Abstract

The purpose of this paper is to present a literature review of the theoretical and experimental work that describes the blood-oxygen-level-dependent or BOLD effect using quantitative T2 relaxation mechanism. The BOLD effect is mediated by paramagnetic deoxyhemoglobin which generates magnetic field inhomogeneities around erythrocytes; T2 is modulated by diffusion of spins in and around erythrocytes. Early work in blood describing the mechanisms of T2 relaxation as a function of oxygen saturation, field strength and cell integrity paved the way towards in vivo determination of tissue oxygenation state. Theoretical modeling in blood and tissue microcirculation further shed light into underlying mechanisms of deoxyhemoglobin, leading the way for interrogating myocardial oxygenation state non invasively. From a clinical standpoint, ongoing pre-clinical studies indicate that quantitative T2 may potentially be more specific than signal intensity measures, allowing regional, longitudinal and cross-subject comparison. Furthermore, the T2-based BOLD technique offers greater sensitivity on a 3T scanner, compared to 1.5T, allowing reliable detection of serial changes in perfusion reserve following acute coronary syndrome. We have thus reviewed the theoretical formulations and experimental observations made over several years by many investigators who have given significant insight into the oxygen-sensitive nature of T2 relaxation in blood and hence into the T2-based BOLD effect in tissue microcirculation.