"Sparse" temporal sampling in auditory fMRI

The use of functional magnetic resonance imaging (fMRI) to explore central auditory function may be compromised by the intense bursts of stray acousti c noise produced by the scanner whenever the magnetic resonance signal is r ead out. We present results evaluating the use of one method to reduce the effect of the scanner noise: "sparse" temporal sampling. Using this techniq ue, single volumes of brain images are acquired at the end of stimulus and baseline conditions. To optimize detection of the activation, images are ta ken near to the maxima and minima of the hemodynamic response during the ex perimental cycle. Thus, the effective auditory stimulus for the activation is not masked by the scanner noise. In experiment 1, the course of the hemodynamic response to auditory stimula tion was mapped during continuous task performance. The mean peak of the re sponse was at 10.5 sec after stimulus onset, with little further change unt il stimulus offset. In experiment 2, sparse imaging was used to acquire act ivation images. Despite the fewer samples with sparse imaging, this method successfully delimited broadly the same regions of activation as convention al continuous imaging. However, the mean percentage MR signal change within the region of interest was greater using sparse imaging. Auditory experime nts that use continuous imaging methods may measure activation that is a re sult of an interaction between the stimulus and task factors (e.g., attenti ve effort) induced by the intense background noise. We suggest that sparse imaging is advantageous in auditory experiments as it ensures that the obta ined activation depends on the stimulus alone. (C) 1999 Wiley-Liss, Inc.