Multi‐slice MRI with the dynamic multi‐coil technique

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. Copyright © 2015 John Wiley & Sons, Ltd.     

Requirements for static and dynamic higher order B0 shimming of the human breast at 7 T

The increased magnetic susceptibility effects at higher magnetic fields increase the demands for shimming of the B₀ field for in vivo MRI and MRS. Both static and dynamic techniques have been developed to compensate for susceptibility‐induced field inhomogeneities. In this study, we investigate the impact of and need for both static and dynamic higher order B₀ shimming of magnetic field homogeneities in clinical breast MRI at 7 T. Both global and local field variations at lipid–tissue interfaces were observed in the magnetic field using TE‐optimized B₀ mapping at 7 T. With static B₀ shimming, a field homogeneity of 39 ± 11 Hz (n = 48) was reached in a single breast using second‐order shimming. Further compensation of the residual local field inhomogeneities caused by lipid–tissue interfaces does not seem to be feasible with shallow spherical harmonic fields. For bilateral shimming, the shimming quality was significantly less at 62 ± 15 Hz (n = 22) over both breasts, even after (simulated) fourth‐order shimming. In addition, a substantial time‐dependent field instability of 30 Hz peak to peak, with significant higher order field contributions, was observed during regular breathing. In conclusion, TE‐optimized B₀ field mapping reveals substantial field variations in the lipid‐rich environment of the human breast, in both space and time. The static field variations could be partially minimized by third‐order B₀ shimming, providing sufficient lipid suppression. However, in order to fully benefit from the increased spectral dispersion at high fields, the significant magnetic field variations during breathing need to be considered. Copyright © 2014 John Wiley & Sons, Ltd.     

Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart

Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field (B₁⁺) inhomogeneities. Parallel transmission MRI offers the ability to correct for both issues at the level of individual radiofrequency (RF) pulses, but must operate within strict hardware and safety constraints. The constraints are themselves affected by sequence parameters, such as the RF pulse duration and TR, meaning that an overall optimal operating point exists for a given sequence. This work seeks to obtain optimal performance by performing a ‘sequence‐level’ optimization in which pulse sequence parameters are included as part of an RF shimming calculation. The method is applied to balanced steady‐state free precession cardiac MRI with the objective of minimizing TR, hence reducing the imaging duration. Results are demonstrated using an eight‐channel parallel transmit system operating at 3 T, with an in vivo study carried out on seven male subjects of varying body mass index (BMI). Compared with single‐channel operation, a mean‐squared‐error shimming approach leads to reduced imaging durations of 32 ± 3% with simultaneous improvement in flip angle homogeneity of 32 ± 8% within the myocardium.     

Interleaved 31P MRS imaging of human frontal and occipital lobes using dual RF coils in combination with single‐channel transmitter–receiver and dynamic B0 shimming

In vivo ³¹P magnetic resonance spectroscopy (MRS) provides a unique tool for the non‐invasive study of brain energy metabolism and mitochondrial function. The assessment of bioenergetic impairment in different brain regions is essential to understand the pathophysiology and progression of human brain diseases. This article presents a simple and effective approach which allows the interleaved measurement of ³¹P spectra and imaging from two distinct human brain regions of interest with dynamic B₀ shimming capability. A transistor–transistor logic controller was employed to actively switch the single‐channel X‐nuclear radiofrequency (RF) transmitter–receiver between two ³¹P RF surface coils, enabling the interleaved acquisition of two ³¹P free induction decays (FIDs) from human occipital and frontal lobes within the same repetition time. Linear gradients were incorporated into the RF pulse sequence to perform the first‐order dynamic shimming to further improve spectral resolution. The overall results demonstrate that the approach provides a cost‐effective and time‐efficient solution for reliable ³¹P MRS measurement of cerebral phosphate metabolites and adenosine triphosphate (ATP) metabolic fluxes from two human brain regions with high detection sensitivity and spectral quality at 7 T. The same design concept can be extended to acquire multiple spectra from more than two brain regions or can be employed for other magnetic resonance applications beyond the ³¹P spin.     

