Abstract
An acoustic control scheme is proposed in this paper through the process of gradient coil design for magnetic resonance imaging (MRI). With a finite-difference-based method, the stream function and coil displacement caused by fast gradient switching can be unified by a simplified momentum equation, which can be incorporated into the conventional gradient coil design. A three-dimensional transverse gradient coil with an edge-connected cylindrical structure is used as a design example to verify the proposed design method. In addition, an acoustic model is established to simulate the sound pressure level (SPL).In the model, two hemispherical air volumes are added flush with the ends of the cylindrical main magnet to mimic the free propagation of sound waves on the boundaries. The simulation results show that by optimizing coil displacement, the overall SPL can be attenuated by 4 dB over the frequency range from 0 to 3000 Hz with the displacement reduced by about 50%, at the cost of a figure of merit (FOM) loss by about 8%. Therefore, the proposed acoustic control scheme can be used as a complement to conventional acoustic control methods for further noise reduction.
Highlights
During magnetic resonance imaging (MRI) scanning, significant Lorentz forces are produced by fast gradient switching, which leads to vibration of the gradient assembly and induces loud acoustic noise inside the patient bore [1, 2]. e sound pressure level (SPL) of this acoustic noise can reach 130 dB in some MRI scanners [3]
The design results obtained by solving the optimization problem (16) are shown to validate the effectiveness of the proposed method in reducing coil displacement and controlling noise levels. e design parameters ε1 and ε2 were chosen to be 0.05 and 0.03, respectively, which were frequently used values in gradient coil design. e thickness of the epoxy layer w is equal to 5 mm, and the frequency of the sinusoidal gradient pulse was set to be 1750 Hz in equation (11), which is near the root mean square (RMS) of the frequency range 0 to 3000 Hz, to find an optimized solution over the whole frequency range
In order to describe coil performance, several parameters are chosen in this paper, including max field deviation ΔBmz ax, gradient efficiency η, and figure of merit (FOM)
Summary
During MRI scanning, significant Lorentz forces are produced by fast gradient switching, which leads to vibration of the gradient assembly and induces loud acoustic noise inside the patient bore [1, 2]. e sound pressure level (SPL) of this acoustic noise can reach 130 dB in some MRI scanners [3].is serious noise may cause patient discomfort and affect image quality and damage the patient’s auditory system. erefore, acoustic noise control has become an important issue in MRI system design.To mitigate the effect of acoustic noise, some engineers and scholars attempted to block the noise propagation. In addition to blocking the noise propagation, attenuating noise level from the noise source is another scheme of noise elimination One method of this category is active noise control [10,11,12,13,14], which generates active acoustic noise with equal amplitude and opposite phase to counteract the original noise. Considering the positive correlation between acoustic noise and gradient pulse, Hennel et al proposed soft gradient pulses, using sinusoidal components to replace linear varying gradient pulse and reduce noise level [15, 16]. This method was efficient, different pulse shapes may degrade the image quality
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