High Field Functional MRI in Humans: Applications to Cognitive Function

Abstract

Magnetic resonance imaging (MRI) technology at field strengths up to 1.5 T is mature after many years of experience by manufacturers in providing a clinically acceptable level of performance. The MRI technique is the accepted modality of choice for neuroimaging when the question of structural abnormality is raised. The development of functional MRI (fMRI) offers potential for developing clinical applications in which brain function as well as anatomy are examined [1]. The presumed mechanism on which most current fMRI is based is the increased MR signal that arises from the increased blood flow that occurs with increased neuronal activity. The increased flow, out of proportion to the tissue oxygen utilization, increases the net tissue blood oxygenation [2] thereby decreasing the magnetic susceptibility-induced transverse relaxation caused by deoxygenated blood. This has been termed blood oxygenation level dependent (BOLD) contrast [3]. Such signal changes in the microvasculature are small (1–5%), and therefore always limited by the available signal-to-noise ratio (SNR). Strategies to improve the reliability of such measurements for clinical applications would be useful.