Regional neurochemical profiles in the human brain measured by 1H MRS at 7 T using local B1 shimming

Increased sensitivity and chemical shift dispersion at ultra‐high magnetic fields enable the precise quantification of an extended range of brain metabolites from ¹H MRS. However, all previous neurochemical profiling studies using single‐voxel MRS at 7 T have been limited to data acquired from the occipital lobe with half‐volume coils. The challenges of ¹H MRS of the human brain at 7 T include short T₂ and complex B₁ distribution that imposes limitations on the maximum achievable B₁ strength. In this study, the feasibility of acquiring and quantifying short‐echo (TE = 8 ms), single‐voxel ¹H MR spectra from multiple brain regions was demonstrated by utilizing a 16‐channel transceiver array coil with 16 independent transmit channels, allowing local transmit B₁ (B₁⁺) shimming. Spectra were acquired from volumes of interest of 1–8 mL in brain regions that are of interest for various neurological disorders: frontal white matter, posterior cingulate, putamen, substantia nigra, pons and cerebellar vermis. Local B₁⁺ shimming substantially increased the transmit efficiency, especially in the peripheral and ventral brain regions. By optimizing a STEAM sequence for utilization with a 16‐channel coil, artifact‐free spectra were acquired with a small chemical shift displacement error (<5% /ppm/direction) from all regions. The high signal‐to‐noise ratio enabled the quantification of neurochemical profiles consisting of at least nine metabolites, including γ‐aminobutyric acid, glutamate and glutathione, in all brain regions. Significant differences in neurochemical profiles were observed between brain regions. For example, γ‐aminobutyric acid levels were highest in the substantia nigra, total creatine was highest in the cerebellar vermis and total choline was highest in the pons, consistent with the known biochemistry of these regions. These findings demonstrate that single‐voxel ¹H MRS at ultra‐high field can reliably detect region‐specific neurochemical patterns in the human brain, and has the potential to objectively detect alterations in neurochemical profiles associated with neurological diseases. Copyright © 2011 John Wiley & Sons, Ltd.     

A radiofrequency coil to facilitate B₁⁺ shimming and parallel imaging acceleration in three dimensions at 7 T

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.     

Simultaneous bilateral hip joint imaging at 7 Tesla using fast transmit B(1) shimming methods and multichannel transmission - a feasibility study

The objective of this study was to demonstrate the feasibility of simultaneous bilateral hip imaging at 7 Tesla. Hip joint MRI becomes clinically critical since recent advances have made hip arthroscopy an efficacious approach to treat a variety of early hip diseases. The success of these treatments requires a reliable and accurate diagnosis of intraarticular abnormalities at an early stage. Articular cartilage assessment is especially important to guide surgical decisions but is difficult to achieve with current MR methods. Because of gains in tissue contrast and spatial resolution reported at ultra high magnetic fields, there are strong expectations that imaging the hip joint at 7 Tesla will improve diagnostic accuracy. Furthermore, there is growing evidence that the majority of these hip abnormalities occur bilaterally, emphasizing the need for bilateral imaging. However, obtaining high quality images in the human torso, in particular of both hips simultaneously, must overcome a major challenge arising from the damped traveling wave behaviour of RF waves at 7 Tesla that leads to severe inhomogeneities in transmit B1 (B₁⁺) phase and magnitude, typically resulting in areas of low signal and contrast, and consequently impairing use for clinical applications. To overcome this problem, a 16‐channel stripline transceiver RF coil was used, together with a B1 shimming algorithm aiming at maximizing B₁⁺ in six regions of interest over the hips that were identified on axial scout images. Our successful results demonstrate that this approach effectively reduces inhomogeneities observed before B1 shimming and provides high joint tissue contrast in both hips while reducing the required RF power. Critical to this success was a fast small flip angle B₁⁺ calibration scan that permitted the computation of subject‐specific B1 shimming solutions, a necessary step to account for large spatial variations in B₁⁺ phase observed in different subjects. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrocardiogram‐triggered,higher order,projection‐based B0 shimming allows for fast and reproducible shim convergence in spinal cord 1H MRS

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. Copyright © 2012 John Wiley & Sons, Ltd.     

Development of a 7 T RF coil system for breast imaging

In ultrahigh‐field MRI, such as 7 T, the signal‐to‐noise ratio (SNR) increases while transmit (Tx) field (B₁⁺) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B₁⁺ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. An RF coil system composed of an eight‐channel Tx‐only array based on a tic‐tac‐toe design (can be combined to operate in single‐Tx mode) in conjunction with an eight‐channel Rx‐only insert was developed. Characterizations of the B₁⁺ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made. At 7 T, about 20% B₁⁺ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx‐only array enhances the SNR by a factor of about three. High‐quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B₁⁺ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High‐quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.     

Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate,tissue temperature,and thermal dose

In this study, the performance of an integrated body-imaging array for 7 T with 32 radiofrequency (RF) channels under consideration of local specific absorption rate (SAR), tissue temperature, and thermal dose limits was evaluated and the imaging performance was compared with a clinical 3 T body coil. Thirty-two transmit elements were placed in three rings between the bore liner and RF shield of the gradient coil. Slice-selective RF pulse optimizations for B₁ shimming and spokes were performed for differently oriented slices in the body under consideration of realistic constraints for power and local SAR. To improve the B₁⁺ homogeneity, safety assessments based on temperature and thermal dose were performed to possibly allow for higher input power for the pulse optimization than permissible with SAR limits. The results showed that using two spokes, the 7 T array outperformed the 3 T birdcage in all the considered regions of interest. However, a significantly higher SAR or lower duty cycle at 7 T is necessary in some cases to achieve similar B₁⁺ homogeneity as at 3 T. The homogeneity in up to 50 cm-long coronal slices can particularly benefit from the high RF shim performance provided by the 32 RF channels. The thermal dose approach increases the allowable input power and the corresponding local SAR, in one example up to 100 W/kg, without limiting the exposure time necessary for an MR examination. In conclusion, the integrated antenna array at 7 T enables a clinical workflow for body imaging and comparable imaging performance to a conventional 3 T clinical body coil.     

High-resolution simulation of B0 field conditions in the human heart from segmented computed tomography images

B₀ inhomogeneity leads to imaging artifacts in cardiac magnetic resonance imaging (MRI), in particular dark band artifacts with steady-state free precession pulse sequences. The limited spatial resolution of MR-derived in vivo B₀ maps and the lack of population data prevent systematic analysis of the problem at hand and the development of optimized B₀ shim strategies. We used readily available clinical computed tomography (CT) images to simulate the B₀ conditions in the human heart at high spatial resolution. Calculated B₀ fields showed consistency with MRI-based B₀ measurements. The B₀ maps for both the simulations and in vivo measurements showed local field inhomogeneities in the vicinity of lung tips with dominant Z3 spherical harmonic terms in the field distribution. The presented simulation approach allows for the derivation of B₀ field conditions at high spatial resolution from CT images and enables the development of subject- and population-specific B₀ shim strategies for the human heart.     

High‐resolution MRI encoding using radiofrequency phase gradients

Although MRI offers highly diagnostic medical imagery, patient access to this modality worldwide is very limited when compared with X‐ray or ultrasound. One reason for this is the expense and complexity of the equipment used to generate the switched magnetic fields necessary for MRI encoding. These field gradients are also responsible for intense acoustic noise and have the potential to induce nerve stimulation. We present results with a new MRI encoding principle which operates entirely without the use of conventional B₀ field gradients. This new approach – ‘Transmit Array Spatial Encoding’ (TRASE) – uses only the resonant radiofrequency (RF) field to produce Fourier spatial encoding equivalent to conventional MRI. k‐space traversal (image encoding) is achieved by spin refocusing with phase gradient transmit fields in spin echo trains. A transmit coil array, driven by just a single transmitter channel, was constructed to produce four phase gradient fields, which allows the encoding of two orthogonal spatial axes. High‐resolution two‐dimensional‐encoded in vivo MR images of hand and wrist were obtained at 0.2 T. TRASE exploits RF field phase gradients, and offers the possibility of very low‐cost diagnostics and novel experiments exploiting unique capabilities, such as imaging without disturbance of the main B₀ magnetic field. Lower field imaging (<1 T) and micro‐imaging are favorable application domains as, in both cases, it is technically easier to achieve the short RF pulses desirable for long echo trains, and also to limit RF power deposition. As TRASE is simply an alternative mechanism (and technology) of moving through k space, there are many close analogies between it and conventional B₀‐encoded techniques. TRASE is compatible with both B₀ gradient encoding and parallel imaging, and so hybrid sequences containing all three spatial encoding approaches are possible. Copyright © 2013 John Wiley & Sons, Ltd.     

Slice‐selective FID acquisition,localized by outer volume suppression (FIDLOVS) for 1H‐MRSI of the human brain at 7 T with minimal signal loss

