PRODUCING IMAGES BY IONOGRAPHY

Abstract: A method and apparatus for producing images from an electrically insulated film (6) on which a latent ionographic image has been formed by supporting the film between a pair of electrodes in a gas ionisation chamber exposed to x-rays. The film (6) is advanced on a support surface beneath at least one scanning electrode (13) signals derived from the said at least one scanning electrode (13) are digitised, and an image representative of the digitised signals is displayed. The film (6) may be supported in a cassette and advanced beneath said at least one scanning electrode (13) on the surface of a rotating drum (11). A predetermined voltage is applied between the support surface and said at least one scanning electrode (13) such that the potential difference between the film (6) and said at least one scanning electrode (13) is less than the potential difference between regions of maximum and minimum potential on the surface of the film (6).

Description: PRODUCING IMAGES BY IONOGRAPHY The present invention relates to a method and apparatus for producing images by ionography. Ionography is one of a number of alternative imaging systems which may be used in radiology to replace silver halide photography. Ionography depends upon the ionisation of a gas by x-rays which have passed through a subject under consideration. Ions which are produced in the gas are collected on an insulating film so as to form a latent ionographic image on the film. A visible image is then produced by scanning the latent image so as to derive signals representative of the charge on the film at various points across the latent image and then subjecting those signals to appropriate processing. A known method for producing latent ionographic images and deriving a visible image therefrom is described in British Patent Specification No. 2101447. In accordance with that known method an electrically insulated film is supported on one of a pair of electrodes positioned within an ionisation chamber. After the formation of a latent ionographic image on the film an array of scanning electrodes also located within the chamber is moved across the stationary film and signals derived from those electrodes are digitised to generate digital information from which a visible image corresponding to the latent image can be generated. With the known method a latent image can be scanned relatively quickly, for example in less than 90 seconds, and this scanning period can be reduced by increasing the complexity of the scanning electrode structure. The production of a series of images does however require alternate x-ray irradiation and latent image scanning stages and it is therefore difficult to process a large number of images rapidly. Furthermore it is difficult to provide an electrode traversing mechanism within the chamber of the required high accuracy. Another problem with the known method is that relatively high voltages are built up on the film, especially in regions of the film which receive ions resulting from exposure to x-rays which have not passed through the subject under investigation. For example, when images of a woman's breast are to be produced, the x-ray beam is directed parallel to the patient's chest and a proportion of the beam passes around the outside of the breast and is therefore not significantly attenuated. Furthermore, particularly high resolution images are required, and therefore high intensity x-ray beams are used. As a result, the potential difference between different portions of the film after exposure can be as much as 500 volts or more. If a scanning electrode is positioned close to a region carrying a high voltage a discharge can be initiated, destroying the film and possibly seriously damaging the scanning system. A further problem with the known method is that the film is generally planar during exposure. As the x-rays are generated from effectively a point source the x-ray beam is diverging. Ideally the two electrodes and the film would be arranged such that their surfaces were at all points perpendicular to the x-rays so that every point on the path of a single x-ray passing through the chamber would lie on the same electric field line. With such an arrangement all the ions produced by one ray would be automatically focussed at the same point on the film. Such an arrangement requires three concentric part-spherical surfaces which is difficult to achieve in practice. It is an object of the present invention to obviate or mitigate one or more of the above problems. According to the present invention there is provided a method for producing images from an electrically insulated film on which a latent ionographic image has been formed by supporting the film between a pair of electrodes in a gas - ionisation chamber exposed to x-rays, wherein the film is advanced on a support surface beneath at least one scanning electrode, signals derived from the said at least one scanning electrode are digitised, and an image representative of the digitised signals is displayed. Preferably, the film is advanced beneath said at least one scanning electrode on the surface of a rotating drum. The film may be supported in a cassette and advanced beneath said at least one scanning electrode such that each of a plurality of spaced apart latent ionographic images is scanned in turn, and the resultant digitised signals are stored. Alternatively, the film may be secured on the surface of the rotating drum and rotated therewith. Preferably, a predetermined voltage is applied between the support