Patent Publication Number: US-3881104-A

Title: Storing optical relay for converting x-ray images which comprises an electro-optic material

Description:
United States Patent 1191 Donjon et al.  
 14 1 Apr. 29, 1975 [73] Assignee: U.S. Philips Corporation, New  
 York, NY.  
 [22] Filed: July 25, I973 [21] Appl.-No.:382,441  
 3.383,5l4 5/I968 Dolon er al 250/213 VT 3,749.920 7 1973 Sheldon 250/213 VT FOREIGN PATENTS OR APPLICATIONS 2.098.522 2/1972 France l.526l69 4/1968- France 250/213 VT Primary Examiner-James W. Lawrence Assistant Examiner-T. N. Grigsby Attorney, Agent, o&#39;r FirmFrank R. Trifari; Simon L. Cohen [57] ABSTRACT The invention relates to an arrangement for convert- [30] Foreign Application Priority a ing X-ray images into images of a given spectrum. The Au 7 France 72 77914 X-ray image, which is projected on a photosensitive layer and converted into an image of electric charges which are stored in the vicinity of an electro-optic g or ifigi fibi i625 panel, modulates the birefringence of the latter. This birefringence, and hence in actual fact the X-ray im- 7 [58] held of Search 50/213 313/65 101 age, is read by means of polarized light the wavelength [561 References Cited and the power of which are chosen in accordance with th v UNITED STATES PATENTS 6 use 3.015731 1 /1%2 Van Santcn et al 250 213 VT 9 Clam, 7 D&#39;awmg Fgures TRANSPARENT HEATER SUPPORT PHOTOCONDUCTIVE 21 17 2 3 1 4 LAYER CONDUCTIVE I LAYER w1NDow f /-W|NDOW I I I l I 1s 5L T RADIATION v I 3 SOURCE v CONDUCTlVE 3 0 31 20 LAYER INSULATING MIRROR ELECTROOPTIC LA YER JENIEDavaestrs SHEET 3 OF X-RAY SENSITIVE PHOSPHOR ELECTRON FOCUSSING SYSTEM 3 1 4 5 ELECTRON SENSITIVE PHOSPHOR PHOTOCATHODE VACUUM -TlGHT WALL LOW VACUUM A Fig.6 ENVELOPE ENVELOPE Pmmmmmms,  
 SHEET 4 BF 4 ALIGNED FIBER OPTIC SLABS STORING OPTICAL RELAY FOR CONVERTING X-RAY IMAGES WHICH COMPRISES AN ELECTRO-OPTIC MATERIAL The present invention relates to a device for converting X-ray images into images of a given spectrum, usually in the visible range. The X-ray image which is pro jected on a photosensitive layer and is converted into an image of electric charges which are stored in the vicinity of an electro-optical panel, modulates the birefringence of the latter. This birefringence and hence in actual fact the charge image or the X-ray image is read by means of suitably polarized light the wavelength and intensity of which may be chosen in accordance with the instantaneous use.  
  Among the devices of the above mentioned kind mention may be made of the relays described in French Pat. Specification No. 2,098,522 and applicants French patent application No. 71/11319 of Mar. 31, 1971. These devices ensure the relay of images in the visible spectrum. a  
  One of the objects of the invention is to also ensure this role of relay in the region of the spectrum which corresponds to the X-rays.  
  The advantages of such a device are comparable to those of the prior art devices used for the same purposes.  
  The said devices are the ones which comprise an Xray image intensifying tube in combination with a television camera.  
  The major drawback of such devices is due to the fact that in television cameras generally a number of images per second is used which is fixed according to universally accepted standards. Thus, in many cases the integrating (or storing) time of the X-ray image which determines the amount of X-radiation received per image for a given level of irradiation is determined by the said number of images per second instead of by medical requirements. It often is less than would be desirable to enable the radiation dose to be reduced or the signalto-noise ratio to be increased.  
  Moreover the use of a television system produces appreciable deterioration of the resolution of the images.  
  Another disadvantage of such a device is that because it has no storage properties the observation of the image and the irradiation are simultaneous.  
  Hence, prolonged observation of the person being examined may result in his being subjected to a comparatively large amount of radiation, especially if, because there is no storage, after visual examination it is decided to make a photograph of the image, so that the duration of the exposure of the patient to X-rays is prolonged.  
  The addition of a video tape recorder to the said device permits prolonged observation without simultaneous irradiation of the patient; however, the recorder produced additional deterioration of the images; moreover it is not possible to examine the same image for a period exceeding about 1 minute.  
