Active control of the volume acquisition noise in functional magnetic resonance imaging: Method and psychoacoustical evaluation

Abstract

Functional magnetic resonance imaging (fMRI) provides a noninvasive tool for observing correlates of neural activity in the brain while a subject listens to sound. However, intense acoustic noise is generated in the process of capturing MR images. This noise stimulates the auditory nervous system, limiting the dynamic range available for displaying stimulus-driven activity. The noise is potentially damaging to hearing and is distracting for the subject. In an active noise control (ANC) system, a reference sample of a noise is processed to form a sound which adds destructively with the noise at the listener's ear. We describe an implementation of ANC in the electromagnetically hostile and physically compact MRI scanning environment. First, a prototype system was evaluated psychoacoustically in the laboratory, using the electrical drive to a noise-generating loudspeaker as the reference. This system produced 10-20 dB of subjective noise-reduction between 250 Hz and 1 kHz, and smaller amounts at higher frequencies. The system was modified to operate in a real MR scanner where the reference was obtained by recording the acoustic scanner noise. Objective reduction by 30-40 dB of the most intense component in scanner noises was realized between 500 Hz and 3500 Hz, and subjective reduction of 12 dB and 5 dB in tests at frequencies of 600 Hz and at 1.9 kHz, respectively. Although the benefit of ANC is limited by transmission paths to the cochlea other than air-conduction routes from the auditory meatus, ANC achieves worthwhile attenuation even in the frequency range of maximum bone conduction (1.5-2 kHz). ANC should, therefore, be generally useful during auditory fMRI.

Simulation study on active noise control for a 4-T MRI scanner

“…a feedback control that has a feedback controller K(s) embedded with a feedforward control loop in which an adaptive FIR filter W(z) is used, as illustrated in Figure 2. (a) Feedforward control-The feedforward loop is an adaptive controller which uses the filtered-x least mean square (FXLMS) algorithm to adjust the filter weights. The FXLMS control algorithm, an extended version of the LMS algorithm typically used for dynamic systems with phase delay secondary paths, is well studied and has been widely applied to many active vibration and noise control applications [14,16,[19][20][23][24]. This controller requires detailed information about the secondary path transfer function.…”

“…For low frequency noises, the passive materials and structures are bulky as well as uncomfortable, making them impractical for MRI application. In general, frequencies above 1 kHz are adequately attenuated by 30 dB using conventional ear protection devices, but lower frequencies around 250 Hz are attenuated by about 10 dB which may not be enough for MRI noise reduction [14]. Except for methods involving the modification of gradient pulse sequences, all other approaches require the MRI scanner to be redesigned, which poses other major challenges.…”

“…This method has already been applied to numerous problems where acoustic noise was severe and traditional passive approaches were not suitable or efficient. However, only a few studies using active control for MRI noise reduction have been reported in the literature [14][15][16][17][18][19][20]. To apply the ANC technique on MRI acoustic noise suppression, two things need to be addressed.…”

“…Using this approach, they achieved an average sound power reduction of approximately 19dB. Chambers et al [14] implemented and evaluated an ANC prototype system for MRI noise reduction. In the most intense component of the scanner noises an objective reduction of 30 −40dB was obtained for the frequency between 500Hz and 3500Hz.…”

Efficacy of passive acoustic screening: Implications for the design of imager and MR‐suite

“…Another potential solution may be in the application of active noise control; that is, the reduction of MR-generated acoustic noise by superposition of an exactly inversed sound (20). Currently, we are investigating the application of an analogous technique called "active structural acoustic control."…”