Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-12-27
2001-11-06
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000, C324S322000
Reexamination Certificate
active
06313633
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to a magnetic resonance imaging system (hereinafter called “MRI”), and more particularly to a short head coil apparatus for imaging the human head or brain.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that the coils can be operated in persistent mode, that is, a power supply can be connected for a short time to start current flowing through the coils, then a superconducting switch can be closed, the power supply removed, and the current will continue to flow, thereby maintaining the coil current and resultant magnetic field. Superconducting magnets find wide application in the field of MRI.
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrical cryogen pressure vessel, contained within a vacuum vessel and forming an imaging bore in the central region. The main magnet coils develop a strong magnetic field in the imaging bore.
However, it is necessary for acceptable quality imaging to provide and maintain a strong homogeneous magnetic field in the imaging region. Typical field strengths of 0.5 to 1.5 T (Tesla) require homogeneity of 10parts per million (ppm) over volumes on the order of a 45 cm diameter sphere.
In MRI imaging a plurality of additional magnetic fields are added in order to shape the imaging magnetic field and to conduct MRI operation. These include radio frequency (hereinafter called “RF”) signals and include pulsed magnetic field gradients to spatially encode the nuclear magnetic resonance (NMR) signal from various portions of an anatomical region of interest. The pulsed magnetic field gradients together with RF excitation of the nuclear spins. and acquisition of signal information are commonly referred to as pulse sequences. Pulsing current through conductors generate pulsing magnetic fields external to the conductors which can interfere with other magnetic fields in the superconducting magnet including the homogeneity of the main magnetic field in the imaging volume, and adversely affect imaging quality.
It is known in the imaging of the human head and brain to utilize a head coil or additional coil assembly, positioned around the head during imaging. The separated parallel conductors of such head coils has led to their being called birdcages.
RF head coils are applied to generate a uniform RF magnetic field over the brain. In general these head coils are very long, and continue past the apex of the patient's head. Since the signal to noise ratio is proportional to the square root of the Quality factor of the loaded coil divided by the effective volume, which is a volume integral of the RF energy stored in the coil, it is important to limit the size of RF coils and keep them as small as possible while at the same time providing the necessary MRI signals. However, this has to be done while maintaining the RF magnetic field homogeneity of the coils for equal flip angle distribution during transmit pulses and equal sensitivity during receive.
Attempts to shorten the RF head coil have not proven entirely satisfactory, providing for example hot spots or areas of high B
1
amplitude which disrupt the RF magnetic field homogeneity and the imaging quality.
SUMMARY OF INVENTION
Thus, there is a particular need for a short RF head coil which overcomes the aforementioned problems and which truly maintains homogeneity.
In accordance with one embodiment of the invention a short radio frequency coil for surrounding and magnetic resonance imaging of the human head includes a birdcage structure formed by a plurality of parallel conductors. The conductors first form a cylindrical portion about the axis followed by a tapered portion in the shape of a frustum of a cone having an opening at the apex of the frustum which is smaller than that of the cylindrical portion about the axis. The conductors then extend at a different angle to form a second smaller diameter cylindrical portion. Conductive circular end rings support the parallel conductors at the open ends of the cylinders.
An asymmetrical coil assembly may surround the first cylinder and extend to the tapered portion of the conductors. The coil in the central region of the asymmetrical coil assembly is of a larger diameter than the end coils of the coil assembly. The resultant improved homogeneity in the magnetic field provides improved quality imaging. The head coil may be pulsed by the transmit coil of the magnetic resonance imager or alternatively may act as a receiver coil for transmit pulses from the transmit coil surrounding the interior of the bore of the superconducting magnet.
REFERENCES:
patent: 5309104 (1994-05-01), Frederick
patent: 5602479 (1997-02-01), Srinivasan
patent: 5619996 (1997-04-01), Bernstein
patent: 5689189 (1997-11-01), Morich et al.
patent: 5999000 (1999-12-01), Srinivasan
patent: 6029082 (2000-02-01), Srinivasan et al.
patent: 6100691 (2000-08-01), Yeung
Cabou Christian G.
Freedman Irving M.
General Electric Company
Patidar Jay
Price Phyllis Y.
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