surface and said at least one scanning electrode such that the potential difference between the film and said at least one scanning electrode is less than the potential difference between regions of maximum and minimum potential on the surface of the film. Furthermore, an inert gas may be supplied to the gap defined between the said at least one scanning electrode and the adjacent surface of the film. The invention also provides an apparatus for deriving signals from a latent ionographic image formed on an electrically insulating film, comprising at least one scanning electrode, means for advancing the film on a support surface beneath the said at least one scanning electrode, and means for digitising signals derived by the said at least one scanning electrode and displaying an image representative of the digitised signals. The support surface may be defined by the surface of a rotating drum, and the rotating drum may define a pair of slots spaced apart in the circumferential direction for receiving the ends of the film, and means for gripping the ends of and tensioning the film. The gripping and tensioning means comprise two pairs of ' nip rollers each pair being positioned adjacent a ' respective slot. Alternatively, the rotating drum may have a porous surface and means to evacuate the interior of the drum such that a film placed on the surface is secured thereon by suction. Alternatively or in addition, the surface of the drum may be resilient. Preferably, means are provided for applying a voltage between the support surface and said at least one scanning electrode such that the potential difference between said at least one scanning electrode and an irradiated film mounted on the support surface is less than the potential difference between regions of maximum and minimum potential on the surface of the irradiated film. Alternatively or in addition, means may be provided for supplying an inert gas to the gap defined between the said at least one scanning electrode and the adjacent surface of an irradiated film mounted on the support surface. The invention also provides an apparatus for use in the production of a latent ionographic image on an electrically insulating film, comprising a cassette on which a sheet of insulating film may be supported, and a support body for the cassette, the cassette and support body when assembled defining a pair of spaced apart concentric part-spherical surfaces between which the film is received such that the film is pressed against and conforms to the shape of one of the part-spherical surfaces. Preferably, the cassette comprises a C-shaped frame over which the insulating film can be laid, and means for securing the film around the perimeter- of the C-shaped frame, and the support body defines a slot between the said part-spherical surfaces into which slot the cassette may be inserted. The insertion of the film causes the film to slide over and cohform to the shape of a convex one of said part-spherical surfaces. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a schematic perspective view of a first apparatus for forming latent ionographic images on an electrically insulating film; Fig. 2 illustrates the positional relationship between the electrically insulated film of Fig. 1 and an adjacent electrode; Fig. 3 is a schematic illustration of a drum reader for deriving signals representative of a latent ionographic image formed on a film by using the apparatus of Fig. 1; Fig. 4 is a sectional view through the drum reader of Fig. 3 illustrating the structure of one scanning electrode and its positional relationship to a film wound around the drum; Fig. 5 is a schematic illustration of a cassette for receiving a film for exposure to x-rays to produce an image of a woman's breast; Fig. 6 is a schematic illustration of the insertion of the cassette of Fig. 5 into a breast imaging plate; Fig. 7 is a schematic illustration of the spatial relationship between the breast imaging plate and cassette of Fig. 6 and an x-ray source during exposure; Fig. 8 illustrates the opening of the cassette after exposure and the positioning of the exposed film in a drum reader; Fig. 9 illustrates an arrangement for securing the film to the drum reader of Fig. 8; and Fig. 10 is a schematic illustration of a scanning and reading system for use with the drum reader of Fig. 8. Referring to Fig. 1, an ionisation chamber indicated schematically by a housing 1 encloses an upper electrode 2 and a lower electrode 3. An x-ray source 4 directs x-rays in the direction of arrow 5 via a subject to be radiographed (not shown) towards the electrodes 2 and 3. A gas, preferably having a high atomic number and a high density and capable of ionisation as a result of incident x-rays fills the space between the electrodes 2 and 3. A roll of electrically insulating plastics film 6 is supported on rollers 7 and 8 so that a portion of the film is in contact with the lower electrode 3. Charges formed by ionisations occurring in the ionisation chamber are collected on the film 6 so as to form a latent ionographic image. A side view of the roll 6 of film shown in Fig. 2 illustrates the positional relationship between the film 6 and the electrode 3. A series of latent ionographic images can be formed on the film 6 by alternately irradiating the chamber so as to form a latent image on the portion of the film 6 positioned above the electrode 3, advancing the film so that a fresh portion of the film is positioned above the electrode 3, re-irradiating the chamber, and so on in a series of successive steps. Thus a large