Professor Sir Peter Mansfield FRS
University of Nottingham
The principle of active magnetic screening of gradient coils introduced by Mansfield and Chapman in 1986 was rapidly assimilated by the major manufacturers of MRI equipment and is now standard on most MRI scanners. This principle allows rapid switching of field gradients within the close confines of a superconductive magnet without inducing extraneous, unwanted currents in the surrounding magnet structures. However, in this work and also in all gradient coil structures constructed to date, an important aspect of gradient design has been so far overlooked. This is the acoustic noise which accompanies high rapidly switched gradient coils. In some experimental gradient structures we have measured noise output in excess of 110 dB. At these levels acoustic noise can become dangerous unless ear defenders and earplugs are used to protect patients and scanner personnel. However, there are classes of patient where it is difficult if not impossible to use ear defenders, for example very young children and babies, and especially foetuses in utero. Other classes of subject include animal studies.
Some manufacturers have been concerned with noise levels and have attempted to quieten their products by including sound absorbing materials. Many of these approaches are really treating the symptoms rather than going to the root cause of the problem.
At Nottingham over the last five years we have been attempting to tackle the acoustic problem in a more fundamental way. This talk will be a review of an approach we have tried, together with an update on the current theory of sound propagation in simplified model systems.
Active Magnetic Screening of Gradient Coils in NMR Imaging. P Mansfield and B Chapman, J Mag Res 66, 573-576 (1986).
Sound Generation in Gradient Coil Structures for MRI. P Mansfield, P M Glover and J Beaumont, Mag Res Med 39 (1998), in press.