GB2126727A

GB2126727A - Method and apparatus for imaging substances in biological samples by nuclear magnetic resonance

Info

Publication number: GB2126727A
Application number: GB08225695A
Authority: GB
Inventors: Derek SHAW
Original assignee: Oxford Research Systems Ltd
Current assignee: Oxford Research Systems Ltd
Priority date: 1982-09-09
Filing date: 1982-09-09
Publication date: 1984-03-28
Also published as: GB2126727B

Abstract

A method of determining the distribution in a sample, of non-proton nuclei having a magnetic moment, comprises subjecting the sample to a magnetic field, irradiating the sample with RF radiation at a proton magnetic resonance frequency, deriving a first NMR signal, indicative of electromagnetic absorption of the sample at the proton magnetic resonance frequency, deriving from the sample a second such NMR signal at the proton resonance frequency in the presence of RF radiation at the nuclear magnetic resonance frequency of the said non-proton nuclei so as to decouple protons in the sample from the said non-proton nuclei, and applying an imaging technique to produce an image indicative of the spatial variation of the difference between the said first and second signals. Imaging may be performed on the difference between the two NMR signals, or on each NMR signal followed by subtraction of the images. The method can be used to trace how a <13>C. labelled material introduced into a patient, and its breakdown products, become distributed.

Description

SPECIFICATION Method and apparatus for imaging substances in biological samples by nuclear magnetic resonance Nuclear Magnetic Resonance (NMR) is based on the phenomenon that when a substance is subjected to a static magnetic field B and an oscillatory electro-magnetic field of angular frequency c9 then a condition of resonance occurs when w=y B, where y is the gyromagnetic ratio for a particular nucleus present in the substance.

The resonance can be detected and is indicative of the presence of a particular element comprising that nucleus.

Where the material includes complex molecules the localised fields produced by molecular electrons have a screening effect which causes identical nuclei in different chemical or molecular environments to resonate at slightly different frequencies, that is to say the value of B is slightly modified for the same nucleus in different environments. This effect is known as the chemical shift. The magnitude of such shifts is very small but it can often be detected and can be used for distinguishing between different compounds containing the same element.

The nuclear magnetic resonance technique may be used only on nuclei which have a magnetic moment, and this places severe restriction on the circumstances in which the technique can be used. Examples of nuclei having magnetic moment are 1H, 31P and 13C.

In recent times, great interest as been expressed in utilising nuclear magnetic resonance in conjunction with imaging techniques, to form a two or three dimensional representation of the distribution of particular materials in a sample.

Various imaging techniques that use NMR have been proposed in which a specimen is subjected to non-homogeneous magnetic fields varying in time or space, for example by superimposing a time-varying magnitude field component or a linear magnetic field gradient on the main magnetic field, to induce localised resonances in parts of the specimen and thus enable an image to be built up based on the presence of a particular nucleus. Examples of such prior art proposals are the following techniques: (1) Projection-reconstruction procedures, e.g.

P.C. Lauterbur (Nature,242,190 (1973)), (2) Selective excitation methods, e.g.: (i) Garroway A.N., Grannel P, and Mansfield P., J. Phys. C., 7, L457 (1974), (ii) Lauterbur P.C., Dulcey Cos., Lai C.M., Feiler M.A., House W.V., Kramer D.M., Chen C.N.

and Dias R., Proc. XVIII Ampere Congress (eds. P.S. Allen, E.R. Andrew C.A. Bates), Amsterdam: North Holland, P27 (1974), (iii) Hutchison J.M.S., Proc. 7th L.H. Gray Conf., Chichester: Wiley pl 35, (1976)), 3) Modifications of 2D Fourier Transform NMR techniques, e.g.:- (i) Kumar A., Welti D. and Ernst R.R., Natur wissenschaften 62, 34 (1975), (ii) Kumar A., Welti D. and Ernst R.R., J. Magn.

Reson., (1975) and (4) Fonar techniques, e.g.: Damadian R., Minkoff L., Goldsmith M., Stanford M. Koutcher J., Physiol.

