A Study of NMR Relaxation in Blood Mechanistic Considerations and Implications for Quantitative Functional MRI
Year: 2002
Type: dissertation
Abstract: Nuclear magnetic resonance (NMR) imaging (MRI) and spectroscopy (MRS) are invaluable tools in medical imaging, providing information on both the structure and the function of living systems in a noninvasive manner.The NMR signal and image contrast are product of several physical and physiological factors, the most important being NMR relaxation of the tissue.In the present study, blood was chosen as a model tissue, since it accounts for 7-8 % of body weight and approximately 4 % of gray matter.This study focused on the quantification of dynamic relaxation properties (relaxation times T 1 , T 1ρ , T 2 and T 2 *) of isolated blood as well as on the consequences of these properties on quantitative functional MRI (fMRI) applications.In transverse NMR relaxation (CPMG-T 2 , single-echo (Hahn) T 2 and T 2 *), the expected blood oxygen level dependent (BOLD) behavior was observed and quantified at several hematocrits at 1.5 and 4.7 T. The blood CPMG results were utilized in the determination of oxygen extraction ratio in vivo in humans at 1.5 T. Moving from single spin echoes to gradient echoes (T 2 *) was shown to add an oxygenation-independent term to the spin echo relaxation rate.The single and gradient echo data were used to simulate intravascular BOLD signal changes in fMRI experiments with neuronal activation both at 1.5 T and at 4.7 T.The longitudinal relaxation in blood at 4.7 T was found to depend on hematocrit.Interestingly, some dependence on blood oxygenation also in both T 1 and T 1ρ was observed, possibly arising from intramolecular relaxation in hemoglobin.The consequences of physiological variation in the blood parameters, and thus in T 1 , in the determination of cerebral blood flow in vivo were evaluated and found to induce significant bias under conceivable conditions.The study provides a thorough quantitative description of relaxation processes in blood and addresses their significance in different fMRI applications.In addition, a set of reference values are given for blood, at body temperature and over a wide range of oxygen saturation levels as well as hematocrits, for fMRI models of hemodynamics in vivo.
Author: Johanna Närväinen
Language: English
Subfield: Radiology, Nuclear Medicine and Imaging
Field: Medicine
Domain: Health Sciences
Open Access status: green
Funder: Johns Hopkins University
OpenAlex ID: W3096296966