Hellmut Merkle

7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images

Signal‐to‐noise ratio (SNR), RF field (B1), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B1 magnitude was nearly twofold higher at 7T than at 4T (∼42% compared to ∼23%). The power required for a 90° pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwells equations. Magn Reson Med 46:24–30, 2001.

Simultaneous in vivo spectral editing and water suppression

Water suppression is typically performed in vivo by exciting the longitudinal magnetization in combination with dephasing, or by using frequency‐selective coherence generation. MEGA, a frequency‐selective refocusing technique, can be placed into any pulse sequence element designed to generate a Hahn spin‐echo or stimulated echo, to dephase transverse water coherences with minimal spectral distortions. Water suppression performance was verified in vivo using stimulated echo acquisition mode (STEAM) localization, which provided water suppression comparable with that achieved with four selective pulses in 3,1‐DRYSTEAM. The advantage of the proposed method was exploited for editing J‐coupled resonances. Using a double‐banded pulse that selectively inverts a J‐coupling partner and simultaneously suppresses water, efficient metabolite editing was achieved in the point resolved spectroscopy (PRESS) and STEAM sequences in which MEGA was incorporated. To illustrate the efficiency of the method, the detection of γ‐aminobutyric acid (GABA) was demonstrated, with minimal contributions from macromolecules and overlying singlet peaks at 4 T. The estimated occipital GABA concentration was consistent with previous reports, suggesting that editing for GABA is efficient when based on MEGA at high field strengths.

Imaging brain function in humans at 7 Tesla.

This article describes experimental studies performed to demonstrate the feasibility of BOLD fMRI using echo‐planar imaging (EPI) at 7 T and to characterize the BOLD response in humans at this ultrahigh magnetic field. Visual stimulation studies were performed in normal subjects using high‐resolution multishot EPI sequences. Changes in R  *2 arising from visual stimulation were experimentally determined using fMRI measurements obtained at multiple echo times. The results obtained at 7 T were compared to those at 4 T. Experimental data indicate that fMRI can be reliably performed at 7 T and that at this field strength both the sensitivity and spatial specificity of the BOLD response are increased. This study suggests that ultrahigh field MR systems are advantageous for functional mapping in humans. Magn Reson Med 45:588–594, 2001.

In vivo 1H NMR spectroscopy of the human brain at 7 T

In vivo 1H NMR spectra from the human brain were measured at 7 T. Ultrashort echo‐time STEAM was used to minimize J‐modulation and signal attenuation caused by the shorter T2 of metabolites. Precise adjustment of higher‐order shims, which was achieved with FASTMAP, was crucial to benefit from this high magnetic field. Sensitivity improvements were evident from single‐shot spectra and from the direct detection of glucose at 5.23 ppm in 8‐ml volumes. The linewidth of the creatine methyl resonance was at best 9 Hz. In spite of the increased linewidth of singlet resonances at 7 T, the ability to resolve overlapping multiplets of J‐coupled spin systems, such as glutamine and glutamate, was substantially increased. Characteristic spectral patterns of metabolites, e.g., myo‐inositol and taurine, were discernible in the in vivo spectra, which facilitated an unambiguous signal assignment. Magn Reson Med 46:451–456, 2001.

Functional magnetic resonance imaging of Broca's area during internal speech

Conventional gradient-echo magnetic resonance imaging (MRI) at 4 Tesla was used successfully to study the activity of Brocas area during internal speech word generation in healthy right-handed volunteers. Activity was demonstrated in the internal gray matter surrounding the ascending ramus of the lateral sulcus, deep to the cortical surface representation of Brocas area, in all the subjects. These studies demonstrate the capability of functional MRI to non-invasively map language related cognitive functions. Such functional mapping has value for both the study of basic neuroscience and neurosurgical planning.

Layer-specific variation of iron content in cerebral cortex as a source of MRI contrast

Recent advances in high-field MRI have dramatically improved the visualization of human brain anatomy in vivo. Most notably, in cortical gray matter, strong contrast variations have been observed that appear to reflect the local laminar architecture. This contrast has been attributed to subtle variations in the magnetic properties of brain tissue, possibly reflecting varying iron and myelin content. To establish the origin of this contrast, MRI data from postmortem brain samples were compared with electron microscopy and histological staining for iron and myelin. The results show that iron is distributed over laminae in a pattern that is suggestive of each region’s myeloarchitecture and forms the dominant source of the observed MRI contrast.

The effects of electrical microstimulation on cortical signal propagation

Electrical stimulation has been used in animals and humans to study potential causal links between neural activity and specific cognitive functions. Recently, it has found increasing use in electrotherapy and neural prostheses. However, the manner in which electrical stimulation–elicited signals propagate in brain tissues remains unclear. We used combined electrostimulation, neurophysiology, microinjection and functional magnetic resonance imaging (fMRI) to study the cortical activity patterns elicited during stimulation of cortical afferents in monkeys. We found that stimulation of a site in the lateral geniculate nucleus (LGN) increased the fMRI signal in the regions of primary visual cortex (V1) that received input from that site, but suppressed it in the retinotopically matched regions of extrastriate cortex. Consistent with previous observations, intracranial recordings indicated that a short excitatory response occurring immediately after a stimulation pulse was followed by a long-lasting inhibition. Following microinjections of GABA antagonists in V1, LGN stimulation induced positive fMRI signals in all of the cortical areas. Taken together, our findings suggest that electrical stimulation disrupts cortico-cortical signal propagation by silencing the output of any neocortical area whose afferents are electrically stimulated.

Sensitivity of MRI resonance frequency to the orientation of brain tissue microstructure

Recent advances in high-field (≥7 T) MRI have made it possible to study the fine structure of the human brain at the level of fiber bundles and cortical layers. In particular, techniques aimed at detecting MRI resonance frequency shifts originating from local variation in magnetic susceptibility and other sources have greatly improved the visualization of these structures. A recent theoretical study [He X, Yablonskiy DA (2009) Proc Natl Acad Sci USA 106:13558–13563] suggests that MRI resonance frequency may report not only on tissue composition, but also on microscopic compartmentalization of susceptibility inclusions and their orientation relative to the magnetic field. The proposed sensitivity to tissue structure may greatly expand the information available with conventional MRI techniques. To investigate this possibility, we studied postmortem tissue samples from human corpus callosum with an experimental design that allowed separation of microstructural effects from confounding macrostructural effects. The results show that MRI resonance frequency does depend on microstructural orientation. Furthermore, the spatial distribution of the resonance frequency shift suggests an origin related to anisotropic susceptibility effects rather than microscopic compartmentalization. This anisotropy, which has been shown to depend on molecular ordering, may provide valuable information about tissue molecular structure.

Proton T2 relaxation study of water, N-acetylaspartate, and creatine in human brain using Hahn and Carr-Purcell spin echoes at 4T and 7T.

Carr‐Purcell and Hahn spin‐echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T  †2 ) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T  †2 values of proton resonances (water, N‐acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T  †2 decay can be substantially reduced using a Carr‐Purcell‐type SE sequence. Magn Reson Med 47:629–633, 2002.

3-D FLASH imaging using a single surface coil and a new adiabatic pulse, BIR-4.

A new adiabatic pulse, which can induce uniform and arbitrary flip angles despite the presence of transmitter coil magnetic field (B1) inhomogeneities, is employed for 3-D fast imaging using a single surface coil for pulse transmission and signal detection. Computer calculations and phantom, rat, and human surface coil imaging experiments demonstrate the utility of this adiabatic pulse for T1-weighted imaging with a transmitter coil which generates a highly inhomogeneous B1 field profile.