Characterization of vibration and acoustic noise in a gradient-coil insert

Abstract

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. To characterize the vibration properties as well as the acoustic noise properties of the gradient coil, a finite-element (FE) model was developed using the dimensional design specifications of an available gradient-coil insert and the concentration of the copper windings in the coil. This FE model was then validated using experimentally collected vibration data. A computational acoustic noise model was then developed based on the validated FE model. The validation of the finite-element analysis results was done using experimental modal testing of the same gradient coil in a free-free state (no boundary constraints). Based on the validated FE model, boundary conditions (supports) were added to the model to simulate the operating condition when the gradient-coil insert is in place in an MRI machine. Vibration analysis results from the FE model were again validated through experimental vibration testing with the gradient-coil insert installed in the MRI scanner and excited using swept sinusoidal time waveforms. The simulation results from the computational acoustic noise model were also validated through experimental noise measurement from the gradient-coil insert in the MRI scanner using swept sinusoidal time waveform inputs. Comparisons show that the FE model predicts the vibration properties and the computational acoustic noise model predicts the noise characteristic properties extremely accurately.

Influence of selecting EPI readout-encoding bandwidths on arterial spin labeling perfusion MRI

“…Echo-planar imaging (EPI) [1,2], which facilitates a speedy acquisition of the magnetic resonance signal, is nowadays widely used for generating maps of various signal contrasts, such as the blood oxygenation level dependent (BOLD) contrast in functional MRI (fMRI), diffusion tensor imaging (DTI), and cerebral blood flow (CBF) using arterial spin labeling (ASL) MRI. However, in addition to factors of eddy current, echo misalignment, or gradient/receiver miscalibration, the rapidly switching magnetic field gradients of EPI can result in considerable mechanical vibrations which lead in turn to magnetic field fluctuations causing Nyquist (N2) ghosting in the EPI data [1][2][3].…”

“…However, in addition to factors of eddy current, echo misalignment, or gradient/receiver miscalibration, the rapidly switching magnetic field gradients of EPI can result in considerable mechanical vibrations which lead in turn to magnetic field fluctuations causing Nyquist (N2) ghosting in the EPI data [1][2][3]. Although many strategies have been invented to reduce N2 ghosts, including gradient compensation [4], timing corrections [5], reference phase corrections [6], and post-processing image restoration [7], none of these methods can entirely eliminate this artifact.…”

Compensation of gradient-induced magnetic field perturbations

“…Gradient coil vibrations and acoustic noise are caused by Lorentz forces acting on the gradient coil windings during a time-varying current (4). The gradient coil windings, and as a consequence the entire gradient coil former, are deformed during a gradient pulse and relax back to their original state in an damped, oscillatory fashion (4).…”

“…The gradient coil windings, and as a consequence the entire gradient coil former, are deformed during a gradient pulse and relax back to their original state in an damped, oscillatory fashion (4). While the exact mechanism is not well understood, the gradient coil vibrations do lead to damped sinusoidal perturbations of the main magnetic field B 0 .…”

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

“…Upon image reconstruction, the phase asymmetry produces a ghost that is displaced by half the field of view (FOV) in the EPI phase encode direction relative to the correct image position. While the mechanical vibrations cannot be avoided, their amplitude and frequency are linked to the oscillatory magnetic field gradients of EPI [5,6]. Hence, the bandwidth (BW) and frequency of oscillations [i.e., echo spacing (ES)] of EPI acquisitions should be chosen carefully to avoid triggering resonant mechanical vibrations and to reduce acoustic noise [7].…”