Ravicz M E, Melcher J R, Kiang N Y
Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston 02114, USA.
J Acoust Soc Am. 2000 Oct;108(4):1683-96. doi: 10.1121/1.1310190.
Functional magnetic resonance imaging (fMRI) enables sites of brain activation to be localized in human subjects. For studies of the auditory system, acoustic noise generated during fMRI can interfere with assessments of this activation by introducing uncontrolled extraneous sounds. 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. Peak noise levels were 123 and 138 dB re 20 microPa in a 1.5-tesla (T) and a 3-T imager, respectively. 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). 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 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 gradient noise waveform was highly repeatable. 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. 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.
功能磁共振成像（fMRI）能够在人类受试者中定位大脑激活部位。对于听觉系统研究而言，fMRI过程中产生的声学噪声会引入不受控制的外部声音，从而干扰对这种激活的评估。作为降低fMRI噪声的第一步，本文描述了在典型fMRI研究条件下，两种具有不同静磁场强度的成像仪所产生噪声的时间和频谱特性。在1.5特斯拉（T）和3T成像仪中，峰值噪声水平分别为相对于20微帕的123分贝和138分贝。噪声频谱（在与最高幅度噪声重合的10毫秒窗口内计算）显示，1.5T成像仪在1千赫处有一个突出的最大值（115分贝声压级），3T成像仪在1.4千赫处有一个突出的最大值（131分贝声压级）。最强噪声成分的频率内容和时间表明，噪声主要归因于成像脉冲序列中的读出梯度。梯度活动停止后，噪声持续高于背景水平300 - 500毫秒，这表明成像仪中的共振结构或成像仪房间内的噪声回响也是因素之一。梯度噪声波形具有高度可重复性。此外，成像仪永磁体的冷却泵和房间空气处理系统是持续噪声的来源，其噪声水平和频率均低于梯度线圈噪声。了解噪声的来源和特性有助于研究可用于在fMRI扫描过程中减少不必要噪声的通用噪声控制方法。
