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MR spectroscopic imaging (MRSI) noninvasively maps the metabolism of human brains. In particular, the imaging of D-2-hydroxyglutarate (2HG) produced by glioma isocitrate dehydrogenase (IDH) mutations has become a key application in neuro-oncology. However, the performance of full field-of-view MRSI is limited by B0 spatial nonuniformity and lipid artifacts from tissues surrounding the brain. Array coils that multiplex RF-receive and B0 -shim electrical currents (AC/DC mixing) over the same conductive loops provide many degrees of freedom to improve B0 uniformity and reduce lipid artifacts. AC/DC coils are highly efficient due to compact design, requiring low shim currents (

Purpose To investigate inter‐subject variability of B 1 +, SAR and temperature rise in a database of human models using a local transmit array for 7 T cardiac imaging. Methods Dixon images were acquired of 14 subjects and segmented in dielectric models with an eight‐channel local transmit array positioned around the torso for cardiac imaging. EM simulations were done to calculate SAR distributions. Based on the SAR distributions, temperature simulations were performed for exposure times of 6 min and 30 min. Peak local SAR and temperature rise levels were calculated for different RF shim settings. A statistical analysis of the resulting peak local SAR and temperature rise levels was performed to arrive at safe power limits. Results For RF shim vectors with random phase and uniformly distributed power, a safe average power limit of 35.7 W was determined (first level controlled mode). When RF amplitude and phase shimming was performed on the heart, a safe average power limit of 35.0 W was found. According to Pennes' model, our numerical study suggests a very low probability of exceeding the absolute local temperature limit of 40 °C for a total exposure time of 6 min and a peak local SAR of 20 W/kg. For a 30 min exposure time at 20 W/kg, it was shown that the absolute temperature limit can be exceeded in the case where perfusion does not change with temperature. Conclusion Safe power constraints were found for 7 T cardiac imaging with an eight‐channel local transmit array, while considering the inter‐subject variability of B 1 +, SAR and temperature rise., 14 subject specific dielectric body models were constructed based on dixon MRI scans. SAR and thermal simulations were performed in all these for a 7T body array of 8 fractionated dipole antennas to find distributions of peak SAR and temperature. Based on our model database, safe power limits were found for 7T cardiac imaging, while considering inter‐subject variation of SAR, temperature and B1+.

Fast, automatic shimming by mapping along projections (FASTMAP) is an elegant analytical method developed to quantify three-dimensional first- and second-order spherical harmonic B0 shapes along six one-dimensional column projections. The straightforward application of this theoretical concept to B0 shimming, however, neglects crucial aspects of sequence implementation and shim hardware, commonly necessitating multistep iterative adjustments. We demonstrate a software package, referred to as FASTMAP Shim Tool (FAMASITO), which is composed of a FASTMAP pulse sequence, automated calibration and analysis routines, and a script for automatically performing experiments. With FAMASITO we demonstrate optimal single-step adjustment of first- and second-order terms (with potential

Long acquisition times due to intrinsically low signal-to-noise and the need for highly homogeneous B(0) field make magnetic resonance spectroscopy (MRS) particularly susceptible to motion or scanner instability compared to MRI. Motion induced changes in both localization and shimming (i.e., B(0) homogeneity) degrade MRS data quality. To mitigate the effects of motion three approaches can be employed: 1) subject immobilization, 2) retrospective correction, and 3) prospective real-time correction using internal and/or external tracking methods. Prospective real-time correction methods can simultaneously update localization and the B(0) field to improve MRS data quality. While localization errors can be corrected with both internal (navigators) and external (optical camera, NMR probes) tracking methods, the B(0) field correction requires internal navigator methods to measure the B(0) field inside the imaged volume and the possibility to update the scanner shim hardware in real time. Internal and external tracking can rapidly update the MRS localization with sub-millimeter and sub-degree precision, while scanner frequency and 1(st) order shims of scanner hardware can be updated by internal methods every sequence repetition. These approaches are most well-developed for neuroimaging, for which rigid transformation is primarily applicable. Real-time correction greatly improves the stability of MRS acquisition and quantification as shown in clinical studies on subjects prone to motion, including children and patients with movement disorders, enabling robust measurement of metabolite signals including those with low concentrations, such as gamma-aminobutyric acid (GABA) and glutathione (GSH). Thus, motion correction is recommended for MRS users and calls for tighter integration and wider availability of such methods by MR scanner manufacturers.

