Introduction to BOLD fMRI

Abstract

Over the past 20 years, functional magnetic resonance imaging (fMRI) has been widely used to measure the brain activity. The primary form of fMRI uses the blood oxygenation level dependent (BOLD) contrast, in which hemoglobin in a red blood cell is used as an endogenous contrast agent, based on the difference in magnetic properties of oxyhemoglobin (diamagnetic) and deoxyhemoglobin (paramagnetic). In the physical basis, the presence of paramagnetic deoxyhemoglobin in blood vessels causes changes in the local magnetic field gradients that decrease MR signal. In the physiological basis, when neuronal activity increases, the blood flow increases to overcompensate the increased oxygen metabolism. The mismatch between the blood flow and the oxygen consumption leads to the paradoxical increase in MR signal with increased neuronal activity. In this tutorial lecture, we will explain the complex mechanism underlying the BOLD signal, which is needed to correctly interpret the fMRI data.