Dynamic characteristics of oxygenation-sensitive MRI signal in different temporal protocols for imaging human brain activity

The temporal characteristics of cerebral blood oxygenation during human bra in activation were monitored with dynamic echo-planar imaging (EPI) using t he blood oxygenation level dependent (BOLD) fMRI. We investigated oxygenati on-sensitive signal changes: 1. during repetitive block stimuli, to determi ne the latency of the activation-induced signal change in the primary visua l cortex; 2. on shortening the rest periods between constant stimulated pha ses, to investigate the limitations that this latency poses in temporal res olution of the technique; and 3. on sustained steady-state stimulation, to characterise oxygenation changes during prolonged brain activation using di fferent stimuli. Delayed intrinsic haemodynamic response and a finite signa l-to-noise ratio limit the temporal resolution achieved with BOLD fMRI. Sep arate activation periods were resolved when the delay between consecutive s timulations was at least 2 s. In this study oxygenation remained elevated t hroughout sustained activation, suggesting a constant rate of oxygen consum ption by the primary cortical neurones during activation. Characterisation of fMRI signal dynamics in dynamic temporal protocols is significant both i n terms of optimising stimulation protocols and the potential to gain insig ht into the physiological mechanisms underlying neuronal activation which c ould increase the clinical applicability of the technique.