US2667588A - Beam-limiting diaphragm for x-ray tubes - Google Patents

Description

Jan. 26, 1954 K. OSWALD BEAM-LIMITING DIAPHRAGM FOR X-RAY TUBES Filed March 4, 1952 INVENTOR Karl swo d By Agent Patented Jan. 26, 1954 UNITED STATES PATENT OFFICE BEAM-LIIHIT-ING DIAPHRAGM FOR X-RAY TUBES Karl Oswald, 'Eindhoven, Hartford National Ba Hartford, Conn., as tr Frequently only part of the X-rays emerging from the window provided in the sheath of an X-ray tube is used efiectively. In these cases it is advantageous to restrict the sectional area of the beam with the use of a diaphragm having a variable aperture.

Use is preferably made of two or more diaphragms, which are difierently spaced from the focus of the tube. Constructions are known which comprise a driving mechanism, with the use of which the apertures of all diaphragms are varied simultaneously, When the angle of divergence of the emerging X-ray beam must be varied.

For controlling the aperature of so-called sliding diaphragms comprising two pairs of parallel plates, the plates of each pair of plates are displaced in opposite sense in a plane at right angles to the beam axis. For the simultaneous control of diaphragms arranged one after the other, the corresponding plates have been connected by rods which are rotatable about points in the plane through the focus of the tube, at right angles to the axis of the emerging beam and about two axes intersecting one another at right angles. of these axes coincides with the axis of the X-ray tube and. in that the points of rotation are located inside the tube. From a structural point of view this limitation can be obviated, it is true, but this involves complications and enlarges the size of the device. In a known embodiment use is made of circular guide members to provide a correct guidance for the rods.

In a further known construction, the larger aperture is limited by plates rotatable about axes intersecting with one another at right angles in a plane outside the tube in front of the exit window in the tube sheath. The smaller aperture is limited by a second set of plates rotatable about points in a plane farther away from the focus of the tube. They are moved simultana ously with the first-mentioned plates.

The plates limiting the smaller aperture must be preferably arranged at a small distance from the focus of the X-ray tube. The invention relates to the construction of a limiting diaphragm having at least two successive apertures, in which the said distance can be minimized.

According to the present invention, the plates limiting the larger aperture are coupled each with a plate of the set of plates limiting the smaller aperture in a manner such that at a A difiiculty arises herein that one Netherlands, assignor to nk and Trust Company,

ustee Application March 4, 1952,, Serial No. 274,765

Claims priority, application Netherlands March 10, 1951 2 Claims. (01. 250 -105) variation of the apertures the coupled plates are shifted in opposite sense relative to the beam axis. .For this purpose one plate of the pairs of plates bounding the smaller aperture may be linked by a coupling rod to a plate of the set lim iting the larger aperture, located on the other side of the beam axis and the coupling rod may be rotatable about points in a plane intersecting with the beam axis at right angles at a point located between the two said platesystems.

In order that the invention may be readily can ried into eifect, one embodiment thereof will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows diagrammatically the course or the X-rays and the arrangement of the limiting plates and Figs. 2 and 3 show in two views a simple construction according to the invention.

Fig. 1 shows the anode l of an X-ray tube, viewed in the direction of the tube axis" from the cathode. A part 2 of the anode surface is intended to be struck by electrons and is termed the focus of the X-ray tube. It constitutes the source of X-rays. This is not the only part of the anode surface which is struck by electrons. Secondary and reflected electrons strike also the further surface and produce the so-called scattered radiation. Substantially the entire front of the anode participates in this radiation and although these rays are less penetrating than those produced in the focus and although their internsity is smaller, the luminescent material of an image surface intended to convert the image produced by the X-rays into visible radiation is ac tivated in a manner such that this scattered radiation produces a visible luminescence.

The figure shows two sets of closing plates 3 and 4. The set of plates 3 is located at asmall distance from the anode and the distance of the second set 4 from the anode is considerably larger. The apertures between the plates are chosen such that they exactly permit the effective X-ray beam to pass.

If onl the plates 3 are provided, not only the efiective ray beam but also scattered rays emerge" from the aperture, forming a wide beam which is limited as indicated by the broken lines 5. This beam may cover the entire image surface and continue to cause troublesome additional light around the detail to be observed, even if the effective beam of rays is further limited.

This undesirable radiation has considerably less effect if only the plates 4 are used. The beam width of the unwanted radiation, indicated by the broken lines 6, differs in this case little from the width of the beam of effective X- rays between the full lines 7. Now only at the edge of the image produced on the image surface a certain amountof additional light is produced over a small width.

