Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-10-02
2002-08-20
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S319000, C324S322000
Reexamination Certificate
active
06437568
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a magnetic resonance imaging (MRI) scanner and more particularly to a low-noise MRI scanner.
MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field that makes nuclei, such as hydrogen atomic nuclei, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatial-gradient magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during the imaging pulse sequence. The radiofrequency coil creates an excitation frequency pulse that temporarily creates an oscillating transverse magnetization which is detected by the radiofrequency coil and used by the computer to create the image. Typically, there is a radiofrequency coil and a gradient coil assembly within the magnet.
Magnets for MRI scanners include superconductive-coil magnets, resistive-coil magnets, and permanent magnets. Known superconductive magnets include liquid-helium cooled and cryocooler-cooled superconductive magnets. Known superconductive magnet designs include cylindrical magnets and open magnets.
Cylindrical magnets typically are cylindrically shaped and have an axially-directed static magnetic field. In MRI systems based on cylindrical magnets, the radiofrequency coil, the gradient coil assembly and the magnet are generally annularly-cylindrical shaped and are generally coaxially aligned, wherein the gradient coil assembly circumferentially surrounds the radiofrequency coil and wherein the magnet circumferentially surrounds the gradient coil assembly.
Open magnets typically employ two spaced-apart magnetic assemblies with the space between the assemblies allowing for access by medical personnel for surgery or other medical procedures during MRI imaging. The open space helps the patient overcome feelings of claustrophobia that may be experienced in a cylindrical magnet design.
Generally, the various components of the MRI scanner represent sources and pathways of acoustic noise that can be objectionable to the patient being imaged and to the user of the scanner. For example, the gradient coil assemblies of MRI scanners generate loud acoustic noises which many medical patients find objectionable. The acoustic noises occur internal to the patient bore of the scanner as well as outside of the scanner. Active noise control techniques have been used to reduce gradient coil assembly noise including noise-canceling patient earphones. Known passive noise control techniques include locating the gradient coil assembly in a vacuum enclosure.
The rf coil structure is also another source of vibration land acoustic noise. An MRI system employs electrically excited gradient coils to impose time varying magnetic fields on the primary or B
0
magnetic field. These time varying fields tend to induce eddy currents in the conductors of the rf coil, which in turn may cause mechanical motion of the rf coil.
Yet another source and pathway of acoustic noise is due to vibration of mechanical components in the scanner. It is known in the mechanical arts area to design and use isolation mounts so that vibrations from machinery supported by the isolation mounts are not transmitted to surrounding structure that supports the isolation mounts. Conventional isolation mounts include those of the elastromeric type and those of the spring type. Such isolation mounts are designed by the artisan so that the natural frequency of vibration of the mounts and the machinery is less than the important excitation frequencies of the machinery in order to provide effective vibration isolation.
These techniques or measures to reduce acoustic noise due to the various components in the MRI scanner have been partially effective, but patients and technicians still find the noise in and about a MRI scanner to be problematic. What is needed is a lower noise MRI scanner that addresses the multiple sources and pathways of acoustic noise in and about the scanner.
SUMMARY OF THE INVENTION
A low noise imaging apparatus for producing Magnetic Resonance (MR) images of a subject and for substantially minimizing acoustic noise generated during imaging is provided. The imaging apparatus comprises a magnet assembly, a gradient coil assembly, and a rf coil assembly, wherein at least one of the magnet assembly, the gradient coil assembly and the rf coil assembly are configured to reduce the generation and transmission of acoustic noise in and about the imaging apparatus.
A radiofrequency (rf) coil assembly for a Magnetic Resonance Imaging (MRI) system comprises a plurality of conductors wherein each of the conductors has a width selected for transmitting a radiofrequency pulse, for receiving an MR signal induced in the subject and for reducing eddy current excitation contributing to acoustic noise in and about the imaging apparatus. Further, a layer of acoustic absorptive material may be disposed between the plurality of conductors and a patient bore tube.
A magnet assembly for an imaging apparatus for a Magnetic Resonance Imaging (MRI) system comprises an outer surface and a plurality of suspension members for attaching a magnet to the outer surface. The suspension members are configured to reduce generation and transmission of acoustic noise.
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Ackermann Robert Adolph
Amm Bruce Campbell
Dean David Edward
Edelstein William Alan
El-Hamamsy Sayed-Amr
Breedlove Jill M.
Lefkowitz Edward
Shrivastav Brij B.
Testa Jean K.
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