Modified from Essentials of MRI Safety
One of the principal bugbears of patients undergoing MRI, along with being squeezed into a narrow tunnel, claustrophobia and general anxiety about their health, is the noise. UK Department of Health figures indicate nine noise-related incidents reported from 21 million scans between 1994 and 2011 – a rate of 0.00004%. In the USA FDA figures show acoustic noise as being responsible for 6% of MRI-related incidents reported. With such a small number relating to so great a nuisance, it appears everyone accepts that MRI is loud. However, we must not become complacent as acoustic noise is a potential source of injury as well as discomfort. Here we review sequence and parameter changes that help to reduce the overall noise.
For a given RF pulse waveform the slice width is inversely proportional to the slice select gradient Gss. Increasing slice width in 2D acquisitions will reduce the acoustic noise.
Low SAR RF pulses
Low SAR RF pulses are longer than standard RF pulses and consequently have a narrower bandwidth. For a given slice thickness, a lower slice select gradient will be required. The selection of low SAR RF pulses may reduce the noise.
Field of view
Field-of-view (FOV) also depends upon the gradient amplitude. Increasing FOV without changing the receive bandwidth will decrease the gradient amplitude and noise.
An increase in in-plane resolution or smaller pixel size will result in higher gradient amplitude and more acoustic noise.
TE and receive bandwidth
Increasing TE may allow a reduction in receive bandwidth, resulting in a reduction in FE-gradient amplitude and lower noise. For TSE (FSE) sequences noise reduction may be achieved by increasing the inter-echo spacing (IES) or reducing the turbo factor (echo train length), whilst minimising the receive bandwidth. These changes increase the overall scan time.
Increasing TR without any other parameter changes reduces the noise level.
Number of slices, echoes
Reducing the number of slices or echoes will mean there is less gradient activity per TR period and will reduce the overall noise.
In diffusion-weighted imaging (DWI) or diffusion tensor imaging (DTI), the use of lower diffusion weighting or b-value and less diffusion directions will reduce the acoustic noise.
Consult your applications specialist or MRSE for more information. Some scanners offer specialist low-noise sequences (e.g. “zero-TE”, “silent scan”) or low noise gradient pulses (e.g. “whisper” or “softone”).
McJury, M. (2014). Acoustic noise and MRI procedures. In: MRI bioeffects, safety, and patient management (Ed. F.D. Shellock and J.V. Crues III) pp. 256–281. Los Angeles, CA: Biomedical Research Publishing Group. Note: 2nd edition coming soon.