The effect of MR scanner noise on auditory cortex activity using fMRI

Abstract

Auditory functional magnetic resonance imaging (fMRI) studies are limited by the presence of noise produced by echo planar imaging (EPI). The current study quantifies the effect of MR scanner noise on psychophysical measures of the perception of loudness and on measures of tonotopy, the representation of auditory frequencies within the auditory cortex. Seven normal hearing adults were examined using tones of 5 different auditory frequencies (250, 500, 1,000, 2,000, 4,000 Hz) of equal loudness. Using an imaging protocol with peak MR scanner noise at 1,460 Hz, the perception of loudness and detectable fMRI activity in response to a 1-kHz tone was less compared to other frequencies. When the imaging protocol was changed such that peak MR scanner noise occurred at 2,080 Hz, the perception of loudness and detectable fMRI activity in response to a 2-kHz tone was less compared to other frequencies. The reduction in the measured fMRI activity for tones near scanner frequencies may be due to an inflated scanner-induced baseline at those frequencies. In addition, fMRI activity decreased with increasing frequency, possibly due to the upward spread of masking of low-frequency, high-intensity tonal stimuli or the proximity of low-frequency core and belt areas of the auditory cortex. These results demonstrate the direct effect of scanner noise and high-intensity tonal stimuli on measurements of auditory cortex tonotopy.

Figure 1

Frequency spectra of MR scanner noise produced during imaging Protocol 1 (TR/TE = 500/30 msec; 64 × 64; 62.5‐kHz receiver bandwidth; FOV = 240 × 240 mm; 6 slices; slice thickness = 5 mm) and Protocol 2 (TR/TE = 750/30 msec; 96 × 96; 100‐kHz receiver bandwidth; FOV = 240 × 240 mm; 9 slices; slice thickness = 5 mm). The highest peaks in each spectrum occur at 1,460 Hz (Protocol 1) and 2,080 Hz (Protocol 2) (normalized to 0 dB at 1,000 Hz).

Figure 2

Behavioral data showing the percentage of trials in which an audible tone was determined to be louder than a paired tone in the presence of magnet noise for (A) Protocol 1 (peak =1,460 Hz) and (B) Protocol 2 (peak = 2,080 Hz). Data points represent the average responses of single participants at a particular frequency. The curve is a fitted spline through the average of all participants at each frequency. The dashed line indicates the frequency location of MR scanner noise.

Figure 3

A,B: Activity within the auditory cortex (number of activated voxels, t > 4.0, times percentage increase in MR signal above baseline) for all subjects using (A) imaging Protocol 1 (peak = 1,460 Hz) and (B) imaging Protocol 2 (peak = 2,080 Hz). Data points represent the fMRI response from single participants at a particular frequency. The curve is a fitted spline through the average of all participants at each frequency. C,D: Proportion of “best” frequency voxels in fMRI maps of auditory cortex activity obtained using imaging Protocol 1 (C) (peak = 1,460 Hz) and Protocol 2 (D) (peak = 2,080 Hz). Data points represent single participants at a particular frequency. The curve is a fitted spline through the average of all participants at each frequency. The dashed line indicates the frequency location of MR scanner noise.

Figure 4

Representative functional maps of tonotopicity for two participants during Protocol 1 (A) and for four participants during Protocol 2 (B,C) overlaid on oblique coronal structural MR images through the superior temporal lobe, which are in the same imaging planes as the prescribed functional images. Colored voxels represent the frequency of highest activation (right hemisphere is on left side of image).