X-Ray Collimator, Particularly for Use in Computerized Axial Tomography Apparatus

This invention relates to X-ray collimators. More particularly, this invention relates to laminar X-ray collimation apparatus for producing wedge-shaped X-ray beams of adjustable thickness.

Computerized apparatus for producing cross-section images of the body by the method of X-ray axial tomography are known, for example from U.S. Pat. Nos. 3,778,614 and 3,866,047 to Hounsfield. In one form of such apparatus a divergent beam of X-ray photons is directed from an X-ray source in a plane which passes through a body undergoing examination and thereafter impinges on an array of X-ray detectors which lie in the plane of examination. The X-ray source and detector array rotate (and in some embodiments also translate) about the body to produce a series of one-dimensional X-ray shadowgraphs which are combined in a digital computer, using well-known computational algorithms, to yield cross-section images of the examination plane.

Common X-ray sources, that is X-ray tube anodes or radioisotope sources, generally produce X-ray beams which diverge through substantial solid angles. In computerized axial tomography (CAT) equipment mechanical collimation is generally utilized in conjunction with the X-ray source to limit the divergence of the X-ray beam to a wedge or fan-shaped swath which is confined to the examination plane and to the included angle of the detector array. Unnecessary radiation dose to the patient and system noise from scattered X-ray photons is thereby reduced. The mechanical constraints of CAT equipment generally require that such X-ray source collimators have minimum weight, to permit rapid motion, and minimum length, to reduce the overall size of the rotating components and maximize the X-ray flux at the detector array.

Means are generally provided for adjusting the thickness of the X-ray beam, and thus the examination plane, in CAT apparatus. In prior art collimators, which generally comprised one or more long channels through bodies of X-ray absorbing material, two sets of moveable jaws were generally utilized to control the thickness of the X-ray swath. Such multiple jaw sets were required to limit the X-ray penumbra which would otherwise be produced if a single set of jaws were utilized with an X-ray source of finite dimensions.

An X-ray collimator for producing a planar, wedge-shaped swath of X-ray photons comprises a plurality of substantially parallel X-ray absorbing laminae disposed in the plane of the X-ray beam and spaced one from the other. The extent and relative displacement of the laminae along the X-ray beam as well as the number of such laminae are determined to individually maximize the ratio of X-ray flux within a defined beam thickness to X-ray flux outside the defined beam. X-ray photons at all points within the beam are generally confined to paths lying substantially parallel to the beam plane so that the beam thickness may be adjusted with a single set of moveable jaws with minimum X-ray flux in the penumbra. Collimators of the present invention are generally shorter and lighter than prior art, conventional collimators and are thus ideally suited for use in CAT scanning apparatus.

It is, therefore, an object of this invention to produce short, lightweight X-ray collimators for use in computerized axial tomography apparatus.

Another object of this invention is to allow adjustment of the thickness of a planar X-ray swath by means of a single set of moveable jaws.

The novel features believed to be characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may be best understood by reference to the following detailed descriptions, taken in connection with the appended drawings in which:

FIG. 1 schematically represents computerized axial tomography apparatus which includes collimating components of the present invention;

FIG. 2 is a sectional view of a laminar collimator of the present invention;

FIG. 3 illustrates the design of a collimator; and

FIG. 4 is an enlarged sectional view of a portion of the collimator of FIG. 2 illustrating the distribution of laminae therein.

FIG. 1 is an X-ray measurement system of the present invention for use with computerized axial tomography apparatus. Electrons in an X-ray tube 10 impinge upon an anode 11 to produce a divergent X-ray beam 13 which emerges through a window 12. A collimator 14, more particularly described below, is disposed in the X-ray beam 13 to produce a planar, wedge-shaped exit beam 15. The thickness of the wedge-shaped beam 15 is determined by the overall internal thickness of the collimator 14. The collimator 14 further functions to confine photon propagation in the beam 15 to directions substantially parallel to the plane of the X-ray beam. The exit beam 15 then passes through a slot 16 between a set of parallel X-ray absorbing jaws 17 and 18 (which may comprise tungsten plates) which are adjustable via a gear drive 19 to produce a wedge-shaped X-ray beam 20 having an effective thickness determined by the width of the slot 16. The X-ray beam 20 is directed through a body 21 where it is selectively absorbed by tissues of varying density to produce a one-dimensional shadowgraph image of X-ray intensities. X-ray photons emerging from the body 21 pass through a slot 22 between a second set of parallel, X-ray absorbing jaws 23 and 24 which are adjusted by a gear mechanism to the same spacing as were the jaws 17 and 18. The jaws 23 and 24 function to absorb X-ray photons which are scattered by the body 21 or by other objects in the beam path and thus reduce noise in the X-ray measurement data.

