Erwin L. Hahn Institute for Magnetic Resonance Imaging

With the increasing number of clinically oriented studies at 7T, scientists face the challenge of providing new coil concepts for high-field MRI in body parts other than the head. Large field-of-view imaging is important for assessing patients with metastases or multiple sclerosis lesions in the spinal cord, for example. A multi-channel radiofrequency coil for 7T MRI of the human spine with an extensive imaging area of 40 cm has been developed and evaluated in healthy volunteers, and Erwin L. Hahn scientists were able to demonstrate that imaging even over such a large region can be successful at 7T. Currently, work is underway to assess a variety of pathologies in patients to further elucidate the clinical impact of this technology.
As already pointed out, an important precondition for obtaining excellent image quality at 7 Tesla is a uniform excitation of the body tissue by the high-frequency (300 MHz) magnetic field. Antenna structures based on metamaterials developed by the Department of General and Theoretical Electrical Engineering (ATE) appear to hold great potential in this regard. In resonance, such antennas present a spatially constant current distribution and are therefore especially well suited for uniform excitation of large and/or long body regions. The employment of metamaterials for constructing radiofrequency coils is an internationally unique approach to solving the challenges of high-field MRI. The photo on the first page of this article shows the prototype of a spine coil consisting of three antenna elements based on metamaterials (coil in the front).
With regard to spectroscopic applications, the promise of ultra-high magnetic field systems is two-fold: not only does the sensitivity for the detectable metabolites increase, but the spectral resolution also increases. The resulting spectra contain metabolic fingerprints of the tissue at hand. In the prostate, this fingerprint can be used to discriminate between cancer and non-cancer tissue. Using a small loop transmit/receive coil mounted inside an endorectal balloon, Institute scientists were able to obtain unprecedented spatial resolution (3.5 mm)3 of the MR spectral imaging matrix at 7T; this work indicates that smaller cancer foci can possibly be characterised. In future work with external transmit array coils, MR spectral imaging will be combined with structural MRI for better anatomical reference.
An ultra-high magnetic field such as 7 Tesla provides great opportunities for functional MRI. This is thanks to the increased image signal-to-noise ratio, and more importantly the supra-linear increase of the blood oxygenation level dependent (BOLD) signal which can be used to detect brain activation during an experimentally controlled task. However, severe image artifacts can arise at 7T with the commonly employed method known as echo-planar imaging (EPI). Institute scientists were able to apply and evaluate a multi-echo EPI technique to reduce said image artifacts and also increase the sensitivity to activation-induced signal changes at the same time. Further improvements are expected when a new 32-channel head coil becomes available in early 2010.

Sagittal view of thoracolumbosacral spine (upper left). The high spatial resolution is reflected by the depiction of the posterior longitudinal ligament (grey arrow) and by the delineation of the entry points of veins into the vertebrae (white arrow). The latter are also shown in an axial view (bottom, grey arrow), with the spinal cord marked by the white arrow. For compliance, Specific Absorption Rate (SAR) calculations revealing potential tissue warming were performed (upper right) in a numerical human body model (voxel-based SAR, voxel size 2 mm3).