Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-06-30
2001-07-24
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06265876
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for improving the efficiency of coil systems, particularly in devices for Nuclear Magnetic Resonance image acquisition, and in particular in which the coil system is a quadrature coil system.
RELATED ART
Currently, quadrature coil systems are used both to capture the echoes of nuclear spins induced by at least one exciting pulse, and to transmit the exciting pulses of nuclear spins.
The receptive sensitivity and the transmission efficiency are both related with the characteristics of the magnetic field associated with the coil system.
The expression magnetic field associated with each coil of the system, refers to the component thereof which is orthogonal to a polarizing static magnetic field. The relevant component of the quadrature coil system consists of an appropriate vector addition of the orthogonal components of the magnetic fields associated with the single coils of the system, at least one of the said components being multiplied by a predetermined weighting factor.
As for the reception of the resonance echoes of nuclear spins, thanks to the sensitivity of the different orthogonal components of the magnetic fields associated with the two quadrature coils, the noise components that the coils receive are incoherent with each other, so the sum of the signals produces a total noise which is only {square root over (2)} and not twice the noise, with a consequent improvement of the signal-to-noise ratio.
On transmission, the power of the transmitted radio-frequency signal will be higher, the more intense the magnetic field associated with the configuration of the coils is.
Particularly in receiving coil systems, though the reachable theoretical gain of the signal-to-noise ratio through quadrature coils is about 41%, only smaller gains are obtainable in practice, and currently, for certain field configurations, even considerably smaller than the theoretical value.
This is caused by additional restrictions that the quadrature coil system must meet. First, in order to obtain valuable images for diagnostic purposes from the Nuclear Magnetic Resonance signals, the magnetic field resulting from the coil system is to be substantially homogeneous, within a predetermined tolerance range, at least in one predetermined area in which the part, limb or body under examination are to be placed. Further, the form and size of the coil system are to be designed in such a way as to enable the parts of the bodies under examination or the relevant areas thereof to enter the areas of the magnetic field of the quadrature coil systems, having a predetermined sensitivity and a sufficient homogeneity. In addition to all this, the quadrature coil systems are to be as little bulky and expensive as possible.
Currently, the coils in quadrature coil systems are conformed in such a manner that the magnetic field associated therewith have a predetermined sufficient sensitivity, that is a predetermined sufficient intensity of the magnetic field, and with a predetermined homogeneous distribution of the sensitivity throughout the examined area of the body, that is throughout the area enclosed inside the coil. This requires forms and configurations of coils ensuring both the sensitivity, i.e., intensity level of the magnetic field associated thereto, and the predetermined homogeneous distribution of the said sensitivity in the relevant volume, to obtain serviceable image information.
In the attempt to reduce the space requirement of coil systems, advantageous results have been obtained by the production of different types of the systems, specifically dimensioned for the treatment of one type or some types of limbs. However, the need to have magnetic fields which ensure, inside a predetermined volume, the necessary sensitivity, homogeneously distributed therein, and to account for the different sizes that the limbs may have, always involves huge dimensions of receiving coil systems.
OBJECTS AND SUMMARY
An object of the invention is to implement a method for improving the efficiency of a coil system of the so-called quadrature type, allowing it to get nearer to theoretical gains, with no considerable cost increase and, at the same time, to limit the space required by the system, and to provide reliable and valuable images for diagnostic purposes.
The invention solves the above problem with a method of the type described hereinbefore, in which the following steps are provided:
a) The deformation of at least one, preferably both coils of the system, according to the peculiar geometrical characteristics of the parts, i.e. limbs, or bodies to be examined;
b) and in such a way that the magnetic fields individually associated to each of the two coils have, within the relevant volume, being meant to contain the body under examination or a part thereof, different and substantially complementary areas of higher concentration, hence of higher intensity, i.e., of higher sensitivity or efficiency, of the magnetic field, the combination of the said complementary areas generating a comprehensive magnetic field with a predetermined higher intensity and homogeneity in the relevant volume, which is meant to contain the body under examination or a part thereof.
According to an enhancement, the method according to the invention provides the deformation of at least one and/or both the quadrature coils, in such a way as to obtain, within predetermined areas of the volume being meant to contain the body under examination or a part thereof, a concentration of the magnetic field with a predetermined homogeneity, the boundaries between the different complementary concentration areas being defined along lines of equal intensity of the magnetic field, corresponding to a predetermined attenuation with respect to the maximum value.
Particularly, the boundaries between the different areas of concentration of the magnetic field are defined along lines of equal intensity corresponding to a 10% to 30% reduction with respect to the maximum intensity of the field in the complementary areas, especially along lines of equal intensity corresponding to a 20% reduction of the intensity of the magnetic field.
The values for the definition of the line of equal intensity which defines the boundaries of the complementary areas of higher concentration of the magnetic field are determined, for at least one of the coils, through appropriate weighting by means of factors calculated as a function of the signal-to-noise ratios obtainable with the two coils.
According to a further characteristic of the method and of the coil system, at least one of the quadrature coils may be deformed in such a manner, that the magnetic field associated thereto and/or the comprehensive one are strongly attenuated or even null, in certain predetermined areas of the relevant volume, corresponding to one or more parts of the body, whose examination is of no interest.
The receiving quadrature coil system according to the invention provides at least one pair of coils, at least one of the said coils, preferably both of them, being deformed according to the geometrical/morphological characteristics of the body under examination and in such a way that the magnetic fields associated thereto contain areas of higher concentration and intensity of the magnetic field in the volume being meant to contain at least one relevant part of the body under examination, for the detection of Nuclear Magnetic Resonance images of that part, the said areas of higher concentration, i.e. intensity of the said magnetic fields complementing each other to obtain a comprehensive magnetic field with a predetermined intensity and with a predetermined homogeneity, which permeates the whole volume being meant to contain the body under examination or a part thereof.
Advantageously, the coils are deformed in such a way as to concentrate the area of higher intensity of the associated magnetic field in one portion of the volume being meant to contain the body under examination and corresponding to a part thereof, strongly reducing or annulling the magnetic fi
Arana Louis
Burns Doane Swecker & Mathis L.L.P.
Esaote S.p.A.
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