Arthur W. Magill

Are glutamate and lactate increases ubiquitous to physiological activation? A 1H functional MR spectroscopy study during motor activation in human brain at 7Tesla

Recent studies at high field (7Tesla) have reported small metabolite changes, in particular lactate and glutamate (below 0.3μmol/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 7T. Small but sustained increases in lactate (0.17μmol/g±0.05μmol/g, p<0.001) and glutamate (0.17μmol/g±0.09μmol/g, p<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 ΔCMRO2 that is transiently below that of ΔCMRGlc during the first 1 to 2min of the stimulation.

SA2RAGE: A new sequence for fast B1+-mapping

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  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  1+ field in the organ of interest. In this work, a new phase‐sensitive three‐dimensional B  1+ ‐mapping technique is introduced that allows the acquisition of a 64 × 64 × 8 B  1+ ‐map in ∼20 s, yielding an accurate mapping of the relative B  1+ with a 10‐fold dynamic range (0.2–2 times the nominal B  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  1+ ‐shimming procedures at high magnetic fields. The proposed methodology was validated in phantom experiments and demonstrated good results in phantom and human B  1+ ‐shimming using an 8‐channel transmit‐receive array. Magn Reson Med, 2011.

Digit somatotopy in the human cerebellum: a 7T fMRI study.

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 7T 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.

Dielectric pads and low- B1+ adiabatic pulses: Complementary techniques to optimize structural T1w whole-brain MP2RAGE scans at 7 tesla

To evaluate the combination of low‐B1+ adiabatic pulses and high permittivity (εr ≈ 165) dielectric pads effectiveness to reproducibly improve the inversion efficiency for whole‐brain MP2RAGE scans, at ultra‐high field.

An improved trap design for decoupling multinuclear RF coils

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 1H 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 1H blocking.

A double-quadrature radiofrequency coil design for proton-decoupled carbon-13 magnetic resonance spectroscopy in humans at 7T.

Carbon‐13 magnetic resonance spectroscopy (13C‐MRS) is challenging because of the inherent low sensitivity of 13C detection and the need for radiofrequency transmission at the 1H frequency while receiving the 13C signal, the latter requiring electrical decoupling of the 13C and 1H radiofrequency channels. In this study, we added traps to the 13C coil to construct a quadrature‐13C/quadrature‐1H surface coil, with sufficient isolation between channels to allow simultaneous operation at both frequencies without compromise in coil performance.

Feasibility of direct mapping of cerebral fluorodeoxy-D-glucose metabolism in situ at subcellular resolution using soft X-ray fluorescence

Glucose metabolism is difficult to image with cellular resolution in mammalian brain tissue, particularly with 18fluorodeoxy‐D‐glucose (FDG) positron emission tomography (PET). To this end, we explored the potential of synchrotron‐based low‐energy X‐ray fluorescence (LEXRF) to image the stable isotope of fluorine (F) in phosphorylated FDG (DG‐6P) at 1 μm2 spatial resolution in 3‐μm‐thick brain slices. The excitation‐dependent fluorescence F signal at 676 eV varied linearly with FDG concentration between 0.5 and 10 mM, whereas the endogenous background F signal was undetectable in brain. To validate LEXRF mapping of fluorine, FDG was administered in vitro and in vivo, and the fluorine LEXRF signal from intracellular trapped FDG‐6P over selected brain areas rich in radial glia was spectrally quantitated at 1 μm2 resolution. The subsequent generation of spatial LEXRF maps of F reproduced the expected localization and gradients of glucose metabolism in retinal Müller glia. In addition, FDG uptake was localized to periventricular hypothalamic tanycytes, whose morphological features were imaged simultaneously by X‐ray absorption. We conclude that the high specificity of photon emission from F and its spatial mapping at ≤1 μm resolution demonstrates the ability to identify glucose uptake at subcellular resolution and holds remarkable potential for imaging glucose metabolism in biological tissue.

Simultaneous and interleaved acquisition of NMR signals from different nuclei with a clinical MRI scanner

Modification of a clinical MRI scanner to enable simultaneous or rapid interleaved acquisition of signals from two different nuclei.

Ultra-high field birdcage coils: A comparison study at 14.1T

An essential feature of magnetic resonance (MR) probes for magnetic resonance imaging and spectroscopy is the ability to generate uniform B1+ excitation in a volume of interest. When the magnetic field strength is increased, leading to an increase in resonance frequency, the constraints on the MR probes size, the sample size and the associated radiation losses caused by conductor elements are higher. In this study we simulate, test and construct two birdcage coils for imaging rodents operated at 14.1 T. Bench experiments and imaging tests show that at 14.1 T dielectric resonance effect is the dominant factor accounting for B1+ field inhomogeneity but remained achievable for imaging rodent brains.

Continuous arterial spin labeling of mouse cerebral blood flow using an actively-detuned two-coil system at 9.4T

Among numerous magnetic resonance imaging (MRI) techniques, perfusion MRI provides insight into the passage of blood through the brains vascular network non-invasively. Studying disease models and transgenic mice would intrinsically help understanding the underlying brain functions, cerebrovascular disease and brain disorders. This study evaluates the feasibility of performing continuous arterial spin labeling (CASL) on all cranial arteries for mapping murine cerebral blood flow at 9.4T. We showed that with an active-detuned two-coil system, a labeling efficiency of 0.82±0.03 was achieved with minimal magnetization transfer residuals in brain. The resulting cerebral blood flow of healthy mouse was 99±26mL/100g/min, in excellent agreement with other techniques. In conclusion, high magnetic fields deliver high sensitivity and allowing not only CASL but also other MR techniques, i.e. 1H MRS and diffusion MRI etc, in studying murine brains.