GB2175778A

GB2175778A - Radiographic apparatus

Info

Publication number: GB2175778A
Application number: GB08611398A
Authority: GB
Inventors: James Leslie Lees; Anthony Reynolds
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1985-05-18
Filing date: 1986-05-08
Publication date: 1986-12-03
Also published as: GB2175778B; GB8512641D0; GB8611398D0

Abstract

In some systems, where an X- ray image of a subject is produced on photographic film, the X-ray emitter (4) has to be positioned precisely relative to the photographic film (2) and the subject (3) under inspection, the distance between the X-ray emitter and the film needing to be accurately set. This distance is known as the film-to-focus distance (FFD) and is related to the intensity of the X-ray beam. Deviations from an accurate FFD may result in over- or under-exposure of the film. An accurate FFD can be obtained by mounting a laser alignment system within the examination head (1) together with the emitter (4). A beam (12) from the laser (11) is split into two beams (14, 16) which are directed onto the surface (15) of the subject (3) under inspection. For a given FFD, the angle one beam (16) makes with the other (14) is directly related to the FFD and hence coincidence of the two beams (14, 16) on the surface (15) results in the FFD being accurately aligned to provide the correct exposure of the film (2). <IMAGE>

Description

SPECIFICATION Radiographic apparatus This invention relates to radiographic apparatus and is particularly although not exclusively concerned with X-ray systems in which the Xray emitter needs to be accurately positioned relative to the subject under examination or inspection.

In some systems, where an X-ray image of a subject is produced on photographic film, the X-ray emitter has to be positioned precisely relative to the photographic film and the subject under inspection. In the majority of cases, the X-ray beam is directed towards an area of interest with the centre-line of the beam passing through the centre of the film placed behind the subject, the distance between the X-ray emitter and the film needing to be accurately set. This distance is known as the film-to-focus distance (FFD) and is related to the intensity of the X-ray beam, the intensity of an X-ray beam being inversely proportional to the square of the distance the beam travels. Deviations from an accurate FFD may result in over- or under-exposure of the film.

A known set-up procedure used to obtain alignment of the centre-line of the X-ray beam with the film includes using a telescopic foldaway pointer to mark the centre-line, the FFD is adjusted, and then the distance is checked using a tape measure. This procedure, however, has several drawbacks particularly when a large subject is to be examined in several positions as the telescopic pointer provides only a rough guide to the centre-line alignment. In some cases, the set-up procedure may consist of a tedious adjustment-correction-recheck routine. In other cases, the Xray emitter may not be easily accessible.

It is therefore an object of the invention to provide an alignment system for adjusting the FFD of X-ray inspection apparatus which is both accurate and simple to use.

According to one aspect of the invention, there is provided a radiographic system which emits radiation along a given axis to irradiate a a subject, and which includes a ranging system comprising laser beam generating means for generating a beam and beamsplitting means for providing two beams from said beam which converge to provide a single spot of light on the subject when the radiation emitter is at a desired distance from the subject, said beamsplitter means being transparent to said emitter radiation and being permanently mounted such that said emitted radiation passes through the beamsplitter means and one of said beams is projected along said given axis.

According to a second aspect of the invention, there is provided radiographic apparatus for inspecting a component comprising: support means which is relatively movable with respect to the component; radiation producing means mounted within the support means and operable for producing a high intensity radiation beam which beam is directed towards the component along a given axis; detector means positioned relative to the component for receiving radiation from the component and operable for providing an indication of said received radiation; and position alignment means mounted within the support means and operable for determining an optimum distance between the radiation producing means and the component, said optimum distance being related to the intensity of the radiation beam incident on the component necessary to provide said output response, said alignment means including laser beam generating means for producing a laser beam, beamsplitter means for splitting said laser beam into a first and a second beam, and deflector means, said first beam being directed onto said surface along said axis to form a first light spot and said second beam being deflected by said deflector means at an angle to said axis onto said surface to form a second light spot, said angle being related to said optimum distance;; whereby the support means is moved relative to the component until said first and second light spots coincide on said surface, indicating that said optimum distance has been attained.

Preferably, said radiation producing means includes an X-ray emitter mounted on said axis and collimating means for collimating Xrays produced by said tube. In this case, the detector means is sensitive to X-rays and may be in the form of photographic film or a video system.

