row transmit array provides whole-brain coverage at 9.4 T, which, in combination with the helmet-shaped receive array, is a valuable radio frequency configuration for ultra-high field magnetic resonance imaging of the human brain.

Um die räumliche und zeitliche Auflösung der Magnetresonanztomografie zu verbessern, werden immer höhere Magnetfeldstärken verwendet: So wird das zu detektierende Signal verstärkt. Hierfür müssen technologische Herausforderungen, wie zum Beispiel die Entwicklung neuartiger Radiofrequenzspulen, bewältigt werden. Erste klinische Studien mit selbstentwickelten Spulen wurden bereits an Tumoren im menschlichen Gehirn bei einer Feldstärke von 9,4 Tesla durchgeführt. Verglichen wurden die Spektren von gesundem und tumorösem Gewebe. Die Ergebnisse zeigen großes Potenzial für medizinische Anwendungen.

PURPOSE: The increased signal-to-noise ratio and blood oxygen level dependent signal at ultra-high field can only help to boost the resolution in functional MRI studies if the spatial specificity of the activation signal is improved. At a field strength of 9.4 T, both gradient-echo and spin-echo based echo-planar imaging were implemented and applied to investigate the specificity of human functional MRI. A finger tapping paradigm was used to acquire functional MRI data with scan parameters similar to standard neuroscientific applications. METHODS: Spatial resolution, echo, and readout times were varied to determine their influence on the distribution of the blood oxygen level dependent signal. High-resolution co-localized images were used to classify the signal according to its origin in veins or tissue. RESULTS: High-quality activation maps were obtained with both sequences. Signal contributions from tissue were found to be smaller or slightly larger than from veins. Gradient-echo echo-planar imaging yielded lower ratios of micro-/macro-vascular signals of around 0.6 than spin-echo based functional MRI, where this ratio varied between 0.75 and 1.02, with higher values for larger echo and shorter readout time. CONCLUSION: This study demonstrates the feasibility of human functional MRI at 9.4 T with high spatial specificity. Although venous contributions could not be entirely suppressed, venous effects in spin-echo echo-planar imaging are significantly reduced compared with gradient-echo echo-planar imaging.

Purpose: Subject motion can severely degrade MR images. A retrospective motion correction algorithm, Gradient-based motion correction, which significantly reduces ghosting and blurring artifacts due to subject motion was proposed. The technique uses the raw data of standard imaging sequences; no sequence modifications or additional equipment such as tracking devices are required. Rigid motion is assumed. Methods: The approach iteratively searches for the motion trajectory yielding the sharpest image as measured by the entropy of spatial gradients. The vast space of motion parameters is efficiently explored by gradient-based optimization with a convergence guarantee. Results: The method has been evaluated on both synthetic and real data in two and three dimentions using standard imaging techniques. MR images are consistently improved over different kinds of motion trajectories. Using a graphics processing unit implementation, computation times are in the order of a few minutes for a full three-dimentional volume. Conclusion: The presented technique can be an alternative or a complement to prospective motion correction methods and is able to improve images with strong motion artifacts from standard imaging sequences without requiring additional data.

The purpose of this study was to investigate the potential of a novel targeted contrast agent (CA) for the in vivo visualization of single native pancreatic islets, the sites of insulin production, in the pancreas of mice using magnetic resonance imaging (MRI). The CA for intravenous administration was composed of the β‐cell‐specific single‐chain antibody fragment, SCA B1, and ferromagnetic carbon‐coated cobalt nanoparticles. MRI experiments were performed at 7, 9.4 and 16.4 T in excised organs (pancreas, liver, kidney, spleen), at 7 T in mice fixed in formalin and at 9.4 and 16.4 T in living mice. Image contrast in untreated control animals was compared with images from mice treated with unspecific and specific CA. For the validation of MRI results, selected pancreases were subjected to immunohistochemical staining and numerical contrast simulations were performed. Ex vivo results and the outcome of immunohistochemistry suggest that islets are marked only by the CA containing SCA B1. Strong accumulation of particles was found also in other investigated organs owing to the uptake by the reticuloendothelial system, but the contrast in the MR images is clearly distinguishable from the islet specific contrast in pancreases and numerical predictions. In vivo experiments based on averaged dynamic sampling with 66 × 66 × 100 µm3 and triggered acquisition with 90 × 90 × 200 µm3 nominal resolution resulted in similar particle contrast to in in vitro measurements. The newly developed CA and MRI strategies have the potential to be used for studying mouse diabetes models by visualizing single native pancreatic islets.

Functional quantitative susceptibility mapping was performed on high resolution time-series acquired at 9.4T. Two alternative pulse sequences, two functional stimulation paradigms and three susceptibility mapping pipelines were evaluated. In addition to the conventional statistical parametric mapping of brain activation, also multivariate processing was performed in order to investigate the effect of physiological variations on the signal, and to learn about possible differences between these effects in the evolution of signal modulus and the evolution of calculated susceptibilities. The results provide useful information on the potential and the technical limitations of functional quantitative susceptibility mapping at 9.4T.

