Technical developments for in-vivo atomic hydrogen magnetic resonance spectroscopic imaging

Magnetic Resonance Spectroscopic Imaging (MRSI) combines the technical features of both imaging and spectroscopy by providing the metabolite information over a spatial distribution. To date, most clinical applications of MRSI have concentrated towards regions of the brain. Recently however, there has been increased attention towards clinical applications beyond the brain. Some of the difficulties associated with MRSI outside of the brain include main field inhomogeneity, efficient spatial coverage schemes, strong lipid signals, and motion related artifacts.;This work describes several technical developments for improving in vivo proton MRSI. The developments are primarily targeted for applications beyond the brain but are also useful for brain studies as well.;The first development involves improving main field homogeneity, which plays a crucial role in MRSI. This work presents a method for improving the field homogeneity via a fast field mapping sequence and using a regularization algorithm to correct for field inhomogeneities. The regularization algorithm prevents adverse situations, which can commonly occur when correcting the field for off-centered regions of interest such as in applications outside of the brain. In vivo examples demonstrate the increased homogeneity using the method.;The second development relates to designing a pulse sequence optimized for MRSI applications beyond the brain. Spectral-spatial excitation pulses and very selective saturation pulses are used for suppression of lipids and/or any other unwanted signals. Spiral based readout gradients are used for efficient full volumetric coverage. In addition, a method for designing variable density spiral trajectories using analytic solutions is given. Using variable density spirals, lipid ringing can be reduced by improving the spatial impulse response of the signal.;The third development involves a method for motion correction in MRSI. Using the characteristics of spiral based readout trajectories, data can be post-processed to correct for motion-induced artifacts similar to techniques used in single voxel spectroscopic studies. Phantom and in vivo examples from regions of the body susceptible to motion show that the signal-to-noise ratio (SNR) of the metabolites can be improved using this correction scheme. Finally, a spiral based readout trajectory that is insensitive to motion is introduced.