Echo planar imaging at 4 Tesla with minimum acoustic noise.

Area of Science:

• Medical Imaging
• Acoustics
• Biophysics

Background:

• High magnetic field MRI scanners, particularly during echo planar imaging (EPI) acquisitions, generate significant acoustic noise.
• The noise is primarily caused by vibrations in the gradient coils, which are excited by the rapidly switching magnetic field gradients.

Purpose of the Study:

• To investigate and minimize the acoustic sound pressure levels generated during single-shot EPI sequences on high magnetic field MRI scanners.
• To reduce patient discomfort and improve the quality of MRI examinations.

Main Methods:

• Measured the resonance frequencies of gradient coil vibrations using piezoelectric transducers.
• Identified the dependence of resonance frequencies on coil length and material elastic properties.
• Adjusted the frequency of the EPI-readout train to avoid identified mechanical resonance frequencies.

Main Results:

• Identified two sharp mechanical resonances in the MRI system at 720 Hz and 1220 Hz, which can amplify vibrations up to six-fold.
• A minor adjustment in the EPI-readout frequency successfully reduced the sound pressure level of EPI-based perfusion and functional MRI scans by 12 dB.
• Demonstrated a significant reduction in acoustic noise without compromising image quality.

Conclusions:

• The normal vibrational modes of MRI gradient coils are a major contributor to high sound pressure levels during EPI scans.
• Diverting EPI-readout frequencies away from the gradient coil's mechanical resonance frequencies is an effective strategy for minimizing acoustic noise.
• This approach offers a practical method for enhancing patient comfort and improving MRI scan quality.