Abstract
Detecting sound-related activity using functional MRI requires the auditory stimulus to be more salient than the intense background scanner acoustic noise. Various strategies can reduce the impact of scanner acoustic noise, including "sparse" temporal sampling with single/clustered acquisitions providing intervals without any background scanner acoustic noise, or active noise cancelation (ANC) during "continuous" temporal sampling, which generates an acoustic signal that adds destructively to the scanner acoustic noise, substantially reducing the acoustic energy at the participant's eardrum. Furthermore, multiband functional MRI allows multiple slices to be collected simultaneously, thereby reducing scanner acoustic noise in a given sampling period. Isotropic multiband functional MRI (1.5 mm) with sparse sampling (effective TR = 9000 ms, acquisition duration = 1962 ms) and continuous sampling (TR = 2000 ms) with ANC were compared in 15 normally hearing participants. A sustained broadband noise stimulus was presented to drive activation of both sustained and transient auditory responses within subcortical and cortical auditory regions. Robust broadband noise-related activity was detected throughout the auditory pathways. Continuous sampling with ANC was found to give a statistically significant advantage over sparse sampling for the detection of the transient (onset) stimulus responses, particularly in the auditory cortex (P < .001) and inferior colliculus (P < .001), whereas gains provided by sparse over continuous ANC for detecting offset and sustained responses were marginal (p ~ 0.05 in superior olivary complex, inferior colliculus, medial geniculate body, and auditory cortex). Sparse and continuous ANC multiband functional MRI protocols provide differing advantages for observing the transient (onset and offset) and sustained stimulus responses.
Highlights
Functional MRI of the auditory pathways requires a neural response to a delivered auditory stimulus that exceeds that of the scanner acoustic noise,[1] otherwise stimulus-related neuronal activity will be compromised
At least one study has reported that turning the coolant pumps off during acquisition can reduce some of the background scanner acoustic noise,[4,5] but this is not a solution to the major source of acoustic noise produced by rapid gradient switching
Available on any scanner, to reduce the impact of background scanner acoustic noise is sparse temporal sampling, in which single or clustered brain volumes are acquired interspersed with periods of no data acquisition during the stimulus presentation in a long TR.6-12 This has been shown to enhance the detection of sound-related activity in the auditory cortex (AC) compared with more conventional continuous sampling.[6,13]
Summary
Functional MRI (fMRI) of the auditory pathways requires a neural response to a delivered auditory stimulus that exceeds that of the scanner acoustic noise,[1] otherwise stimulus-related neuronal activity will be compromised. The intense bursts of acoustic noise generated by rapidly switching the electrical current through the gradient coil generate sound-related activity in the kilohertz range, which stimulates the central auditory pathways.[1,2] This acoustic noise confounds the detection of an fMRI response to the sound of interest, as the resting (nonstimulus periods) are never silent during conventional scanning.[3] At least one study has reported that turning the coolant pumps off during acquisition can reduce some of the background scanner acoustic noise,[4,5] but this is not a solution to the major source of acoustic noise produced by rapid gradient switching. Determining the most favorable acquisition method for the assessment of both transient and steady-state auditory responses is paramount for studies requiring sensitivity to both cortical and subcortical activity, such as in the assessment of the effect of noise exposure on the entire ascending auditory pathway.[17]
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