... the auditory pathways. Uwe Baumann, Dept. of Audiology, Lorenz Jäger, Dept. of Radiology. Ludwig-Maximilians-Universität München, Klinikum Großhadern.
Characteristics of MRI acoustic noise in functional imaging of the auditory pathways Uwe Baumann, Dept. of Audiology, Lorenz Jäger, Dept. of Radiology Ludwig-Maximilians-Universität München, Klinikum Großhadern Sound pressure
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Magnet resonance imaging (MRI) can give insights in functional brain properties by applying the BOLD (Blood Oxygenation Level Dependent Contrast) paradigm. Examination of the auditory pathways is difficult due to the intrinsic MRI scanner noise. Special care has to be taken to ensure a sufficient sensation level above masked threshold. An investigation of the MRI noise as well as the determination of the transfer characteristic of the sound delivery system is necessary. Due to the required time/place resolution, the EPI method (Echo Planar Imaging) is employed in functional MRI.
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The custom-made sound delivery system achieves an attenuation of 20 dB at low and 50 dB in the high frequencies. Because of a tube based sound transportation and driver characteristics a rippled band pass transfer function occurs (above). With level adjustment a frequency range from 300 Hz to 3 kHz seems to be applicable for fMRI with this device. A wobbled sinusoid (30 %, 6 Hz) was placed at 500 Hz between fundamental and second harmonic. A level of 90 dB was chosen to obtain a sensation level of approximately 50 dB SL. The overlay of conventional and functional MRI shows activated areas in Heschel’s Gyrus in the right hemisphere for the stimulus presented on the left ear (male, normal hearing subject).
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The time signal (top left, recorded inside the MRI cabinet) displays a sharp pressure pulse caused by the slice selection gradient (112 dB SPL peak) and a mixture of pulses generated by readout- and phase coding gradient. The power spectrum shows a steep high frequency cutoff disrupted by several harmonic peaks ( f0 = 300 Hz). An ear related spectral transformation (FTT) was applied to acquire relevant auditory components (above). The repeated phase coding gradient is responsible for the salient pitch of the sound of the EPI sequence.
