GB2061055A - Imaging system - Google Patents

Abstract

The invention provides a two dimensional imaging system in which a pattern of radiation falling on the system is detected to give electrical signals for each of a plurality of strips across the pattern. The detection is repeated for different orientations of the strips and the whole processed by compensated back projection. For a shadow x-ray system a plurality of strip x-ray detectors (1-10) are rotated on a turntable 11. For lower frequencies the pattern may be rotated with a Dove prism and the strips condensed to suit smaller detectors with a cylindrical lens. <IMAGE>

Description

SPECIFICATION Improvements in or relating to imaging systems This invention relates to systems for imaging a scene by radiation received therefrom. In one embodiment it relates to radiography particularly of the form conventionally used for chest X-rays, in which a source of a pyramidal or conical shaped beam of radiation and a flat, plate-like X-ray detector are disposed on opposite sides of a patient. In such systems 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 for such systems 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 in 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 the 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 gradually to 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.

Similar considerations apply to imaging by radiation of other wavelengths or by particles.

It is an object of this invention to provide an alternative two-dimensional imaging system that generates electrical signals in response to the pattern of radiation including particle beams. The radiation may be x-rays emergent from a patient and in that case it is a further object that the difficulty referred to above is reduced or eliminated without introducing the disadvantages associated with the approach of the aforementioned United States patent.

According to the invention there is provided an imaging system for providing an image of a planar pattern of radiation, the system including a plurality of detectors, sensitive to the radiation, each producing an output signal related to the total intensity of radiation in a respective one of plurality of strips in said pattern, means for producing a relative rotational motion between said detectors and said pattern about an axis perpendicular to the pattern to provide output signals from the detectors for a plurality of orientations of the strips relative to the pattern and means for processing the said output signals for each of the orientations to provide said image.

In order that the invention may be clearly understood and readily carried into effect, embodiments thereof will now be described with reference to the accompanying drawings, of which Figure 1 shows a detector arrangement in accordance with one example of the invention, Figure 2 illustrates a Dove prism used for rotating the radiation pattern at lower frequencies, Figure 3a and 3b show in side and end elevation respectively a system incorporating the Dove prism, and Figures 4a and 4b show side elevation and view on XX respectively of an arrangement allowing the detector size to be reduced.

Referring now to Figure 1, a detector arrangement for an imaging system in accordance with one example of the invention for x-ray examination is formed of a plurality of separate parallel strips 1-10 of X-ray responsive scintillator material such as caesium iodide or sodium iodide, supported on a turntable member 11 formed of electrically insulating material which is preferably also substantially opaque to X-radiation. For simplicity the number of strips shown is much smaller than would be the case in a practical system.Each strip feeds a respective photo-electrical converter device, 1 a-1 0a such as a photodiode or a photomultiplier tube, and the electrical signals provided by each converter are applied to respective integrating circuits 1 b-lOb. The patient is irradiated by a divergent beam of X-rays from a conventional source, not shown, and the detector arrangement is placed to receive the radiation after passage therethrough.The turntable member 11, together with the detector strips and the converters, is rotated through 1800 by a drive motor D during the exposure of the patient to the Xradiation (typically for one second) and during that rotation the integrators are read and reset many times to produce discrete signals, each signal being indicative of the overall amount of radiation incident on one of the various strips during a respective integration period.

Each output signal is thus a line integral of X-ray intensity for a line (i.e. one of the strips) across the pattern of radiation emergent from the body and incident on the detector plane. Signals are provided for line integrals for many parallel strips as defined by the scintillators 1-10 and many different directions in the detector plane as a result of the rotation.