Chem. Phys., 8,61(1976).

A more complete list of imaging techniques useful in NMR may be found in Phil. Trans. Roy.

Soc., B289 (1980).

It can be seen from the above references that imaging techniques are themselves very well known, and the present specification should be considered to incorporate by reference the disclosures of all of the above listed documents.

The imaging techniques described above as applied in biology and medicine have hitherto been almost exclusively directed to the detection of resonance of the proton or 1H nucleus.

Biological material contains large amounts of hydrogen, substantially all of which is the isotope 1H, and thus the technique is relatively sensitive for protons. In order to successfully carry out an imaging experiment, it is desirable to be able to select from the NMR spectrum a single sharp peak which is widely spaced from any neighbouring peaks in comparison with the field gradient required to perform the imaging.

Because all biological material contains very large quantities of protons however, it is in general not possible to follow, using conventional imaging techniques, the progress of individual metabolites in a biological system, for example in the human body.

If the chemical environment of a proton in a molecule includes an atom having a magnetic moment, then the proton magnetic resonance signal derived will be split into a multiplet. For example, the spectrum of a proton adjacent to a '3C atom will in general be split into a doublet, and the splitting can be detected, provided that the magnetic field is sufficiently homogeneous (i.e. homogeneous to at least 1 in 106).

A number of workers have shown that it is possible to obtain a large increase in specificity of proton NMR spectra by utilising a '3C-labelled substrate and observing the difference between proton spectra in the presence and absence of 13C decoupling fields. An example of such a proposal is put forward in a paper by L.O. Sillerud, J.R.

Alger, and R.G. Shulman, (Journal of Magnetic Resonance 45, 142-1 50 (1981)). In this paper, the Authors show that it is possible to differentiate selected proton resonances very easily from background resonances, and thereby distinguish signals obtained from a labelled compound, for example glucose, alanine, or glycerol, to enable kinetic studies to be carried out on metabolism of such compounds, in particular in yeast.

We have now discovered that, by combining a decoupling technique such as that used by Sillerud, Alger, and Shulman with an imaging technique it is possible to form an accurate picture of the distribution of labelled metabolites in living systems, and to follow the way in which the distribution of such metabolites changes with time.

According to a first aspect of this invention, there is therefore provided a method of determining the distribution of non-proton nuclei having a magnetic moment, for example 13C, in a sample, which method comprises subjecting the sample to a magnetic field, irradiating the sample with radiofrequency radiation at a first frequency, corresponding to a proton magnetic resonance frequency, deriving a first signal, indicative of electro-magnetic absorption of the sample at the proton magnetic resonance frequency, deriving from the sample a second signal indicative of electro-magnetic absorption of the sample at the proton resonance frequency in the presence of electromagnetic radiation at a second frequency, corresponding to the nuclear magnetic resonance frequency of the said non-proton nuclei, and being such as to decouple protons in the sample from the said non-proton nuclei, and applying an imaging technique to produce an image indicative of the spatial variation of the difference between the said first and second signals.

The method of the invention is based on the familiar INDOR principle used in conventional high resolution NMR. It can be used to provide an image of a sample, for example a portion of the body of a human patient, with the sensitivity normally associated with proton NMR, but the chemical selectivity which could be obtained if 13C could be imaged directly. The method can therefore be used with great chemical specificity, to determine the progress of a particular metabolite through the various organs of the body.

Preferably, the method is carried out by introducing into a sample a quantity of a chemical compound labelled with 13C. By a conventional INDOR process, for example that described by Sillerhud, Alger, and Shulman, a single sharp peak can be obtained by a differencing technique, whereby a proton signal derived only from those protons coupled with the labelled 13C atom is produced, by subtraction of signals obtained in the presence and absence of a decoupling RF field at the 13C resonance frequency. At a magnetic field strength of approximately 2 Tesla, a suitable proton (first) frequency is approximately 80 MHz, the 13C decoupling (second) frequency being approximately 20 MHz.

