Signal-to-noise ratio (SNR), RF field (B 1 ), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B 1 magnitude was nearly twofold higher at 7T than at 4T (ϳ42% compared to ϳ23%). 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 Max In the last decade, MRI studies conducted at 4T have demonstrated the utility of high magnetic fields in functional and anatomical imaging of the human brain and for spectroscopy studies in the brain and the human body (1-7). These accomplishments and the continued successes at magnetic fields up to 9.4T with animal models have paved the way for the exploration of magnetic fields of higher than 4T for human brain studies (8 -12). Consequently, recent efforts have been undertaken to establish 8T and 7T systems, the latter in our laboratory (13)(14)(15). Now with an operational 7T system, the signal-to-noise ratio (SNR), RF field (B 1 ), and RF power requirement at 7T were compared to the same parameters at 4T. MATERIALS AND METHODSIn this 7T vs. 4T comparison study, we used the same size coils, the same model consoles, identical acquisition parameters, and the same volunteers for six carefully reproduced experiments at each field strength. Hardware SystemsThis experiment was performed on Varian Unity Inova consoles interfaced to 90 cm bore Oxford 4T and Magnex 7T magnets. The noise figures of the two systems were the same, measuring 1.3 dB. Siemens body gradients (65 cm i.d.) and Magnex head gradients (38 cm i.d.) were used in the 4T and 7T systems, respectively. Coils

We used functional magnetic resonance imaging (fMRI) to monitor stimulus-selective responses of the human fusiform face area (FFA) and parahippocampal place area (PPA) during binocular rivalry in which a face and a house stimulus were presented to different eyes. Though retinal stimulation remained constant, subjects perceived changes from house to face that were accompanied by increasing FFA and decreasing PPA activity; perceived changes from face to house led to the opposite pattern of responses. These responses during rivalry were equal in magnitude to those evoked by nonrivalrous stimulus alternation, suggesting that activity in the FFA and PPA reflects the perceived rather than the retinal stimulus, and that neural competition during binocular rivalry has been resolved by these stages of visual processing.

We used high-field (3T) functional magnetic resonance imaging (fMRI) to label cortical activity due to visual spatial attention, relative to flattened cortical maps of the retinotopy and visual areas from the same human subjects. In the main task, the visual stimulus remained constant, but covert visual spatial attention was varied in both location and load. In each of the extrastriate retinotopic areas, we found MR increases at the representations of the attended target. Similar but smaller increases were found in V1. Decreased MR levels were found in the same cortical locations when attention was directed at retinotopically different locations. In and surrounding area MT+, MR increases were lateralized but not otherwise retinotopic. At the representation of eccentricities central to that of the attended targets, prominent MR decreases occurred during spatial attention.

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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.

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.

Purpose: To examine relationships between specific energy absorption rate (SAR) and temperature distributions in the human head during radio frequency energy deposition in MRI. Materials and Methods:A multi-tissue numerical model of the head was developed that considered thermal conductivity, heat capacity, perfusion, heat of metabolism, electrical properties, and density. Calculations of SAR and the resulting temperature increase were performed for different coils at different frequencies.Results: Because of tissue-dependant perfusion rates and thermal conduction, there is not a good overall spatial correlation between SAR and temperature increase. When a volume coil is driven to induce a head average SAR level of either 3.0 or 3.2 W/kg, it is unlikely that a significant temperature increase in the brain will occur due to its high rate of perfusion, although limits on SAR in any 1 g of tissue in the head may be exceeded. Conclusion:Attempts to ensure RF safety in MRI often rely on assumptions about local temperature from local SAR levels. The relationship between local SAR and local temperature is not, however, straightforward. In cases where high SAR levels are required due to pulse sequence demands, calculations of temperature may be preferable to calculations of SAR because of the more direct relationship between temperature and safety.

Radiofrequency (RF) field wave behavior and associated nonuniform image intensity at high magnetic field strengths are examined experimentally and numerically. The RF field produced by a 10-cm-diameter surface coil at 300 MHz is evaluated in a 16-cm-diameter spherical phantom with variable salinity, and in the human head. Temporal progression of the RF field indicates that the standing wave and associated dielectric resonance occurring in a pure water phantom near 300 MHz is greatly dampened in the human head due to the strong decay of the electromagnetic wave. The characteristic image intensity distribution in the human head is the result of spatial phase distribution and amplitude modulation by the interference of the RF traveling waves determined by a given sample-coil configuration. Enhancements in signal-to-noise ratio (SNR) and T* 2 contrast arising from high static magnetic field strengths are desirable for in vivo MR applications. Thus, the number of high-field human MRI systems has increased rapidly in recent years (1-10). The advent of high-field human imaging systems introduces new challenges in radiofrequency (RF) engineering (11,12). Because at high frequencies the wavelength of the RF field is comparable to or less than that of the dimension of the human body, the RF magnetic field (B 1 ) inside a sample exhibits prominent wave behavior (13-16). Additionally, the homogeneity of the B 1 field and source currents in the RF coil are strongly perturbed by sample loading (17-19). The B 1 field distribution inside a sample is important for both specific absorption rate (SAR) assessment and RF coil engineering at high frequency. However, mathematical treatment of the RF field in such systems can be extremely complicated because 1) the quasi-static approximations are no longer valid, and Maxwell's wave equation must be employed; and 2) the geometry of the human body is irregular, and electromagnetic properties of tissues are heterogeneous. Thus, computer numerical calculation becomes an effective and indispensable tool for studying interactions of the RF field with the human body at high field (20 -24). Associated with the RF field wave behavior, the distributions of the B 1 field and its circularly polarized components B ϩ and B -, which are directly responsible for the MR image intensity distribution, become distinctively different from one another. Consequently, the relationship of RF field polarization to coil configuration and sample electric properties needs to be analyzed in order to understand the resultant image intensity distribution. Computer modeling provides an effective way to study this problem, and may provide insight into complex RF field wave behavior and its dependence on the electrical properties of the sample. In this report, we present a study specifically devised to analyze high-frequency wave behavior of the RF field with the aid of numerical calculation and parallel experimental measurements. METHODSThe study was carried out using water and saline phantoms with a 10-cm-diameter sur...