a b s t r a c t a r t i c l e i n f oRecent studies at high field (7 Tesla) have reported small metabolite changes, in particular lactate and glutamate (below 0.3 μmol/g) during visual stimulation. These studies have been limited to the visual cortex because of its high energy metabolism and good magnetic resonance spectroscopy (MRS) sensitivity using surface coil. The aim of this study was to extend functional MRS (fMRS) to investigate for the first time the metabolite changes during motor activation at 7 T. Small but sustained increases in lactate (0.17 μmol/g ± 0.05 μmol/g, p b 0.001) and glutamate (0.17 μmol/g ± 0.09 μmol/g, p b 0.005) were detected during motor activation followed by a return to the baseline after the end of activation. The present study demonstrates that increases in lactate and glutamate during motor stimulation are small, but similar to those observed during visual stimulation. From the observed glutamate and lactate increase, we inferred that these metabolite changes may be a general manifestation of the increased neuronal activity. In addition, we propose that the measured metabolite concentration increases imply an increase in ΔCMR O2 that is transiently below that of ΔCMR Glc during the first 1 to 2 min of the stimulation.© 2014 Elsevier Inc. All rights reserved. IntroductionFunctional MR spectroscopy (fMRS) provides direct insights into brain metabolism by investigating the metabolic response of the brain to a physiological stimulus. The high spectral resolution and signal-tonoise ratio (SNR) of fMRS at high field (N3 Tesla) improve the accuracy and precision of the quantification of many brain metabolites (Mekle et al., 2009;Tkac et al., 2001Tkac et al., , 2009. In addition to the increased chemical shift dispersion, a high time resolution is of advantage for the characterization of the small transient changes observed. Recent fMRS studies (Lin et al., 2012;Mangia et al., 2007b;Schaller et al., 2013) at high field (7 Tesla) reported small metabolite concentration changes during visual stimulation (around 0.2 μmol/g). In particular, a very small lactate concentration increase between 10 and 23% has been observed. Recently, functional diffusion-weighted MRS has been used to investigate metabolite ADC changes as a potential consequence of micro structural changes during activation (Branzoli et al., 2013).The lactate increase observed during visual stimulation has been suggested to explain the mismatch of cerebral metabolic rate of change for glucose (ΔCMR Glc ) (or cerebral blood flow, CBF) and cerebral metabolic rate of change for oxygen (ΔCMR O2 ) during brain activation. However, many studies have reported different results concerning the transient mismatch of ΔCMR Glc , CBF and ΔCMR O2 during functional activity (reviewed in Buxton (2010)). The characterization of these changes has been reported in positron emission tomography (PET) (Fox and