In their configuration (being many sets of projections of the pattern in many different directions in its plane) these signals can be seen to be similar to the line integrals of X-ray absorption provided by computerised tomographic (CT) scanners and described in British Patent No. 1283915. In a similar manner they can therefore be manipulated, by means of a circuit 12, to reconstruct the original pattern. This reconstruction is achieved by a technique known as compensated back projection. A suitable procedure for this is the CT processing technique described in British Patent Specification No. 1,471,531 and it is proposed that circuit 12 be arranged as described therein.This processing provides incident X-ray intensities for a matrix of elements in the detector plane and the evaluated X-ray intensities for the various elements are stored in appropriate locations of a digital store 13. The stored values can be read out in sequence under the influence of addressing circuits 14. They are applied to control circuits such as window width and window level control circuits 15 and thence to a visual display arrangement 16 such as a cathode ray tube device which displays them as a matrix of intensities forming the X-ray picture.

Clearly the detector strips need not be of scintillator material. It is possible, and in some respects preferable, to use strip like hollow chambers containing a noble gas such as xenon. In that case, of course, no photo-electrical conversion devices are called for as the signals indicative of line integrals af X-ray intensity across the pattern can be derived directly from electrodes included in the chambers.

It is not necessary for the detector strips to cover a circular area. They may, for example, cover a square or rectangular area. If they do cover a circular area, however, it can be advantageous to allow for the different lengths of the various strips, for example by relative amplification of the signals derived from the shorter strips, prior to the step of reconstructing the X-ray intensity pattern.

It can be desirable to utilise a commutator device to connect each detector strip output to several different output circuit channels during the aforementioned 180 rotation so that drift of amplifiers and/or other circuits included in such channels can be averaged over the whole image. The processing circuits, of course, have to be adjusted appropriately to take account of such commutation during the reconstruction process.

It will be appreciated that the rotation need not be carried out through 180 . Lesser or greater angles of rotation could be used, depending upon the resolution required and the time available for the scanning.

Although the example of Figure 1 is for an X-ray picture the invention may be used to record pictures of other patterns imposed on radiation, or particle beams such as neutrons which may be considered as radiation, incident on a detector plane. The pattern may be imposed on the radiation by transmission, as for X-rays and neutrons or by reflection and perhaps emission, as for electromagnetic radiation wavelength in the region of visible light, which in this specification is intended to include visible light, infra red radiation and ultra violet radiation. In each case the radiation is detected for strips, generally parallel, of the pattern by appropriate detector to produce line integrals. Relative rotation is achieved between the pattern and the detectors so that the line integrals are obtained in different directions and the CT type of processing is applicable.

Systems using lower frequencies, such as the visible and infra-red, allow manipulation of the light itself so that the pattern can be rotated relative to stationary strip detectors, which is not readily possible for X-rays, at least at this time. It also allows the use of lenses to reduce detector size so that, for example, light for a strip of the pattern can be focussed onto a small area detector.

In considering use with different frequencies it should be noted that at X-ray frequencies quantum effects can be the predominating noise mechanism.

Line integrals and subsequent CT type of reconstruction thereof appear to give about the same signal to noise ratio in the final image or the same number of detectors detecting one picture point each and moved across the pattern in parallel to cover it in the same time. This is due to the subtractive nature of the reconstruction process in which the signal is reduced but the noise is additive in an RMS sense. At lower frequencies where first amplifier noise predominates, integration over a strip will give a better signal to noise ratio since the noise will remain constant as the signal increases and not increase by N2 with N times the signal as in the quantum limited case.

In a preferred embodiment for visible light, infrared, or electromagnetic radiation of similar frequencies, it is proposed to rotate the radiation pattern relative to 9 bank of fixed detector strips. This may be achieved by the use of a Dove or inverting prism as shown in Figure 2. It is known that if the prism is rotated about the direction of the radiation the radiation paths at exit rotate around each other with twice the angular velocity of the prism.

A practical arrangement is shown in Figure 3a in side elevation and in Figure 3b in end elevation. A dove prism 17 is mounted as a turn table 18 which is supported on three roller bearings 19 for rotation about its axis 20. One roller is driven by an electric motor 21 to achieve a rotation which may be either continuous or stepped. The radiation transmitted by prism 17 is incident on a radiation sensitive screen 23 comprising a plurality of parallel radiation sensitive strips similar to 1-10 of Figure 1. By rotation of prism 17 about axis 20 the pattern of radiation projected onto screen 22 is rotated relative to these Strips.