In comparison to 1.5 and 3 T, MR spectroscopic imaging at 7 T benefits from signal‐to‐noise ratio (SNR) gain and increased spectral resolution and should enable mapping of a large number of metabolites at high spatial resolutions. However, to take full advantage of the ultra‐high field strength, severe technical challenges, e.g. related to very short T₂ relaxation times and strict limitations on the maximum achievable B₁ field strength, have to be resolved. The latter results in a considerable decrease in bandwidth for conventional amplitude modulated radio frequency pulses (RF‐pulses) and thus to an undesirably large chemical‐shift displacement artefact. Frequency‐modulated RF‐pulses can overcome this problem; but to achieve a sufficient bandwidth, long pulse durations are required that lead to undesirably long echo‐times in the presence of short T₂ relaxation times. In this work, a new magnetic resonance spectroscopic imaging (MRSI) localization scheme (free induction decay acquisition localized by outer volume suppression, FIDLOVS) is introduced that enables MRSI data acquisition with minimal SNR loss due to T₂ relaxation and thus for the first time mapping of an extended neurochemical profile in the human brain at 7 T. To overcome the contradictory problems of short T₂ relaxation times and long pulse durations, the free induction decay (FID) is directly acquired after slice‐selective excitation. Localization in the second and third dimension and skull lipid suppression are based on a T₁‐ and B₁‐insensitive outer volume suppression (OVS) sequence. Broadband frequency‐modulated excitation and saturation pulses enable a minimization of the chemical‐shift displacement artefact in the presence of strict limits on the maximum B₁ field strength. The variable power RF pulses with optimized relaxation delays (VAPOR) water suppression scheme, which is interleaved with OVS pulses, eliminates modulation side bands and strong baseline distortions. Third order shimming is based on the accelerated projection‐based automatic shimming routine (FASTERMAP) algorithm. The striking SNR and spectral resolution enable unambiguous quantification and mapping of 12 metabolites including glutamate (Glu), glutamine (Gln), N‐acetyl‐aspartatyl‐glutamate (NAAG), γ‐aminobutyric acid (GABA) and glutathione (GSH). The high SNR is also the basis for highly spatially resolved metabolite mapping. Copyright © 2009 John Wiley & Sons, Ltd.     

MR‐based measurements and simulations of the magnetic field created by a realistic transcranial magnetic stimulation (TMS) coil and stimulator

Transcranial magnetic stimulation (TMS) is an emerging technique that allows non‐invasive neurostimulation. However, the correct validation of electromagnetic models of typical TMS coils and the correct assessment of the incident TMS field (B_TMS) produced by standard TMS stimulators are still lacking. Such a validation can be performed by mapping B_TMS produced by a realistic TMS setup. In this study, we show that MRI can provide precise quantification of the magnetic field produced by a realistic TMS coil and a clinically used TMS stimulator in the region in which neurostimulation occurs. Measurements of the phase accumulation created by TMS pulses applied during a tailored MR sequence were performed in a phantom. Dedicated hardware was developed to synchronize a typical, clinically used, TMS setup with a 3‐T MR scanner. For comparison purposes, electromagnetic simulations of B_TMS were performed. MR‐based measurements allow the mapping and quantification of B_TMS starting 2.5 cm from the TMS coil. For closer regions, the intra‐voxel dephasing induced by B_TMS prohibits TMS field measurements. For 1% TMS output, the maximum measured value was ~0.1 mT. Simulations reflect quantitatively the experimental data. These measurements can be used to validate electromagnetic models of TMS coils, to guide TMS coil positioning, and for dosimetry and quality assessment of concurrent TMS‐MRI studies without the need for crude methods, such as motor threshold, for stimulation dose determination.     

Multiband diffusion‐weighted MRI of the eye and orbit free of geometric distortions using a RARE‐EPI hybrid

Diffusion‐weighted imaging (DWI) provides information on tissue microstructure. Single‐shot echo planar imaging (EPI) is the most common technique for DWI applications in the brain, but is prone to geometric distortions and signal voids. Rapid acquisition with relaxation enhancement [RARE, also known as fast spin echo (FSE)] imaging presents a valuable alternative to DWI with high anatomical accuracy. This work proposes a multi‐shot diffusion‐weighted RARE‐EPI hybrid pulse sequence, combining the anatomical integrity of RARE with the imaging speed and radiofrequency (RF) power deposition advantage of EPI. The anatomical integrity of RARE‐EPI was demonstrated and quantified by center of gravity analysis for both morphological images and diffusion‐weighted acquisitions in phantom and in vivo experiments at 3.0 T and 7.0 T. The results indicate that half of the RARE echoes in the echo train can be replaced by EPI echoes whilst maintaining anatomical accuracy. The reduced RF power deposition of RARE‐EPI enabled multiband RF pulses facilitating simultaneous multi‐slice imaging. This study shows that diffusion‐weighted RARE‐EPI has the capability to acquire high fidelity, distortion‐free images of the eye and the orbit. It is shown that RARE‐EPI maintains the immunity to B₀ inhomogeneities reported for RARE imaging. This benefit can be exploited for the assessment of ocular masses and pathological changes of the eye and the orbit.     