At high magnetic field strengths (!3T), the radiofrequency wavelength used in MRI is of the same order of magnitude of (or smaller than) the typical sample size, making transmit magnetic field (B þ 1 ) inhomogeneities more prominent. Methods such as radiofrequency-shimming and transmit SENSE have been proposed to mitigate these undesirable effects. A prerequisite for such approaches is an accurate and rapid characterization of the B þ 1 field in the organ of interest. In this work, a new phase-sensitive three-dimensional B þ 1 -mapping technique is introduced that allows the acquisition of a 64 3 64 3 8 B þ 1 -map in~20 s, yielding an accurate mapping of the relative B þ 1 with a 10-fold dynamic range (0.2-2 times the nominal B þ 1 ). Moreover, the predominant use of low flip angle excitations in the presented sequence minimizes specific absorption rate, which is an important asset for in vivo B þ 1 -shimming procedures at high magnetic fields. INTRODUCTIONAt high magnetic field strengths (!3T), the radiofrequency (RF) wavelength used in MRI becomes smaller than the typical sample size, making transmit magnetic field (B þ 1 ) inhomogeneities more prominent. RF-shimming (1) and transmit SENSE (2,3) or combinations of these methods (4) have been proposed to mitigate these B þ 1 variations. However, a prerequisite of such methods is to accurately map the B þ 1 field in short experimental times.Several approaches had been developed in the past 20 years to determine the B þ 1 distribution. The technique of most straightforward implementation is the double-angle method (DAM), which calculates B þ 1 from the ratio of two images acquired by using flip angles a 1 and a 2 ¼ 2a 1 and a very long repetition time (5). Another approach relies on a sequence made of three RF pulses (a, 2a, a) in order to generate both a spin echo and stimulated echo signal, which encode the transmit magnetic field information (6,7). Recently, a technique dubbed actual flip angle imaging has been introduced where two images are acquired after excitations with the same flip angle a but different repetition times TR 1 and TR 2 (8). An appropriate choice of TR 1 and TR 2 removes the T 1 -sensitivity of ratio between the two signals, which therefore only depends on B þ 1 and TR 2 /TR 1 . Another method uses the 180 signal null to build a flip angle map (9). An innovative approach consists in the measurement of B þ 1 from the phase of the MR signal (10,11). Recently, with the availability of higher magnetic fields (implying increased B þ 1 inhomogeneity) and multiple transmit channels, B þ 1 measurements have become increasingly important, and many new methodologies have been proposed (12-18).One of the main challenges for any B þ 1 -mapping strategy to be used in the context of high static magnetic fields and parallel transmission is the range in which the measured B þ 1 is accurate and precise. A B þ 1 mapping methodology should have a large range where the assumptions made to calculate B þ 1 are valid, and the B þ 1 calculation should in that ran...

The complementary techniques of low-B1 (+) adiabatic RF pulses and high permittivity dielectric pads allow whole-brain structural T1 w images to be reliably acquired at ultra-high field. J. Magn. Reson. Imaging 2014;40:804-812. © 2013 Wiley Periodicals, Inc.

Purpose: Multinuclear magnetic resonance spectroscopy and imaging require a radiofrequency probe capable of transmitting and receiving at the proton and non-proton frequencies. To minimize coupling between probe elements tuned to different frequencies, LC (inductor-capacitor) traps blocking current at the 1 H frequency can be inserted in non-proton elements. This work compares LC traps with LCC traps, a modified design incorporating an additional capacitor, enabling control of the trap reactance at the low frequency while maintaining 1 H blocking. Methods: Losses introduced by both types of trap were analysed using circuit models. Radiofrequency coils incorporating a series of LC and LCC traps were then built and evaluated at the bench. LCC trap performance was then confirmed using 1 H and 13 C measurements in a 7T human scanner. Results: LC and LCC traps both effectively block interaction between non-proton and proton coils at the proton frequency. LCC traps were found to introduce a sensitivity reduction of 562%, which was less than half of that caused by LC traps.

Purpose: Modification of a clinical MRI scanner to enable simultaneous or rapid interleaved acquisition of signals from two different nuclei. Methods: A device was developed to modify the local oscillator signal fed to the receive channel(s) of an MRI console. This enables external modification of the frequency at which the receiver is sensitive and rapid switching between different frequencies. Use of the device was demonstrated with interleaved and simultaneous 31 P and 1 H spectroscopic acquisitions, and with interleaved 31 P and 1 H imaging. Results: Signal amplitudes and signal-to-noise ratios were found to be unchanged for the modified system, compared with data acquired with the MRI system in the standard configuration. Conclusion: Interleaved and simultaneous 1 H and 31 P signal acquisition was successfully demonstrated with a clinical MRI scanner, with only minor modification of the RF architecture. While demonstrated with 31 P, the modification is applicable to any detectable nucleus without further modification, enabling a wide range of simultaneous and interleaved experiments to be performed within a clinical setting.