number of latent images can be rapidly formed on the film 6. Furthermore the film transport mechanism of which the rollers 7 and 8 form a part can be of a relatively simple construction so that no difficult to manufacture mechanisms are required within the ionisation chamber. When it is desired to derive images from the latent images formed on the film 6, the film can be removed from the ionisation Chamber and transported to a drum reader of the type illustrated in Figs. 3 and 4. Referring to Figs. 3 and 4, the film 6 is supported on rollers 9 and 10 (which may be the same rollers as those illustrated in Fig. 1 and given reference numerals 7 and 8) . The film 6 is wound around a drum reader 11 which rotates in the direction of arrow 12 and thus as the film is driven continuously from roller 9 to roller 10 it passes beneath a transverse scanning electrode structure 13. The electrode structure 13 supports a series of individual electrodes spaced apart along its length and is reciprocated in the direction of arrows 14 so that each of the spaced apart electrodes can scan a strip of the film 6 which is considerably wider than the electrode itself. As best seen from Fig. 4, the electrode structure 13 incorporates a series of individual electrodes 15 each surrounded by a cylindrical guard ring 16. The electrodes 15 and guard ring 16 may be connected to circuitry of the form described in the above mentioned British Patent Specification and accordingly the processing of the signals derived by the electrodes 15 will not be further described herein. The film 6 is supported on a resilient foam layer 17 which is laminated onto the surface of the drum. The spacing between the electrode 15 and the film 6 is controlled by connecting the electrode to a support structure which incorporates a wheel that runs lightly against the surface of the film. If desired the actual electrode/film surface distance may also be measured to enable any necessary compensation for spacing variations in accordance with the method described in the above mentioned British Patent Specification.' Thus, a series of images formed on the film 6 may be rapidly and accurately scanned. In the above-described embodiment of the invention the scanning electrodes are provided independently of the ionisation chamber. It would however be possible to mount the scanning electrode structure within the ionisation chamber if it was desired to be able to produce visual images from recorded latent images without any significant delay. In the drum reader described with reference to Figs. 3 and 4, an electrode structure 13 is reciprocated perpendicular to the direction of movement of the film so that each of the spaced apart electrodes of the structure 13 can scan a strip of the film which is considerably wider than the electrode itself. It will be appreciated however that alternative arrangements are possible. For example an electrode structure which is stationary may be used provided a sufficient number of electrodes are supported by the structure. For example 2000 electrodes arranged in a linear array would be capable of deriving a high resolution image from a film which is passed beneath the array. Referring now to Figs. 5 to 10, further features of the invention will be described. The apparatus for forming latent ionographic images illustrated in Fig. 1 is not always convenient for forming images of the extremity of a patient's body, for example a woman's breasts. Furthermore the electrode structure required to form an accurate image on a flat film is relatively complex. An arrangement which obviates both these problems is described with reference to Figs.' 5 to 10. Referring to Fig. 5, a cassette is illustrated which is intended to receive a - rectangular sheet 18 of insulating film for use in forming a single image of a woman's breast. The cassette comprises a frame 19 on which a lid 20 is pivotally mounted. In use the film 18 is placed on top of the frame 19 and the lid 20 is then closed to trap three of the four sides of the film. An appropriate arrangement (not shown) may be provided to clamp the lid shut and an appropriate means, for example a rubber lip, may be provided around the periphery of the frame to improve engagement between the cassette and the inserted film. In use, the cassette and its supported film is inserted as shown in Fig. 6 into a support body 21 which defines a slot 22. The upper and lower walls of the slot 22 each define portions of spheres centered on a common point and the upper surface 23 of the support body is dished to receive the lower portion of a woman's breast. The rear edge 24 of the support body can be pushed up against the patient's chest wall so that x-rays emanating from a shield 25 can penetrate the great majority of the breast tissue as shown in Fig. 7. The x-ray source 26 is positioned at the centre of the two part spherical surfaces defined by the upper and lower walls of the slot 22. These part spherical surfaces are indicated by dotted lines 27 and 28. As can be seen from Fig. 7 because of the disposition of the wall 27 as the cassette is inserted into the support body the sheet 18 which normally assumes a substantially planar position slides beneath the wall 27 which is convex and as a result the sheet 18 follows the contour of the surface 27. The support body 21 may be fabricated from for example carbon fibre which is strong and rigid and can be accurately machined but which represents little obstruction to x-rays.' Each individual x-ray follows a path which is substantially perpendicular to the surfaces 