GB2055526A - X-ray Image Signal Generator - Google Patents

Abstract

The invention relates to a flat plate X-ray detector which uses a plate detector exhibiting so-called permanent induced electric polarization in response to a pattern of radiation emergent from a patient to generate a polarization pattern which is scanned by means of a laser to cause discharge of the polarization through the plate and so generate electric signals representative of the X-ray image of the patient. In addition a second laser operating at a different wavelength e.g. infra-red, also scans or floods the plate detector to move "dark polarisation". The plate detector may be a phosphor screen or a phosphor screen in combination with a scintillator.

Description

SPECIFICATION Improvements in or Relating to Radiography This invention relates to radiography, and it relates more particularly to the form of radiography conventionally used for chest X-rays, in which a source of a pyramidal or conical shaped beam or radiation and a flat, plate-like Xray detector are disposed on opposite sides of a patient. The source is energised for a short time to pass X-rays through the patient and the detector responds to the radiation transmitted through the patient to develop a two-dimensional image of the emergent radiation pattern.

Conventionally, the X-ray detector is constituted by an X-ray sensitive film and the image is produced by processing the film in known manner. There are, however, many advantages to be gained from utilising a detector which responds to the two-dimensional pattern of radiation emergent from the patient to generate electrical signals indicative of the pattern. One advantage of utilising such a detector is that the electrical signals can be subjected to various processing techniques that can improve the usefulness of the image with regard to clinical diagnosis. For example, the signals may be filtered, spatially and/or frequency. Moreover, the signals could be subjected to windowing control, by means of which the extent and/or the mean level of the dynamic range of signals can be adjusted.

These advantages have been recognised for some time, but a difficulty has arisen in constructing a suitable two-dimensional detector which is capable of providing discrete output signals for many locations, distributed over the two dimensions as dictated by the resolution required in the image.

One approach, disclosed in United States Patent No. 3,101,407 has been to utilise a source of a flat fan of radiation and a one-dimensional array of detectors, and to scan both the source and the detector array along the patient so as to gradually build up a two-dimensional image of part of the patient's body. This approach, however, is subject to a number of disadvantages.

Firstly, it is not compatible with existing filmbased systems, as would be desirable to enable existing systems to be up-dated to take advantage of electronic detectors and associated processing. Secondly, it calls for a scanning movement to be imparted synchronously and concomitantly to the source and the detector array. It is undesirable to have to provide such scanning movement. Thirdly, the flat fan-shaped distribution of X-rays is produced by collimating the conical or pyramidal shaped beam that is produced by the X-ray source (e.g. a rotating anode X-ray tube) and this is wasteful in terms of source efficiency and operation.

It is the object of this invention to provide a two-dimensional X-ray detector arrangement that generates electrical signals in response to the pattern of radiation emergent from a patient and in which the //eifficulty referred to above is reduced or eliminated without introducing the disadvantages associated with the approach of the aforementioned United States Patent.

In order that the invention may be clearly understood and readily carried into effect, some embodiments thereof will now be described, by way of example only, with reference to the accompanying drawing, the single figure of which shows, in perspective view, the basic elements of a radiographic system employing one embodiment of the present invention.

This invention resides in utilising an area detector exhibiting so-called permanent induced polarization to respond to the pattern of radiation emergent from a patient to generate a polarization pattern which can be scanned by means of a suitable device to generate electrical signals indicative of the stored pattern.

Referring now to the drawing, a source 1 of a conical or pyramidal beam 2 of penetrating Xradiation is supported and disposed to irradiate a part of a patient's body 3. An area detector of the kind referred to in the immediately preceding paragraph is shown at 4 and, in this example, the detector 4 comprises a phosphor screen.

It has been known for some time that it is possible to selectively polarize a phosphor in the presence of radiation, so producing a stored polarization pattern. This was disclosed, for example, by Kallmann et al in Phys, Rev. 97, 6 March 1 955. In Photographic Sci. and Eng. 6, 2 March 1962, Kallmann further disclosed that the pattern so produced could be read by scanning a radiating source over the surface and measuring the discharge current flow.

In this embodiment of the invention, the phosphor screen, having been polarized in response to the pattern of X-radiation emergent from the body, is scanned with an intense light source such as a Green-Blue Argon laser 5.

