Homogeneous B1 Field Research Articles

High-Bandwidth Q-Band EPR Resonators

The emerging technology of ultra-wide-band spectrometers in electron paramagnetic resonance—enabled by recent technological advances—provides the means for new experimental schemes, a broader range of samples, and huge gains in measurement time. Broadband detection does, however, require that the resonator provides sufficient bandwidth and, despite resonator compensation schemes, excitation bandwidth is ultimately limited by resonator bandwidth. Here, we present the design of three resonators for Q-band frequencies (33–36 GHz) with a larger bandwidth than what was reported so far. The new resonators are of a loop-gap type with 4–6 loops and were designed for 1.6 mm sample tubes to achieve higher field homogeneity than in existing resonators for 3 mm samples, a feature that is beneficial for precise spin control. The loop-gap design provides good separation of the B 1 and E field, enabling robust modes with powder samples as well as with frozen water samples as the resonant behavior is largely independent of the dielectric properties of the samples. Experiments confirm the trends in bandwidth and field strength and the increased B 1 field homogeneity predicted by the simulations. Variation of the position of the coupling rod allows the adjustment of the quality factor Q and thus the bandwidth over a broad range. The increased bandwidth of the loop-gap resonators was exploited in double electron–electron resonance measurements of a Cu(II)-PyMTA ruler to yield significantly higher modulation depth and thus higher sensitivity.

A target field design of open multi-purpose RF coil for musculoskeletal MR imaging at 3 T

Musculoskeletal MR imaging under multi-angle situations plays an increasingly important role in assessing joint and muscle tissues system. However, there are still limitations due to the closed structures of most conventional RF coils. In this study, a time-harmonic target-field method was employed to design open multi-purpose coil (OMC) for multi-angle musculoskeletal MR imaging. The phantom imaging results suggested that the proposed OMC could achieve homogeneously distributed magnetic field and high signal-to-noise ratio (SNR) of 239.04±0.83 in the region of interest (ROI). The maximum temperature in the heating hazard test was 16°C lower than the standard regulation, which indicated the security of the designed OMC. Furthermore, to demonstrate the effectiveness of the proposed OMC for musculoskeletal MR imaging, especially for multi-angle imaging, a healthy volunteer was examined for MR imaging of elbow, ankle and knee using OMC. The in vivo imaging results showed that the proposed OMC is effective for MR imaging of musculoskeletal tissues at different body parts, with satisfied B1 field homogeneity and SNR. Moreover, the open structure of the OMC could provide a large joint movement region. The proposed open multi-purpose coil is feasible for musculoskeletal MR imaging, and potentially, it is more suitable for the evaluation of musculoskeletal tissues under multi-angle conditions.

Design and Testing of Radiofrequency Spherical four Coils

<p>In this paper, we present the design and testing of a radiofrequency prototype coil with good performances in terms of B<sub>1</sub> magnetic field homogeneity and can be utilized for Magnetic Resonance Imaging. It is constituted of four coaxial separately tuned rings of wire and symmetrically located on a spherical surface. Compared to standard Helmholtz pair, which has 2nd-order magnetic field homogeneity, it yields to improvement in field homogeneity, while preserving the simplicity of design. The four coils of proposed structure are tuned to the same frequency. The proposed structure gets at 4th-order magnetic field homogeneity by optimizing the distance between rings and the diameters of outer loops. An electrical modeling of the four-coil system taking into account the coupling effects between all rings permits to determine the resonance frequency in the homogenous mode. Measurements of B1 field homogeneity were introduced in free space. Compared to the Helmholtz coil, the proposed structure presents good performances in terms of B1 homogeneity, quality factor and sensitivity. The design of proposed coil has been optimized for best SNR performances. Globally, this work claims to be a contribution to the study of the four-coil RF systems derived from the Helmholtz pairs.</p>

Assessment of MR-compatibility of SiPM PET insert using shortoptical fiber bundles for small animal research

Simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) can provide new perspectives in human disease research because of their complementary in-vivo imaging techniques. Previously, we have developed an MR-compatible PET insert based on optical fibers using silicon photomultipliers (SiPM). However when echo planar imaging (EPI) sequence was performed, signal intensity was slowly decreased by −0.9% over the 5.5 minutes and significant geometrical distortion was observed as the PET insert was installed inside an MRI bore, indicating that the PET electronics and its shielding boxes might have been too close to an MR imaging object. In this paper, optical fiber bundles with a length of 54 mm instead of 31 mm were employed to minimize PET interference on MR images. Furthermore, the LYSO crystals with a size of 1.5 × 1.5 × 7.0 mm3 were used instead of 2.47 × 2.74 × 20.0 mm3 for preclinical PET/MR applications. To improve the MR image quality, two receive-only loop coils were used. The effects of the PET insert on the SNR of the MR image either for morphological or advanced MR pulse sequences such as diffusion weighted imaging (DWI), functional MRI (fMRI), and magnetic resonance spectroscopy (MRS) were investigated. The quantitative MR compatibility such as B0 and B1 field homogeneity without PET, with `PET OFF', and with `PET ON' was also evaluated. In conclusion, B0 maps were not affected by the proposed PET insert whereas B1 maps were significantly affected by the PET insert. The advanced MRI sequences such as DWI, EPI, and MRS can be performed without a significant MR image quality degradation.

Real-time measurement and correction of both B0 changes and subject motion in diffusion tensor imaging using a double volumetric navigated (DvNav) sequence

Diffusion tensor imaging (DTI) requires a set of diffusion weighted measurements in order to acquire enough information to characterize local structure. The MRI scanner automatically performs a shimming process by acquiring a field map before the start of a DTI scan. Changes in B0, which can occur throughout the DTI acquisition due to several factors (including heating of the iron shim coils or subject motion), cause significant signal distortions that result in warped diffusion tensor (DT) parameter estimates.In this work we introduce a novel technique to simultaneously measure, report and correct in real time subject motion and changes in B0 field homogeneity, both in and through the imaging plane. This is achieved using double volumetric navigators (DvNav), i.e. a pair of 3D EPI acquisitions, interleaved with the DTI pulse sequence. Changes in the B0 field are evaluated in terms of zero-order (frequency) and first-order (linear gradients) shim. The ability of the DvNav to accurately estimate the shim parameters was first validated in a water phantom. Two healthy subjects were scanned both in the presence and absence of motion using standard, motion corrected (single navigator, vNav), and DvNav DTI sequences. The difference in performance between the proposed 3D EPI field maps and the standard 3D gradient echo field maps of the MRI scanner was also evaluated in a phantom and two healthy subjects. The DvNav sequence was shown to accurately measure and correct changes in B0 following manual adjustments of the scanner's central frequency and the linear shim gradients. Compared to other methods, the DvNav produced DTI results that showed greater spatial overlap with anatomical references, particularly in scans with subject motion. This is largely due to the ability of the DvNav system to correct shim changes and subject motion between each volume acquisition, thus reducing shear distortion.

An RF breast coil for 0.2TMRI

ABSTRACTBreast cancer is the most frequently diagnosed cancer in women. High field studies have shown the diagnostic value of breast MRI, but the examination costs greatly exceed those of competing conventional mammography. Low field MRI offers typical MRI contrast at substantially lower cost, but has suffered from lower spatial resolution. Specificity of breast MRI can potentially be increased by acquiring MR imaging with higher spatial or temporal resolution, but the signal‐to‐noise ratio (SNR) achievable in a given imaging time becomes limiting. SNR for the particular pulse sequence and magnet field strength is strongly influenced by the characteristics of the radio‐frequency coil. An optimal breast coil should yield excellent SNR but also generate a homogeneous B1 field, while allowing imaging of the both breasts simultaneously and maintaining patient comfort. RF receiver coil design is a key determinant of image quality, thus to address this we have designed and constructed a low field breast imaging coil. The coil was tested with a 4‐post 0.2T MRI providing high quality breast images. Designed and constructed saddle rf coil allows to obtain good quality image of the breast using low 0.2 T MRI system within 2 minutes. The coil provides patient comfort as breast compression is not required and minimizes artefacts caused by respiration or motion. A high contrast, low‐cost and pain‐free breast examination using optimized low field MRI system has the potential to serve a large patient population for whom current technologies have deficiencies. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 3–7, 2016

MR-compatibility assessment of the first preclinical PET-MRI insert equipped with digital silicon photomultipliers

