US2955205A - Device in which a light beam marks the x-ray beam - Google Patents

Description

Oct. 4, 1960 P. CAMFFERMAN 2,955,205

DEVICE IN WHICH A LIGHT BEAM MARKS THE X-RAY BEAM I Filed July 21, 1958 ////////////////////,7/////AV/J///A7////////fi7/// mrllualllvalvnfll nuuuarll. vnuum (Mann MIIAIII-A 1".

INVENTOR. PAUL US CAMF F ERMAN AGE attachment casting shadows.

United States Patent W DEVICE m WHICH A LIGHT BEAM MARKS THE X-RAY BEAM Paulns Camfierman, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed July 21, 1958, Ser. No. 749,677

Claims priority, application Netherlands July 22, 1957 4 Claims. (Cl. 250-64) Accurate adjustment of the X-ray tube in therapeutic irradiation and in making radiographs can be performed in a simple manner by the use of a device in which a light beam marks the X-ray beam. For this purpose, a source of light is disposed so that the virtual origin of the light rays coincides with the focus of the X-ray tube. The light beam is limited by the screening envelope for the X-rays so that the light rays illuminate the same area which is irradiated by the X-rays. Thus, if the size of the irradiation field is to be changed, the angle of aperture of the X-rays can be adjusted by using the change in size of the illuminated area, without the use of a fluorescent screen.

In order to avoid undesirable shadows in the X-ray image, the source of light cannot be arranged in the X-ray beam. It is disposed at the side of this beam. Provision is further made of an inclined plane mirror to deflect the light in the direction of the beam axis of the X-rays. The mirror is disposed in front of the X-ray window and consists of a thin reflecting metal layer applied to a support made of a material which absorbs the X-rays slightly only, for example, a transparent synthetic resin such as the material which is commercially available under the trademark Plexiglas. The X-ray absorption cannot be entirely avoided so that in this arrangement the amount of radiation supplied by the X-ray tube is slightly diiferent from the amount which is really supplied to a patient or is used for exposing a photographic plate or film.

The amount of radiation produced by the X-ray tube is measured by means of an ionization chamber. This chamber must be arranged in the X-ray beam and, even if a small ionization chamber is used, not only causes a certain absorption but also casts some shadows, mainly of the means of attachment by which the ionization chamber is secured in the screening envelope for the X-rays.

It is known to measure the amount of radiation by means of an ionization chamber which is arranged in front of the window of the X-ray tube and is secured to the tube housing. This arrangement requires the use of an ionization chamber which is suitable for attachment to the tube housing while the housing must be provided with means of attachment.

It is an object of the present invention to avoid these special provisions and the disadvantage of the means of The invention relates to an arrangement in which a light beam marks the beam emitted by an X-ray tube by means of a light source and at least one mirror, the light of the light source, which is arranged at the side of the X-ray window of the tube, being deflected in the direction of the beam axis of the X-rays while provision is made of an ionization chamber for measuring the X-rays. According to the invention, the light rays from the light source are deflected by a reflecting surface the support of which sup ports the ionization chamber also.

The absence of separate securing and supporting means 2,955,205 Patented Oct. 4,

for the ionization chamber renders the construction of such an arrangement simple and cheap. Furthermore, the radiation absorption can be diminished by combining certain components. For this purpose, one of the electrodes of the ionization chamber can be the reflecting layer or part of this layer, the wall which is more remote from the focal spot of the X-ray tube and is coatedwith this layer being transparent.

In order that the invention may readily be carried out, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows an X-ray tube provided with an arrangement in accordance with the invention, and

Fig. '2 shows the support of the reflecting surface.

Fig. 1 is a longitudinal sectional view of part of an X-ray tube 1 provided with an anode 2 and a cathode 3. This cathode includes a filament 4 while the anode is provided with a target 5 for intercepting the electrons which are emitted by the filament 4 and accelerated by the action of an electric potential set up between the anode and the cathode. The electrons concentrate onto the target 5 to forma focal spot and this is the focus of the X-ray beam which passes through an aperture 7 in the wall of the envelope 6. Along the circumference of the aperture 7 the wall of the envelope 6 has a thickened part 8 in which a circular recess 9 is made. A flange 10 of a cone 11 fits in the recess. A threaded securing ring 12 serves to fix the cone 11.