Advantageously, said laser beam generating means is mounted perpendicular to said axis, said laser beam intersecting said axis at a point at an angle of 900 to said axis. The beamsplitter means is positioned at said point and said first beam is formed by reflection at said beamsplitter means.

The deflector means may be an angularly adjustable mirror, the angle said mirror makes with said axis being related to said optimum distance and by varying said angle said distance may be varied.

Naturally, said beamsplitter means is made from a material which is substantially transparent to X-rays.

According to a third aspect of the invention, there is provided an X-ray examination head comprising: an X-ray emitter for producing X-rays, the emitter being mounted on an axis passing from the head to a component which is to be examined, the emitter having a collimator attached to it to direct X-rays onto the component; and an alignment system for aligning the head with the component, the system including a laser mounted to one side of the axis and operable for producing a laser beam, a beamsplitter mounted on the axis at an angle inclined to it and operable for forming a primary and a secondary laser beam from said beam, the primary beam being directed onto the component along the axis to form a first light spot, and reflector means mounted on the other side of the axis diametrically opposite the laser and operable for receiving the secondary beam from the beamsplitter and for directing it onto the component at an angle inclined to the axis to form a second light spot, whereby movement of the head to bring the first and second light spot into coincidence positions the head in an optimum position relative to the component for examination to be undertaken, the angle of the secondary beam being related to the optimum position relative to the component.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, the single figure of which is a schematic part-sectioned view of an X-ray inspection arrangement which incorporates a laser alignment system.

The X-ray inspection arrangement shown in the figure comprises a movable X-ray tube mounting head 1, - a sheet of X-ray-sensitive photographic film 2 and a component 3 which is to be inspected. The mounting head 1 houses both an X-ray generating system and a laser alignment system, the alignment system being used to position the head 1 at its appropriate film-to-focus distance (FFD) ie the distance between the X-ray tube 4 and the centre 5 of the film 2, for a given component, which gives the correct exposure of the film 2. The two systems will now be described separately.

The X-ray system includes an X-ray tube 4 and a hollow collimating cone 6 which is attached to the tube 4. The cone 6 has two holes 7 and 8 formed in its wall to accommodate a portion of the alignment system, the cone having an apex angle of 450 The cone 6 limits the angle of divergence of the X-ray beam 9 to 45 , the beam being symmetrical about a centre-line or axis 10 which coincides with the FFD. Once the correct FFD has been accurately determined using the alignment system, the X-ray inspection of the component 3 can be carried out.

The alignment system includes a laser 11 which generates and directs a beam 12 onto a beamsplitter 13 after the beam has passed through hole 7 in the cone 6. The beamsplitter 13 is mounted on the axis 10 inside the cone 6, at an angle of 450 the axis 10. At the beamsplitter 13, the beam 12 is divided into a primary beam 14 which is reflected onto the upper surface 15 of the component 3 along the axis 10, and a secondary beam 16 which passes undeviated through hole 8 to an adjustable mirror 17. The mirror 17 reflects the beam 16 onto the component surface 15. The primary and secondary beams 14 and 16 form respective spots of light on the surface 15 when the mounting head 1 is not at its optimum distance from the film 2.In practice, the mirror 17 is adjusted to the angle required to provide the correct FFD for a given component-film separation ie when the two light spots are made to coincide before the inspection arrangement is aligned with the component. Sometimes it is more convenient to use the focal spot-to-object distance instead of the FFD ie the distance from the tube 4 to the surface 15. The mounting head 1 is then moved relative to the component 3 along the axis 10 until, the two spots of light produced on the surface 15 by the beams 14 and 16 coincide. The mounting head 1 is now precisely aligned with the centre 5 of the film 2, (or alternatively the surface 15 if the focal spot-to-object distance is used) and the X-ray tube 4 can be operated to produce X-rays which expose the film 2 thereby forming an X-ray image of the component 3.

The beamsplitter 13 is a very fine ellipse of clear mylar film which does not affect the passage of X-rays through it, only acting to divide the laser beam 7.

The mirror 17 may be adjusted by a worm and wheel system (not shown), but because of the backlash present in such a system, direct calibration of the mirror angle with filmto-film distance or focal spot-to-object distance ie removing the need to adjust the angle before setting up the apparatus, cannot be obtained. However, another method of adjustment of the mirror 17 would be to have the mirror hinged at one end and sprung-loaded in permanent compression, against a calibrated micrometer adjuster at the other end.

