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 Maxwell's equations. Magn Reson Med 46:24–30, 2001.

RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as “dielectric resonances,” while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B₁ in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B₁ proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B₁ phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B₁ in user‐defined areas of the brain. Magn Reson Med, 2005.

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 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.

Transceive array coils, capable of RF transmission and independent signal reception, were developed for parallel, ¹H imaging applications in the human head at 7 T (300 MHz). The coils combine the advantages of high‐frequency properties of transmission lines with classic MR coil design. Because of the short wavelength at the ¹H frequency at 300 MHz, these coils were straightforward to build and decouple. The sensitivity profiles of individual coils were highly asymmetric, as expected at this high frequency; however, the summed images from all coils were relatively uniform over the whole brain. Data were obtained with four‐ and eight‐channel transceive arrays built using a loop configuration and compared to arrays built from straight stripline transmission lines. With both the four‐ and the eight‐channel arrays, parallel imaging with sensitivity encoding with high reduction numbers was feasible at 7 T in the human head. A one‐dimensional reduction factor of 4 was robustly achieved with an average g value of 1.25 with the eight‐channel transmit/receive coils. Magn Reson Med 53:434–445, 2005.

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.