27 and 28. The surfaces 27 and 28 are metalised and a DC potential is applied between them. Thus electric field lines extending between the surfaces 27 and 28 are substantially straight and perpendicular to those surfaces. X-rays passing through the space between the surfaces 27 and 28 ionise a gas with which that space is filled and ions are attracted to the surface 27 and impinge upon the film 18 covering that surface. A latent ionographic image is accordingly formed on the film 18. The intensity of that latent image which corresponds to the electrostatic potential on the surface of the film 18 is proportional to the intensity of the local x-ray radiation. Some of the radiation enters the ionisable gas without passing through any breast tissue and accordingly results in a very high intensity pattern around the periphery of the breast. Rays which have passed through the maximum thickness of breast tissue on the other hand are considerably attenuated. Accordingly the potential on the film 18 can vary from several hundred volts around the periphery of the breast, for example, 500 volts, to a few volts beneath the centre of the breast adjacent the chest wall. After a latent ionographic image has been formed on the film 18 it is simply removed from the cassette as shown in Fig. 8. One end is fed into a slot 29 defined in a rotatable drum 30 and the other is inserted into a slot 31. As shown in Fig. 9 the drum 30 is hollow and supports internally two pairs of nip rollers 32 and 33. The ends of the film 18 are pushed between the nip rollers which are rotated to tension the film around the drum 30. As shown in Fig. 10 a scanning electrode 34 is supported adjacent the drum on a mechanism (not shown) which enables the scanning electrode to be moved parallel to the drum axis. Thus if the drum is rotated at a relatively high speed and the scanning electrode 34 is traversed along . the length of the drum the scanning electrode scans a series of adjacent lines extending generally in a circumferential direction to enable the build-up of a high resolution image for display on a television monitor 35. The image information can also be stored on a disc storage system 36. To achieve good results the scanning electrode 34 must be positioned very close to the surface of the film 18. If there is a potential difference between the electrode 34 and the surface of the film 18 amounting to several hundred volts there is a considerable risk that an electrical discharge will occur in the gap between the scanning electrode and the film. Such a discharge would result in the effective destruction of parts of the latent image, particularly around the edges, and could also cause serious damage to the circuitry attached to the scanning head. Thus the scanning head-film voltage must be limited. This can be achieved by reducing the intensity of the latent image ' for example by reducing the intensity of the x-ray beam used to irradiate the subject under investigation. Unfortunately in the case of breast cancer very high resolution images are required as it # is vital to detect even very small irregularities in the breast tissue. In accordance with an aspect of the present invention this problem is overcome by applying a potential between the scanning electrode 34 and the drum 30. This is illustrated schematically in Fig. 10 by the positioning of a D.C. power supply 37 connected on the one hand to the sensing electrode 34 and on the other to the drum 30. The connection to the scanning electrode 34 can be simply achieved by an appropriate connection to the electrode casing. In the case of the drum 30 the connection must be such that the drum voltage is very stable as any variations in the drum voltage would induce variations in the sensed output of the scanning electrode 34. Accordingly, the drum can be mounted on metal bearings to which the power supply 37 is connected, the metal bearings themselves being mounted on insulated supports. The voltage applied between the drum 30 and the scanning electrode 34 is preferably equal to half the difference between the maximum and minimum surface voltages of the film 18. For example if in regions of maximum irradiation the film carries a surface voltage of 500 volts whereas in regions of minimum irradiation the film 18 carries a surface voltage of 10 volts, the drum potential may be reduced by 245 volts relative to the potential of the sensing electrode 34. As a result the potential difference between the sensing head 34 and the film 18 when the sensing head is positioned adjacent areas of maximum surface voltage amounts to 255 volts (500-245) . On the other hand when the sensing head 34 is positioned adjacent regions of minimum surface potential, a potential difference between the sensing head 34 and the film surface is again 255 volts (10 + 245) . The effective potential between the . surface of the film 18 and the sensing electrode 34 is thus substantially halved so that either the risk of a discharge can be greatly reduced or the maximum level of irradiation can be substantially increased. * As an alternative or additional means for reducing the risk of discharges occurring between the surface of the film 18 and the sensing electrode 34 a jet of an inert gas such as freon can be injected into the gap between the sensing head 34 and the film 18. If freon gas is used in addition to controlling the voltage between the sensing electrode 34 and the film 18 very high surface voltages can be accommodated enabling the production of images of much higher resolution than hereto thought possible.

