MRI at Very High Fields

J. Thomas Vaughan

Center for Magnetic Resonance Research, University of Minnesota, USA

Introduction: At this writing, the human body has been imaged to 4.7T (1) and the human head has been imaged at 7T and 8T.(2,3) By June 2004, the first body results from 7T and the first head images from 9.4T will be acquired at the University of Minnesota and the University of Illinois, Chicago. Compared to the present clinical field strengths, MR at 4T and above promises to improve anatomic imaging quality significantly, and to bring metabolic and functional imaging to the forefront of research and diagnostic modalities. While human bore sized magnets as high as 9.4T are now installed and 11.7T magnets are being considered, realization of the potential benefit of these magnets will require more than a simple field, frequency or power scaling of existing approaches. High field imposed constraints, both physical and physiological, will require new technical and methodological developments to reach the unprecedented benefits of MRI at the very highest field strengths. Some of the challenges and possible solutions for high field head and body imaging follow.

Problems: The radiofrequency (RF) subsystem exemplifies many new problems posed and solutions sought for these highest field strengths. At magnetic field strengths of 4T and higher, the Larmor wavelengths become increasingly long compared to coil circuit lengths and anatomic dimensions. Many conventional lumped element coil circuits become increasingly radiative at higher frequencies, and their currents become proportionately less uniform. Additionally, the propagation of RF fields in the human anatomy becomes very lossy and non uniform. In a head for example, the B₁ gradient across a head is 23%, peaking in the middle of the head. At 7T this gradient is 42%, and at 9.4T RF field contours exceeding 60% are predicted.(1) SNR and image uniformity are proportional to these severe B₁ field inhomogeneities. Excitation power requirements increase in proportion to frequency and B₁ field contour to restrict protocol options within SAR limits. How will these problems be solved?

Solutions: Because many of the problems related to high field imaging are Larmor frequency dependent, head and body spectroscopic imaging of the lower gamma nuclei work exceedingly well at the highest fields using many conventional approaches. While these approaches begin to fail above 4T for head and body imaging, new options are showing some promise. One approach is to tailor RF coils, gradients and pulse protocols to specific regions of interest. For example, a TEM volume coil couples very efficiently to the brain stem at 7T, where as a surface coil or array couples more strongly to the cortex. Another promising approach is to use coils with controllable current elements that can be interactively biased in phase and amplitude. Such a coil can be used to interactively optimize signal feedback from an anatomic region of interest to meet specified criteria such as maximum signal to noise and/or uniformity. A multi-channel TEM coil works well for this purpose.(4) This design with independently controlled, mutually decoupled elements lends itself to parallel imaging applications for significant increases in temporal resolution. A high filed MR system incorporating an automatic, negative feedback driven multichannel transceiver is currently under construction.(5) Preliminary results from 7T predict this TEM Parallel Transmit / Receive “TEMPTR” coil driven by a multichannel transceiver system to be a solution for imaging the human head, body and limbs to 7T and possibly to 9.4T.

References:  1) Vaughan JT, et.al., MRM 2004 (in press); 2) Robitaille PM, et.al., NMR Biomed. 11:263-265(1998);  3) Vaughan JT, et.al. Magn. Reson. Med. 46:24-30(2001);

4) Vaughan JT, US Patent Serial No. 09/575,384 (2003); 5) Vaughan JT, et.al. US Patent Appl. #60/378,111.