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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

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.

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 Theoretical and experimental studies have shown at least a linear increase in sensitivity with magnetic field strength (1,2). On the other hand, the transverse relaxation rate is known to increase with magnetic field strength (3,4), which can result in reduced sensitivity in spin-echo (SE) experiments. The apparent transverse relaxation time (T 2 † ) is related to the intrinsic transverse relaxation time (T 2 ) through the following equation:The first term on the right side of Eq.[1] is the inverse of the intrinsic T 2 and is governed by a number of possible mechanisms, including 1) homonuclear dipole-dipole interaction between protons, which is strongly dependent on rotational correlation time c ; 2) hyperfine (contact) interaction, namely, the change of transverse relaxation time due to interaction with a paramagnetic center; and 3) cross-relaxation, which can be significant in dipole-coupled systems. The second and third terms, T 2,Diffusion and T 2,Exchange , are the transverse relaxation times related to diffusion and exchange of spins between regions with different magnetic field strengths, respectively. These contributions describe the dynamic dephasing regime, whereby the net magnetization is reduced by diffusion and exchange between regions with different magnetic field strengths, which causes the phases of the different spin packets to average out. The opposite situation is defined as the static dephasing regime. NMR signal loss due to static dephasing can be refocused by SE sequences and is therefore not considered here.It is important to investigate: 1) how the increase of field strength causes T 2 † shortening, and 2) how the signal loss from T 2 † decay can be compensated for. Key experiments for answering these questions involve measuring T 2 † at different field strengths and attempting to estimate T 2 . The theory of NMR signal formation in the presence of local magnetic field inhomogeneity was first derived by Carr and Purcell (5), and later generalized by Torrey (6), who incorporated the diffusion effects into the Bloch equations to take into account the actual field distribution. The CarrPurcell (CP) method is the most valuable technique for determining transverse relaxation times. CP experiments are performed by applying a /2 pulse followed by a series of pulses spaced with time interval cp . The value of T 2 † determined with a CP technique can vary with cp because dynamic dephasing and homonuclear spin-spin coupling can cause signi...

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...

The use of body coils is favored for homogeneous excitation, and such coils are often paired with surface coils or arrays for sensitive reception in many MRI applications. While the body coil's physical size and resultant electrical length make this circuit difficult to design for any field strength, recent efforts to build efficient body coils for applications at 3T and above have been especially challenging. To meet this challenge, we developed an efficient new transverse electromagnetic (TEM) body coil and demonstrated its use in human studies at field strengths up to 4T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8T. RF shimming was used to remove a high-field-related cardiac imaging artifact in these preliminary studies. Whole-body imaging at field strengths up to 4T was investigated by MR system manufacturers in the late 1980s (1,2). However, it was thought that the RF power requirements were too high, and RF penetration was too low for successful body imaging above 100 MHz (3). As with most "first" results, the earliest images acquired at higher fields were not the "best" results. Skepticism about high-field body imaging followed, and persisted for nearly a decade (4). Not until recently have technology, support, and interest coalesced to produce encouraging results at fields higher than 1.5 T. Preliminary body imaging results acquired with 55-60-cm inner diameter (i.d.) birdcage transmit coils together with phased-array receivers indicate that 3T whole-body systems may have a clinical role (5). However, these developments are still in their infancy, and there are remaining problems to be solved and performance parameters to be improved. Commercial 128-MHz body coils are usually paired with 35-kW RF power amplifiers to compensate for increased RF losses to the patient and the coil. This is a significant peak power increase compared to 1.5 T MRI, where 15-20-kW amplifiers are more typical. Accordingly, the scope and scale of image protocols operating within FDA specific absorption rate (SAR) guidelines for 3T are highly constrained compared to those for 1.5T.By this trend, body imaging at 4T would be more limited still and would require amplifiers approaching a 50-kW peak. Thus, if the human body provided the only significant loss mechanism for imaging at 3T with coil designs requiring 35 kW of peak RF power, body coils and body imaging above 3T would be improbable. However, if it can be shown that RF coil losses contribute a significant fraction of the total loss, then the use of more efficient coil circuits might lessen the total RF losses and make human imaging feasible at 4T and higher.

This work reports preliminary results from the first human cardiac imaging at 7 Tesla (T). Images were acquired using an eightchannel transmission line (TEM) array together with local B 1 shimming. The TEM array consisted of anterior and posterior plates closely positioned to the subjects' thorax. The currents in the independent elements of these arrays were phased to promote constructive interference of the complex, short wavelength radio frequency field over the entire heart. Anatomic and functional images were acquired within a single breath hold to reduce respiratory motion artifacts while a vector cardiogram (VCG) was used to mitigate cardiac motion artifacts and gating. SAR exposure was modeled, monitored, and was limited to FDA guidelines for the human torso in subject studies. Preliminary results including short-axis and four-chamber VCG-retrogated FLASH cines, as well as, short-axis TSE images demonstrate the feasibility of safe and accurate human cardiac imaging at 7T. Magn Reson Med 61:517-524, 2009.

Most high-field MRI systems do not have the actively detuned body coils that are integral to clinical systems operating at 1.5T and lower field strengths. Therefore, many clinical applications requiring homogeneous volume excitation in combination with local surface coil reception are not easily implemented at high fields. To solve this problem for neuroimaging applications, actively detunable transverse electromagnetic (TEM) head coils were developed to be used with receive-only surface coils for signal-to-noise ratio (SNR) gains and improved spatial coverage from homogeneously excited regions. These SNR and field of view (FOV) gains were achieved by application of a detunable TEM volume coil to human brain imaging at 4T.