Making MRI quieter

We have mitigated acoustic noise in a 1.5 T cylindrical MRI scanner equipped with epoxy-potted, shielded gradients. It has been widely assumed that MRI acoustic noise comes overwhelmingly from vibrations of the gradient assembly. However, with vibration-isolated gradients contained in an airtight enclosure, we found the primary sources of acoustic noise to be eddy-current-induced vibrations of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a reduced-acoustic-noise demonstration MRI system. This scanner employed a number of acoustic noise reduction measures including a vacuum enclosure of a vibrationally isolated gradient assembly, a low-eddy-current rf coil and a non-conducting inner bore cryostat. The demonstration scanner reduced, by about 20 dBA, the acoustic noise levels in the patient bore to 85 dBA and below for several typical noisy pulse sequences. The noise level standing near the patient bore is 71 dBA and below. We have applied Statistical Energy Analysis to develop a vibroacoustic model of the MR system. Our model includes vibrational sources and acoustic pathways to predict acoustic noise and provides a good spectral match above 400 Hz to experimentally measured sound levels. This tool enables us to factor acoustics into the design parameters of new MRI systems.     

Acoustic noise simulation and measurement of a gradient insert in a 4 T MRI

High speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. Many studies have already been conducted to characterize the sound field in and around MRIs and various methods have been investigated to attenuate the noise generated. In the work presented here a computational vibro-acoustic model was developed based on an iteratively modified and validated finite element (FE) model to characterize the acoustic noise properties of the gradient coil. The simulation results from the computational model were verified through experimental noise measurement for the gradient coil insert in a 4 T MRI scanner by using swept sinusoidal time waveform inputs. Comparisons show that the computational model predicts the noise characteristic properties extremely accurately. There are three dominant frequency bands where the SPL is much higher than those at other frequencies. The SPL in the horizontal direction is much higher than that in the vertical direction due to the excitation to the horizontally placed X coil. The SPL to the inner surface of the coil is higher than far from the inner surface, which proves that the acoustic noise is radiated from the inner surface and primarily caused by the normal vibration of the inner surface. Further verification was conducted by using two types of trapezoidal sequence inputs usually used, which is to simulate real scanning sequences for small animals. Again the accuracy of the developed model is verified. The validated acoustic computational model could be used as an effective method to predict the noise that would be produced by a coil in the design stage. Modification of the structural design or the excitation pulse could be performed to reduce the acoustic noise when the gradient coil is in scanning.     

Large surface mapping by a unilateral NMR scanner

The nuclear magnetic resonance (NMR) surface scanner, which provides images of sample surfaces larger than the probe dimension, has been realized using a single-sided device. Although conditioned by distortion effects originated by convolution between the sensitive volume of the probe and the space structures to be imaged, the scanner is able to provide images with good spatial resolution. The images obtained by the surface scanner can be made sensitive to relaxation parameters, magnetization or molecular self-diffusion; also, the dimension perpendicular to the sample surface can be scanned by varying the depth from which the probe detects the sample signal. It may scan surfaces arbitrarily large and with some degree of curvature. This aspect, together with the noninvasive characteristic of the apparatus, indicates that the surface scanner could be used profitably in the field of cultural heritage, where it could provide NMR maps of frescos, paintings on wood, marble artifacts, books and others.     

Active control of the volume acquisition noise in functional magnetic resonance imaging: method and psychoacoustical evaluation.

