Effect of an active noise control system on acoustical noise during magnetic resonance imaging diagnosis

摘要

A magnetic resonance imaging (MRI) equipment generates loud sounds during its operation. The sound pressure level depends on the imaging sequences but is generally around 100 dB. This makes patients uncomfortable, and temporary hearing loss is possible if ear protection to decrease the sound pressure level by 20 dB is not used. This situation is so discomforting to the patient that in some cases MRI diagnosis and treatment may be refused. We studied ear protectors with an active noise control (ANC) system. Traditional protectors can decrease sound pressure level for high frequencies, but ANC systems can control low-frequency sounds as well. This study considers improvements to the acoustical environment when using ANC systems for MRI patients. The ANC system we developed is a feedforward system because MRI acoustical noise is unsteady and pulsed. The system uses nonmagnetic devices, ear protectors, and optical microphones because MRI rooms are highly magnetic environments. A computer simulation showed that the transition of the level of MRI acoustical noise was controlled by the ANC system during a diagnosis. As a result, the system reduced the MRI acoustical noise from 110 dB to 70 dB and was capable of changing the imaging sequences during MRI operation.