BACKGROUND OF THE INVENTION

The present invention relates to a method for recording the nuclear magnetic resonance in selected areas of a body for the purpose of representing body cross-sections in the form of images (NMR tomography), in which the nuclear spins of a selected type present in the area of one plane of the body are subjected to a superimposed gradient field varying in the selected cross-sectional plane, and selectively excited, whereupon the induction signals supplied by the excited nuclear spins are processed by calculation to obtain image signals.

The cross-sectional images obtained by NMR tomography represent substantially the density distribution of the nuclear spins of the selective type in the cross-sectional plane. But the signal amplitude responsible for the light intensity of the individual image points depends not only on the density of the excited nuclear spins, but also on their state of excitation. Further, the image quality is affected to a quite considerable degree by unavoidable noise signals. There are, consequently, considerable elements of uncertainity in the information content of images of body cross-sections obtained by NMR tomography, and the images obtained are not of the desirable quality.

Accordingly, it is an object of the present invention to improve the known methods used in NMR tomography to increase the information content and the quality of the images obtained.

SUMMARY OF THE INVENTION

According to the present invention, this object is achieved by a method characterized in that the excitation of the nuclear spins is achieved in the conventional manner by means of a pulse sequence which induces the nuclear spins to produce at each stage a number of chronological induction signals, and that the induction signals corresponding to each other in chronological sequence are processed into separate sets of image signals so that a single meansurement provides a sequence of cross-sectional images differing from each other by the intensity of the excited core spins which vary according to the spin-lattice and spin-spin relaxation times T.sub.1 and T.sub.2 respectively.

By using the method of the invention, one obtains by a single measurement not only the one image usual heretofore, but a number of images of considerably different structures because the excited core spins found in different structures of the body cross-section have different relaxation times T.sub.1 and T.sub.2 so that the signal intensity of the individual structures of the body cross-section varies during the recording time. In the course of such variations, certain details of the chronologically successive images may vanish and be replaced by others which could not be seen before. The particular advantage of the method of the invention is to be seen in the fact that one has not to pay for the additional information thus gained with a notable extention of the measuring time which could not be tolerated in particular in medical applications and which in the case of in vivo measurements could quite generally lead to changes in the position of the object under examination and, thus, a considerable loss of information. Contrary to image sequences that could be obtained by several repeated measurements, the images obtained by the method of the invention distinguish themselves by absolute coincidence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1b, 1c, 1d and 1e are photographs taken using the method of the present invention showing tomographic images of a sagittal cross-sectional plane through a human head with FIG. 1a being a first image obtained, 16 ms following a first excitaton and FIGS. 1b, 2c, 3d and 4e showing images obtained at 32 ms intervals thereafter; and

FIGS. 2a, 2b, 2c, 2d and 2e are photographs taken using the method of the present invention showing tomographic images of a transverse cross-sectional plane through a human head with FIG. 2a being a first image obtained 16 ms following a first excitation and FIGS. 2b, 2c, 2d and 2e showing images obtained at 32 ms intervals thereafter.

DETAILED DESCRIPTION

The pulse sequences used in the method of the invention may be the same as those used in normal NMR spectography for determining the relaxation times. These are in the case of T.sub.1 in particular the DEFT sequence (see E. D. Becker et al in J. Amer. Chem. Soc., vol. 91 (1969), p.7784) or corresponding variangs thereof, and in the case of T.sub.2 a Carr-Purcell sequence with or without Gill-Meiboom modification. But while in classical NMR spectography the signals are intregrally derived from the whole sample volume and, therefore, directly comparable, the successive interferograms excited by such a pulse sequence do not lend themselves for such a comparison because the individual interferograms represent the signals from large volume domains which do not supply directly usable data. Rather, comprehensive processing of the free induction signals is required to obtain the desired information on the individual image points. So, the only measure that suggested itself was to add up the induction signals obtained by such a pulse sequence in the conventional manner, prior to processing them further, in order to improve the signal-to-noise ratio. However, the improvement of the image quality thus obtainable is not nearly as great as that achievable by the method of the invention. In addition, it is a disadvantage of the above method that the information relating to the relaxation times get lost, whereas in the method of the invention this information is gained by the step of processing the individual induction signals corresponding to each other in chronological sequence into separate sets of image signals.