  The apparatus according to the invention obviates these disadvantages. The image is made visible by projecting onto a screen by means of a read light beam; consequently no scanning by a television system is required; thus the resolution is substantially equal to that of the optical relay and the overall number of resolved points per image may be appreciably higher than that of an image produced by a video system using the accepted standards.  
  There is image persistence after irradiation, which permits a prolonged examination of the image without repeated irradiation and enables photographs to be made of the image.  
  In addition, the apparatus enables certain operations to be performed such as addition, integration or subtraction and differentation of images, as described in applicants French patent applications No. 72/03620 of Feb. 3, 1972, and No. 71/11318 ofMar. 3, 1971. Especially addition is advantageous for the superposition of images made with different voltages of the X-ray tube. Integration permits an increase of the signal-to-noise ratio of images subject to non-correlated noise.  
  Finally, because the target plate does not adversely affect the coherence of the light it may be of advantage to process the recorded images by coherent light, with the use of holographic and optical filtering methods.  
  The present invention relates to a storing optical relay apparatus for converting X-ray images which comprises at least one X-ray source, as the case may be means for the intermediate conversion of X-rays into visible light rays, at least one source of light, an optical image relay constituted by at least one evacuated enclosure provided with the necessary windows (at least one window transparent to the display radiation and one window transparent to the read radiation), containing a panel of a material exhibiting electro-optic effects which is raised to a temperature near to its Curie point and has birefringence which is variable in accordance with the voltage difference set up between its faces, the face facing the transparent window being coated with a transparent conductive layer, while the other face is coated with a number of superinposed layers which comprise a dielectric mirror, a layer of a photoconductive material which is sensitive to X-rays or to I the radiation into which the X-rays are converted and an electrically conductive layer which is transparent to the X-rays or to the light into which the X-rays are converted.  
  ln an embodiment of the invention the photoconductive layer is directly sensitive to the X-rays.  
  Further embodiments are characterized in that a phosphor layer is interposed between the X-ray source and the photoconductor, the combinations of the phosphor and the photoconductor and their thicknesses being chosen for optimum suitability for the instantaneous use and for&#39;maximum sensitivity of the apparatus.  
  Another feature of the invention consists in that the conversion of the X-rays into light rays may be effected via a first conversion of X-rays into electrons by means of a phosphor and a photocathode. The apparatus then is suitable for the observation of large-size images reduced to the size of the electro-optical target by the addition of an electron-optical system.  
  In a particular embodiment of the invention the X-ray detector tube and the optical relay proper maintain their independence and their proper role for other uses. They are constructed and arranged for the conditions of the invention so as to be readily combined and to cooperate.  
  The invention will be better understood by means of the following description, given by way of example, with reference to the drawings, in which:  
 FIG. 1 shows a first embodiment,  
  FIG. 2 shows an embodiment derived from the first embodiment with the addition of a phosphor in a first position,  
  FIG. 3 shows another embodiment with a phosphor in a second position,  
  FIG. 4 shows another embodiment in which a slab of optical fibres is used in combination with the phosphor,  
  FIG. 5 shows an embodiment which comprises a first conversion of X-rays into electrons and a second conversion of electrons into light,  
  FIG. 6 shows a modification of the lastmentioned embodiment containing two compartments which are evacuated to different degrees, and  
  FIG. 7 shows another modification of this embodiment comprising the combination of an X-ray detector tube and an optical relay.  
  In a first embodiment of the invention shown schematically in FIG. 1 the relay for X-rays has the same structure as in one of the embodiments described in applicants French Pat. Specification No. 2,098,522; shown again are the following elements: the target plate 1 made of an electro-optic material, glued to a transparent support 2 and having its face which faces the support coated with a transparent conductive layer 3 and its other face with an insulating mirror 4. Shown again also is the device 17 which enables the target plate to be raised to its Curie temperature, the evacuated air-tight enclosure 21, the windows 18 and 19 which transmit the read radiation and the write X-ray radiation respectively. The thickness and the nature of the window 19 are chosen so as to ensure good transmission of the X-rays. The layer 5 is a photoconductive layer sensitive to X-rays. A conductive layer 6 which is transparent to the X-rays is disposed on the photoconductive layer 5.  
  A direct voltage is applied between the said electrodes 3 and 6 by means of terminals 30 and 31; the photoconductive layer 5 illuminated by the X-rays through the elements 6 and 19 becomes conductive and a charges image appears in the vicinity of the electro-optic panel. The resulting birefringence image in the electro-optic material may be read by means of a suitably polarized light-beam which illuminates the panel 1 through the window 18 and is reflected at the mirror 4.  