PET (positron emission tomography) with its high sensitivity in combination with MRI (magnetic resonance imaging) providing anatomic information with good soft-tissue contrast is considered to be a promising hybrid imaging modality. However, the integration of a PET detector into an MRI system is a challenging task since the MRI system is a sensitive device for external disturbances and provides a harsh environment for electronic devices. Consequently, the PET detector has to be transparent for the MRI system and insensitive to electromagnetic disturbances. Due to the variety of MRI protocols imposing a wide range of requirements regarding the MR-compatibility, an extensive study is mandatory to reliably assess worst-case interference phenomena between the PET detector and the MRI scanner. We have built the first preclinical PET insert, designed for a clinical 3 T MRI, using digital silicon photomultipliers (digital SiPM, type DPC 3200-22, Philips Digital Photon Counting). Since no thorough interference investigation with this new digital sensor has been reported so far, we present in this work such a comprehensive MR-compatibility study.Acceptable distortion of the B0 field homogeneity (volume RMS = 0.08 ppm, peak-to-peak value = 0.71 ppm) has been found for the PET detector installed. The signal-to-noise ratio degradation stays between 2–15% for activities up to 21 MBq. Ghosting artifacts were only found for demanding EPI (echo planar imaging) sequences with read-out gradients in Z direction caused by additional eddy currents originated from the PET detector. On the PET side, interference mainly between the gradient system and the PET detector occurred: extreme gradient tests were executed using synthetic sequences with triangular pulse shape and maximum slew rate. Under this condition, a relative degradation of the energy (⩽10%) and timing (⩽15%) resolution was noticed. However, barely measurable performance deterioration occurred when morphological MRI protocols are conducted certifying that the overall PET performance parameters remain unharmed.

Effective arrangement of separated transmit-only/receive-only RF coil for improvement of B1 homogeneity at 7 Tesla

This article presents an effective arrangement with shifted transmit (Tx)-only and receive (Rx)-only (TORO) radiofrequency (RF) coils in a single-channel surface coil for improving the magnetic flux (B 1) homogeneity in an ultra-high field (UHF) magnetic resonance imaging (MRI) scanner. The proposed new methodology for the coil arrangement using the shifted TORO RF coils was demonstrated for coils with 50-mm, 100-mm, and 150-mm-square surfaces and the results were compared to those for general Tx/Rx surface coils with the same dimensions. The computational analysis indicated that a homogeneous B1 field was achieved when the Rx-only coil was shifted in the two-dimensional xy-plane away from the Tx-only coils. Because the proposed coil configuration provides a unique opportunity for increasing the B 1 homogeneity, this feature is likely to increase the feasibility via new coil arrangements of UHF surface design and fabrication.

Reduced dose uncertainty in MRI-based polymer gel dosimetry using parallel RF transmission with multiple RF sources

In this work, we present the feasibility of using a parallel RF transmit with multiple RF sources imaging method (MultiTransmit imaging) in polymer gel dosimetry. Image quality and B1 field homogeneity was statistically better in the MultiTransmit imaging method than in conventional single source RF transmission imaging method. In particular, the standard uncertainty of R 2 was lower on the MultiTransmit images than on the conventional images. Furthermore, the MultiTransmit measurement showed improved dose resolution. Improved image quality and B1 homogeneity results in reduced dose uncertainty, thereby suggesting the feasibility of MultiTransmit MR imaging in gel dosimetry.

Comparing Saddle, Slotted-tube and Parallel-plate Coils for Magnetic Resonance Imaging

Abstract The paper is concerned with a comparison of the properties of RF coils of three configurations for MRI measurements on small animals. In comparison with the classical saddle coil the proposed modification of slotted-tube coil exhibits identical homogeneity of B1 field in a larger space. The parallel-plate coil has a satisfactory homogeneity of B1 field over the whole internal space. The signal-to-noise ratio measured for all three coils is roughly the same and is given by the magnitude of RF pre-amplifier noise. As the slotted-tube and parallel-plate coils have a lower inductance compared with the saddle coil, they can be tuned to resonance on the 200 MHz frequency even at larger dimensions. The results show that the parallel-plate coil has very good properties for the measurement of small animals.

