Slice acquisition order and blood oxygenation level dependent frequency content: an event-related functional magnetic resonance imaging study

Many event-related functional magnetic resonance imaging paradigms performe d so far have been designed to study a limited mis can be exploratory, ther efore requiring whole part of the brain with high temporal resolution. Howe ver, event-related paradigms can be exploratory, therefore requiring whole brain scans and so repetition times (TR) of several seconds, For these larg e TR values, the slice acquisition order may have an important effect on th e detection of event-related activation. Indeed, when the scanning is inter leaved, the temporal delay between the acquisition of two contiguous slices can reach a few seconds. During this time, the subject is likely to move, and the haemodynamic response will vary significantly. In this case, the in terpolation applied between contiguous slices for motion correction induces a temporal smoothing between voxels that are spatially close but temporall y sampled a few seconds apart. This should modify the frequency structure o f the response and may impair the detection of short events. We, therefore tested the effect of three acquisition schemes (sequential, sequential with gap and interleaved, INT) at two repetition times (TR = 3 and 6 s on six a nd seven subjects, respectively) on activation detection and frequency cont ent in a visual motion event-related paradigm. Unexpectedly, for large TR ( 6 s), results were found in favour of the INT acquisition scheme (P < 0.05) . For smaller TR, no strong bias could be found. Generally, intra-subject v ariability (across acquisition schemes) is found to be much smaller than in ter-subject variability, confirming the importance of multi-subjects analys es. Our study also shows that important physiological information is carrie d by high frequency components that should not be filtered out. (C) 2001 El sevier Science B.V. All rights reserved.