Claims: CLAIMS : 1. A method for producing images from an electrically insulated film on which a latent ionographic image has been formed by supporting the film between a pair of electrodes in a gas ionisation chamber exposed to x-rays, wherein the film is advanced on a support surface beneath at least one scanning electrode, signals derived from the said at least one scanning electrode are digitised, and an image representative of the digitised signals is displayed. 2. A method according to claim 1, wherein the film is advanced beneath said at least one scanning electrode on the surface of a rotating drum. 3. A method according to Claim 1 or 2, wherein the film is supported in a cassette and advanced beneath said at least one scanning electrode such that each of a plurality of spaced apart latent ionographic images is scanned in turn, and the resultant digitised signals are stored. 4. A method according to claim 2, wherein the film is secured on the surface of the rotating drum and rotated therewith. 5. A method according to any preceding claim, wherein a predetermined voltage is applied between the support surface and said at least one scanning electrode such that the potential difference between the film and said at least one scanning electrode is less than the potential difference between regions of maximum and minimum potential on the surface of the film. 6. A method according to any preceding claim, wherein an inert gas is supplied to the gap defined between the said at least one scanning electrode and the adjacent surface of the film. 7. An apparatus for deriving signals from a latent ionographic image formed on an electrically insulating film, comprising at least one scanning electrode, means for advancing the film on a support surface beneath the said at least one scanning electrode, and means for digitising signals derived by the said at least one scanning electrode and displaying an image representative of the digitised signals. 8. An apparatus according to claim 7, . wherein the support surface is defined by the surface of a rotating drum. 9. An apparatus according to claim 8, wherein the rotating drum defines a pair of slots spaced apart in the circumferential direction for receiving the ends of the film, and means for gripping the ends of and tensioning the film. 10. An apparatus according to claim 9, wherein the gripping and tensioning means comprise two pairs of nip rollers -each pair being positioned adjacent a respective slot. 11. An apparatus according to claim 8, wherein the rotating drum has a porous surface and means are provided to evacuate the interior of the drum such that a film placed on the surface is secured thereon by suction. 12. An apparatus according to claim 8, wherein the surface of the drum is resilient. 13. An apparatus according to any one of claims 7 to 12, comprising means for measuring the spacing between the or each scanning electrode and the film. 14. An apparatus according to any one of claims 7 to 13, comprising means for controlling the spacing between the or each scanning electrode and the film. 15. An apparatus according to any one of claims 7 to 14, comprising means for applying a voltage between the support surface and said at least one scanning electrode such that the potential difference between said at least one scanning electrode and an irradiated film mounted on the support surface is less than the potential difference between regions of maximum and minimum potential on the surface of the irradiated film. 16. An apparatus according to any one of claims 7 to 15, comprising means for supplying an inert gas to the gap defined between the said at least one scanning electrode and the adjacent surface of an irradiated film mounted on the support surface. 17. An apparatus according to any one of claims 7 to 16, comprising a cassette on which the film is mounted. 18. An apparatus for use in the production of a latent ionographic image on an electrically insulating film, comprising a cassette on which a sheet of insulating film may be supported, and a support body for the cassette, the cassette and support body when assembled defining a pair of spaced apart concentric part-spherical surfaces between which the film is received such that the film is pressed against and conforms to the shape of one of the part-spherical surfaces. 19. An apparatus according to claim 18, wherein the cassette comprises a C-shaped frame over which the insulating film can be laid, and means for securing the film around the perimeter of the C-shaped frame, and the support body defines a slot between the said part-spherical surfaces into which slot the cassette may be inserted. 20. An apparatus according to claim 19, wherein insertion of the film causes the film to slide over and conform to the shape of a convex one of said part-spherical surfaces.

Inventors: BOOLER RUSSEL VINCENT, DOVAS THOMAS, MOORES BARRY MICHAEL, BOOLER, RUSSEL, VINCENT, DOVAS, THOMAS, MOORES, BARRY, MICHAEL
Applicants: UNIV MANCHESTER, THE VICTORIA UNIVERSITY OF MANCHESTER
Application year: 1987
Main classification: G03G15
IPC codes: G03G15