SU‐E‐J‐71: Feasibility Study On MRI‐Based BANG3 Gel Dosimetry Using Dual‐Source Parallel RF Transmission System

Purpose:In this work, we present the feasibility of use of the parallel RF transmission with multiple RF source (MultiTransmit) imaging in MRI‐based polymer gel dosimetry.Methods:The commercially available BANG3 gel was used for gel dosimetry. Spin‐spin relaxation rate R2 was used to quantify the absorbed dose. The image quality (signal‐to‐noise ratio, SNR; image uniformity) and B1 field inhomogeneity between conventional single‐source and MultiTransmit MR imaging were compared. Finally, the estimated R2 uncertainty σ(R2) and dosimetric performance (i.e., dose resolution) between conventional single‐source and MultiTransmit MR imaging were compared.Results:Image quality and B1 field homogeneity within each calibration vial and large phantom was statistically better in MultiTransmit imaging than in conventional single‐source RF transmission imaging (P < 0.005 for all calibration vials). In particular, σ(R2) (defined as the standard uncertainty of R2) was lower on the MultiTransmit images than on the conventional single‐source images. Furthermore, the MultiTransmit measurement gives a lower than that obtained using the conventional single‐source method.Conclusion:The improved image quality and B1 homogeneity resulted in reduced dose uncertainty (i.e., σ(R2) and dose resolution) in MRI‐based polymer gel dosimetry, suggesting that MultiTransmit MR imaging has potential benefits for use in clinical 3D gel dosimetry without the need for the complicated B1 field correction method.

Eight channel transmit array volume coil using on-coil radiofrequency current sources.

At imaging frequencies associated with high-field MRI, the combined effects of increased load-coil interaction and shortened wavelength results in degradation of circular polarization and B1 field homogeneity in the imaging volume. Radio frequency (RF) shimming is known to mitigate the problem of B1 field inhomogeneity. Transmit arrays with well decoupled transmitting elements enable accurate B1 field pattern control using simple, non-iterative algorithms. An eight channel transmit array was constructed. Each channel consisted of a transmitting element driven by a dedicated on-coil RF current source. The coil current distributions of characteristic transverse electromagnetic (TEM) coil resonant modes were non-iteratively set up on each transmitting element and 3T MRI images of a mineral oil phantom were obtained. B1 field patterns of several linear and quadrature TEM coil resonant modes that typically occur at different resonant frequencies were replicated at 128 MHz without having to retune the transmit array. The generated B1 field patterns agreed well with simulation in most cases. Independent control of current amplitude and phase on each transmitting element was demonstrated. The transmit array with on-coil RF current sources enables B1 field shimming in a simple and predictable manner.

Efficiency evaluation of a 13C Magnetic Resonance birdcage coil: Theory and comparison of four methods

Radiofrequency coils in Magnetic Resonance systems are used to produce a homogeneous B1 field for exciting the nuclei and to pick up the signals emitted by the nuclei with high signal-to-noise ratio. Accordingly, coil performance affects strongly the quality of the obtained data and images.Coil efficiency, defined as the B1 magnetic field induced at a given point on the square root of supplied power P, is an important parameter that characterizes coil performance, since by maximizing efficiency will also maximize the signal-to-noise ratio.This work describes and compares four methods for coil efficiency estimation, based on different theoretical approaches. Three methods allow efficiency measurement by using “probe techniques” (perturbing loop, perturbing sphere and pick-up coil), which can be used both on the bench and inside the scanner, while an “NMR technique” has been employed for comparison purpose.Methods were tested on a 13C birdcage coil tuned at 32.13MHz.

On the validity of 3D polymer gel dosimetry: III. MRI-related error sources**Both authors contributed equally to this study.