As in the Figure 1 arrangement the electrical signals provided by the strip, and related to received radiation intensity, are integrated over intervals sufficiently short to distinguish line integrals at different orientations and are processed for display, for example as described in relation to Figure 1.

Using the facility available at lower frequencies of manipulating the radiation paths, the radiation incident on each strip may be directed to a smaller detector element shown at 23 in the simplified arrangement of Figure 4a. In this example the manipulation is achieved by a cylindrical lens 24 whose axis is perpendicular to the long dimensions of the strips. A view on XX of Figure 4a is shown in Figure 4b. In this figure the position of prism 17 is shown full size in broken outline and not as it might be seen through lens 24.

The arrangement of Dove prism and cylindrical lens is optically similar to those parts of the arrangement shown in United States Patent No. 4,135,096.

That arrangement is for a different purpose, being in fact to provide processing for x-ray signals from a CT apparatus. However those optical parts of the arrangement shown therein which are applicable may be adapted to the implementation of this invention.

Clearly other embodiments of the invention may be derived by those with appropriate skills using other detector devices and equipment for directing the radiation thereto.

Claims (19)

1. An imaging system for providing an image of a planar pattern of radiation, the system including a plurality of detectors, sensitive to the radiation, each producing an output signal related to the total intensity of radiation in a respective one of plurality of strips in said pattern, means for producing a relative rotational motion between said detectors and said pattern about an axis perpendicular to the pattern to provide output signals from the detectors for a plurality of orientations of the strips relative to the pattern and means for processing the said output signals for each of the orientations to provide said image.

2. A system according to Claim 1 in which the strips are linear and parallel.

3. A system according to Claim 1 or Claim 2 arranged to provide an image of a pattern of X-radiation transmitted by a body including means for rotating the detectors about said axis relative to the radiation.

4. A system according to Claim 3 in which the detectors are strips ofscintillator material each cooperating with at least one photo-electric converter.

5. A system according to Claim 4 in which the photo-electric converters are photo-diodes.

6. A system according to Claim 3 in which the detectors are linear ionisation detectors.

7. A system according to either Claim 1 or Claim 2 arranged to provide an image of a pattern of electromagnetic radiation of wavelength in the region of visible light.

8. A system according to Claim 7 including means for rotating the pattern of the radiation about said axis relative to the detectors.

9. A system according to Claim 8 in which the means for rotating includes a dove prism and means for rotating said prism about said axis.

10. A system according to any one of Claims 7-9 including means for directing the radiation of strips of the pattern to be incident on a region of a detector smaller than the respective strip.

11. A system according to Claim 10 in which the detectors are each of dimensions smaller than the respective strip.

12. A system according to Claim 10 or Claim 11 in which the means for directing is a cylindrical lens.

13. A system for producing an image of a pattern of radiation, the system being substantially as herein described with reference to Figure 1 oftheaccom- panying drawings.

14. A system for providing an image of a pattern of radiation, the system being substantially as herein described with reference to the accompanying draw ings.

15. A method of providing an image of a pattern of radiation, the method including projecting a plurality of strips in said pattern on to a plurality of radiation sensitive detectors, rotating the strips relative to the pattern to provide a plurality of line integrals of radiation intensity in the pattern at each of a plurality of orientations relative thereto and processing the line integrals to provide said image.

16. A method according to Claim 15 for provid ing an image of a pattern of X-radiation transmitted by a body, in which the relative rotation is achieved by rotating said detectors.

17. A method according to Claim 16 for provid ing an image of a pattern of electromagnetic radia tion of wavelength in the region of visible light.

18. A method according to Claim 17 in which the relative rotation is provided by rotating the pattern relative to the detectors.

19. A method of providing an image of a pattern of radiation, the method being substantially as herein described with reference to the accompanying drawings.