Acoustic noise characteristics of a 4 Telsa MRI scanner

Purpose: To quantify the acoustic noise characteristics of a 4 Tesla MRI scanner, and determine the effects of structural acoustics and gradient pulse excitations on the sound field so that feasible noise control measures can be developed. Materials and Methods: Acoustic noise emissions were measured in the ear and mouth locations of a typical adult. The sound pressure measurements were acquired simultaneously with the electrical current signals of the gradient pulses. Two forms of gradient waveforms (impulsive and operating pulses) were studied. Results: The sound pressure levels (SPLs) emitted by the MRI scanner operating in echo-planar imaging (EPI) mode were in the range of 120-130 decibels. Three types of sound pressure responses were observed in the EPI sequences: 1) harmonic, 2) nonharmonic, and 3) broadband. The frequency-encoding gradient pulses were the most dominant and produced generally odd-number harmonics and non-harmonics. The phase-encoding gradient pulses generated mostly even-number harmonics, and the slice-selection gradient pulses produced primarily a broadband spectrum. Conclusion: The operating condition acoustic spectrum can be predicted from the magnet-structural acoustic transfer functions, which are independent of imaging sequences. This finding is encouraging because it shows that it is possible to treat such noises with an active noise control application.