This invention relates to nuclear spin tomography in general and more particularly to an antenna device for exciting an at least largely homogeneous magnetic high-frequency field and/or for receiving corresponding high-frequency signals in a nuclear spin tomography apparatus.
An antenna device for exciting an at least largely homogeneous magnetic high-frequency field and/or for receiving corresponding high-frequency signals in apparatus for nuclear spin tomography, in which device at least two conductor sections of predetermined length are provided which extend on an imaginary cylinder surface parallel to the direction of the cylinder axis, through which current flows in opposite directions and which are connected to an external energy feeding device is disclosed in DE-OS No. 3133432. The antenna device further contains a tubular enclosure which is arranged at a predetermined distance concentrically with respect to the imagined cylinder surface and around the conductor sections. The enclosure passes at least largely, low-frequency magnetic gradient fields, contains electrically highly conducting material and is likewise connected to the energy feeding or receiving device. The conductor sections are terminated at their respective end which is not connected to the energy feed or receiving device by means reflecting the waves of the high-frequency field so that a high-frequency field which oscillates in phase can be developed by the conductor system formed by the tubular enclosure and the conductor sections.
In the field of medical diagnostics image-forming methods have been developed in which resonance signals integrated by calculation or measurement of nuclei of a given element of, in particular, a human body or part of a body are analyzed. From the spatial spin density and/or relaxation time distribution so obtained, an image similar to an x-ray tomogram can be constructed. Such methods are known under the designation "Nuclear Spin Tomography" (Nuclear Magnetic resonance tomography) or "Zeugmatography."
A requirement in nuclear spin tomography is a strong magnetic field which is generated by a so-called base field magnet, is as homogeneous as possible in a region of predetermined extent and into which the body to be examined is placed along an axis which generally coincides with the orientation axis of the magnetic base field. Superimposed on this base field are stationary and/or pulsed, so-called gradient fields. For exciting the individual atomic nuclei in the body to perform a precession motion, a special antenna device is further required, by which means of a high-frequency magnetic alternating field (RF alternating field) can be excited for a short time and which can also be used for receiving the RF signals connected thereto if a separate measuring coil is not provided for this purpose.
As is well known, the quality of the sectional images in such apparatus for nuclear spin tomography (NMR tomography) depends on the signal-to-noise ratio of the induced nuclear spin resonance signal. Since this signal-to-noise ratio in turn depends on the strength of the magnetic base field and increases with frequency, it is desirable to provide frequencies as high as possible for high base fields (see "Jour. Phys. E: Sci. Instrum.", volume 13, 1980 pages 38 to 44).
With the known RF antenna device mentioned above, RF fields with high frequencies of about 20 MHz or more can be excited and received. To this end, the antenna device contains a tubular antenna part of electrically highly conductive nonmagnetic material. This antenna part represents and envelope around several conductor sections which form at least one pair of conductors which are disposed on an imaginary cylinder surface, around which the envelope is arranged concentrically at predetermined spacings. On the at least one conductor pair and the envelope and wave propagation with very high frequency is then made possible, resonance conditions being adjusted in such a manner that fields oscillating in the same phase are developed in the entire volume of interest in the form of standing waves on the pair of conductors. Since, furthermore, the common envelope around the pair of conductors is designed so that it passes, at least largely, low frequencies, the low-frequency gradient fields can accordingly propagate unimpeded in the volume into which the body to be examined is to be placed.
In this known antenna device, however, the alternating RF field can also cover regions which are located in front of the respective axial end faces of the conductor system formed by the conductor sections and the tubular enclosure. This means that disturbances caused in these regions can possibly falsify the high-frequency measuring signal.
It is, therefore, an object of the present invention to improve this known antenna device in such a manner that it is largely independent of external interference fields.