Acoustic screen

Details Acoustic screen Acoustic screen

1995-12-01

1998-06-09

Arana, Louis M.

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

324319, G01V 300

Patent

active

057640599

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to an apparatus for and a method of acoustic and magnetic screening of coils through which pass rapidly varying currents which interact with a surrounding static magnetic field to produce strong Lorentz forces. The invention, hereinafter referred to as an active acoustic screen or acoustic screen, is particularly well suited for use with Magnetic Resonance Imaging (MRI) also known as Nuclear Magnetic Resonance (NMR) imaging. More particularly the invention relates to an apparatus and method of active acoustic screening of any coil structure which lies in a static magnetic field and which produces unwanted noise by virtue of a variable current drive through the coil. Additionally, there is provided an embodiment in which simultaneous active acoustic and active magnetic screening of a coil structure in a static magnetic field is achieved.
Particular application of the invention is in MRI or NMR equipment where gradient coils are pulsed in the presence of a static magnetic field. Although noise is generated at any field strength, the invention has particular relevance to situations in MRI where field strengths are 0.1 T (Tesla) or above. For consistency NMR equipment is hereinafter referred to as MRI equipment.
Magnetic gradient coils are a prerequisite for MRI (as discussed by Mansfield P. and Morris P. G. in NMR imaging in Biomedicine Academic Press (1982) N.Y.) and also for use in a range of applications including diffusion studies and flow. In medical imaging, acoustic noise associated with rapid gradient switching, is exacerbated at higher static magnetic field strengths, and is at best an irritant and at worst could be damaging to a patient. Some degree of protection can be given to adults and children by using ear defenders. However, for foetal studies and also for studies using animals and for veterinary use in general, acoustic protection is difficult if not impossible.
Several attempts have been made to ameliorate the acoustic noise problem. For example, by lightly mounting coils on rubber cushions, by increasing the mass of the total gradient arrangement and by absorptive techniques in which acoustic absorbing foam was used to deaden the sound. The problem of acoustic noise has been considered in published European Patent Applications EP-A1-431216, EP-A2-350640 and EP-A1-304127. Acoustic screening was also considered in U.S. Pat. No. 4,680,545, U.S. Pat. No. 5,018,359 and published International Patent Application No. WO-A1-8607459. These are briefly considered below.
U.S. Pat. No. 4,680,545 describes a method for shaping a current pulse such that high frequency components are reduced or eliminated. This reduced some high frequency noise.
EP-A1-431216 describes a technique for minimising the pulse rise and fall times by pulse shaping. This removes a high frequency noise component.
EP-A1-304127 describes an arrangement comprising a stack of arc conductors connected to a former in a rigid self-supporting arrangement. The arrangement is such that relative movement between arc conductors is reduced. This reduces the amount of vibration which caused acoustic noise. WO-A1-8607459 describes an arrangement in which coils are embedded in epoxy and/or suspended on rubber dampers. In one embodiment a stainless steel mass is attached to a coil and this is used to lower the characteristic frequency of the system. Noise was reduced by acoustic absorbtion and/or damping of vibrating equipment.
EP-A2-350640 describes an arrangement in which gradient coils are embedded in layer of viscoelastic material. The layer is then sandwiched so as to restrain it.
U.S. Pat. No. 5,018,359 describes a cryogenically cooled apparatus in which a magnetic field produced by a superconductive coil is screened by a magnetic screen and which is connected to a vacuum vessel by a vibration absorbing means.
Techniques have also been proposed in which acoustic noise cancellation is used. In this method earphones or sound pipes are used to transmit sound of equal but opposite a

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