When carried out on a human patient, the method can be used, for example, to determine the way in which a particular labelled substance, which may be given by mouth or injected, is metabolised, and the way in which the distribution of the substance, and its products after breakdown, are distributed between various internal organs of the body. By careful application of the second radiofrequency signal, individual carbon-atoms may be followed through various metabolites.

The imaging may be carried out either by first forming a difference signal corresponding to the difference between the proton resonance in the presence and absence of the decoupling radiation, and then using the difference signal in an imaging experiment. Alternatively, an image may be formed by a conventional proton imaging technique in the absence of decoupling radiation, and the second image formed whilst decoupling radiation is applied. The two image signals may then be subtracted to form the difference image.

The invention also provides apparatus for determining the distribution in a sample of nonproton nuclei, which apparatus comprises means for generating a magnetic field, means for supporting a sample in the magnetic field, means for generating in the sample a radiofrequency electromagnetic field at a first frequency, corresponding to a proton magnetic resonance frequency in the magnetic field, means for generating in the sample a radiofrequency electromagnetic field at a second frequency, corresponding to the nuclear magnetic resonance frequency of a non-proton nucleus, to decouple protons present in the sample from the said nonproton nucleus, means for deriving from the sample first and second signals, indicative respectively of electromagnetic absorption of the sample at the proton magnetic resonance frequency, in the absence and presence respectively of the said second frequency electromagnetic radiation, means for producing a spatial variation in the magnetic field or electromagnetic field, and for determining the spatial variation in the difference between the said first and second signals, to determine the distribution in the sample of the said non-proton nuclei.

The sample support is preferably adapted for supporting a human patient, with at least a portion of the body of the patient, for example the trunk, in the magnetic field, whereby the distribution of labelled substances in the patients internal organs may be determined. The radiofrequency source at the second frequency is preferably phased locked to the means for generating the radiofrequency field at the first frequency. The patient support may take any conventional form, for example a bed or chair in the vicinity of the magnet, to enable the desired part of the body to be located in the field.

The method of the invention has a wide range of applications in the field of medicine and diagnosis, and in addition may be used in mapping of labelled compounds in tissue samples, for example in autopsy, Spin-echo techniques may preferably be utilised in the imaging method, for example the echo planar method, as described by P. Mansfied, A.A.

Maudsley, and T. Baines (J. Phis. E., 9, 271 (1976)).

Care must be taken when carrying out in-vivo measurements to avoid any harmful effects of heating, induced by the radiofrequency irradiation. However, the technique as a whole is reasonably safe to use in-vivo particularly as compared with X-ray imaging techniques.

Claims

1. A method of determining the distribution of non-proton nuclei having a magnetic moment, in a sample, which method comprises subjecting the sample to a magnetic field, irradiating the sample with radiofrequency radiation at a first frequency, corresponding to a proton magnetic resonance frequency, deriving a first signal, indicative of electro-magnetic absorption of the sample at the proton magnetic resonance frequency, deriving from the sample a second signal indicative of electromagnetic absorption of the sample at the proton resonance frequency in the presence of electromagnetic radiation at a second frequency, corresponding to the nuclear magnetic resonance frequency of the said non-proton nuclei, and being such as to decouple protons in the sample from the said non-proton nuclei, and applying an imaging technique to produce an image indicative of the spatial variation of the difference between the said first and second signals.

2. A method as claimed in Claim 1 wherein the non-proton nuclei are 13C nuclei.

3. A method as claimed in Claim 2, wherein the magnetic field strength is approximately 2.0 Tesla, the first frequency is approximately 80 MHz, and the second frequency is approximately 20 MHz.

4. A method as claimed in any one of the preceding Claims, wherein the sample comprises a 13C labelled compound.

5. A method as claimed in any one of the preceding Claims, wherein the sample is a living organism.

6. A method of determining the fate of a metabolisable substance in a living organism, which method comprises introducing into the organism a quantity of the substance which has been labelled with non-proton nuclei having a magnetic moment, and determining the distribution in the organism of the labelled substance by means of a method as claimed in any one of Claims 1 to 3.

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