Mescher-Garwood (MEGA) editing with spin echo full intensity acquired localization (MEGA-SPECIAL, MSpc) is a technique to acquire γ-aminobutyric acid (GABA) without macromolecule (MM) contamination at a TE of 68 ms. However, due to the requirement of multiple shot-localization, it is often susceptible to subject motion and B0 inhomogeneity. A method is presented for real-time shim and motion correction (ShMoCo) using volumetric navigators to correct for motion and motion-related B0 inhomogeneity during MSpc acquisition. A phantom experiment demonstrates that ShMoCo restores the GABA peak and improves spectral quality in the presence of motion and zero- and first-order shim changes. The ShMoCo scans were validated in three subjects who performed up-down and left-right head rotations. Qualitative assessment of these scans indicates effective reduction of subtraction artefacts and well edited GABA peaks, while quantitative analysis indicates superior fitting and spectral quality relative to scans with no correction. Copyright © 2015 John Wiley & Sons, Ltd.

To date, spatial encoding for MRI is based on linear X, Y and Z field gradients generated by dedicated X, Y and Z wire patterns. We recently introduced the dynamic multi-coil technique (DYNAMITE) for the generation of magnetic field shapes for biomedical MR applications from a set of individually driven localized coils. The benefits for B0 magnetic field homogenization have been shown, as well as proof of principle of radial and algebraic MRI. In this study the potential of DYNAMITE MRI is explored further and the first multi-slice MRI implementation in which all gradient fields are purely DYNAMITE based is presented. The obtained image fidelity is shown to be virtually identical to that of a conventional MRI system with dedicated X, Y and Z gradient coils. Comparable image quality is a milestone towards the establishment of fully functional DYNAMITE MRI (and shim) systems.

Reconstruction of MRI data is based on exact knowledge of all magnetic field dynamics, since the interplay of RF and gradient pulses generates the signal, defines the contrast and forms the basis of resolution in spatial and spectral dimensions. Deviations caused by various sources, such as system imperfections, delays, eddy currents, drifts or externally induced fields, can therefore critically limit the accuracy of MRI examinations. This is true especially at ultra-high fields, because many error terms scale with the main field strength, and higher available SNR renders even smaller errors relevant. Higher baseline field also often requires higher acquisition bandwidths and faster signal encoding, increasing hardware demands and the severity of many types of hardware imperfection. To address field imperfections comprehensively, in this work we propose to expand the concept of magnetic field monitoring to also encompass the recording of RF fields. In this way, all dynamic magnetic fields relevant for spin evolution are covered, including low- to audio-frequency magnetic fields as produced by main magnets, gradients and shim systems, as well as RF pulses generated with single- and multiple-channel transmission systems. The proposed approach permits field measurements concurrently with actual MRI procedures on a strict common time base. The combined measurement is achieved with an array of miniaturized field probes that measure low- to audio-frequency fields via (19) F NMR and simultaneously pick up RF pulses in the MRI system's (1) H transmit band. Field recordings can form the basis of system calibration, retrospective correction of imaging data or closed-loop feedback correction, all of which hold potential to render MRI more robust and relax hardware requirements. The proposed approach is demonstrated for a range of imaging methods performed on a 7 T human MRI system, including accelerated multiple-channel RF pulses. Copyright © 2015 John Wiley & Sons, Ltd.

Spectral degradations as a result of temporal field variations are observed in MRSI of the human prostate. Moving organs generate substantial temporal and spatial field fluctuations as a result of susceptibility mismatch with the surrounding tissue (i.e. periodic breathing, cardiac motion or random bowel motion). Nine patients with prostate cancer were scanned with an endorectal coil (ERC) on a 7-T MR scanner. Temporal B0 field variations were observed with fast dynamic B0 mapping in these patients. Simulations of dynamic B0 corrections were performed using zero- to second-order shim terms. In addition, the temporal B0 variations were applied to simulated MR spectra causing, on average, 15% underestimation of the choline/citrate ratio. Linewidth distortions and frequency shifts (up to 30 and 8 Hz, respectively) were observed. To demonstrate the concept of observing local field fluctuations in real time during MRSI data acquisition, a field probe (FP) tuned and matched for the 19 F frequency was incorporated into the housing of the ERC. The data acquired with the FP were compared with the B0 field map data and used to correct the MRSI datasets retrospectively. The dynamic B0 mapping data showed variations of up to 30 Hz (0.1 ppm) over 72 s at 7 T. The simulated zero-order corrections, calculated as the root mean square, reduced the standard deviation (SD) of the dynamic variations by an average of 41%. When using second-order corrections, the reduction in the SD was, on average, 56%. The FP data showed the same variation range as the dynamic B0 data and the variation patterns corresponded. After retrospective correction, the MRSI data showed artifact reduction and improved spectral resolution. B0 variations can degrade the MRSI substantially. The simple incorporation of an FP into an ERC can improve prostate cancer MRSI without prior knowledge of the origin of the dynamic field distortions. Copyright © 2014 John Wiley & Sons, Ltd.

