Quantitative assessment of auditory cortex responses induced by imager acoustic noise

A clustered volume acquisition functional MRT pulse sequence was modified t o assess the response to the acoustic noise of echo-planar imaging in the a uditory cortex and to determine whether it is possible to obtain data which is relatively free of acoustic contamination. The spatial location and str ength (percent signal change) of cortical responses to the imager noise wer e examined by introducing extra gradient readouts, without slice excitation , to provide acoustic stimulation immediately prior to acquisition of a cer ebral volume. The duration of acoustic stimulation was controlled by varyin g the number of extra gradient readouts. Slice acquisitions were clustered at the end of the repetition time (TR) period to prevent a response from be ing induced by the volume acquisition itself ("Intra-Acquisition Response") . The cerebral volumes were acquired using a long TR in order to limit the integration of the cortical response across volume acquisitions ("Inter-Acq uisition Response"). Cortical responses were observed to be largest and mos t significant on the medial two-thirds of Heschl's gyrus, the location of p rimary auditory cortex. Mean signal changes induced by the imager noise wer e observed to be as high as 0.95%. A 2 sec delay prior to onset of the BOLD response was empirically determined. These results demonstrate that cluste red volume acquisitions may be utilized for up to 2 sec of volume acquisiti on without inducing an appreciable Intra-Acquisition Response and can be us ed, with a sufficiently long TR, to provide data which are similarly free o f any Inter-Acquisition Response. (C) 1999 Wiley-Liss, Inc.