QUANTITATIVE STUDY OF LONGITUDINAL RELAXATION (<i>T<sub>1</sub></i>) CONTRAST MECHANISMS IN BRAIN MRI

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dc.contributor.advisorAnlage, Steven M.en_US
dc.contributor.authorJiang, Xuen_US
dc.contributor.departmentPhysicsen_US
dc.contributor.publisherDigital Repository at the University of Marylanden_US
dc.contributor.publisherUniversity of Maryland (College Park, Md.)en_US
dc.date.accessioned2017-06-22T05:43:57Z
dc.date.available2017-06-22T05:43:57Z
dc.date.issued2017en_US
dc.description.abstractLongitudinal relaxation (<i>T<sub>1</sub></i>) contrast in MRI is important for studying brain morphology and is widely used in clinical applications. Although MRI only detects signals from water hydrogen (<sup>1</sup>H) protons (WPs), <i>T<sub>1</sub></i> contrast is known to be influenced by other species of <sup>1</sup>H protons, including those in macromolecules (MPs), such as lipids and proteins, through magnetization transfer (MT) between WPs and MPs. This complicates the use and quantification of <i>T<sub>1</sub></i> contrast for studying the underlying tissue composition and the physiology of the brain. MT contributes to <i>T<sub>1</sub></i> contrast to an extent that is generally dependent on MT kinetics, as well as the concentration and NMR spectral properties of MPs. However, the MP spectral properties and MT kinetics are both difficult to measure directly, as the signal from MPs is generally invisible to MRI. Therefore, to investigate MT kinetics and further quantify <i>T<sub>1</sub></i> contrast, we first developed a reliable way to indirectly measure the MP fraction and their exchange rate with WPs, with minimal dependence on the spectral properties of MPs. For this purpose, we used brief, high-power radiofrequency (RF) NMR excitation pulses to almost completely saturate the magnetization of MPs. Based on this, both MT kinetics and the contribution of MPs to <i>T<sub>1</sub></i> contrast through MT were studied. The thus obtained knowledge allowed us to subsequently infer the spectral properties of MPs by applying low-power, frequency-selective off-resonance RF pulses and measuring the offset-frequency dependent effect of MPs on the WP MRI signal. A two-pool exchange model was used in both cases to account for direct effects of the RF pulse on WP magnetization. Consistent with earlier works using MRI at low-field and post-mortem analysis of brain tissue, our novel measurement approach found that MPs constitute an up to 27% fraction of the total <sup>1</sup>H protons in human brain white matter, and their spectrum follows a super-Lorentzian line with a <i>T<sub>2</sub></i> of 9.6±0.6 μs and a resonance frequency centered at -2.58±0.05 ppm, at 7 T. <i>T<sub>1</sub></i> contrast was found to be dominated by MP fraction, with iron only modestly contributing even in the iron-rich regions of brain.en_US
dc.identifierhttps://doi.org/10.13016/M2V87T
dc.identifier.urihttp://hdl.handle.net/1903/19315
dc.language.isoenen_US
dc.subject.pqcontrolledPhysicsen_US
dc.subject.pqcontrolledMedical imagingen_US
dc.subject.pqcontrolledBiophysicsen_US
dc.subject.pquncontrolledcontrast mechanismen_US
dc.subject.pquncontrolledlongitudinal relaxationen_US
dc.subject.pquncontrolledmacromolecular protonen_US
dc.subject.pquncontrolledmagnetization transferen_US
dc.subject.pquncontrolledMRIen_US
dc.subject.pquncontrolledt1 relaxationen_US
dc.titleQUANTITATIVE STUDY OF LONGITUDINAL RELAXATION (<i>T<sub>1</sub></i>) CONTRAST MECHANISMS IN BRAIN MRIen_US
dc.typeDissertationen_US

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