Model gradient coil employing active acoustic cont

Authors: B Haywood B Chapman P Mansfield

Publish Date: 2007/11/15

Volume: 20, Issue: 5-6, Pages: 223-

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Abstract

Results are presented for a model threeaxis gradient coil incorporating active acoustic control which is applied to the switched read gradient during a singleshot rapid echoplanar imaging EPI sequence at a field strength of 30 T The total imaging acquisition time was 106 ms Substantial noise reduction is achieved both within the magnet bore and outside the magnet Typical internal noise reduction over the specimen area is 40 dBA whereas outside the acoustic chamber the noise level is reduced by 60–77 dBA However these results are relative to a control winding which is switched in phase adding 6 dBA in its nonoptimized mode which is included in the quoted figuresWith the advent of morepowerful magnets and faster imaging regimes in magnetic resonance imaging MRI a major concern is the high levels of noise generated during scanning This noise arises within the cylindrical structure of the gradient coil and is proportional to the driving current and the strength of the static magnetic field This current generates Lorentz forces that induce vibrations in the coil structure thereby generating substantial noiseThe general trend for increased static field strengths and faster imaging techniques have made the noise situation worse The acoustic noise problem has now reached the point where it poses a significant health risk not only to the patient but also to medical staff working in close proximity to scanners For example in ultrahighspeed imaging such as echoplanar imaging EPI and echovolumar imaging EVI because of the high maximum gradient amplitudes employed and extremely rapid slewrates used in these techniques noise levels can approach 130 dBA 1 2 3 which is well above the safety levels permitted in the workplace While adult patients can be protected to some extent against the dangers of acoustic noise infants and foetuses in utero cannot be so easily protected The same is true for animals in veterinary practiceVarious approaches have been explored to try to ameliorate the acoustic noise problem including the development of special pulses for use in EPI 45 and also gradient shielding techniques 6 Presently the most common approach is to seal the gradient coil in a vacuum chamber 7As an alternative approach active acoustic control for gradient coils was introduced by Mansfield and Haywood 8 in an attempt to ameliorate the acoustic noise problem by controlling the vibrational modes generated in the coil structure In this novel approach the gradient coil structure comprises flat rectangular plates or flat arcuate sectors in which the gradient field winding and an additional control winding are embedded in the flat plate structure in order to control the vibrational mode of the structure thereby reducing the noise The use of either rectangular or arcuate sectors simplifies somewhat the theory of noise propagationThe purpose of the control winding is to prevent the production of compressional and bending vibration modes in the coil structure that result from the opposing forces applied to the coil former segments by the opposite currents in the gradient and return arcs of the coil It achieves this by decoupling these two currents Each coil segment consists of two plates separated by a small gap Around this gap a second control winding is placed which carries a current I 1 ′ e 1 iϕ for the plates with a gradient field winding current I 1 and I 2 ′ e 2 iϕ for the plates with a gradient field winding current I 2 The two halves of the former can then move independently thereby reducing considerably the compressive and bending forces These forces cannot be eliminated entirely as the former is not infinitely rigid To assist this process further the control winding is activated so that the currents flowing on each side of both sections are acting in unison Simple theory indicates that the current and phase in the control windings should be equal to those in the gradient windings Due to the finite velocity of sound in the formers greater acoustic attenuation is achieved by modifying slightly the amplitude and phase of the current in the control windingsIn the work presented here as a first step to demonstrating the principle of active acoustic control a scaleddown model threeaxis gradient coil was constructed and used to obtain a singleshot image using EPI The coil consists of a number of sectors each comprising flat rectangular plates with embedded straight wire windings for the transverse gradient G y used as the read gradient together with embedded straight wire secondary control windings for active acoustic control of this switched gradient The same method of flat rectangular plates with embedded straight wire gradient windings was used for the G x gradient but without the control winding The longitudinal gradient G z used a traditional Maxwell pair

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