Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1991-03-29
1993-05-04
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324309, G01V 300
Patent
active
052085333
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nuclear magnetic resonance (NMR) machine with low field and dynamic polarization. The NMR machines concerned are more specifically NMR imaging machines which find application in particular in the medical field. The object of the machine in accordance with the invention is to improve the signal-to-noise ratio of the detected signal in order to achieve enhanced sharpness of detail of the images obtained. Moreover, the invention can make a significant contribution to reduction in cost of such machines by simplification of their homogeneity correction coils.
2. Description of the Prior Art
Classically, an NMR machine essentially comprises a magnet, or a coil which performs the same function, in order to subject a body to be examined to an intense and permanent orienting magnetic field. When subjected to this influence, the body is then excited electromagnetically by a high-frequency electromagnetic wave. On completion of the excitation, a measurement is performed on a de-excitation electromagnetic wave which is emitted by the body and provides information on the intimate nature of said body. It is known that the amplitude of the detectable electromagnetic signal in such machines is of the type .chi. B.sub.O.sup.2. In this expression, .chi. is the magnetic susceptibility of the body to be examined and B.sub.O is the intensity of the orienting field of the machine.
Now the amplitude of the noise re-emitted by a body is proportional to .omega., where .omega. is the resonant angular frequency of the magnetic moments of the body particles when the body is subjected to the influence of the orienting field. The immediate result thereby achieved is that the signal-to-noise ratio of the detectable electromagnetic wave is proportional to B.sub.O since .omega. is itself proportional to B.sub.O. In the present state of the art, this observation has favored the construction of NMR machines with an orienting field B.sub.O which is as large as possible. This incentive is greater by virtue of the fact that, with a high field, the resistance provided by the antenna in parallel with the resistance offered by the body becomes negligible.
In a high-field machine, the magnetization given to the particles of a body to be scanned is substantial. This magnetization is related to the polarizing force of said field and is stronger as the field is higher. Furthermore, at the moment of excitation, the spins of the particles begin to resonate at a frequency which is also proportional to the orienting field B.sub.O. At the moment of excitation, the orienting field therefore performs in addition to its polarizing field function, a second function which is that of a resonance field. Just as the use of a high-intensity orienting field is useful in regard to polarization and magnetization since it increases the amplitude of the detectable signal, so the use of a high field at the moment of resonance is a difficult matter.
In fact, if the orienting field is not perfectly homogeneous, two types of disadvantages accordingly arise. Minor consequences first appear in regard to magnetization (on condition that inhomogeneity is nevertheless not excessive). In fact, this different magnetization at different locations of the machine produces a defect in homogeneity of luminosity of the image which can be arranged. But other more critical consequences affect the detectable electromagnetic signal. In fact, if the resonance field is of the order of 1 Tesla and if the particles examined are hydrogen particles (present in water and therefore in a large quantity in human bodies), the resonance frequency of the spins is of the order of 40 MHz. An inhomogeneity of one millionth of the value of the resonance field therefore produces a resonance deviation of approximately 40 Hz. This means that, at the end of a period of 12.5 ms, contributions to the total NMR signal made by adjacent particles but subjected to resonance fields which differ from each other by one millionth a
REFERENCES:
patent: 4887034 (1989-12-01), Smith
patent: 5057776 (1991-10-01), Macovski
Patent Abstracts of Japan, vol. 10, No. 389 (P-531) (2446), Dec. 26, 1986, & JP, A, 61176842 (Mitsubishi Electric Corp.) Aug. 8, 1986.
Arana Louis
General Electric CGR S.A.
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