Stimulus-modulated approach to steady state (SASS): a flexible paradigm for event-related fMRI.

Functional magnetic resonance imaging (fMRI) studies discard the initial volumes acquired during the approach of the magnetization to its steady-state value. Here we leverage the higher temporal signal-to-noise ratio of these initial volumes to increase the sensitivity of event-related fMRI experiments. We introduce acquisition-free periods (AFPs) that permit the full recovery of the magnetization, followed by an acquisition block of fMRI volumes. An appropriately placed stimulus in the AFP produces a blood oxygenation level-dependent response that peaks during the initial high temporal signal-to-noise ratio phase of the acquisition block. Using humans and monkeys (Callithrix jacchus) at different field strengths, we demonstrate up to a ~50% reduction in the number of trials needed to achieve a given statistical threshold relative to conventional fMRI. The silent AFP can be exploited for the presentation of auditory stimuli or uncontaminated electrophysiological recording and its variable duration allows aperiodic stimulus or response-locked signal averaging as well as gating to physiology or motion.