  The inscribed image is erased by illuminating the photoconductive layer and simultaneously removing the voltage applied between the electrodes 3 and 6. For this purpose the layer 5 is uniformly illuminated by means of X-rays or any other radiation which falls within the sensitivity spectrum of the photoconductive material. The layer 5 may be illuminated through one of the windows 18 and 19 or, for example, by means of a source which may be accommodated within the envelope 21. Obviously all the elements disposed between the photoconductive layer and the erasing source must be transparent to the radiation of this source. In the embodiments to be described hereinafter erasion may be effected by any of the aforementioned means.  
  Furthermore, although the absorption of the X-rays increases with increase in the thickness of the photoconductive layer, the maximum photo-current can only be obtained in the case of a thickness which gives rise to low absorption of the X-rays.  
  In order simultaneously to increase the absorption of the X-rays and the photo-current it may be advantageous for a layer of phosphor sensitive to X-rays to be interposed between the source and the photoconductive layer. as is described in the following three embodiments.  
  In one of these embodiments which is shown in FIG. 2 the mirror 4 is coated with a photoconductive material 5 which is sensitive to the light emitted by a layer of a phosphor 7 sensitive to X-rays. The layer 7 is applied to the transparent conductive layer 6. This arrangement provides greater liberty in selecting the na ture of the photoconductive material and its thickness, for the optimum thickness of the photoconductive layer for visible radiation need not necessarily be the same as for X-rays.  
  For practical reasons it may be advantageous to arrange the phosphor on an individual support, as is the case in the embodiment shown in FIG. 3, in which the elements 1, 2, 3, 4, 5, 6, 17, 18, 19, 21, 30 and 31 are equal to those in FIG. 2. The phosphor 7 which faces the photoconductive layer 5 is disposed on a support 8 which is transparent to X-rays and may be the window 19.  
  The spacing between the phosphor and the conductive layer 6 which is transparent to the light emitted by the phosphor is small, of the order of some tens of microns, to maintain satisfactory resolution at the level of the photoconductor.  
  In another embodiment illustrated in FIG. 4 an optical fiber slab 9 is used as a support for the phosphor 7, the face of the slab not coated with the phosphor facing the photoconductive layer 5 and being spaced therefrom by a small distance in order to maintain satisfactory resolution at the level of the photoconductive layer. It should be noted that in the case of a spacing equal to that which exists between the phosphor and the photoconductive layer in the preceding embodiment the loss of resolution here is smaller owing to the reduction of the aperture of the light beam by the optical fibers.  
  Image conversion by means of the preceding arrangements is limited to X-ray images the size of which is substantially equal to that of the target plate and hence is comparatively small.  
  In other embodiments of the invention conversion of larger-size X-ray images is considered; for this purpose the X-rays are previously converted into electronic radiation, and the use of an electron-optical system of less than unity magnification enables the size of the image to be reduced to that of the target made of electro-optic material.  
  A first one of these embodiments is shown schematically in FIG. 5.  
  To the elements 1, 2, 3, 4, 5, 6,17,18,19, 21, 30 and 31 of the preceding embodiments are added, for example by deposition on the window 19, a phosphor 12 sensitive to X-rays and a photocathode 11 sensitive to the radiation emitted by the phosphor. The electrons emitted by the photocathode are accelerated and focussed by means of an electron-optical system 13 onto a second phosphor 10 which is capable of being excited by these electrons and is arranged on the conductive coating 6 and emits a radiation in the band of spectral sensitivity of the photoconductor. In the same manner as in image intensifier tubes and for the same reasons a metallic layer 14, for example an aluminum layer, is deposited on the phosphor 10. The potential of this layer may be the same as that of the transparent conductive layer 6.  
  A second one of these embodiments in comparison to the preceding one has the advantage that the optical relay part proper needs a much lower vacuum than does the photocathode, so that the envelope of the tube may be divided in two parts, as is shown in H6. 6.  
  The part of the envelope in which a high vacuum obtains is designated by A and that in which a lower vacuum prevails is designnated by B. A vacuum-tight wall 22 separates the two differently evacuated parts of the enveloper. In this partition a window is made which is constituted by the optical fiber slab 9 on which is deposited the phosphor 10 coated with the conductive layer 14. Similarly to the embodiment shown in FIG. 4, the slab 9 is arranged near the layer 6 to maintain good resolution.  
  In another embodiment the X-ray detector tube and the optical relay may maintain, for other applications, their independence and their separate roles; they are associated and constructed in a manner such as to permit this association and cooperatioin. This association and the relative adjustment of the two devices are shown in FIG. 7.  