As mentioned before, it is a particular advantage of the method of the invention that all images representing chronologically successive states are gained by one and the same measurement so that they provide absolute conicidence and an exact comparison. This then offers the possibility to relate the image signals associated with the individual image points to each other and to derive from the variation of the image signals associated with the individual image points the relaxation times T.sub.1 and/or T.sub.2 for the individual volume elements of the body cross-section related to the respective image points. Considering that the method of the invention can supply a relatively large number, for instance 24, of chronological parameters for each image point, it is possible to derive therefrom the e-functions characteristic of the relaxation times T.sub.1 and/or T.sub.2 with great accuracy. The same degree of accuracy is achieved in determining the relaxation times related to the individual image points. Since it has been found that the relaxation times T.sub.1 and T.sub.2 of excited nuclear spins in cancer tissue are considerably greater than those of nuclear spins in similar sound tissue, the exact determination of the relaxation times T.sub.1 and T.sub.2 is of considerable importance. The fact that the method of the invention offers the possibility to derive from the images obtained different relaxation times should open up interesting perspectives for cancer diagnosis as it is no longer necessary to carry out separate in vitro examinations of individual tissue samples.

The fact that the method of the invention permits to determine the relaxation times T.sub.1 and T.sub.2 for the individual volume elements of the body cross-section with great accuracy opens up the possibility to produce, by way of a further improvement of the invention, a set of image signals represeting only the density of nuclear spins having a selected relaxation time T.sub.1 or T.sub.2.

If, for instance, the spin-spin relaxation time T.sub.2 of a typical cancer tissue were known, there would be the possibility to produce with the method of the invention images representing exclusively those body areas which contain cancer tissue. So, one could for instance determine the relaxation time T.sub.2 of cancer tissue removed in the course of an operation and examine the patient later with the aid of the method of the invention to determine if tissue presenting the same relaxation time T.sub.2 develops again in his body, and in particular in the neighbourhood of the operation site.

The fact that due to the exact coincidence of the individual points of the image set obtained the relaxation times T.sub.1 and T.sub.2 related to the individual image points can be determined with great accuracy opens up an experimental way to correct all image points of the individual images so as to bring their intensity exactly to the value which it must have according to the e-function obtained from the determined relaxation time. In this manner, any disturbance by noise signals and the like can be considerably reduced so that it is possible, without any extension of the measuring time, to obtain images of a quality which is necessarily better than that achievable by the usual methods forimproving the signal-to-noise ratio.

It is a precondition of the method of the invention that the nuclear spins to be excited by means of the pulse sequence in the area of the selected body plane have not been previously subjected to influence which disturbed their orientation relative to the homogeneous static magnetic field so that when a pulse sequence consisting of 90.degree. and 180.degree. pulses is applied, their orientation in parallel to the static magnetic field is constantly restored so that they can supply a plurality of successive induction signals. In the simplest of all cases, the method can be used for examining a body which exhibits from the very beginning the shape of a disk, whose entire volume supplies the image to be produced and which, accordingly, need not be subjected to any particular steps for preparing a particular cross-sectional plane for the image-forming process. The method of the invention is also of definite interest for examinations of organic and anorganic bodies which can without any problems be cut mechanically into disks suited for producing NMR images which give the desired information on the structure of the bodies.

But there definitely also exists the possibility to prepare a body by special excitation steps to ensure that only the nuclear spins obtained in the selected body plane are oriented in parallel to the static magnetic field and, thus, available for an excitation supplying a defined signal. In a further improvement of the invention, the selection of the body plane is, therefore, effected by application of a gradient field extending in parallel to the homogeneous magnetic field and varying in a direction perpendicular to the selected plane, and selected pulse signals which provide both, a selective excitation of the nuclear spins of the selected type contained in the selected plane and an excitation of the entirety of the selected nuclear spins in a manner such that only the nuclear spins contained in the selected plane resume the orientation in the sense of the static magnetic field. An excitation method of this type is the subject-matter of a prior U.S. patent application Ser. No. 474,699 which describes the method in detail so that this description need not be repeated here.

To demonstrate the value and importance of the method of the invention, two sets of photographs are attached which represent a sequence of cross-sectional images obtained by the method of the invention. The photographs show tomographic images of a sagittal (FIGS. 1a, 2b, 3c, 4d, 5e) and a transverse (FIGS. 2a, 2b; 2c, 2d, 2e) cross-sectional plane through a human head. The selection of the cross-sectional planes was effected with the aid of the method described by the patient application just mentioned. The exitation of the selected spins of protons was achieved by applying a Carr-Purcell sequence in such a manner that the first image was obtained 16 ms following the first excitation, and the other images at intervals of 32 ms each thereafter. The differences in structure of the successive images are striking. Structures clearly visible in the first image vanish gradually and give room to other structures. One will note, for instance, that a small light circular bow develops in the lower section of the back of the head in the sagittal images whereas a large dark bow to be seen a little lower vanishes gradually. Similarly, the central structure of the transverse cross-sectional image changes considerably. In both series of images. The image of the brain substance is gradually superseded by the image of the brain structure.

It should be noted that the attached photos have by no means the quality that can be achieved with the method of the invention. This is due to the fact that on the one hand each image has been obtained by summing up four successive images and, on the other hand, the before-described correction by means of the determined T.sub.2 values has not carried out yet.