Transceive array coils, capable of RF transmission and independent signal reception, were developed for parallel, 1 H 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 1 H 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 eightchannel arrays, parallel imaging with sensitivity encoding with high reduction numbers was feasible at 7 T in the human head.

High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B 1 (B 1 ؉ ) shimming to minimize B 1 ؉ losses resulting from destructive interferences. B 1 ؉ shimming was performed by altering the input phase of individual RF channels based on relative B 1 ؉ phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B 1 ؉ coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B 1 ؉ shimming provided a gain in transmit efficiency of 4.2 ؎ 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B 1 ؉ shimming decreased B 1 ؉ nonuniformity within the prostate from

This work reports the preliminary results of the first human images at the new high-field benchmark of 9.4T. A 65-cmdiameter 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. The impetus for this work originated with the first 9.4T primate images obtained from a large male cynomolgus macaque at the University of Minnesota's Center for Magnetic Resonance Research in 1995 (1). In that early study, the signal-to-noise ratio (SNR) at 9.4T more than doubled the SNR reported at the time for 4T human results (2-5). The RF uniformity and specific absorption rate (SAR) for the monkey study were high and low, respectively. These data also validated the technology and methodology developed to achieve successful 9.4T results in larger laboratory animals. Based on these preliminary results and the need to double the SNR and spectral resolution in laboratory primate studies from the commonly used 4.7T field strength, an initial "seed" grant was awarded in 1997 for what would eventually become the 9.4T system of this report (6). Significant support from the Keck Foundation, the University of Minnesota, and other sources made the present 9.4T system and facility possible.As reported, the macaque head examined in Ref. 1 had only one-seventh the volume of an adult human head. While the SNR and spectral resolution from that study should predict human results, the uniform RF field contours in the monkey head at 9.4T would not predict the B 1 field in the human head of significantly greater electrical dimension. Prior to the present study, predictions of human images at 400 MHz Larmor frequencies were extrapolated upward from 7T data (7), downward from 11.1T images of fixed brains ex vivo (8), or directly from headcoil bench studies (9) and numerical models at 400 MHz, as presented in this article. Data from all four sources predicted severe RF artifacts for images achieved by the conventional means of placing a human head inside a homogeneous, circularly polarized volume coil that is resonant at 400 MHz. These high-frequency RF artifacts were first reported as a "dielectric resonance" (2). Since those first observations were made, high-field-dependent head imaging artifacts hav...