For human brain imaging at ultrahigh fields, the traveling wave concept can provide a more uniform B1+ field over a larger field of view with improved patient comfort compared to conventional volume coils. It suffers, however, from limited transmit efficiency and receive sensitivity and is not readily applicable in systems where the radiofrequency shield is too narrow to allow for unattenuated wave propagation. Here, the near field of a capacitively adjustable patch antenna for excitation is combined with a receive-only array at 9.4 T. The antenna is designed in compact size and placed in close proximity to the subject to improve the transmit efficiency in narrow bores. Experimental and numerical comparisons to conventional microstrip arrays reveal improved B1+ homogeneity and longitudinal coverage, but at the cost of elevated local specific absorption rate. High-resolution functional and anatomical images demonstrate the use of this setup for in vivo human brain imaging at 9.4 T.

With the increasing use of ultrahigh-field MR with multiple transmit channels, mapping of the B1+ field has become a critical factor in many studies, leading to the publication of a large number of sequences for the measurement of the flip angle in recent years. In this article, the accuracy, precision and practicability of some of the most prominent of these techniques are investigated both theoretically, using error propagation computations and Monte-Carlo simulations, and experimentally for different settings. For an exemplary experiment, which is typical for high-field applications, the flip angle uncertainty is calculated and measured for two- and three-dimensional acquisitions for techniques based on both magnitude and phase data. Simulated and measured results show good agreement. An experimental assessment of T1 and B0 dependence yields weak variations with these parameters for only a few of the sequences. Measurements on human scanners show crucial influences of specific absorption rate limitations, especially at ultrahigh field.

The accuracy and precision of the quantification of metabolite concentrations in in vivo1H NMR spectroscopy are affected by linewidth and signal-to-noise ratio (SNR). To study the effect of both factors in in vivo1H NMR spectra acquired at ultrahigh field, a reference spectrum was generated by summing nine in vivo1H NMR spectra obtained in rat brain with a STEAM sequence at 16.4 T. By progressive deterioration of linewidth and SNR, 6400 single spectra were generated. In an accuracy study, the variation in the mean concentrations of five metabolites was mainly dependent on SNR, whereas 11 metabolites were predominantly susceptible to the linewidth. However, the standard deviations of the concentrations obtained were dependent almost exclusively on the SNR. An insignificant correlation was found between most of the heavily overlapping metabolite peaks, indicating independent and reliable quantification. Two different approaches for the consideration of macromolecular signals were evaluated. The use of prior knowledge derived by parameterization of a metabolite-nulled spectrum demonstrated improved fitting quality, with reduced Cramér–Rao lower bounds, compared to the calculation of a regularized spline baseline.

For MRI at 16.4T, with a proton Larmor frequency of 698 MHz, one of the principal RF engineering challenges is to generate a spatially homogeneous transmit field over a larger volume of interest for spin excitation. Constructing volume coils large enough to house a receive array along with the subject and to maintain the quadrature symmetry for different loading conditions is difficult at this frequency. This calls for new approaches to RF coil design for ultra-high field MR systems. A remotely placed capacitively tunable patch antenna, which can easily be adjusted to different loading conditions, was used to generate a relatively homogeneous excitation field covering a large imaging volume with a transversal profile similar to that of a birdcage coil. Since it was placed in front of the animal, this created valuable free space in the narrow magnet bore around the subject for additional hardware. To enhance the reception sensitivity, the patch antenna was combined with an actively detunable 3-channel receive coil array. In addition to increased SNR compared to a quadrature transceive surface coil, we were able to get high quality gradient echo and spin-echo images covering the whole rat brain.

Localized in vivo1H NMR spectroscopy is playing an increasing role in preclinical studies, because of its ability to quantify the concentrations of up to 20 metabolites in rat brain. To assess the differences between often-used rat strains, the neurochemical profiles of Sprague-Dawley, Wistar and Fischer rats were determined at ultrashort TE at 16.4 T. To ascertain high-qualitative quantification, a first experiment examined the dependence of the measuring time on the quantification results and precision by precisely the number of averages between 16 and 320. It was shown that most metabolites can be quantified accurately within a short scan time, yielding Cramér–Rao lower bounds below 20 and stable concentrations for 16 metabolites with as few as 32 or 64 averages in the thalamus and hippocampus, respectively. Interstrain differences in metabolite concentrations were shown to be moderate, with taurine varying significantly between Sprague-Dawley and Wistar rats, and slightly more distinct differences from Fischer rats, including variations in glutamate and myo-inositol. The high spectral quality and quantification precision of all data again demonstrated the potential of in vivo1H NMR spectroscopy at ultrahigh field.