Also shown is an infra-red laser 6. This laser can either be arranged to flood the entire surface of thescreen 4 with infra-red energy, or it may be scanned over the surface of the screen 4. In either event, the said surface is exposed to the infra-red radiation from laser 6 prior to being scanned by the laser 5 in order to remove the so-called "dark polarization" which exists even in areas of the phosphor screen that received little X-radiation. If the laser 6 is to be scanned over the surface of detector 4 it can, of course, be advantageous to scan it simultaneously with the scanning of laser 5, with the beam from laser 6 leading, in the direction of scanning, the beam from laser 5.In some circumstances, however, depending upon the relative power outputs and the relative beam shapes and sizes available from the two lasers, it might be preferable to impose different scanning patterns on the two lasers. For example, one laser might execute a spiral scan and the other a raster scan. A Nd:YAG laser may be especially suitable as the laser source 6 infra-red radiation.

The scanning of laser 5 over the surface of the phosphor screen 4 generates electrical signals indicative of the polarization produced at various locations of the screen by the aforementioned pattern of X-radiation. These signals are applied to processing circuits 7, such as windowing and/or filtering circuits, and thence to a display 8 which can take any suitable form capable of producing a visual display of the X-ray pattern. A store 9 is shown connected to receive the electrical signals in parallel with the processing circuits 7 so that the signals can be stored in their virgin state, unaffected by the processing, for future use. Store 9 can take any convenient form such as a magnetic tape, disc or core store.

The scanning of the laser 5, and (if necessary) of the laser 6, is effected under the control of a timing citcuit 1 0. The timing circuit 10 preferably controls the operations of the store 9 and the display 8, thus synchronising the operations of those components with the scanning and, moreover, permitting any standards conversion necessary to be properly implemented and supervised. If desired, the timing circuit 10 can also control the operations of the processing circuits 7, enabling signals relating to different areas of the screen 4 to be subjected to different processing if required.

The phosphors normally used for the purposes described above have a disadvantage in that they are relatively transparent to X-rays and are thus not very efficient collectors of X-radiation. They are, in addition, optically opaque so that, if thick sections were built up to increase the X-ray collection efficiency, the efficiency of the laser scanning is seriously impaired. Thus it can be advantageous to dispose a Csl or other scintillator in the X-ray path, closely adjacent the surface of the screen 4 facing the X-ray source 1. The scintillator will intercept virtually all the X-rays. If the scintillator is suitably doped with sodium or tantalum, it will radiate light photons which will be absorbed by the Phosphor screen, producing an-increase in polarization. If such a scintillator is employed, it would be disposed so that it is electrically insulated from, but in close optical contact with, the phosphor screen 4.

Claims (10)

Claims

1. An X-ray detector arrangement capable of responding to a two-dimensional pattern of Xradiation emergent from a body under examination to generate electrical signals indicative of said pattern, the arrangement including a two-dimensional detector device for receiving said pattern of X-radiation and for generating, in response thereto, a corresponding pattern of induced polarization means including a scannable laser device for scanning a laser beam over the detector device to discharge the polarization pattern, thereby to generate said electrical signals, and means including a further laser device effective to illuminate said detector device, prior to its illumination by means of said scannable laser device, the scannable laser device and the further laser device producing radiation of different wavelengths and the wavelength of the radiation produced by said further laser device being suitable for the removal of dark polarization from said detector device.

2. An arrangement according to Claim 1 wherein said further laser device is not scannable and produced a flood beam of sufficient dimensions to irradiate the whole of said detector device.

3. An arrangement according to Claim 1 wherein said further laser device is scannable.

4. An arrangement according to Claim 3 wherein both of said laser devices are constrained to follow the same scanning pattern with respect to said detector device.

5. An arrangement according to any preceding claim wherein the first-mentioned laser device comprises a green-blue Argon laser.

6. An arrangement according to any preceding claim wherein the further laser device comprises an infra-red laser.

7. An arrangement according to any preceding claim wherein said detector device comprises a phosphor screen.

8. An arrangement according to Claim 7 wherein a scintillator device is disposed in the path of X-radiation to said screen.

9. An arrangement according to Claim 8 wherein said scintillator device comprises a caesium iodide scintillator disposed closely adjacent the surface of the screen upon which the x-radiation would otherwise be incident.

10. An X-ray detector arrangement substantially as herein described with reference to the drawing.