Technical Developments of in vivo Proton Magnetic Resonance Spectroscopy

摘要

Proton magnetic resonance spectroscopy (MRS) is a powerful non-invasive technique to probe the biochemistry of the brain and body. Because MRS signals are collected from metabolites in concentrations on the order of a few mM in biological tissues, (rather than from water at approximately 40 M concentration, as in magnetic resonance imaging) low signal-to-noise ratio (SNR) poses a major challenge. In the face of this challenge, there is an ongoing need for improved techniques to enhance the clinical applicability of MRS. One area of interest, for example, involves using MRS data as a biomarker for the assessment of early response in radiation therapy.;This thesis focuses on the development of three MRS techniques, with the ultimate goal of this work being used in early radiation detection response protocols. First, a technique is developed to extend single voxel MRS to MRS of a small number of voxels (eg. two) without the need for spatial frequency encoding, using customized selective radiofrequency (RF) excitation and the localized sensitivity of multichannel receiver coils. The SNR efficiency of this technique is demonstrated in phantoms, healthy adult volunteers and patients with brain cancer. Second, a novel inversion and saturation recovery sequence is developed in conjunction with a spectral editing module to estimate the longitudinal relaxation time (T1) of lactate in vivo at 3 T. Lactate is of significant interest due to its role in cellular metabolism, and particularly its elevation in tumors. The resulting T1 estimate enables further optimization of MRS protocols and assists in estimating the absolute concentration of lactate from MRS data. Third, a diffusion-weighted two-dimensional spectroscopy sequence is developed, subsequently referred to as DW-JPRESS. This sequence, as well as an optimized processing pipeline, is demonstrated to provide estimates of the apparent diffusion coefficient (ADC) of brain metabolites beyond those typically accessible at 3 T, namely glutamate, myo-inositol and scyllo-inositol. The DW-JPRESS data provide unique information concerning the local diffusion environment of each measured metabolite, with the potential to characterize microstructural changes from different brain pathologies including cancer. Collectively, the technical development undertaken in this thesis promises to enhance future clinical applications of MRS, such as its use in distinguishing between neoplastic and nonneoplastic lesions, or for assessing tumor recurrence.