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1. WO1998013821 - ACTIVE CONTROL OF ACOUSTIC OUTPUT IN GRADIENT COILS

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[ EN ]

CLAIMS
1. Apparatus for active acoustic screening of a coil system mounted in a block of acoustic material comprising an active electro mechanical transducer mounted in said block of material and means for energising die electro

5 mechanical transducer to oppose die vibratory noise generated by said coil when energised.

2. An active acoustically controlled magnetic coil system comprising at least one current carrying conductor configured in a loop to provide outward and lo remrn paths and being mounted in support blocks of material having predetermined acoustic transmission characteristics, said support blocks comprising first and second portions separated by a gap said outward and remrn padis being mounted respectively in said first and second portions and furdier including at least one active transducer mounted in said gap, and comprising i5 means for supplying a predetermined alternating current to said conductor and energisation means for supplying the at least one active transducer with selected energisation and selected phase widi respect to die said alternating current, said energisation being selected to generate mechanical movement in said transducer to actively oppose vibrations created in said support block by said conductor to

20 reduce die acoustic noise generated by die coil system.

3. An acoustically controlled magnetic coil system as claimed in claim 2 in which there are provided a plurality of transducers, each being provided widi energisation to create mechanical movement within the transducer to reduce d e

25 noise generated by the coil system.

4. An acoustically controlled magnetic coil system as claimed in claim 3 in which a means for determining the selected energisation supplied to the transducers comprises a microphone mounted to receive noise signals generated

3o by d e at least one current carrying conductor, said microphone being connected to a display means and including a control circuit supplying relative phase and amplitude control signals to said energisation means to energise die transducers to reduce die noise generated by die coil system.

5. An acoustically controlled magnetic coil system as claimed in any one of claims 2 to 4 in which me active transducer comprises a piezoelectric transducer, die piezoelectric material of which may be a quartz crystal, barium titanate, lead zirconium titanate or other such piezoelectric materials.

6. An acoustically controlled magnetic coil system as claimed in any one of claims 2 to 4 in which the active transducers may comprise magneto strictive devices of a size small enough so as not to significantly affect the required gradient field.

7. An acoustically controlled magnetic coil system as claimed in any one of claims 2 to 4 in which the active transducer comprises a thin current loop enclosing a rubber block, its dimensions chosen so as to make the device non-resonant at the operating frequency.

8. An acoustically controlled magnetic coil system as claimed in claim 7 in which die first and second portions of die support block comprise the same material and die position of die gap is equidistant between die outward and remrn padis.

9. An acoustically controlled magnetic coil system as claimed in claim 7 in which the first and second portions of me support block comprise different materials having different acoustic compressional wave velocities and different attenuation constants, the position of the gap then being selectable to be non-equidistant between the outward and remrn paths of said current carrying conductor.

10. An acoustically controlled magnetic coil system as claimed in claim 7 in which die transducer is provided wid a matching unit to enable it to produce different mechanical movements on each side of the gap.

5 11. An acoustically controlled magnetic coil system as claimed in any one of claims 2 to 10 in which die outward and remrn padis comprise separate conductors wid separate drive means forming a coil system comprising a primary coil and a magnetic screening coil for said coil system, die transducer means being energised to reduce the acoustic output of the coil system.
0
12. An acoustically controlled magnetic coil system as claimed in any one of claims 2 to 11 in which ie coil system comprises x,y and z gradient coils and screening coils for said x,y and z gradient coils and in which die transducers are mounted between said coils and said screening coils and s energised to reduce die acoustic output of the system.

13. An acoustically controlled magnetic coil system as claimed in claim 12 in which the transducers are spatially distributed widi common zones used to cancel die acoustic wave for the x,y and z gradient coils and a logic matrix 0 drive system used to selectively energise a plurality of drive units for me transducers, diose transducers widiin any common zone area being driven at an amplitude and phase which may be different to diose in non-common zones.

14. An acoustically controlled magnetic coil system as claimed in any one 5 of claims 11 to 13 in which the conductors for each primary coil are arranged in arcs.

15. An acoustically controlled magnetic coil system as claimed in claim 12 or 13 in which the conductors for each primary coil and magnetic screening o coil and die acoustic control transducers are each distributed on individual cylindrical geometries, each of the aforesaid coils being arranged in die form of a fingerprint coil, each cylinder being arranged in a nested concentric and coaxial set.

16. An acoustically controlled magnetic coil system as claimed in claim 12 or 13 in which the coils form a gradient coil system for an MRI apparatus.