The advancement of clinical applications of ultrahigh field (UHF) MRI depends heavily on advances in technology, including the development of new radiofrequency (RF) coil designs. Currently, the number of commercially available 7 T head RF coils is rather limited, implying a need to develop novel RF head coil designs that offer superior transmit and receive performance. RF coils to be used for clinical applications must be robust and reliable. In particular, for transmit arrays, if a transmit channel fails the local specific absorption rate may increase, significantly increasing local tissue heating. Recently, dipole antennas have been proposed and used to design UHF head transmit and receive arrays. The dipole provides a unique simplicity while offering comparable transmit efficiency and signal-to-noise ratio with the conventional loop design. Recently, we developed a novel array design in our laboratory using a folded-end dipole antenna. In this work, we developed, constructed and evaluated an eight-element transceiver bent folded-end dipole array for human head imaging at 7 T. Driven in the quadrature circularly polarized mode, the array demonstrated more than 20% higher transmit efficiency and significantly better whole-brain coverage than that provided by a widely used commercial array. In addition, we evaluated passive dipole antennas for decoupling the proposed array. We demonstrated that in contrast to the common unfolded dipole array, the passive dipoles moved away from the sample not only minimize coupling between the adjacent folded-end active dipoles but also produce practically no destructive interference with the quadrature mode of the array.

Purpose To develop a framework for 3D sodium (23Na) MR fingerprinting (MRF), based on irreducible spherical tensor operators with tailored flip angle (FA) pattern and time‐efficient data acquisition for simultaneous quantification of T1, T2normall∗, T2normals∗, and T2∗ in addition to ΔB0. Methods 23Na‐MRF was implemented in a 3D sequence and irreducible spherical tensor operators were exploited in the simulations. Furthermore, the Cramér Rao lower bound was used to optimize the flip angle pattern. A combination of single and double echo readouts was implemented to increase the readout efficiency. A study was conducted to compare results in a multicompartment phantom acquired with MRF and reference methods. Finally, the relaxation times in the human brain were measured in four healthy volunteers. Results Phantom experiments revealed a mean difference of 1.0% between relaxation times acquired with MRF and results determined with the reference methods. Simultaneous quantification of the longitudinal and transverse relaxation times in the human brain was possible within 32 min using 3D 23Na‐MRF with a nominal resolution of (5 mm)3. In vivo measurements in four volunteers yielded average relaxation times of: T1,brain = (35.0 ± 3.2) ms, T2normall,brain∗ = (29.3 ± 3.8) ms and T2normals,brain∗ = (5.5 ± 1.3) ms in brain tissue, whereas T1,CSF = (61.9 ± 2.8) ms and T2,CSF∗ = (46.3 ± 4.5) ms was found in cerebrospinal fluid. Conclusion The feasibility of in vivo 3D relaxometric sodium mapping within roughly ½ h is demonstrated using MRF in the human brain, moving sodium relaxometric mapping toward clinically relevant measurement times.

The representation of the human body in the human cerebellum is still relatively unknown, compared to the well-studied homunculus in the primary somatosensory cortex. The investigation of the body representation in the cerebellum and its somatotopic organisation is complicated because of the relatively small dimensions of the cerebellum, compared to the cerebrum. Somatotopically organised whole-body homunculi have previously been reported in both humans and rats. However, whether individual digits are represented in the cerebellum in a somatotopically organised way is much less clear. In this study, the high spatial resolution and high sensitivity to the blood oxygenation level dependent (BOLD) signal of 7 T fMRI were employed to study the BOLD responses in the human cerebellum to the stroking of the skin of individual digits, the hand and forearm. For the first time, a coarse somatotopic organisation of the digits, ordered from D1-D5, could be visualised in individual human subjects in both the anterior (lobule V) and the posterior (lobule VIII) lobes of the cerebellum using a somatosensory stimulus. The somatotopic gradient in lobule V was found consistently in the posterior to anterior direction, with the thumb most posterior, while the direction of the somatotopic gradient in lobule VIII differed between subjects. No somatotopic organisation was found in Crus I. A comparison of the digit patches with the hand patch revealed that the digit regions are completely covered by the hand region in both the anterior and posterior lobes of the cerebellum, in a non-somatotopic manner. These results demonstrate the promise of ultra-high field, high-resolution fMRI for studies of the cerebellum.