Functional magnetic resonance imaging (fMRI) is one of the principal neuroimaging techniques for studying human audition, but it generates an intense background sound which hinders listening performance and confounds measures of the auditory response. This paper reports the perceptual effects of an active noise control (ANC) system that operates in the electromagnetically hostile and physically compact neuroimaging environment to provide significant noise reduction, without interfering with image quality. Cancellation was first evaluated at 600Hz, corresponding to the dominant peak in the power spectrum of the background sound and at which cancellation is maximally effective. Microphone measurements at the ear demonstrated 35dB of acoustic attenuation [from 93to58dB sound pressure level (SPL)], while masked detection thresholds improved by 20dB (from 74to54dB SPL). Considerable perceptual benefits were also obtained across other frequencies, including those corresponding to dips in the spectrum of the background sound. Cancellation also improved the statistical detection of sound-related cortical activation, especially for sounds presented at low intensities. These results confirm that ANC offers substantial benefits for fMRI research.

Cross-talk cancellation is a method for synthesizing virtual auditory space using loudspeakers. One implementation is the "Optimal Source Distribution" technique [T. Takeuchi and P. Nelson, J. Acoust. Soc. Am. 112, 2786-2797 (2002)], in which the audio bandwidth is split across three pairs of loudspeakers, placed at azimuths of +/-90 degrees, +/-15 degrees, and +/-3 degrees, conveying low, mid, and high frequencies, respectively. A computational simulation of this system was developed and verified against measurements made on an acoustic system using a manikin. Both the acoustic system and the simulation gave a wideband average cancellation of almost 25 dB. The simulation showed that when there was a mismatch between the head-related transfer functions used to set up the system and those of the final listener, the cancellation was reduced to an average of 13 dB. Moreover, in this case the binaural interaural time differences and interaural level differences delivered by the simulation of the optimal source distribution (OSD) system often differed from the target values. It is concluded that only when the OSD system is set up with "matched" head-related transfer functions can it deliver accurate binaural cues.

Magnetic resonance imaging (MRI) scanners can produce noise measuring over 130 dB SPL. This noise stimulates the auditory nervous system, limiting the dynamic range for stimulus driven activity in functional MRI (fMRI) experiments and can influence other brain functions. Even for structural scans it causes subject anxiety and discomfort in addition to the impediment to communications. Here we describe the realization and validation of a sound system for sound presentation inside an MRI scanner and the modifications to a standard active noise control technique for use in the noisy and compact environment of the scanner. This paper provides a review of the technology available for the presentation of audio stimuli in an MRI environment and the modifications required for the active control of scanner noise. Some of the content has been previously published [Chambers J, Akeroyd MA, Summerfield AQ, Palmer AR. Active control of the volume acquisition noise in functional magnetic resonance imaging: method and psychoacoustical evaluation. J Acoust Soc Am 2001;110(6):3041–54; Levitt H. Transformed up–down methods in psychoacoustics. J Acoust Soc Am 1971;49:467–77], but this paper goes further in describing the stages of development as the system performance was optimised. The performance of the system and both the objective and subjective reduction of the scanner noise are reported. Finally, we discuss recent improvements to the system that are currently being evaluated and describe the theory of opto-acoustical transducers that operate on the principle of light modulation. These are immune from, and do not create, electro-magnetic interference (EMI) and radio-frequency interference (RFI).

Magnetic resonance imaging (MRI) has become the dominant technique for noninvasive measurements of central auditory function in humans, but its operation generates intense acoustic noise (some 115 decibels). Although this can typically be attenuated by about 30 decibels with ear defenders, there is considerable scope for additional benefit. We took advantage of two features of the sound: (i) its predictable temporal onset, and (ii) its fixed power spectrum (which is dominated by intense harmonic components at low frequencies) to engineer an active noise cancellation (ANC) system to achieve further noise reduction. We measured its perceptual effectiveness by collecting detection thresholds for a tonal signal, masked by the image acquisition noise of the scanner (Philips 3 Tesla Intera) with and without the ANC operating. We presented signal frequencies centered at the peaks of the objective cancellation (150, 300, and 600 Hz), plus 450 Hz (a control frequency achieving no significant acoustic cancellation). Across four listeners, the ANC system gave average benefits of 4, 11, 15 dB, respectively, at the peaks and 10 dB at 450 Hz. These results confirm that the system offers substantial benefits for audibility in MR research. Moreover, the ANC equipment does not interfere with image quality.

In this short review, we outline the challenges presented by the hostile environment for auditory fMRI. There are two main elements to this: the high level magnetic fields and the intense sound generated by the scanner operation. We then describe a series of techniques that have been employed to minimize the effects of the scanner noise on the subject and upon the subject's evoked responses to sound stimuli. These include scanner designs, passive attenuation, use of specifically designed scanning sequences, specially designed sound delivery systems and active noise control. Finally, we briefly describe the use of some of these in combination to allow measurement of sound activation of different brain areas non-invasively using fMRI.