At the field strength of 9.4 T, the highest field currently available for human MRI, the wavelength of the MR signals is significantly shorter than the size of the examined structures. Even more than at 7 T, constructive and destructive interferences cause strong inhomogeneities of the B1 field produced by a volume coil, causing shading over large parts of the image. Specialized radio frequency hardware and B1 management methods are required to obtain high-quality images that take full advantage of the high field strength. Here, the design and characteristics of a radio frequency front-end especially developed for proton imaging at 9.4 T are presented. In addition to a 16-channel transceiver array coil, capable of volume transmit mode and independent signal reception, it consists of custom built low noise preamplifiers and TR switches. Destructive interference patterns were eliminated, in virtually the entire brain, using a simple in situ radio frequency phase shimming technique. After mapping the Bmath image profile of each transmit channel, a numerical algorithm was used to calculate the appropriate transmit phase offsets needed to obtain a homogeneous excitation field over a user defined region. Between two and three phase settings are necessary to obtain homogeneous images over the entire brain.

In vivo 1H NMR spectroscopy was used to obtain the neurochemical profile in the posterior parts of the brain, the cerebellum and the medulla oblongata in comparison to the hippocampus and the thalamus. Using small voxel sizes between 16 and 32 μl to avoid partial volume effects, most metabolites demonstrated significant regional differences except acetate, γ-aminobutyric acid, and phosphorylcholine. Noticeable regional differences in metabolite concentrations were the significant increase of total creatine in the cerebellum and the substantial decrease of taurine in thalamus and medulla oblongata. In particular, the glycine concentration in the medulla oblongata was determined to be 4.37 ± 0.68 μmol/g (Cramér-Rao lower bounds 7) and thus significantly higher than in the other regions, consistent with findings reported in both in vivo 1H NMR spectroscopy and in vitro biochemical assays. Intraindividual reproducibility and interindividual variability were investigated by acquiring spectra from the thalamus of the same rats in two sessions. No prominent influence on measurement session was observed in metabolite concentrations with coefficients of variations being below 20 in 16 metabolites.

The effect of susceptibility differences on an MR image is known to increase with field strength. Magnetic field inhomogeneities within the voxels influence the apparent transverse relaxation time T2*, while effects due to different precession frequencies between voxels caused by local field variations are evident in the image phase, and susceptibility-weighted imaging highlights the veins and deep brain structures. Here, these three contrast mechanisms are examined at a field strength of 9.4 T. The T2* maps generated allow the identification of white matter structures not visible in conventional images. Phase images with in-plane resolutions down to 130 amp;amp;956;m were obtained, showing high gray/white matter contrast and allowing the identification of internal cortical structures. The susceptibility-weighted images yield excellent visibility of small venous structures and attain an in-plane resolution of 175 mikro-m.

Die Magnetresonanz-Bildgebung beruht auf dem Verhalten von Atomkernen in einem starken Magnetfeld. Diese Felder erreichen für Anwendungen am Menschen bis zu 9,4 Tesla, an Tieren sogar über 20 T. Die Gründe für die Verwendung immer stärkerer Magneten liegen in der Steigerung der Signalstärke und damit der Ortsauflösung, in der Eröffnung neuer Kontrastmechanismen sowie in der besseren Quantifizierbarkeit von Metabolitenkonzentrationen in der MR-Spektroskopie. Aufgrund der geänderten physikalischen Eigenschaften im Ultrahochfeld müssen neue Messtechniken entwickelt werden.

Laminar-specific functional magnetic resonance imaging (fMRI) has been widely used to study circuit-specific neuronal activity by mapping spatiotemporal fMRI response patterns across cortical layers. Hemodynamic responses reflect indirect neuronal activity given limit of spatial and temporal resolution. Previous gradient-echo based line-scanning fMRI (GELINE) method was proposed with high temporal (50 ms) and spatial (50 µm) resolution to better characterize the fMRI onset time across cortical layers by employing 2 saturation RF pulses. However, the imperfect RF saturation performance led to poor boundary definition of the reduced region of interest (ROI) and aliasing problems outside of the ROI. Here, we propose α (alpha)-180 spin-echo-based line-scanning fMRI (SELINE) method to resolve this issue by employing a refocusing 180° RF pulse perpendicular to the excitation slice. In contrast to GELINE signals peaked at the superficial layer, we detected varied peaks of laminar-specific BOLD signals across deeper cortical layers with the SELINE method, indicating the well-defined exclusion of the large drain-vein effect with the spin-echo sequence. Furthermore, we applied the SELINE method with 200 ms TR to sample the fast hemodynamic changes across cortical layers with a less draining vein effect. In summary, this SELINE method provides a novel acquisition scheme to identify microvascular-sensitive laminar-specific BOLD responses across cortical depth.

[This corrects the article DOI: 10.1371/journal.pbio.3000923.]., (Copyright: © 2022 Chen et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)