Real-time motion and dynamic receiver sensitivity correction for CEST-MRI in the human brain at 7T

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Title: Real-time motion and dynamic receiver sensitivity correction for CEST-MRI in the human brain at 7T
Author: Auno, Sami
Other contributor: Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta, Fysiikan laitos
University of Helsinki, Faculty of Science, Department of Physics
Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten, Institutionen för fysik
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
Thesis level: master's thesis
Discipline: Physics
Abstract: Chemical Exchange Saturation Transfer (CEST) is a novel Magnetic Resonance Imaging (MRI) technique that utilises exchange reactions between metabolites and tissue water to map metabolite concentration or tissue pH noninvasively. Similarly to Magnetic Resonance Spectroscopy (MRS), CEST is able to detect many endogenous metabolites, but unlike MRS, CEST is based on imaging and thus enjoys the speed of modern MR imaging. On the other hand, CEST also suffers from the same difficulties as MRI and MRS. One of the most common source of image artifacts in MRI is subject motion during imaging. Many different motion correction methods have been devised. Recently, a novel real-time motion correction system was developed for MRS. This method is based on volumetric navigators (vNav) that are performed multiple times interleaved with the parent measurement. Navigator image comparison, affine matrix calculation, and acquisition gradient correction to correct the field of view to match subject head motion are done online and in real-time. The purpose of this thesis is to implement this real-time motion correction method to CEST-MRI and study its efficacy and correction potential in phantoms and in healthy volunteers on 7T MR scanner. Additionally, it is hypothesised that the vNav images may be used to correct for motion related receiver sensitivity (B1-) inhomogeneities. Glutamate was chosen as the metabolite of interest due to it being the most abundant neurotransmitter in the human brain and due to its involvement in both normal cognitive function as well as many brain pathologies. Since glutamate has an amine group, it undergoes chemical exchange with water and is thus a usable metabolite for CEST imaging. A glutamate phantom was constructed to show the glutamate concentration sensitivity of CEST and to test and optimise the CEST sequence. Seven healthy volunteers were imaged over a period of two months. All but one volunteer were imaged more than once (2-4 times). Subjects were measured without voluntary head motion and with controlled left-right and up-down head movements. All measurements were performed with and without motion correction to test the motion and B1- -correction methods. Additionally, three volunteers were measured with a dynamic CEST experiment to assess the reproducibility of CEST. The real-time motion correction method was found to be able to correct for small, involuntary head movements. 18 % of the CEST maps measured without motion correction were found to have motion artifacts whereas the equivalent number for maps with motion correction was 0 % (4/22 maps versus 0/18 maps). Larger (>0.7◦ or >0.7 mm in one coregistration step), voluntary head movements could not be corrected adequately. The vNav images could be used to correct for B1- -inhomogeneities. This was found to improve CEST spectra quality and to remove lateral inhomogeneities from the CEST maps. The reproducibility of the CEST-MRI could not be established, however dynamic CEST measurements were found to be stable with only small contrast fluctuation of 4 % between consecutive maps due to noise.

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