Functional magnetic resonance imaging (fMRI) provides a noninvasive tool for observing correlates of neural activity in the brain while a subject listens to sound. However, intense acoustic noise is generated in the process of capturing MR images. This noise stimulates the auditory nervous system, limiting the dynamic range available for displaying stimulus-driven activity. The noise is potentially damaging to hearing and is distracting for the subject. In an active noise control (ANC) system, a reference sample of a noise is processed to form a sound which adds destructively with the noise at the listener's ear. We describe an implementation of ANC in the electromagnetically hostile and physically compact MRI scanning environment. First, a prototype system was evaluated psychoacoustically in the laboratory, using the electrical drive to a noise-generating loudspeaker as the reference. This system produced 10-20 dB of subjective noise-reduction between 250 Hz and 1 kHz, and smaller amounts at higher frequencies. The system was modified to operate in a real MR scanner where the reference was obtained by recording the acoustic scanner noise. Objective reduction by 30-40 dB of the most intense component in scanner noises was realized between 500 Hz and 3500 Hz, and subjective reduction of 12 dB and 5 dB in tests at frequencies of 600 Hz and at 1.9 kHz, respectively. Although the benefit of ANC is limited by transmission paths to the cochlea other than air-conduction routes from the auditory meatus, ANC achieves worthwhile attenuation even in the frequency range of maximum bone conduction (1.5-2 kHz). ANC should, therefore, be generally useful during auditory fMRI.     

一种辐射声场近似计算方法——单元辐射叠加法

提出一种基于表面振速预报辐射声场的近似方法——单元辐射叠加法。研究表明,声传递向量中的每项元素等于对应单元以单位速度振动、其它单元振速都为零时的辐射声压,即刚性障板上活塞面以单位速度振动时的辐射声压。在此基础上,利用刚性障板上单位速度振动活塞面的辐射声压直接建立表面振速与辐射声压之间的传递关系,根据这个传递关系对表面振速进行加权、求和便得到总的辐射声压。计算时采用规则形状障板面去拟合实际障板面,以规则形状障板上振动活塞面的辐射声压去近似实际障板上振动活塞面的辐射声压。相对于边界元方法,该方法在计算速度与存储空间上具有很大的优势。计算表明该近似方法是合理的、可行的。     

An approximate method to acoustic radiation problems: element radiation superposition method

An approximate method is brought forward to predict the acoustic pressure based on the surface velocity. It is named Element Radiation Superposition Method （ERSM）. The study finds that each element in Acoustic Transfer Vector （ATV） equals the acoustic pressure radiated by the corresponding surface element vibrating in unit velocity and other surface elements keep still, that is the acoustic pressure radiated by the corresponding baffled piston vibrating in unit velocity. So, it utilizes the acoustic pressure radiated by a baffled piston to establish the transfer relationship between the surface velocity and the acoustic pressure. The total acoustic pressure is obtained through summing up the products of the surface velocity and the transfer quantity. It adopts the regular baffle to fit the actual baffle in order to calculate the acoustic pressure radiated by the baffled piston. This approximate method has larger advantage in calculating speed and memory space than Boundary Element Method. Numerical simulations show that this approximate method is reasonable and feasible.     

水下声辐射激励水表面声波的特征参数测量

采用激光干涉方法对水下声辐射激励水表面声波的特征参数频率和振幅进行了测量研究。从理论上对水表面声波激光相干测量信号的频谱构成进行了分析,在此基础上提出了水表面声波两个重要声学参数频率和振幅的解算方法,并通过数值仿真进行了验证。搭建了一套简单的激光干涉测量实验系统,对不同频率和声压激励的水表面声波进行了测量实验,验证了水表面声波频率和振幅解调方法的准确性。对水表面声波横向传播的振幅衰减现象进行了初步的实验研究,结果表明水表面声波的频率越高,振幅的横向衰减越快。研究表明激光相干检测方法能够准确地实现水表面声波振幅和频率的测量。     

基于声辐射模态讨论声能量辐射与传递

声辐射模态表示了振动声源表面的辐射模式。基于声辐射模态讨论了振动声源表面声能量辐射和声场中声能量传递的性质,给出声能量辐射和传递的模式。采用表面声强描述振动声源表面的声能量辐射,采用声强描述声能量在声场中的传递。表面声强和声强可分解成两部分,一部分将辐射的声能量传递到远场;另一部分表现为振动声源与声场之间的能量交换。针对矩形板分析了声能量辐射和传递,数值计算结果与理论结果一致。结果表明基于声辐射模态讨论声能量辐射与传递是可行的和有效的。     

