Far Out Experiments in MRI using FM pulses

Michael Garwood, PhD
Center for Magnetic Resonance Research and Department of Radiology
University of Minnesota, Minneapolis, Minnesota 55455

Abstract:
Nuclear magnetic resonance (NMR) imaging and spectroscopy requires radiofrequency (RF) irradiation to excite and manipulate nuclear spins. In multiple disciplines of NMR, significant gains in experimental precision and new experimental capabilities are made possible by RF pulses that are frequency modulated (FM).  This presentation will briefly introduce basic principles of FM methods and will show clinically relevant examples of how they are being exploited for imaging and spectroscopy. 
As one example, FM pulses can be used not only to improve data quality, but also to reveal nuclear spin dynamics, such as dipole-dipole interactions and exchange between spins on different molecules. The ability to modulate the pulse frequency ?RF(t), as well as the pulse amplitude ?1(t), creates almost limitless possibilities to sensitize the NMR signal to molecular motions happening on a slow time scale (i.e., motions having correlation times ?c in the microsecond to millisecond range).  This presentation will show how this novel approach can create contrast for better delineating structure and tissue abnormalities with MRI.
By exploiting unique features of FM pulses, we have also developed a radically different approach to produce MR images. The technique is called SWIFT (sweep imaging with Fourier transformation). The FM pulse used in SWIFT makes possible simultaneous or time-shared excitation and acquisition; thus, preserving signal from spins with extremely short transverse relaxation times, T2 and T2*. In SWIFT, FM excitation minimizes peak RF power needed and generates a uniform spin response as a function of offset frequency (e.g., in an encoding gradient). The smooth change of gradient orientation used in SWIFT produces negligible acoustic noise, which makes image acquisition close to silent. SWIFT avoids bias from T2 or T2*-weighting because there is no echo time (TE). The latter feature is exploited in dynamic contrast-enhanced (DCE) MRI to minimize T2*-bias in the shape of the T1-weighted time-intensity curve and thus to improve estimates of tissue perfusion using pharmacokinetic modeling. Furthermore, SWIFT preserves frequency-shifted signals in the vicinity of magnetic objects. For example, magnetically labeled nanoparticles (e.g., SPIOs), which cause signal voids in GRE images, give rise to positive contrast (bright spots) in SWIFT images. The latter capability is exploited to track cells, identify therapeutic targets (e.g., cancer-specific receptors), and to predict therapeutic efficacy using magnetic fluid hyperthermia for cancer treatment. Lastly, and rather unexpectedly, small magnetic susceptibility variations in tissues give rise to frequency shifts that manifest as phase contrast, which is utilized to visualize brain calcifications following injury.
Finally, FM irradiation in the presence of a field gradient provides an alternative to conventional Fourier-encoded MRI, known as spatiotemporal encoding. We have shown that by modulating the gradient in concert with FM irradiation, the resonance condition can be constrained to a local region or volume. In a technique called STEREO, spins in 2D-localized regions are sequentially brought into resonance following a curved trajectory through physical space. This method offers a unique opportunity to adjust for spatial variation in static (B0) and RF fields.
ACKNOWLEDGEMENT: funded by NIH P41 EB15894

Biography:
Michael Garwood, Ph.D. is a Professor in the Department of Radiology and Associate Director of the Center for Magnetic Resonance Research, at the University of Minnesota, where he has been for the past 27 years.  He holds the Lillian Quist – Joyce Henline Chair in Biomedical Research.  Dr. Garwood was educated at the University of California, Santa Cruz, where he received bachelors degrees in biology and chemistry in 1981, and a Ph.D. in chemistry in 1985.  As an undergraduate, he had his first exposure to nuclear magnetic resonance (NMR), which fascinated him and henceforth became the focus of his scientific career. In his time at the UMN, he has made many significant contributions to the field of biomedical NMR, mostly involving MRI technology development for better detection and assessment of therapies for cancer, Alzheimer’s disease, and other disorders.  He has many awards and honors, including the Gold Medal from the International Society of Magnetic Resonance in Medicine.  Dr. Garwood has published more than 170 scientific papers and is an inventor on 15 patents.