Gregor Adriany

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.

Sustained negative BOLD, blood flow and oxygen consumption response and its coupling to the positive response in the human brain

Most fMRI studies are based on the detection of a positive BOLD response (PBR). Here, we demonstrate and characterize a robust sustained negative BOLD response (NBR) in the human occipital cortex, triggered by stimulating part of the visual field. The NBR was spatially adjacent to but segregated from the PBR. It depended on the stimulus and thus on the pattern of neuronal activity. The time courses of the NBR and PBR were similar, and their amplitudes covaried both with increasing stimulus duration and increasing stimulus contrast. The NBR was associated with reductions in blood flow and with decreases in oxygen consumption. Our findings support the contribution to the NBR of (1) a significant component of reduction in neuronal activity and (2) possibly a component of hemodynamic changes independent of the local changes in neuronal activity.

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.

B1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

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.

Transmit and receive transmission line arrays for 7 Tesla parallel imaging.

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 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.

Spin-Echo fMRI in Humans Using High Spatial Resolutions and High Magnetic Fields

The Hahn spin‐echo (HSE)‐based BOLD effect at high magnetic fields is expected to provide functional images that originate exclusively from the microvasculature. The blood contribution that dominates HSE BOLD contrast at low magnetic fields (e.g., 1.5 T), and degrades specificity, is highly attenuated at high fields because the apparent T2 of venous blood in an HSE experiment decreases quadratically with increasing magnetic field. In contrast, the HSE BOLD contrast is believed to arise from the microvasculature and increase supralinearly with the magnetic field strength. In this work we report the results of detailed and quantitative evaluations of HSE BOLD signal changes for functional imaging in the human visual cortex at 4 and 7 T. This study used high spatial resolution, afforded by the increased signal‐to‐noise ratio (SNR) of higher field strengths and surface coils, to avoid partial volume effects (PVEs), and demonstrated increased contrast‐to‐noise ratio (CNR) and spatial specificity at the higher field strengths. The HSE BOLD signal changes induced by visual stimulation were predominantly linearly dependent on the echo time (TE). They increased in magnitude almost quadratically in going from 4 to 7 T when the blood contribution was suppressed using Stejskal‐Tanner gradients that suppress signals from the blood due to its inhomogeneous flow and higher diffusion constant relative to tissue. The HSE signal changes at 7 T were modeled accurately using a vascular volume of 1.5%, in agreement with the capillary volume of gray matter. Furthermore, high‐resolution acquisitions indicate that CNR increased with voxel sizes < 1 mm3 due to diminishing white matter or cerebrospinal fluid‐space vs. gray matter PVEs. It was concluded that the high‐field HSE functional MRI (fMRI) signals originated largely from the capillaries, and that the magnitude of the signal changes associated with brain function reached sufficiently high levels at 7 T to make it a useful approach for mapping on the millimeter to submillimeter spatial scale. Magn Reson Med 49:655–664, 2003.

9.4T human MRI: preliminary results.

This work reports the preliminary results of the first human images at the new high‐field benchmark of 9.4T. A 65‐cm‐diameter bore magnet was used together with an asymmetric 40‐cm‐diameter head gradient and shim set. A multichannel transmission line (transverse electromagnetic (TEM)) head coil was driven by a programmable parallel transceiver to control the relative phase and magnitude of each channel independently. These new RF field control methods facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.4T head images or localize anatomic targets. Prior to FDA investigational device exemptions (IDEs) and internal review board (IRB)‐approved human studies, preliminary RF safety studies were performed on porcine models. These data are reported together with exit interview results from the first 44 human volunteers. Although several points for improvement are discussed, the preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T. Magn Reson Med, 2006.

In vivo 1H NMR spectroscopy of the human brain at high magnetic fields: metabolite quantification at 4T vs. 7T.

A comprehensive comparative study of metabolite quantification from the human brain was performed on the same 10 subjects at 4T and 7T using MR scanners with identical consoles, the same type of RF coils, and identical pulse sequences and data analysis. Signal‐to‐noise ratio (SNR) was increased by a factor of 2 at 7T relative to 4T in a volume of interest selected in the occipital cortex using half‐volume quadrature radio frequency (RF) coils. Spectral linewidth was increased by 50% at 7T, which resulted in a 14% increase in spectral resolution at 7T relative to 4T. Seventeen brain metabolites were reliably quantified at both field strengths. Metabolite quantification at 7T was less sensitive to reduced SNR than at 4T. The precision of metabolite quantification and detectability of weakly represented metabolites were substantially increased at 7T relative to 4T. Because of the increased spectral resolution at 7T, only one‐half of the SNR of a 4T spectrum was required to obtain the same quantification precision. The Cramér‐Rao lower bounds (CRLB), a measure of quantification precision, of several metabolites were lower at both field strengths than the intersubject variation in metabolite concentrations, which resulted in a strong correlation between metabolite concentrations of individual subjects measured at 4T and 7T. Magn Reson Med, 2009.

Whole-body imaging at 7T: Preliminary results

The objective of this study was to investigate the feasibility of whole‐body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the “landscape” of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development. Magn Reson Med 61:244–248, 2009.