![]() ![]() The highest-level noise, however, is intermittent and is produced whenever an image is acquired. This equipment often includes a pump for liquid helium used to supercool the imager’s permanent magnet, a fan for supplying ventilation to the patient, and the air-handling equipment for the imager room. Continuous noise can originate from ancillary equipment located in the room that houses the imager. Although some modifications to the timing of imaging acquisition have been shown to reduce the influence of the noise on brain activity, these modified paradigms compromise either the temporal resolution of measurements or the efficiency with which data are acquired ( Edmister et al., 1999 Hall et al., 1999 see Melcher et al., 1999 for a discussion).Īcoustic noise in most imaging environments arises from various sources. 1999), but they are insufficient to achieve acceptably quiet conditions ( Ravicz and Melcher, 1998a, b). Earmuffs or earplugs are commonly used to reduce noise levels heard by subjects (e.g., Savoy et al. ![]() If the noise can be heard, then the auditory system presumably is in a different state than during quiet conditions more typical of physiological or psychophysical experiments on hearing. For example, the background noise can mask the stimuli ( Shah et al., 1999 Eden et al., 1999), and the noise itself can produce brain activity that is not related to the intended stimuli ( Bandettini et al., 1998 Ulmer et al., 1998 Talavage et al., 1999 Edmister et al., 1999). These unwanted sounds, or “acoustic noise,” pose particular difficulties for functional MRI (fMRI) studies that measure brain activation in response to sound stimuli. However, an undesirable aspect of present-day MRI is the high-level sounds produced by the imager and associated equipment. Magnetic resonance imaging (MRI) permits mapping of bodily structure and function and is now used routinely for both clinical and basic research studies. Knowledge of the sources and characteristics of the noise enabled the examination of general approaches to noise control that could be applied to reduce the unwanted noise during fMRI sessions. In addition, the coolant pump for the imager’s permanent magnet and the room air handling system were sources of ongoing noise lower in both level and frequency than gradient coil noise. The gradient noise waveform was highly repeatable. The noise persisted above background levels for 300-500 ms after gradient activity ceased, indicating that resonating structures in the imager or noise reverberating in the imager room were also factors. The frequency content and timing of the most intense noise components indicated that the noise was primarily attributable to the readout gradients in the imaging pulse sequence. The noise spectrum (calculated over a 10-ms window coinciding with the highest-amplitude noise) showed a prominent maximum at 1 kHz for the 1.5-T imager (115 dB SPL) and at 1.4 kHz for the 3-T imager (131 dB SPL). Peak noise levels were 123 and 138 dB re 20 μPa in a 1.5-tesla (T) and a 3-T imager, respectively. As a first step toward reducing the noise during fMRI, this paper describes the temporal and spectral characteristics of the noise present under typical fMRI study conditions for two imagers with different static magnetic field strengths. For studies of the auditory system, acoustic noise generated during fMRI can interfere with assessments of this activation by introducing uncontrolled extraneous sounds. ![]() ![]() Functional magnetic resonance imaging (fMRI) enables sites of brain activation to be localized in human subjects. ![]()
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