In MRI (PAGAT) polymer gel dosimetry, there exists some controversy on the validity of 3D dose verifications of clinical treatments. The relative contribution of important sources of uncertainty in MR scanning to the overall accuracy and precision of 3D MRI polymer gel dosimetry is quantified in this study. The performance in terms of signal-to-noise and imaging artefacts was evaluated on three different MR scanners (two 1.5 T and a 3 T scanner). These include: (1) B0-field inhomogeneity, (2) B1-field inhomogeneity, (3) dielectric effects (losses and standing waves) and (4) temperature inhomogeneity during scanning. B0-field inhomogeneities that amount to maximum 5 ppm result in dose deviations of up to 4.3% and deformations of up to 5 pixels. Compensation methods are proposed. B1-field inhomogeneities were found to induce R2 variations in large anthropomorphic phantoms both at 1.5 and 3 T. At 1.5 T these effects are mainly caused by the coil geometry resulting in dose deviations of up to 25%. After the correction of the R2 maps using a heuristic flip angle–R2 relation, these dose deviations are reduced to 2.4%. At 3 T, the dielectric properties of the gel phantoms are shown to strongly influence B1-field homogeneity, hence R2 homogeneity, especially of large anthropomorphic phantoms. The low electrical conductivity of polymer gel dosimeters induces standing wave patterns resulting in dose deviations up to 50%. Increasing the conductivity of the gel by adding NaCl reduces the dose deviation to 25% after which the post-processing is successful in reducing the remaining inhomogeneities caused by the coil geometry to within 2.4%. The measurements are supported by computational modelling of the B1-field. Finally, temperature fluctuations of 1 °C frequently encountered in clinical MRI scanners result in dose deviations up to 15%. It is illustrated that with adequate temperature stabilization, the dose uncertainty is reduced to within 2.58%.

A Stripline-Type Inductor Used for NMR Detection

This paper tries to design a stripline-type inductor with high sensitivity that is used for micro-fluid in downhole nuclear magnetic resonance (NMR) detection. It discusses the design theory for the probe, and does analysis and simulation to guide the optimal geometric parameters. Radiofrequency B1 field homogeneity is evaluated. Finally, the prototype is made.

Investigation of the Asymmetric Distributions of RF Transmission and Reception Fields at High Static Field

When radiofrequency (RF) transmission field represents B(1)(+), the reception field represents B(1)(-)*. The distribution of those maps demonstrates asymmetric features at high field magnetic resonance (MR) imaging. Both maps are in mirror symmetry to one another. Almost symmetric distribution of the B(1) field was expected on the laboratory frame in a symmetric sample loaded inside the RF coil designed to achieve a homogeneous B(1) field. Then, a simple change was made in the coordinate transformation equation of RF fields between the rotating and laboratory frames in both linear and quadrature modes to investigate the source of this feature of asymmetry. The magnitude of rotating frame components, B(1)(+) and B(1)(-), consists of the magnitude and the phase difference of the laboratory frame components. The rotating frame components differ in the sign of the sinusoidal phase difference. B(1)(+) is equal to B(1)(-) at lower field because phase changes that depend on position can be ignored. At higher fields, the magnitude component has a symmetric profile, and distribution in the phase component is antisymmetric. Thus, the distributions of B(1)(+) and B(1)(-) maps demonstrate mirror symmetry. Maps of magnitude and phase components were examined in the laboratory frame. Their maps were computed from B(1)(+) and B(1)(-) maps of the human brain and of a spherical saline phantom measured at 4.7T. It was concluded from these analytical and experimental results that the asymmetric and mirror symmetric distributions in B(1)(+) and B(1)(-) are derived from the phase difference in the laboratory frame.

A generalized strategy for designing 19F/1H dual‐frequency MRI coil for small animal imaging at 4.7 Tesla

To propose and test a universal strategy for building (19) F/(1) H dual-frequency RF coil that permits multiple coil geometries. The feasibility to design (19) F/(1) H dual-frequency RF coil based on coupled resonator model was investigated. A series capacitive matching network enables robust impedance matching for both harmonic oscillating modes of the coupled resonator. Two typical designs of (19) F/(1) H volume coils (birdcage and saddle) at 4.7T were implemented and evaluated with electrical bench test and in vivo (19) F/(1) H dual-nuclei imaging. For various combinations of internal resistances of the sample coil and secondary resonator, numerical solutions for the tunable capacitors to optimize impedance matching were obtained using a root-seeking program. Identical and homogeneous B1 field distribution at (19) F and (1) H frequencies were observed in bench test and phantom image. Finally, in vivo mouse imaging confirmed the sensitivity and homogeneity of the (19) F/(1) H dual-frequency coil design. A generalized strategy for designing (19) F/(1) H dual-frequency coils based on the coupled resonator approach was developed and validated. A unique feature of this design is that it preserves the B1 field homogeneity of the RF coil at both resonant frequencies. Thus it minimizes the susceptibility effect on image co-registration.