Furthermore the geometric unsharpness produced by the finite dimensions of the focus from which the X-rays emerge must be considered. The angle between two rays coinciding at point 8 and emerging from the edges ofrthe focus 2 is a measure for the unsharpness produced by the effective X-rays. The unsharpness produced by the scattered radiation is determined by the angle under which the rays from the periphery of the anode surface coincide at point 8. If the diaphragm is near the focus, the influence of the last-mentioned radiation is least troublesome. This influence increases proportionally to the increase in distance between the diaphragm and the focus. Since the detail sharpness of the image is strongly reduced at an increase in geometrical unsharpness, a diaphragm will preferably be arranged as near as possible to the :focus. These considerations lead to the arrange- .ment of two sets of closing plates at different distances from the focus.

Referring to Figs. 2 and 3, such a device is secured to the sheath 9-of the X-ray tube 10. The metal wall of the X-ray tube is provided with a window H to allow the X-rays to pass, which are produced in the focus 2 of the anode i. The window H maybe made of glass or of a metal having a low atom numeral, for example, beryllium. The sheath 9 has an aperture in which a piece 13 is fitted. The shape of this piece is chosen such that it intimately engages the tube wall. It furthermore closes the aperture in the sheath 9, so that the latter may be filled out with a liquid or gaseous insulating'substance to increase the break-down voltage and to withdraw heat developed in the anode.

' The closing plates 14 and i5 limit the X-ray beam. They are made of material, for example, lead, which absorbs X-rays. The plates M are connected by way of rods it with plates [5,- these rods being rotatably secured at IT to a ring l2. The latter is connected with the fitting piece 13, so that the device for limiting the X-ray beam maybe removed without detaching the fitting piece I3. If the distance between the closing plates i5 is decreased or increased, the distance between th closing plates 14 is also decreased or increased, so that by moving one set of plates the other set of plates varies the beam width to a 5- corresponding extent.

The device may be extended by one or more sets 'of plates, which are arranged between the said two sets of plates. They may also be displaced by means of the crossing rods I6. They may, for example, serve to prevent secondary X- rays emerging from the edges of the plates 14 from reaching the image surface.

The connection of the plates with the crossing rods 16 is constituted by two studs protruding at right angles to the direction of displacement, on either side of the plates [8 and extending in slots IQ of the rods [6. The studs 18 must be displaceable in the slots 19, since at a variation of the aperture size the distances from the rotary points of the rods vary.

When arranging the moving parts, it must be considered that the movability of the inner plate pairs is not hampered by the rods connecting the outer plate pairs. For this reason the plates of these two pairs are provided with long studs 18, so that the rods co-operating herewith may be arranged outside the range covered by the inner plates.

What I claim is:

1. A limiting diaphragm for the X-ray beam of an X-ray tube, comprising a first two pairs of displaceable radiation-opaque plates disposed adjacent said tube and defining a small aper ture, a second two pairs of displaceable radiation-opaque plates disposed on the side of said first two pairs remote from said tube and defining a larger aperture than said small aperture, and a plurality of coupling rods each interconnecting a plate of said first two pairs located on one side of said Y-ray beam to a plate of said second two pairs located on the other side of said X-ray beam, said coupling rods being pivotable about points lying in a plane at right angles to the axis of the X-ray beam and disposed between said first and second two pairs of plates, whereby the interconnected plates move in opposite senses relative to the beam axis when the size of the apertures is varied.

2. A limiting diaphragmfor the X-ray beam of an'X-ray tube, comprising a first two pairs of displaceable radiation-opaque plates disposed adjacent said tube and defining a small aperture, a second two pairs of displaceable radiation-opaque plates disposed on the side ofsaid first two pairs remote from said tube and defining a larger. aperture than said small aperture, a plurality of coupling rods each-interconnecting a plate of said first two pairs located on one side. of said X-ray beam to a plate of said second two pairs located on the other sideof said X-ray beam, and a ring-like member disposed between said first two pairs of plates and said second two pairs of plates and lying in a plane at right angles to the axis of the X-ray beam, each of said coupling rods being pivotable about points on said ring-like member, the coupling rods connected to the plates of each pair of plates being pivotable about the same point on the ring-like member, whereby the interconnected plates move in opposite senses relative to the beam axis when the size of the apertures is varied.

v KARL OSWALD.

References Cited in the file of this patent UNITED STATES PATENTS Haupt Feb. 20, 1951

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