Gerald B. Matson - US grants

DESCRIPTION (provided by applicant): A major problem in quantification of proton metabolites obtained by high field magnetic resonance (MR) spectroscopic imaging (MRSI) at short TE is separating the metabolite resonances from the uneven and varying baseline Some of the baseline fluctuations are due to out-of-volume water and lipid signals, while other variations are due to the presence of macromolecules We propose a series of measures to improve the ability to separate metabolite and baseline signals, the most important of which involves obtaining a series of moderate signal-to noise (S/N) spectra at differing TE times with a sequence that includes closely spaced 180 degrees pulses (CP pulse train) to reduce J evolution for strongly coupled spins prior to acquisition This preserves the spectral patterns for strongly coupled resonances at the different TE values even though the signal amplitudes decay due to T2 relaxation Simultaneous fitting of the suite of spectra is expected to enable improved separation of metabolite signals from baseline, and to provide more reliable estimates of metabolite resonance areas A number of additional measures including 1) improved RF pulses, including improvement of hyperbolic secant pulses and a spin echo pulse cascade with immunity to B1 inhomogenicity for the CP pulse train, 2) improved methods of shimming, 3) improved MRSI acquisition sequences, 4) improved methods for processing of metabolite spectra, and 5) improvements for metabolite quantification in institutional units, based on coanalysis with tissue segmented structural MRI data, are planned Finally, a metabolite atlas of normal brain, reflecting regional metabolite levels and variations, will be developed This proposal is coordinated with the partner IRPG proposal for sharing of programs, pulses, data, and information to enhance the research at both sites.

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Overview: The promise of improved MRI results at high field strength is compromised by the difficulties encountered at high field, including: i) Non-uniform excitation, due to the non-uniform B1 field inherent at high field. Typically, the non-uniform excitation produces non-uniform tissue contrast, although other deleterious effects can be produced as well. ii) Large susceptibility gradients, which can distort slice positions unless large slice-select gradients are used. However, the limited RF power available on high field systems severely limits the gradient strength that can be used for T2-weighted images. The specific aims propose the further development and refinement of two new RF pulse designs to ameliorate these deleterious effects. In addition, further development of software for simulating MRI experiments is proposed to aid in effective implementation of these new RF pulses into suitably re-designed MRI experiments. Specific aim 1: Pulses with immunity to B1 inhomogeneity. The new B1-insensitive design is based on optimized concatenations of rectangular pulses applied along different axes in the rotating frame, where the optimization is for both uniform tip and immunity to resonance offset. The design focuses on excitation pulses, but includes extension of the method to spin echo and inversion pulses. Specific aim 2: Lowered peak voltage spin echo frequency-selective pulses. The new, lowered peak voltage design method consists of concatenation of conventional, frequency-selective pulses with gradients of alternating sign. The design includes spoiler gradients incorporated into the spin echo pulse to shorten the overall length of the pulse. Operation of these pulses in inhomogeneous B1 fields is also considered. Specific aim 3: Further development of MRI simulation software with inclusion of "inadvertent" magnetization transfer (MT) effects. The further development builds on software already developed for MP RAGE MRI experiments, and will include extended phase graph (EPG) algorithms to cover a wide range of MRI experiments. These simulations will aid in effective implementation of the new RF pulses, and avoid deleterious MT effects. A further use of these simulations is expected to be in the optimization of MRI sequences for 4.0 Tesla.