Acoustic fMRI noise: Linear time-invariant system model
Acoustic fMRI noise: Linear time-invariant system model
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DOI：
10.1109/tbme.2008.923112
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发表时间：
2008-09-01
影响因子：
4.6
通讯作者：
Duifhuis, Hendrikus (Diek)
中科院分区：
文献类型：
--
作者：
Sierra, Carlos V. Rizzo;Versluis, Maarten J.;Duifhuis, Hendrikus (Diek)
功能性磁共振成像（fMRI）能够定位人类受试者的大脑激活部位。然而，对于听觉系统研究，扫描仪产生的噪声往往会干扰这种激活的评估。理解和建模fMRI的声学噪声是一个有用的步骤，以减少它。为了研究声学噪声，MR扫描器被建模为线性电声系统，其产生与输入梯度电流的时间导数成比例的声压信号。针对两种不同的输入规范确定一台MR扫描仪的传递函数：1）使用扫描仪软件计算的梯度波形，2）使用梯度电流记录。在4 kHz以下，第一种方法与第二种方法一样可靠，当无法直接测量梯度电流时，鼓励使用这种方法。此外，通过脉冲响应分析确定梯度线圈系统的线性阶数和平均阻尼特性。由于fMRI通常基于回波平面成像（EPI）序列，因此可以通过计算EPI序列的声学输出来获得传递函数预测能力的有用验证。我们发现EPI序列的预测声压级（SPL）为104 dB SPL，而测量值为102 dB SPL。到目前为止，预测的EPI压力波形显示出与直接测量的EPI压力波形的相似性以及一些差异。
Functional magnetic resonance imaging (fMRI) enables sites of brain activation to be localized in human subjects. For auditory system studies, however, the acoustic noise generated by the scanner tends to interfere with the assessments of this activation. Understanding and modeling fMRI acoustic noise is a useful step to its reduction. To study acoustic noise, the MR scanner is modeled as a linear electroacoustical system generating sound pressure signals proportional to the time derivative of the input gradient currents. The transfer function of one MR scanner is determined for two different input specifications: 1) by using the gradient waveform calculated by the scanner software and 2) by using a recording of the gradient current. Up to 4 kHz, the first method is shown as reliable as the second one, and its use is encouraged when direct measurements of gradient currents are not possible. Additionally, the linear order and average damping properties of the gradient coil system are determined by impulse response analysis. Since fMRI is often based on echo planar imaging (EPI) sequences, a useful validation of the transfer function prediction ability can be obtained by calculating the acoustic output for the EPI sequence. We found a predicted sound pressure level (SPL) for the EPI sequence of 104 dB SPL compared to a measured value of 102 dB SPL. As yet, the predicted EPI pressure waveform shows similarity as well as some differences with the directly measured EPI pressure waveform.