Whole‐body radiofrequency coil for 31P MRSI at 7 T

Widespread use of ultrahigh‐field ³¹P MRSI in clinical studies is hindered by the limited field of view and non‐uniform radiofrequency (RF) field obtained from surface transceivers. The non‐uniform RF field necessitates the use of high specific absorption rate (SAR)‐demanding adiabatic RF pulses, limiting the signal‐to‐noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body‐sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick‐up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7‐T MRI scanner. The accuracy of power calibration with pick‐up probes is analyzed at a clinical 3‐T MR system with a close to identical ¹H body coil integrated at the MR system. Finally, we demonstrate high‐quality three‐dimensional ³¹P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.     

19 F MRSI of capecitabine in the liver at 7 T using broadband transmit–receive antennas and dual‐band RF pulses

Capecitabine (Cap) is an often prescribed chemotherapeutic agent, successfully used to cure some patients from cancer or reduce tumor burden for palliative care. However, the efficacy of the drug is limited, it is not known in advance who will respond to the drug and it can come with severe toxicity. ¹⁹ F Magnetic Resonance Spectroscopy (MRS) and Magnetic Resonance Spectroscopic Imaging (MRSI) have been used to non‐invasively study Cap metabolism in vivo to find a marker for personalized treatment. In vivo detection, however, is hampered by low concentrations and the use of radiofrequency (RF) surface coils limiting spatial coverage. In this work, the use of a 7T MR system with radiative multi‐channel transmit–receive antennas was investigated with the aim of maximizing the sensitivity and spatial coverage of ¹⁹ F detection protocols. The antennas were broadband optimized to facilitate both the ¹H (298 MHz) and ¹⁹ F (280 MHz) frequencies for accurate shimming, imaging and signal combination. B₁⁺ simulations, phantom and noise measurements showed that more than 90% of the theoretical maximum sensitivity could be obtained when using B₁⁺ and B₁^? information provided at the ¹H frequency for the optimization of B₁⁺ and B₁^? at the ¹⁹ F frequency. Furthermore, to overcome the limits in maximum available RF power, whilst ensuring simultaneous excitation of all detectable conversion products of Cap, a dual‐band RF pulse was designed and evaluated. Finally, ¹⁹ F MRS(I) measurements were performed to detect ¹⁹ F metabolites in vitro and in vivo. In two patients, at 10 h (patient 1) and 1 h (patient 2) after Cap intake, ¹⁹ F metabolites were detected in the liver and the surrounding organs, illustrating the potential of the set‐up for in vivo detection of metabolic rates and drug distribution in the body. Copyright © 2015 John Wiley & Sons, Ltd.     

Localized 1H NMR spectroscopy in different regions of human brain in vivo at 7 T: T2 relaxation times and concentrations of cerebral metabolites

At the high field strength of 7 T, in vivo spectra of the human brain with exceptional spectral quality sufficient to quantify 16 metabolites have been obtained previously only in the occipital lobe. However, neurochemical abnormalities associated with many brain disorders are expected to occur in brain structures other than the occipital lobe. The purpose of the present study was to obtain high‐quality spectra from various brain regions at 7 T and to quantify the concentrations of different metabolites. To obtain concentrations of metabolites within four different regions of the brain, such as the occipital lobe, motor cortex, basal ganglia and cerebellum, the T₂ relaxation times of the singlets and J‐coupled metabolites in these regions were measured for the first time at 7 T. Our results demonstrate that high‐quality, quantifiable spectra can be obtained in regions other than the occipital lobe at 7 T utilizing a 16‐channel transceiver coil and B₁⁺ shimming. Copyright © 2011 John Wiley & Sons, Ltd.     

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 B₁ 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 B₀ 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 B₀ 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. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimized 31P MRS in the human brain at 7 T with a dedicated RF coil setup

The design and construction of a dedicated RF coil setup for human brain imaging (¹H) and spectroscopy (³¹P) at ultra‐high magnetic field strength (7 T) is presented. The setup is optimized for signal handling at the resonance frequencies for ¹H (297.2 MHz) and ³¹P (120.3 MHz). It consists of an eight‐channel ¹H transmit–receive head coil with multi‐transmit capabilities, and an insertable, actively detunable ³¹P birdcage (transmit–receive and transmit only), which can be combined with a seven‐channel receive‐only ³¹P array. The setup enables anatomical imaging and ³¹P studies without removal of the coil or the patient. By separating transmit and receive channels and by optimized addition of array signals with whitened singular value decomposition we can obtain a sevenfold increase in SNR of ³¹P signals in the occipital lobe of the human brain compared with the birdcage alone. These signals can be further enhanced by 30 ± 9% using the nuclear Overhauser effect by B₁‐shimmed low‐power irradiation of water protons. Together, these features enable acquisition of ³¹P MRSI at high spatial resolutions (3.0 cm³ voxel) in the occipital lobe of the human brain in clinically acceptable scan times (~15 min). © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.