The in vivo rat model is a workhorse in neuroscience research, preclinical studies and drug development. A repertoire of MR tools has been developed for its investigation; however, high levels of B0 magnetic field homogeneity are required for meaningful results. The homogenization of magnetic fields in the rat brain, i.e. shimming, is a difficult task because of a multitude of complex, susceptibility-induced field distortions. Conventional shimming with spherical harmonic (SH) functions is capable of compensating for shallow field distortions in limited areas, e.g. in the cortex, but performs poorly in difficult-to-shim subcortical structures or for the entire brain. Based on the recently introduced multi-coil approach for magnetic field modeling, the DYNAmic Multi-coIl TEchnique (DYNAMITE) is introduced for magnetic field shimming of the in vivo rat brain and its benefits for gradient-echo echo-planar imaging (EPI) are demonstrated. An integrated multi-coil/radiofrequency (MC/RF) system comprising 48 individual localized DC coils for B0 shimming and a surface transceive RF coil has been developed that allows MR investigations of the anesthetized rat brain in vivo. DYNAMITE shimming with this MC/RF set-up is shown to reduce the B0 standard deviation to a third of that achieved with current shim technology employing static first- through third-order SH shapes. The EPI signal over the rat brain increased by 31%, and a 24% gain in usable EPI voxels could be realized. DYNAMITE shimming is expected to critically benefit a wide range of preclinical and neuroscientific MR research. Improved magnetic field homogeneity, together with the achievable large brain coverage of this method, will be crucial when signal pathways, cortical circuitry or the brain's default network are studied. Together with the efficiency gains of MC-based shimming compared with SH approaches demonstrated recently, DYNAMITE shimming has the potential to replace conventional SH shim systems in small-bore animal scanners.

In this study, mapping of ultrashort T2 and T2* of acutely isolated porcine menisci at B0 = 9.4 T was investigated. Maps of T2 were measured from a slice through the pars intermedia with a spin echo-prepared two-dimensional ultrashort-TE T2 mapping technique published previously. T2* mapping was performed by two-dimensional ultrashort-TE MRI with variable acquisition delay. The measured signal decays were fitted by monoexponential, biexponential and Gaussian-exponential fitting functions. The occurrence of Gaussian-like signal decays is outlined theoretically. The quality of the curve fits was visualized by mapping the value δ = abs(1 – χ2red). For T2* mapping, the Gaussian-exponential fit showed the best performance, whereas the monoexponential and biexponential fits showed regionally high values of δ (δ > 20). Interpretation of the Gaussian-exponential parameter maps was found to be difficult, because a Gaussian signal component can be related to mesoscopic (collagen texture) or macroscopic (slice profile, shim, sample geometry) magnetic field inhomogeneities and/or residual 1H dipole–dipole couplings. It seems likely that an interplay of these effects yielded the observed signal decays. Modulation of the T2* signal decay caused by chemical shift was observed and addressed to fat protons by means of histology. In the T2 measurements, no modulation of the signal decay was observed and the biexponential and Gaussian-exponential fits showed the best performance with comparable values of δ. Our results suggest that T2 mapping provides the more robust method for the characterization of meniscal tissue by means of MRI relaxometry. However, mapping of ultrashort T2, as performed in this study, is time consuming and provides less signal-to-noise ratio per time than the mapping of T2*. If T2* mapping is used, pixel-wise monitoring of the fitting quality based on reduced χ2 should be employed and great care should be taken when interpreting the parameter maps of the fits. Copyright © 2013 John Wiley & Sons, Ltd.

¹H MRS allows insight into the chemical composition of the central nervous system. However, as a result of technical challenges, it has rarely been applied to the spinal cord. In particular, the strong susceptibility changes around the spinal cord and the pulsatile flow of the cerebrospinal fluid lead to distinct B₀ field distortions which often considerably degrade the spectral quality. Hence, B₀ shimming is one of the main challenges in ¹H MRS of the spinal cord. Electrocardiogram (ECG)-triggered, higher order, projection-based B₀ shimming was introduced and compared with both conventional projection-based B₀ shimming and B₀ shimming based on ECG-triggered, three-dimensional B₀ field mapping. The linewidth of the unsuppressed water peak was used to evaluate the reproducibility and the potential improvement to B₀ homogeneity. The use of ECG-triggered projection-based B₀ shimming in combination with ECG triggering during preparation phases and triggering during acquisition of the spectra is the most robust method and thus helps to improve the spectral quality for MRS of the spinal cord.