The laser used in the alignment system need not be powerful as its only function is to provide beams which enable the head 1 to be correctly positioned, for example a heliumneon laser may be used.

The arrangement could be improved by incorporating two gravity-operated tilt indicators into its design to enable pitch and roll attitudes to be adjusted directly without having to use separate clinometers.

Using such an inspection arrangement, setup times may be greatly reduced and the quality of the X-ray image greatly improved ie consistent film densities may be obtained without edge fade-out and undesirable shadows.

Claims

1. A radiographic system which emits radiation along a given axis to irradiate a subject, and which includes a ranging system comprising laser beam generating means for generat ing a beam and beamsplitting means for providing two beams from said beam which converge to provide a single spot of light on the subject when the radiation emitter is at a desired distance from the subject, said beamsplitter means being transparent to said emitted radiation and being permanently mounted such that said emitted radiation passes through the beamsplitting means and one of said beams is projected along said given axis.

2. Radiographic apparatus for inspecting a component comprising: support means which is relatively movable with respect to the component; radiation producing means mounted within the support means and operable for producing a high intensity radiation beam which beam is directed towards the component along a given axis; detector means positioned relative to the component for receiving radiation from the component and operable for providing an indication of said received radiation; and position alignment means mounted within the support means and operable for determining an optimum distance between the radiation producing means and the component, said optimum distance being related to the intensity of the radiation beam incident on the component necessary to provide said output response, said alignment means including laser beam generating means for producing a laser beam, beamsplitter means for splitting said laser beam into a first and a second beam, and deflector means, said first beam being directed onto said surface along said axis to form a first light spot and said second beam being deflected by said deflector means at an angle to said axis onto said surface to form a second light spot, said angle being related to said optimum distance;; whereby the support means is moved relative to the component until said first and second light spots coincide on said surface, indicating that said optimum distance has been attained.

3. Apparatus according to claim 2, wherein said radiation producing means includes an Xray emitter mounted on said axis and collimating means for collimating X-rays produced by said emitter.

4. Apparatus according to claim 3, wherein said collimating means is a hollow cone attached to said X-ray emitter, the cone being symmetrical about said given axis.

5. Apparatus according to claim 4, wherein said cone has an apex angle of 450 thereby limiting the angle of divergence of said radiation beam to 450

6. Apparatus according to claims 2 to 5, wherein said laser beam generating means is mounted perpendicular to said axis, said laser beam intersecting said axis at a point at an angle of 90 to said axis.

7. Apparatus according to claim 6, wherein said beamsplitter means is positioned at said point and said first beam is formed by reflection at said beamsplitter means.

8. Apparatus according to claim 7, wherein said beamsplitter means is made from a material which is substantially transparent to Xrays.

9. Apparatus according to claim 7 or 8, wherein said beamsplitter means is mounted within said cone at 450 to said axis, and said cone has holes formed in its wall to allow said laser beam to be incident on said beamsplitter means and said second beam to be incident on said deflector means.

10. Apparatus according to claim 9, wherein said beamsplitter means is an ellipse of mylar film.

11. Apparatus according to anyone of claims 2 to 10, wherein said deflector means is an angularly adjustable mirror, the angle said mirror makes with said axis being related to said optimum distance and by varying said angle said distance is varied.

12. An X-ray examination head comprising: an X-ray emitter for producing X-rays, the emitter being mounted on an axis passing from the head to a component which is to be examined, the emitter having a collimator attached to it to direct X-rays onto the component; and an alignment system for aligning the head with the component, the system including a laser mounted to one side of the axis and operable for producing a laser beam, a beamsplitter mounted on the axis at an angle inclined to it and operable for forming a primary and a secondary laser beam from said beam, the primary beam being directed onto the component along the axis to form a first light spot, and reflector means mounted on the other side of the axis diametrically opposite the laser and operable for receiving the secondary beam from the beamsplitter and for directing it onto the component at an angle inclined to the axis to form a second light spot, whereby movement of the head to bring the first and second light spot into coincidence positions the head in an optimum position relative to the component for examination to be undertaken, the angle of the secondary beam being related to the optimum position relative to the component.

13. Radiographic apparatus substantially as hereinbefore described with reference to the accompanying drawing.

14. An X-ray examination head substantially as hereinbefore described with reference to the accompanying drawing.