声表面波在厚金属栅阵中的耦合模参数

提出了一种研究声表面波在压电晶体厚金属栅阵中传播特性的理论方法。将有限元和声表面波在周期栅阵中的变分原理分析方法相结合,在陈东培和H.A.Haus理论基础上、用有限元分析金属短路栅对声表面波传输特性的影响,将力学负载贡献的耦合模参数用有限元矩阵表示,使其适用于声表面波在厚金属或任意形状栅条中传输情况,给出了具体理论分析方法和相应的理论表达式。最后,具体研究了几种压电晶体上金、铝或银栅阵中声表面波的传输特性,通过数值计算给出了声表面波的耦合模参数。     

声表面波在圆弧处反射及透射的数值研究*

为了研究直达表面波在圆弧过渡面处传播性能的变化,采用有限元方法模拟了热弹机制下,线性脉冲激光辐照金属铝块表面时激发的表面波在近表面传播过程中,在不同曲率半径的圆弧处发生的反射及透射现象,建立了圆弧半径与反射表面波以及透射表面波时域信号特征之间的联系。计算结果表明:曲率半径与表面波中心波长的数值关系对表面波在圆弧处的传播有显著的影响;同时证明了根据透射表面波信号的到达时间可以反演圆弧半径的大小,为之后利用表面波信号定量检测材料表面圆弧凹痕的深度提供了理论依据。     

声表面波渡越时间法检测黏弹性介质表面凹痕深度的研究

研究声表面波渡越时间法检测黏弹性介质表面凹痕的深度。基于频域动力学平衡方程,采用有限元方法分别在有表面凹痕的弹性和黏弹性介质中,建立激光激发声表面波的数值模型。研究材料的黏性引起的声表面波波速和频散的变化,对表面凹痕检测的影响,在此基础上,根据反射和透射声表面波的传播路径及到达时间,利用渡越时间法检测黏弹性介质中的表面凹痕。结果表明:黏性引起的声表面波频散对表面凹痕检测的影响极小,采用渡越时间法检测得到的表面凹痕的位置及深度,与数值模型的设置值符合很好,从而为黏弹性介质表面凹痕的检测提供有效的理论方法。     

通过形变势弱耦合表面极化子的性质

采用Huybrechts线性组合算符和幺正变换法,导出了晶体中电子与表面光学声子和表面声学声子均为弱耦合极化子的有效哈密顿量,并对两种极限情况进行了讨论。     

Pair annihilation effects on dust ion acoustic surface wave in semi-bounded magnetized pair–ion–dust plasmas

The electron–positron pair annihilation effects on the dust ion acoustic surface wave are investigated in semi-bounded magnetized electron–positron–ion–dust plasmas. The dispersion relation of the low frequency dust ion acoustic surface wave is obtained by the plasma dielectric function with the specular reflection boundary condition. The results show that the frequency of the dust ion acoustic surface wave is found to be increased with increasing the annihilation of the electron–positron pair. In addition, the group velocity of the dust ion acoustic surface wave is also found to be increased with the annihilation of the electron–positron pair.     

Probing of laser-induced crack modulation by laser-monitored surface waves and surface skimming bulk waves

All-optical monitoring of the nonlinear motion of a surface-breaking crack is reported. Crack closing is induced by quasi-continuous laser heating, while Rayleigh surface acoustic pulses and bulk longitudinal surface skimming acoustic pulses are also generated and detected by lasers. By exploiting the strong dependence of the acoustic pulses reflection and transmission efficiency on the state-open or closed-of the contacts between the crack faces, the parametric modulation of ultrasonic pulses is achieved. It is observed that bulk acoustic waves skimming along the surface can be more sensitive to crack motion than Rayleigh surface waves.     

Visualization of acoustic radiation from a vibrating bowling ball.