Dynamic B0 shimming at 7 T

Dynamic slice-wise shimming improves B0 field homogeneity by updating shim coil currents for every slice in a multislice acquisition, producing better field homogeneity over a volume than can be obtained by a single static global shim. The first aim of this work was to evaluate the performance of slice-wise field-map-based second-order dynamic shimming in a human high-field 7 T clinical scanner vis-à-vis image based second order static global shimming. Another goal was to characterize eddy currents induced by second and third order shim switching. A final aim was to compare global and dynamic shimming through shim orders to elucidate the relative benefits of going to higher orders and to dynamic shim updating from a static shimming regime. An external hardware module was used to store and dynamically update slice-optimized shim values during multislice data acquisition. High-bandwidth multislice gradient echo scans with B0 field mapping and low-bandwidth single-shot echo planar scans were performed on phantoms and humans using second-order dynamic and static global shims. For the measurement of second and third order shim induced eddy currents, step response temporal phase changes of individual shims were measured and fit to shim harmonics spatially and to multiexponential decay functions temporally. Finally, an order-wise field-map-based comparison was performed with first, second and third order global static shimming, first and second order dynamic shimming, as well as combined second or third order global and first order dynamic shim. Dynamic shimming considerably improved B0 homogeneity compared to static global shimming both in phantoms and in human subjects, reducing image distortion and signal dropout. The unshielded second and third order shims generated strong B0 and self and cross-term eddy fields, with multiple time constants ranging from milliseconds to seconds. Field homogeneity improved with increasing order of shim, with dynamic shimming performing better than global shimming. Hybrid global and dynamic shimming approach yielded field homogeneity better than global static shims but worse than dynamic shims.

A double-tuned 1H/23Na dual resonator system for tissue sodium concentration measurements in the rat brain via Na-MRI

A method for quantifying the tissue sodium concentration (TSC) in the rat brain from 23Na-MR images was developed. TSC is known to change in a variety of common human diseases and holds considerable potential to contribute to their study; however, its accurate measurement in small laboratory animals has been hindered by the extremely low signal to noise ratio (SNR) in 23Na images. To address this, the design, construction and characterization of a double-tuned 1H/23Na dual resonator system for 1H-guided quantitative 23Na-MRI are described. This system comprises an SNR-optimized surface detector coil for 23Na image acquisition, and a volume resonator producing a highly homogeneous B1 field (<5% inhomogeneity) for the Na channel across the rat head. The resonators incorporated channel-independent balanced matching and tuning capabilities with active decoupling circuitry at the 23Na resonance frequency. A quantification accuracy of TSC of <10 mM was achieved in Na-images with 1.2 µl voxel resolution acquired in 10 min. The potential of the quantification technique was demonstrated in an in vivo experiment of a rat model of cerebral stroke, where the evolution of the TSC was successfully monitored for 8 h after the stroke was induced.

13C MRS of occipital and frontal lobes at 3 T using a volume coil for stochastic proton decoupling

Previously, we devised a novel strategy for in vivo 13C MRS using [2-13C]glucose infusion and low-power proton decoupling, and proposed that this strategy could be used to acquire 13C MR spectra from the frontal lobe of the human brain. Here, we demonstrate, for the first time, in vivo 13C MRS of human frontal lobe acquired at 3 T. Because the primary metabolites of [2-13C]glucose can be decoupled using very-low-radiofrequency power, we used a volume coil for proton decoupling in this study. The homogeneous B(1) field of the volume coil was found to significantly enhance the decoupling efficiency of the stochastic decoupling sequence. Detailed specific absorption rates inside the human head were analyzed using the finite difference time domain method to ensure experimental safety. In vivo 13C spectra from the occipital and frontal lobes of the human brain were obtained. At a decoupling power of 30 W (time-averaged power, 2.45 W), the spectra from the occipital lobe showed well-resolved spectral resolution and excellent signal-to-noise ratio. Although frontal lobe 13C spectra were affected by local B(0) field inhomogeneity, we demonstrated that the spectral quality could be improved using post-acquisition data processing. In particular, we showed that the frontal lobe glutamine C5 at 178.5 ppm and aspartate C4 at 178.3 ppm could be spectrally resolved with effective proton decoupling and B(0) field correction. Because of its large spatial coverage, volume coil decoupling provides the potential to acquire 13C MRS from more than one brain region simultaneously.