The X-ray beam emerging from the slot 22 then passes through an X-ray absorbing aperture plate 25, where it is separated into beams 26 of equal cross section, width and angular spacing; which serves to define the spatial resolution of the measurement system. The individual beams 26 exiting the aperture plate 25 then pass through a detector collimator 27, which functions to reject scattered radiation, and impinge on an X-ray detector array 28 which may comprise scintillation crystals or any other type of radiation detectors which are commonly used in X-ray tomography apparatus.

Although any type of X-ray tube or other radiation source may be used, the tube 10 is most advantageously a lightweight X-ray tube having a fixed anode 11 which is disposed at an angle to both the electron beam and the perpendicular window 12 of the tube. The anode focal spot produced by an electron beam of generally circular cross section is thus projected as an ellipse in the plane of the window 13 and is ideally suited for producing a fan or wedge-shaped X-ray swath.

A beam path is defined by lines connecting points on the focal spot with points on the detector array. A central line connects the center of the focal spot (X-ray source) with the center of the detector array. Herein, and in the claims which follow, the length, extent, and position of the beam path and of associated structures are measured along lines which are parallel to the central line.

The source collimator 14 (FIGS. 1 and 2) comprises an open ended hollow housing shell 32 having side walls 33 which act as a baffle and are spaced to define the maximum thickness of the emergent X-ray beam 15 and divergent end walls 34 which serve to limit the divergence of the beam 15 to the width detector array 28. The housing shell 32 contains a plurality of substantially parallel, X-ray absorbing laminae 30 which are supported within the housing and spaced one from the other in a plurality of slots on the interior surface of the end walls 34. The length of each lamina 30 as well as its position within the housing and its spacing from adjacent laminae are fixed to maximize the ratio of X-ray flux emerging within the defined exit beam 15 to flux falling within a penumbra of that beam. This design is most effectively achieved with a digital computer following an iterative procedure which functions to maximize the ratio for each lamina.

FIGS. 1 and 3 illustrate the design principles for the collimator 14. The width of the channel between the walls 34 of the collimator housing is found by connecting the outermost ends at each side of the focal spot 41 with the corresponding outermost ends 44 of the detector array 28; the position of the collimator 14 being fixed by such constraints as the dimensions of the X-ray tube window and available space in the scanning mechanism. The spacing between the laminae 30 is then assumed and the central line A--A of the focal spot is divided into a great number of equidistant points. Each point is assumed to radiate X-rays in all directions within the plane containing the central line of the focal spot and the detector. For each point the radiative flux RD reaching the detector is calculated. Then the flux RP in the penumbra (i.e. the flux passing through the collimator but not reaching the detector) is also calculated. ##EQU1## is maximized by selecting values for the length and position of each lamina.

As an example, collimators having equally spaced, symetrically disposed laminae were optimally designed for a system wherein the beam originated at a 15 mm thick focal spot and terminated at a 20 mm thick detector array approximately 1.2 meters from the focal spot. The collimator lay between 59.5 and 139.7 millimeters from the focal spot. Table I summarizes the calculated performance of collimators containing various numbers of laminae as compared to a conventional collimator with the same exterior dimensions.

In all cases the laminae 30 comprised 0.1 mm depleted uranium foil. Tungsten laminae are also suitable and allow less expensive fabrication. The collimator housing may, for example, comprise brass plate. A Fortran language computer program which is useful for calculating laminae configurations is set forth in Appendix A to enable others to more easily practice the invention.

FIG. 4 is an enlarged section of a collimator of FIG. 2 showing the spacing and extent of the laminae 30A-30K. By way of example Table II sets forth the spacing X, the distance between the lower end of the lamina and the housing 32 exit end, Y.sub.1, and the distance between the top of the lamina and the housing exit end Y.sub.2, for an optimal collimator comprising 21 equally spaced laminae for the source and detector spacing above which produces a convergent beam from a 15 mm focal spot to an 8 mm detector.

The present invention provides a low weight, short structure producing a wedge-shaped planar swath of X-rays used in computerized axial tomography apparatus. X-ray photon flux in the beam emerging from the collimator is substantially parallel to the X-ray beam plane so that beam thickness may be effectively controlled with a single set of moveable jaws and minimal penumbra effects.

The invention has been described in detail herein in accord with certain embodiments thereof, yet many modifications and changes therein may be effected by those skilled in the art.

Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention. ##SPC1##