This paper presents visualization of acoustic radiation from a vibrating bowling ball using the Helmholtz equation least squares (HELS) method. In conducting the experiments, the ball is excited by a vibration shaker using stationary random signals. The radiated acoustic pressures are measured using two microphones and taken as input to the HELS formulations. The reconstructed acoustic pressures on the bowling ball surface are compared with those measured at the same locations. Also shown are comparisons of the reconstructed and measured acoustic pressure spectra at various locations on the bowling ball surface. Results demonstrate that the accuracy of reconstruction based on measurements over a conformal surface is much higher than that over a finite planar surface. This is because the latter often extends beyond the near-field region, making the accuracy of measurements inconsistent. Nevertheless, satisfactory reconstruction of acoustic pressure fields over the entire bowling ball surface can still be obtained based on the measurements taken over a finite planar surface on one side of the source. In a similar manner, the normal component of the surface velocity is reconstructed. Once these acoustic quantities are determined, the time-averaged acoustic intensity is calculated. Also presented are the formulations for estimating a priori the numbers of expansion functions and measurements required by the HELS method and the guidelines for determining the reconstruction error and optimum measurement locations, given the overall dimensions of the source and the highest frequency of interest in reconstruction.     

SiO2波导布拉格器件中声表面波及声光相互作用分析

针对SiO2光波导声光布拉格器件,计算了SiO2非对称平板波导TE模式的横向场分布;给出了SiO2/ZnO/Air层状介质结构的性能方程、运动方程和麦克斯韦方程,推导出这种层状结构的特征方程,并结合所满足的边界条件,得到了各层介质的位移及电磁场分布;计算了声表面波所引起的光学相对介质隔离率张量的变化,最后讨论了声光衍射效率和光场与声场的重叠积分、声功率、声频率、声孔径和光波导参数之间的关系。结果表明,在低频范围内光场与声表面波场重叠良好;低阶模的重叠积分始终大于高阶模重叠积分,最低阶模与声表面波相互作用最强,所需声功率最小;当声功率一定时,增加声孔径可以提高衍射效率。     

水下低频声信号的激光探测及波的衰减

为了探测几十赫兹的低频水下声信号,建立了水下低频声信号的光学探测系统,得到了稳定、清晰的衍射图样.得到了衍射图样的宽度与声源距离的变化关系,声源距离越小,衍射图样越宽.当水下声波传至水表面后,实验上得到了表面声波的衰减特性,理论上得到了衍射图样的角宽度和液体表面波振幅的解析关系式.发现表面波振幅的衰减随距离是指数型衰减.并研究了衰减系数随频率的变化,频率不同衰减系数也不同,而且频率越大,衰减系数越小.     

Influence of acoustic leakage at fluid–solid interface on interferometric detection of surface acoustic wave

When using laser interferometer to detect surface acoustic wave at fluid–solid interface, there are two factors which will cause the optical path length variation of the probe laser beam: interface deformation, and refractive index changes in fluid induced by acoustic leakage. Influence of acoustic leakage on laser interferometric detection for surface acoustic wave is researched here. A metal plate immersed in an infinite fluid is used as a physical model. Interface deformation due to laser-induced acoustic wave and pressure in fluid due to acoustic leakage are computed for select cases by finite element method. The optical path length variation caused by the two factors are calculated respectively and compared. The results show that the influence of acoustic leakage increases with the increasing acoustic impedance matching of fluid and solid, the peak-to-peak of influence degree increases linearly with the increasing acoustic impedance of fluid, and that decreasing the distance between the interferometer and interface can effectively reduce the influence of acoustic leakage.     

The acoustic emission of a distributed mode loudspeaker near a porous layer

Experimental and theoretical modeling of the vibro-acoustic performance of a distributed mode loudspeaker (DML) suggest that their acoustic emission can be significantly affected by the presence of a porous layer. The amplitude of the surface velocity of the panel and the acoustic pressure on the porous surface are reduced largely in the vicinity of structural resonances due to the additional radiation damping and visco-thermal absorption phenomenon in the porous layer. The experimental results suggest that a porous layer between a rigid base and a DML panel can considerably alter its acoustic emission in the near field and in the far field. This is illustrated by a reduction in the level of fluctuations in the emitted acoustic pressure spectra. These fluctuations are normally associated with the interference between the sound emitted by the front surface of the speaker and that emitted from the back. Another contribution comes from the pronounced structural resonances in the surface velocity spectrum. The results of this work suggest that the acoustic boundary conditions near a DML can be modified by the porous layer so that a desired acoustic output can be attained.