High-Resolution Intermolecular Multiple Quantum Coherence Magnetic Resonance Spectroscopy: Technical Improvements For In Vivo Applications

In vivo magnetic resonance spectroscopy(MRS) offers a noninvasive method to obtain molecular information in biological tissues.However,broadened spectral lines due to magnetic field inhomogeneities induced by variations in magnetic susceptibilities among various tissues and cellular structures can degrade substantially the quality of proton MRS.The field gradients caused by magnetic susceptibility differences at air-tissue boundaries can hardly be homogenized by high-order shims available on current magnetic resonance(MR) systems.Several high-resolution methods have been proposed,utilizing spin-spin interactions such as intramolecular multiple quantum coherences(MQCs) and intermolecular nuclear Overhauser effects(NOEs) to obtain spectra independent of intrinsic field inhomogeneities.The signals based on intermolecular multiple-quantum coherences(iMQCs) come from distant dipolar coupling between spins.It is capable of improving spectral resolution when magnetic susceptibilities vary over distances much larger than the correlation distance between coupled spins.The work of this thesis focuses on technical improvements of high-resolution iMQC spectroscopy under inhomogeneous fields and its feasibility for single-voxel MRS study of inhomogeneous tissues.The main results of this thesis are summarized as follows:1.The characters and mechanisms of echo-based high-resolution methods were analyzed.The principle of echo-based high-resolution spectroscopy was revealed here:the key point of echo-based high-resolution nuclear magnetic resonance methods is the chemical shift information of observed spins should not be lost at the echo peaks in time-domain.Comparisons between high-resolution methods were made,and the preponderance of iMQC method over other high-resolution methods was revealed.This part of work provides the theoretical bases and experimental guides for the researches of high-resolution spectroscopy.2.With the intermolecular zero-quantum coherence(iZQC) high-resolution methods proposed previously,strong residual conventional single-quantum coherence(SQC) signal originating mainly from solvent resonance result in strong t₁ ridge noises.A modified HOMOGENIZED with an intermolecular double-quantum filter(iDQF),named iDQF-HOMOGENIZED,is presented to suppress the residual conventional SQC signals as well as solvent iZQC signals. The solvent-suppression efficiency of the iDQF-HOMOGENIZED is analyzed and a thorough comparison of the new sequence with several relevant pulse sequences is made.Dramatic resolution enhancement and solvent suppression in the measurements of a piece of grape sarcocarp suggest potential applications of the method in in vivo spectroscopy.3.Previous iMQC high-resolution methods all require 2D spectra sampling of the full ranges of chemical shifts of solute evolutions in both F1 and F2 dimensions, resulting in a prolonged scanning time for data acquisition.In this work,sparse sampling in the t₁ dimension and subsequent fold-over correction are utilized to speed up the iZQC spectroscopy by up to 50 times on high-field MR systems. Furthermore,three types of spectra with homo-decoupling,original J-coupling constants,and doubled J-coupling constants respectively are obtained with manipulation of the t₁ period.The water suppression is also improved by the combined use of iDQF and excitation sculpting.The feasibilities of this group of new sequences are demonstrated by experiments using an agar gel phantom with an air bubble,in vitro pig brain tissues and an intact post mortem mudskipper.