Parallel Transmission Methods for High-Field MRI

Elfar Adalsteinsson
Magnetic Resonance Imaging Group
Research Laboratory of Electronics

Engineering in magnetic resonance imaging (MRI) is constantly striving for higher resolution images and shorter imaging times for improved diagnostic ability and better patient comfort. Our efforts toward this goal are focused on MRI at very high field strength, i.e. where the main magnetic field is much stronger than in current clinical systems. While MRI systems in clinical use today operate at fields of 1.5 Tesla or 3.0T, we are investigating the use of 7.0T MRI for human imaging. While the 7.0T system promises improved resolution and shorter scan times, obstacles remain in realizing these gains. Prime among the challenges in high-field MRI is a dramatic spatial variation in signal over the anatomy of interest (e.g. the brain) when we apply conventional methods of so-called radio-frequency (RF) excitation. This means that some regions of the brain do not realize the full benefits of the high-field image platform. This inhomogeneity problem drives our research of new MRI excitation methods at the ultra-high-field (7T) human MRI scanner at the HST Athinoula A. Martinos Center for Biomedical Imaging at MGH. Our research is collaborative with groups at MGH (Prof. L. Wald) and Siemens Medical Solutions, and requires the development of new hardware, software, and algorithms for RF excitation in MRI.

Figure 1. Comparison of excitation field mitigation in human imaging where the top row shows axial slice selection for brain excitation with conventional birdcage mode, and the bottom row shows the proposed parallel transmission method, so-called 2-spoke design. Note the much more uniform image in the bottom row (almost all of the brain is red in the “Flip-Angle Map” and the line profiles are much flatter than above).

Figure 1. Comparison of excitation field mitigation in human imaging where the top row shows axial slice selection for brain excitation with conventional birdcage mode, and the bottom row shows the proposed parallel transmission method, so-called 2-spoke design. Note the much more uniform image in the bottom row (almost all of the brain is red in the “Flip-Angle Map” and the line profiles are much flatter than above).

At lower field strengths common in clinical imaging (e.g. 1.5T), conventional birdcage designs for RF excitation offer excellent homogeneity of signal generation. In 7T human neuroimaging, wavelength effects cause partial field cancellations within the brain when conventional birdcage coils are used for RF excitation. Our group has recently demonstrated the use of arrays of independent RF coils to mitigate this field inhomogeneity and achieve very high degree of excitation uniformity. Fig. 1 summarizes the findings of a recent Ph.D. graduate from our group, Dr. Kawin Setsompop, who scanned six volunteer subjects in order to demonstrate the power of his parallel RF pulse design methods for field inhomogeneity mitigation in slice-selective excitation (Setsompop et al, Magnetic Resonance in Medicine 60, p. 1422, 2008). The top row shows the outcome of a conventional excitation of an axial brain section at 7T, where the field inhomogeneity is readily apparent, while the bottom row shows an essentially perfectly uniform excitation due to RF excitation where 8 independent RF power amplifier channels were used to drive the 8 most significant modes of a 16-channel RF coil array. The pulse design methods incorporate prior knowledge of measured field patterns, and through an optimization procedure, calculate the appropriate modulation and combination of these component fields that yield the most uniform excitation flip angle while incorporating constraints on available power.

EECS students Borjan Gagoski, Joonsung Lee (in the left room), and Trina Kok working with Prof. Adalsteinsson (at the computer, right) test their imaging developments on a Siemens Tim Trio MRI at the Martinos Imaging Center, McGovern Institute for brain research at MIT.

EECS students Borjan Gagoski, Joonsung Lee (in the left room), and Trina Kok working with Prof. Adalsteinsson (at the computer, right) test their imaging developments on a Siemens Tim Trio MRI at the Martinos Imaging Center, McGovern Institute for brain research at MIT.

While the primary motivation for this development is to gain high image resolution and short imaging times at 7T for brain imaging, other potential applications of parallel RF transmission include novel methods that may selectively excite prescribed anatomy such as vessels or the beating heart. Such methods that have been impractical may now have clinical impact on research applications for the diagnosis and monitoring of disease.

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