  The X-ray detector tube, which has the same structure as in FIG. 6, is terminated downstream of the electron-optical system by a glass fiber slab 9 which serves as an exit window for the light emitted by the phosphor 10.  
  The entrance window of the optical relay also comprises a glass fiber slab arranged in optical contact with the panel 9 of the X-ray detector tube. Furthermore the relay proper again comprises the other elements 1, 2, 3, 4, 5, 6, l7, 18, 21, 30 and 31 shown in the preceding Figures.  
 What is claimed is:  
  l. Storing optical relay arrangement for converting X-ray images, comprising at least one source of X-rays, at least one source of visible light, an optical image relay constituted by at least one evacuated enclosure provided with at least one window transparent to X- rays and one window transparent to the visible radiation, a panel of a material which exhibits electro-optic effects, means for raising the panel to a temperature near its Curie point whereby the panel exhibits birefringence which is variable in accordance with a voltage difference applied between its faces, a transparent conductive layer coated on that face of the panel which faces the transparent window, a dielectric mirror on the other face of the panel, a layer of a photoconductive material sensitive to X-rays on said mirror, and an electrically conductive layer on said photoconductive layer which is transparent to X-rays.  
  2. A storing optical relay arrangement for converting X-ray images, comprising a source of X-rays, a source of visible light, an optical image relay constituted with an evacuated enclosure provided with a window transparent to the visible radiation and a window transparem to radiation of a predetermined frequency band, a panel of material which exhibits electro-optic effects,  
 means for maintaining the panel at a temperature near its Curie point, whereby the panel exhibits birefringence which is variable in accordance with a voltage difference applied between its faces, a transparent conductive layer on that face of the panel which faces the window transparent to visible radiation, a dielectric mirror on the other face of the panel, a layer of photoconductive material sensitive to a radiation of said predetermined frequency band on said mirror, an electrically conductive layer on said photoconductive layer that is transparent to radiation of said predetermined frequency band, and a phosphor system means having one side facing the window transparent to said predetermined frequency band and&#39;facing the source of X- rays on another side thereof for converting the X-rays from said source into radiation in said predetermined frequency band.  
  3. Arrangement as claimed in claim 2 wherein the phosphor is deposited on a light-guide glass-fiber slab disposed between the phosphor and the photoconductive coating at a slight distance from the latter.  
  4. Arrangement as claimed in claim 2, wherein said phosphor system means comprises means for converting the X-rays into electrons and means for converting the electrons into radiation in said predetermined frequency band.  
  5. Arrangement as claimed in claim 4, wherein said means for converting the electrons into radiation comprises a phosphor sensitive to X-rays and a photocathode.  
  6. Arrangement as claimed in claim 4, further comprising an electron-optical system including a target and means for reducing the X-rays image substantially to the size of the electro-optic target.  
  7. Arrangement as claimed in claim 4, wherein the evacuated enclosure consists of two parts which are evacuated in different degrees and are separated by a window which is transparent to visible light, the less highly evacuated part (vacuum of the order of from 10 to 10 Torr) containing only the relay part proper, while the more highly evacuated part (vacuum of the order of 10&#39; Torr) is used for the conversion of X-rays into electrons, for the electron-optical system and for the conversion of electrons into visible light.  
  8. Arrangement as claimed in claim 7, wherein the window through which the two parts of the enclosure which are evacuated in different degrees are in optical communication consists of a slab of optical fibers which is coated at the side of the conversion of X-rays into electrons with the phosphor which enables the electrons to be converted into visible light.  
  9. Arrangement as claimed in claim 4, wherein it consists of an association of an X-ray detector tube and an optical relay the exit and entrance windows of which respectively are each constituted by a slab of optical fibers, the X-ray detector and optical relay each being in separate unconnected envelopes, the X-ray detector tube and the optical relay being disposed in contact with one another.  
  UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.  
 Dated April 29, 1975 Inventor(s) JACQUES DONJON ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 ON THE TITLE PAGE Section [75] Inventors &#34;Jean- Pierre Hazen&#34; should read -Jean-Pierre Hazan-;  
 IN THE SPECIFICATION Col. line 58, &#34;produced&#34; should be produces;  
 Col. 5, line 12, &#34;enveloper&#34; should be -envelope;  
 line 22, &#34;cooperatioin&#34; should be --cooperation;  
 En&#39;gncd and Sealed-this RUTH&#39;C. MASON .-1I!esling Officer C. MARSHALL DANN (&#39;mnmissimwr Hflalenls um! Trmlcmurkx