HIGH SPEED COMPUTER ASSISTED TOMOGRAPHY

Technical Field

This invention relates to transaxial X-ray tomography systems, and more particularly, to high-speed computer assisted tomography. Background of the Invention

In medical applications .of computer assisted tomography (CAT) it is desirable to achieve scan time of about 0.1 sec in order to image portions of the body during live motion such as heartbeat or respiration. The usual source of X-rrays in conventional CAT systems is the Coolidge tube. The entire X-.ray tube is translated or orbited or both as necessary to provide X-ray projections for planar image reconstruction. One such design using a conventional X-ray tube mechanically orbited about the patient area on a gantry is set forth in U. S. Patent 3,940,625 by Hounsfield issued February 24, 1976. The Coolidge X-ray tube has relatively high mass and is not designed to withstand the strong accelerating forces necessary for rapid movement and fast scanning. Thus, use of such X-rray tube limits scan time to at least a few seconds, too slow for live stop-faction images. An ultrafast CAT scanner has been proposed which is intended to reduce scan time to 0.01 sec. (See "Proposed System for Ultrafast Computer Tomography," by Iinuma, Tateno, Umegaki, and Watanabe, in Journal of Computer Assisted Tomography, Vol. 1_, No. 4, 1977, pp. 494-498.) The ultrafast system uses a large bellr- shaped X_?ray tube containing an annular anode surrounding the patient area. A system of deflection coils directs an electron beam to the anode and sweeps the beam around the anode to generate an X-ray beam which emanates from successive points along the annular anode. The

X-rays are directed through the patient area to a circular array of stationary detectors disposed around the patient area and in the path of the orbiting X-ray beam. Focusing

of the electron beam over the relatively long beam path between the cathode electron source and the anode is difficult and contributes to the complexity and expense of the ultrafast system. Further, in order not to block the X-ray beam from the anode, the detector array is disposed to one side of the anode, and the X-ray beam follows a path which is slightly non-perpendicular to the tube and patient long axis. This results in X-^-ray projection data for a slightly conic surface and not for a true transaxial plane through the patient. This produces images of lower resolution than true planar measurements would produce. Summary

The present invention provides for fast CAT scanning with speeds suitable for imaging a beating heart or respiratory motion but without the complexity of a long electron beam path for X-ray generation. Further, the present invention provides for true planar measurements of X-^-ray projections. The present invention achieves those goals, in accordance with one embodiment of the invention, by providing a hollow, cylindrical X-ray tube which contains therewithin an annular anode which surrounds the patient area (within the hollow space enclosed by the tube) and an electron source (cathode) which orbits within the tube along a circular path generally parallel to and spaced from the anode. The anode is shaped to direct beams of X-.rays, originating from successive points around the anode, through the patient area in directions lying within a plane perpendicular to the Xτ-ray tube central axis. X- _^ -ray detectors are arranged in a circular array disposed within the plane of the X-=-ray beams, the detectors being movable, in sections, for allowing unimpeded passage of the X-ray beams. Description of the Drawing

The Figure shows a computer assisted tomography system constructed according to the present invention.

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Detailed Description

The Figure shows a cut away side view of an X-ray ^' generation and detection arrangement constructed according to the present invention. Detected X-ray signals are processed by computer to reconstruct an image of a transaxial slice through the patient in conventional manner. These matters are not further discussed.

X-ray generation takes place in a hollow cylindrical shaped vacuum chamber 1. Vacuum supporting inner wall 2 surrounds the patient area. Annular anode 3 is supported within the vacuum chamber by high voltage insulators 4. Feedthrough insulator 5 carries current through the. wall of the vacuum chamber to maintain a suitably high voltage at anode 3 for X-ray generation. The outer-periphery of the anode 3 projects in an axial direction forming a cylindrical projection 6. The end surface 7 of the cylindrical projection is bombarded by an electron source, described hereinafter, to generate a beam of X-rays. To this end, the .surface 7 is coated with a suitable Xrrray generating material, such as tungsten. Also, to properly aim or direct the X-ray beam, the end surface 7 is angled or beveled rearwardly as shown. The generation and aiming of an X-ray beam in this manner is well known. Electron source or gun 8 includes a hot filament electron emitter 9 positioned adjacent to anode surface 7. Current is applied to emitter 9 through brush and contact arrangement 18. Rings of conducting material extend around the circumference of rotational structure 10 with sliding brush contacts for conducting filament current through the vacuum supporting wall from a suitable power supply. Heating current supplied to the filament of emitter 9 supports thermionic emission of electrons which are accelerated to strike anode surface 7 causing X-ray emission.

Electron gun 8 is attached to rotational structure 10 supported by bearings 11. Structure 10 is in the form of an annular ring rotatable concentrically

with anode 3. As structure 10 rotates, the axial and radial relationship between electron gun 8 and anode surface 7 remains constant, thereby providing a relatively constant intensity of X-rays emitted from successive points of the stationary anode surface 7 as the electron gun passes. Rotational motion of structure 10 is imparted by motor, shaft, and gear arrangement 12, which extends through the vacuum supporting wall with a vacuum supporting seal. X-rays produced at anode surface 7 pass through an X-ray transparent window 20 in the inner wall 2 of the chamber. This X-ray window may be of any suitable mechanically rigid material such as beryllium. X-rays emerging from anode surface 7 are shaped into a fan-rrshaped beam by circular annular delimiters 21 made of a suitable X-ray opaque material such as lead.

The X-rray beam impinges upon X-ray detector array 22. The output of array 22 is conveyed to the input of an image reconstruction computer, not shown. Detectors in array 22 form a complete circle around the patient area in the plane of the X-ray fan beam, i.e., in a plane perpendicular to the central axis of the chamber. Detectors are axially movable, either individually or in convenient mechanically connected detector subarrays such as subarray 23, each subarray comprising a fraction of the total array circle. For illustrative purposes, subarray 23 comprises onerrfourth of the detectors. Each subarray is movable in the axial direction by a withdrawing element such as solenoid 24. When energized, the solenoid moves subarray 23 out of the path of the X-ray fan beam, thus allowing the fan beam to pass through the patient area and illuminate detectors positioned on the opposing side. Subarray 23 is restored to its position in the beam plane by the action of a suitable restoring element such as compression spring 25. A subarray may comprise any number of one or more detectors. In some applications it may be necessary to move or restore more than one subarray at a time to allow the fan beam to pass unobstructed and to assure that

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detectors are timely returned to the beam plane.

In operation, motion of the detector subarrays is synchronized with the rotation of-structure 10. To this end, suitable position detector 17 is associated with rotational structure 10. Detector 17 may be of mechanical, electrical, optical or other conventional design for determining the rotational position of structure 10.

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