A method is presented to correct the effects of motion and motion-related B0 perturbations on spectroscopic imaging in real-time through the use of a volumetric navigator (vNav). It is demonstrated that for an axial slice, lifting the chin significantly disrupts the B0 homogeneity in the zero-order (frequency), first-order Y (coronal) axis, and in the second-order ZY term. This vNav is able to measure and correct in real time both head pose and zero- to first-order B0 inhomogeneities. The vNav has been validated in six volunteers who deliberately lifted and then dropped their chin during the scan. These scans show that motion correction alone is not enough to recover the spectral quality. By applying real-time shim adjustments, spectral quality was fully recovered to line widths under 0.08 ppm and SNR to within acceptable limits in five out of six subjects. In the sixth subject 83% of the spectra within the VOI were recovered, compared to the worst case non-shim corrected scan where none of the voxels fell within these quality bounds. It is shown that the use of a vNav comes at no additional cost to the scan time or spectral SNR.

A 15-channel transmit–receive (transceive) radiofrequency (RF) coil was developed to image the human brain at 7 T. A hybrid decoupling scheme was implemented that used both capacitive decoupling and the partial geometric overlapping of adjacent coil elements. The decoupling scheme allowed coil elements to be arrayed along all three Cartesian axes; this facilitated shimming of the transmit field, B, and parallel imaging acceleration along the longitudinal direction in addition to the standard transverse directions. Each channel was independently controlled during imaging using a 16-channel console and a 16 × 1-kW RF amplifier–matrix. The mean isolation between all combinations of coil elements was 18 ± 7 dB. After B shimming, the standard deviation of the transmit field uniformity was 11% in an axial plane and 32% over the entire brain superior to the mid-cerebellum. Transmit uniformity was sufficient to acquire fast spin echo images of this region of the brain with a single B shim solution. Signal-to-noise ratio (SNR) maps showed higher SNR in the periphery vs center of the brain, and higher SNR in the occipital and temporal lobes vs the frontal lobe. Parallel imaging acceleration in a rostral–caudal oblique plane was demonstrated. The implication of the number of channels in a transmit–receive coil was discussed: it was determined that improvements in SNR and B shimming can be expected when using more than 15 independently controlled transmit–receive channels. Copyright © 2010 John Wiley & Sons, Ltd.

Subject motion during MRS investigations is a factor limiting the quality and the diagnostic value of the spectra. The possibility of using external motion tracking data to correct for artefacts in MR imaging has been demonstrated previously. In this paper the utility of prospective motion correction for single-voxel proton MRS is investigated. The object motion data are used in real time to update the position of the spectroscopy voxel during the acquisition prior to every sequence repetition cycle. It is not, however, sufficient to update the voxel position alone due to shim changes accompanying subject motion. Adverse effects of frequency shifts induced by subject motion are effectively suppressed by the interleaved reference scan method.

The spurious echo artefact, not uncommon in (1)H MRS in the brain, comes from refocusing outer volume signal. Application of MRS in small volumes in susceptibility-affected regions often results in large shim gradients. The artefact problem is accentuated when the global effect of the shim gradient shifts the water resonance outside the water suppression band in the outer volume. This scenario brings the issue of spurious echoes once again to the fore. In this paper, spurious signals of the point-resolved spectroscopy (PRESS) sequence are analysed using the concept of k-space. This new approach facilitates a more geometrical view of the problem, well suited for studying the effect of gradient spoiling and refocusing of signal. Several spoiling options are shown, and the probability of the global effects of shimming being a primary cause of the artefact is discussed. Fourier transform analysis of realistic slice profiles, combined with the k-space description of spurious echoes, shows that unsuppressed water signal in outer regions greatly increases the demands on spoiling. Gradient spoiling adequate for artefact suppression at a given size of MRS volume may not be sufficient at a smaller size. Several ways to improve PRESS measurements with regard to suppression of spurious signal are discussed.