An aperture 13 of the cone 11 irnits the cross-sectional area of the X-ray beam. In the space enclosed by the cone wall provision is made of an ionization chamber 14. The chamber is secured to supports 15 and 16. The way in which the ionization chamber 14 is secured to these supports is not of importance. A choice can be made from a variety of arrangements, for example attachment by screws, springs or clamps. The ionization chamber 14 is a flat member comprising two plane Walls 17 and 18 spaced from each other by a narrow space. This space is bounded along the circumference of the plates 17 and 18 by a rim 19. The chamber can be filled with air at atmospheric pressure. The chamber walls are made from a material which has a slight radiation absorption, preferably a synthetic resin of the kind made from phenolic resins.

At the side more remote from the source of radiation the wall 17 of the chamber 14 is provided with a smooth reflecting metal layer 22, for example a deposit of a metal having a high reflective power, for example silver or aluminium. Aluminium has a smaller X-ray absorption than silver and is also more attractive because the reflecting layer is less readily damaged. Preferably the reflecting layer 22 is applied to the inner side of the wall 17, and thus lies within the ionization chamber, the surface engaging the wall 17 being reflecting.

The ionization chamber 14 is inclined with respect to the axis of the X-ray beam so that a light ray from a light source 20 which is directed to the point at which the axis of the X-ray beam intersects the reflecting layer, is reflected in the direction of the beam axis 21. If the mirror is provided internally on the wall 17, which consequently must be made from a transparent material, for example Plexiglas, some light losses occur since the light rays twice penetrate the wall. This is less objectionable than the additional X-ray losses produced by the use of a mirror provided externally since in this event the X-rays must pass through the two electrodes of the ionization chamber and subsequently through the reflecting metal layer. The opposite wall 18 of the ionization chamber 14 is coated with a conductive layer which is also used as an electrode. Since this layer does not serve as a mirror for the light rays, it can be made A 72,955,205 a Y a .of a substance which has a slighter absorption for X-rays than metal, for example carbon. 7

In Fig. 2 the wall 17 of the ionization chamber is shown which presents its inner surface to the direction of view. "Phat surface is coated with a-metalilayter-ZZ which $11 the side engagingythe wall .is reflecting. An annular portion 23 lot the .metal layer is removed. In order .toobtain correct measuring results independent of the area which is radiated by the ,X-rays a relatively small portionof the reflecting layer functions ,as .an electrade -;of the ionizationchamber. It should preferably be smaller than the smallest area to which the X-ray beam will :16 eflectively reduced. Therefore a centre portion 26, which is not distinguished from the reflecting layer, is one ,;electrode of the ionization chamber. A conductive strip 26, which is also separated from the remaining metal coating 22 by an intermediate space 25, is conductively connected to the electrode 24 and serves for the connection ofqone terminalof a voltage source which supplies the potential difference required for the ionization chamber.

Whatisclaimed is:

:17. X-ray apparatus comprising an X-ray tube including a target from which Xerays emanate and a Window for transmission of the X-rays, van ionization chamber disposed between the windowofthe X-ray tubeand an object ,to be irradiated by the X-rays for measuring the intensity of X-rays to which said object is exposed, a light source for illuminating thetpath of X-rays between the ionization chamber and the object, and a light reflector at leasta portion of which is an electrode of said ionization chamber constituting one wall of said ionization chamber for reflecting light from said light source into said X-ray path, said light reflector comprising a supporting member transparent to X-rays and a thin light reflecting metal layer transparent to X-rays but opaque to light.

2. X-ray apparatus as claimed in claim 1 in which the light reflector is the wall of the ionization chamber remote from the target of the X-ray tube.

3. "X-ray apparatus comprising an X-ray tube including a target from which X-rays emanate and a window for transmission ,of the ,X-rays, an ionization chamber disposed between the windowof the X-ray tube and an object to be irradiated by the X-rays for measuring the intensity of X-rays to which said object is exposed, a light source for illuminating the path of X-rays between the ionization :chamber and the object, said ionization chamber having .one wall remote from the target and in the path of X-rays constituted of a light-transparent material, and a light-reflective metal layer on the inner surface of said wall constituting an electrode for said ionization chamber and a reflecting surface for said light source whereby the path of X-rays to said object is illuminated by light reflected by said metal layer.

'4. X-ray apparatus as claimed in claim 3 in which the said Wall of the ionization chamber is constituted of a transparent synthetic resin.

References Cited in the file of this patent UNITED STATES PATENTS 1,976,179 Manul Oct. 9, 1934 2,190,200 Victoreen Feb. 13, 1940 2,225,044 George Dec. 17, 1940 2,699,505 ;Zieler Ian. 11, 1955

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