Conclusion and Outlook

Although ultrahigh field imaging offers great promise for a number of clinical applications, the hardware and software used will require careful design and modification compared to that of standard field strength MRI. For example, ultrahigh field MRI systems will require highly stable shims and gradients with minimal coupling between coils. Moreover, because susceptibility effects near air/tissue interfaces introduce severe B0 inhomogeneity, more complex, higher-order shim coils will be needed, together with associated software to control these shims based on the magnetic field of individual acquisitions. Likewise, more complicated and expensive RF hardware, such as multiple transmitters with precise timing and phase control, will be needed for Bi inhomogeneity correction. Again, additional software will be needed to control the multi-channel RF chain based on specific coil and head geometries.

Pulse sequences must be carefully designed and parameters adjusted, specifically for high-quality T1-weighting, which is no longer achieved with simple gradient or spin echoes, but only with IR sequences. Overall, initial studies at 8 T showed that the most promising and successful image contrast behavior was related to tissue susceptibility effects. Due to the increased SNR at ultrahigh field strength, gradient echo images with in-plane resolution of 200-250 mm showed exquisite depiction of venous vasculature based on susceptibility effects from paramagnetic deoxyhemoglobin, providing very promising results for the imaging of brain tumors and cerebrovascular disease. Moderately T2-weighted SE images may reflect paramagnetic tissue iron, while the advantages of ultrahigh field MRI for fMRI and MR spectroscopy has already been demonstrated (Ugurbil et al., 2003). Again, pulse sequences for spectroscopy and EPI sequences for fMRI must be optimized to provide satisfactory performance.

Finally, the early results obtained with a commercial 7-T MRI (Philips Achieva, Cleveland, USA) have shown great promise. For example, both time-of-flight and phase-contrast MR angiography were seen to be feasible at 7 T, with initial findings suggesting that vessel conspicuity is improved compared to lower field strength. Another highly promising sequence was that of the 3D IR magnetization-prepared fast gradient sequence, which combines excellent Ti contrast with depiction of bright arteries with short TE (2-3 ms), and/or dark veins with long TE (7-12 ms). The use of these and many other MRI methods have yet to be fully explored over a wide range of clinical applications.

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416 | 3.3 Modern Applications of MRI in Medical Sciences 3.3.5

Interventional Magnetic Resonance Imaging: Concepts, Systems, and Applications

Clifford R. Weiss and Jonathan S. Lewin

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