Quantitative assessment of auditory cortex responses induced by imager acoustic noise.

A clustered volume acquisition functional MRI pulse sequence was modified to assess the response to the acoustic noise of echo-planar imaging in the auditory cortex and to determine whether it is possible to obtain data which is relatively free of acoustic contamination. The spatial location and strength (percent signal change) of cortical responses to the imager noise were examined by introducing extra gradient readouts, without slice excitation, to provide acoustic stimulation immediately prior to acquisition of a cerebral volume. The duration of acoustic stimulation was controlled by varying the number of extra gradient readouts. Slice acquisitions were clustered at the end of the repetition time (TR) period to prevent a response from being 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-Acquisition Response"). Cortical responses were observed to be largest and most significant on the medial two-thirds of Heschl's gyrus, the location of primary auditory cortex. Mean signal changes induced by the imager noise were 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 clustered volume acquisitions may be utilized for up to 2 sec of volume acquisition without inducing an appreciable Intra-Acquisition Response and can be used, with a sufficiently long TR, to provide data which are similarly free of any Inter-Acquisition Response.