Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1984-02-27
1986-11-04
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324318, G01R 3320
Patent
active
046212370
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a radiofrequency transducer and is concerned in particular, but not exclusively, with the use of such a transducer in conjunction with a known technique of nuclear magnetic resonance.
Nuclear magnetic resonance (NMR) is a well known laboratory technique, which can be used, inter alia for the determination and characterization of chemical species. In the classic NMR experiment, a substance is subjected to a static magnetic field ("B") and an oscillatory electromagnetic field of angular frequency .omega.. A condition of resonance occurs when .omega.=.gamma.B, where .gamma. is the gyromagnetic ratio which is characteristic of a particular nucleus present in the substance. Although the oscillatory field is normally and electromagnetic one for practical reasons, it should be pointed out that it is only the magnetic component which interacts with the test substance to produce the NMR effect. The resonance condition can be detected by absorption of, or absorption and re-emmision of, the radiofrequency field applied, and is indicative of the presence of a particular element comprising that nucleus. In general, the way of coupling the radiofrequency source to the particular substance has been to provide a simple loop or coil transducer, designed to generate a radiofrequency magnetic field connected to the RF source by a coaxial cable or other type of RF transmission line.
It is necessary to tune the coil so as to be resonant at the required frequency for the particular nucleus under investigation at the particular magnetic field employed, and to match the impedance of the coil to that of the transmission line (eg. coaxial cable) employed. Examples of typical circuits for tuning and matching are shown in FIGS. 1a and 1b, which will be described in more detail hereinafter. These are the well known "half-T" and PI networks respectively.
At the high frequencies commonly used in NMR techniques, very low values of variable capacitors are required in the arrangement shown in FIGS. 1a and 1b for capacitors C1 and C3, to successfully tune and match the simple coil. In particular, when large coil sizes are employed, for example loops greater than 5 cm or so, and at high field strengths, for example of 1.5 Tesla and above, the stray capacitances of the coil become significant compared with the values required to tune and match the coil. The self capacitance of the coil is even more significant if the sample being analyzed is conductive, or if it has a high dielectric constant. Under these conditions, the tune and match methods become inefficient and unworkable. This is particularly true for so-called "surface coils" for example of the kind suggested by J. J. H. Ackermann et al (Nature 283,167 (1980)), which are often used in medical and other in vivo applications in which there is physical difficulty in locating the sample at the centre of a loop transducer.
One method of overcoming this problem has been to introduce into the region of the coil one or more series capacitors to lower the effective inductance of the coil. An example of such a solution is that proposed by D. W. Alderman and D. M. Grant (J. Magn. Res. 36, 447, (1979)). As well as reducing the effective inductance of the coil, the introduction of series capacitance in this way means that the high RF voltages employed are divided across more components. A schematic diagram of such an arrangement as shown in FIG. 2, which will be described in more detail hereinafter.
A difficulty with such proposals has been that they are relatively fragile and prone to break when applied to large rf coils.
It is an object of the present invention to provide a radiofrequency transducer, particularly but not exclusively for use in nuclear magnetic resonance techniques, which overcomes or reduces the problems set out above where a relative large coil size is utilised at high frequencies and the stray capacitances of the coil become significant compared with the capacitance values required to tune and match the coil to a source or receiver.
According to
REFERENCES:
patent: 2116734 (1938-05-01), Reinartz
patent: 3184746 (1965-05-01), Chatelain
patent: 3402346 (1968-09-01), Baker
patent: 3783419 (1974-01-01), Lafond
patent: 4095168 (1978-06-01), Hlavka
Lowe et al, "A Fast Recovery Pulsed Nuclear Magnetic Resonance Probe Using a Delay Line", Rev Sci Instrum, vol. 45, No. 5, May 74, pp. 631-639.
Oxford Research Systems Limited
Tokar Michael J.
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