T2* mapping offers access to a number of important structural and physiological tissue parameters. It is robust against RF field variations and overall signal scaling. However, T2* measurement is highly sensitive to magnetic field errors, including perturbations caused by breathing motion at high baseline field. The goal of this work is to assess this issue in T2* mapping of the brain and to study the benefit of field stabilization by feedback field control. T2* quantification in the brain was investigated by phantom and in vivo measurements at 7 T. Repeated measurements were made with and without feedback field control using NMR field sensing and dynamic third-order shim actuation. The precision and reliability of T2* quantification was assessed by studying variation across repeated measurements as well as fitting errors. Breathing effects were found to introduce significant error in T2* mapping results. Field control mitigates this problem substantially. In a phantom it virtually eliminates the effects of emulated breathing fluctuations in the head. In vivo it enhances the structural fidelity of T2* maps and reduces fitting residuals along with standard deviation. In conclusion, feedback field control improves the fidelity of T2* mapping in the presence of field perturbations. It is an effective means of countering bulk susceptibility effects of breathing and hence holds particular promise for efforts to leverage high field for T2* studies in vivo.

Ultra‐high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B1+/√P) in comparison with Tx‐only arrays, which are usually larger to fit receive (Rx)‐only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx‐only/Rx‐only RF system, a single‐row eight‐element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx‐only array, which could also be used in combination with an 18‐channel Rx‐only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B1+/√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.

Single-shot echo planar imaging (EPI) of a mouse brain at high field is very challenging. Large susceptibility-induced gradients affect much of the brain volume, causing severe image deformations and signal loss. Segmented EPI and other conventional multi-shot approaches alleviate these problems but suffer from lower temporal resolution and motion artifacts. We demonstrate that interleaved snapshot EPI represents a simple and robust alternative approach and one that is particularly suitable for high-field T2*-weighted functional imaging of a mouse brain. Similarly to segmented multi-shot techniques, it significantly reduces the susceptibility-related artifacts. At the same time, it preserves the high temporal resolution and the snapshot capability of a conventional EPI by acquiring entire image within a single TR period. We discuss implementation details of the interleaved snapshot EPI sequence and the trade-offs involved between the imaging efficiency, the number of interleaved excitation-acquisition blocks and the artifact reduction. To document the sequence utility, murine brain in vivo imaging with the interleaved snapshot EPI method was compared with a conventional EPI. We found that at least five interleaved blocks were necessary to restore the signal in most cortical areas. We also show that a standard global shimming procedure provides sufficient homogeneity for multi-slice interleaved snapshot EPI acquisition. In contrast, the conventional EPI of comparable image quality would be limited to a single slice with highly optimized local shim. Finally, an in vitro comparison with turbo FLASH acquisition shows the interleaved snapshot EPI to have superior time resolution and signal-to-noise ratio.

The aim of this work was to use post-processing methods to improve the data quality of metabolite maps acquired on the human brain at 9.4 T with accelerated acquisition schemes. This was accomplished by combining an improved sensitivity encoding (SENSE) reconstruction with a B0 correction of spatially over-discretized magnetic resonance spectroscopic imaging (MRSI) data. Since MRSI scans suffer from long scan duration, investigating different acceleration techniques has recently been the focus of several studies. Due to strong B0 inhomogeneity and strict specific absorption rate (SAR) limitations at ultra-high fields, the use of a low-SAR sequence combined with an acceleration technique that is compatible with dynamic B0 shim updating is preferable. Hence, in this study, a non-lipid-suppressed ultra-short TE and TR1 H free induction decay MRSI sequence is combined with an in-plane SENSE acceleration technique to obtain high-resolution metabolite maps in a clinically feasible scan time. One of the major issues in applying parallel imaging techniques to non-lipid-suppressed MRSI is the presence of strong lipid aliasing artifacts, which if not thoroughly resolved will hinder the accurate quantification of the metabolites of interest. To achieve a more robust reconstruction, an over-discretized SENSE reconstruction (with direct control over the shape of the spatial response function) was combined with an over-discretized B0 correction. This method is compared with conventional SENSE reconstruction for seven acceleration schemes on four healthy volunteers. The over-discretized method consistently outperformed conventional SENSE, resulting in an average of 23 ± 1.2% higher signal-to-noise ratio and 8 ± 2.9% less error in the fitting of the N-acetylaspartate signal over a whole brain slice. The highest achievable acceleration factor with the proposed technique was determined to be 4. Finally, using the over-discretized method, high-resolution (97 μL nominal voxel size) metabolite maps can be acquired in 3.75 min at 9.4 T. This enables the acquisition of high-resolution metabolite maps with more spatial coverage at ultra-high fields.