FR2794565A1 - LARGE-DIMENSIONAL X-RAY LUMINOUS PHOTON CONVERSION SCREEN AND RADIOLOGY SYSTEM INCLUDING THIS SCREEN - Google Patents

Abstract

The invention relates to an X-ray to light photon converter screen comprising a rigid carbide-type support plate (20) coated on one side with a layer of sodium iodide CsI. The support plate is made of a material transparent to X-rays and it has a first face (20a) capable of receiving and transmitting X-rays and a second face (20b), covered with sodium iodide, ensuring the conversion of the radiations. X transmitted by the support plate in visible light photons. The invention also relates to a radiology system comprising such a screen.

Description

X-RAY CONVERSION SCREEN

  IN LARGE LIGHT PHOTONS

  AND RADIOLOGY SYSTEM COMPRISING THIS SCREEN

DESCRIPTION

  Field of the Invention The invention relates to a screen for converting X-rays into visible light photons, of large dimension. This screen can be associated with X-ray emission means and with image acquisition and display means to perform a

digital radiology system.

  The invention finds applications in all fields requiring the conversion of X-rays into visible light photons and, in particular, in the field of medical radiology or in the field

non-destructive testing.

  STATE OF THE ART In conventional X-ray radiology systems, the main medium for acquiring radiography is generally an X-ray film. In these conventional radiology systems, the acquisition device consists of an X-ray generator associated with an X-tube, a support (on which is a patient or an object), a

  radiographic film and its associated selector.

  To perform an "x-ray", an X-ray flux is emitted by the X-tube. This X-ray flux is attenuated by the patient or the object; the residual flux transmitted through the patient or the object is measured by the radiographic film. There is then interaction of the non-attenuated X photons with the radiographic film. In recent years, X-ray systems have evolved: radiographic films are increasingly replaced by digital detectors. These digital detectors convert the fluxes of X photons into a two-dimensional image made up of pixels. These digital detector radiology systems allow the image to be obtained directly, unlike the radiographic film which

requires development.

  Different types of digital detectors for radiology systems have been produced in recent years and are described, in particular, in the technical note entitled "Real-time X-ray Imaging

  Systems "of the product" VARIAN VIP - 9 - SR / I ".

  One of these digital detectors consists, in particular, of a screen converting X photons into light photons, associated with coupling optics and with one or more CCD cameras. Such a digital detector is a so-called "indirect" conversion detector; in fact, with this detector, the X photons which arrive on the conversion screen are converted into light photons according to an isotropic emission in space. Part of the light photons is then transmitted through a coupling optic to one (or more) CCD camera (s) which converts these light photons into electrical charges in order to create an image. An example of such a digital detector, applied to medical imaging, is sold by the company SWISSRAY and described in the technical sheet entitled "Addon -Multi-System". However, this device has the drawback of using a conventional conversion screen of the Gd202S type, which is moderately efficient. There are other types of conversion screens. However, for certain applications, and in particular X in the field of medical imaging, large-scale conversion screens are necessary. For example, 43 cm x 43 cm screens are required to make "wide field" acquisitions, such as

chest x-rays.

  In addition to their large dimensions, the conversion screens intended for medical imaging must be precise and as efficient as possible, that is to say, they must provide the greatest possible number of light photons, for each photon.

X interacting with the screen.

  Such screens are known and produced by means of a rigid glass plate, covered on one of its faces with a layer of sodium iodide (CsI). Indeed, sodium iodide is a material which ensures the conversion of X-rays into visible light. This material must be deposited on a sufficiently rigid substrate, because it is the substrate which ensures the rigidity of the conversion screen. Such a conversion screen, consisting of a glass plate covered on one side with sodium iodide, is placed in the radiology system so that the glass side covered with sodium iodide is facing X-rays, and the uncoated glass face is facing the coupling optics. Glass has the advantage of neither attenuating the incident X photons, nor the visible light emitted by sodium iodide. However, the glass has internal light reflections which

  make such a screen difficult to use.

  Patent application US-5 723 865, entitled "X-ray imaging system", from R. TRISSEL et al. presents a conversion screen somewhat similar to that described above, but in which the glass plate is replaced by a polycarbonate plate. The rigid polycarbonate plate is covered on one side with sodium iodide. Polycarbonate has less good transparency to visible light than glass, but the problems of internal light reflection are also less important than in glass. This document presents various ways to remedy these reflections. One of these means is an anti-reflection system, which has the disadvantage of having poor transparency and therefore poor transmission of light photons through the polycarbonate substrate. Another means consists of a fluid placed between the conversion screen and the coupling optics; but this means is difficult to implement. None of the conversion screens mentioned above allow acquisitions

  "large field" with optimal quality.

  DESCRIPTION OF THE INVENTION The object of the invention is precisely to remedy the drawbacks of the techniques previously described. To this end, it offers a large-scale conversion screen intended, in particular, for radiology systems ensuring optimum conversion of X-rays into visible light photons. To be of optimal quality, the “wide field” conversion screen of the invention comprises a substrate which is rigid enough to be covered with CsI, non-attenuating by X-rays and capable of withstanding high temperatures, this screen having

relatively low cost.

  More specifically, the invention relates to a screen converting X-rays into light photons comprising a rigid support plate covered, on one side, with a layer of sodium iodide. The support plate is made of a material transparent to X-rays and it has a first face capable of receiving and transmitting X-rays and a second face, covered with sodium iodide, ensuring the conversion of X-rays transmitted by the plate. support in

visible light photons.

  Advantageously, the layer of sodium iodide is covered with a waterproof, transparent film.

with visible light rays.

  Preferably, the support plate is made of a carbide. It can be, for example, made in a silicon carbide or in a carbide

boron.

  According to one embodiment of the invention, the converter screen comprises a layer of visible light reflecting material, located between

  the support plate and the layer of sodium iodide.

  According to a particular application of the invention, the conversion screen is associated with means for emitting X-rays towards an object or a body, and with digital means for acquiring and displaying the image determined by the conversion screen, to achieve a system of

digital radiology.

Brief description of the figures

  - Figure 1 shows schematically a digital radiology system comprising the conversion screen of the invention; and - Figure 2 shows schematically

  the invention conversion screen.

Detailed description of

  embodiment of the invention In FIG. 1, a large field digital radiology system is shown diagrammatically. This radiology system includes means for emitting X-rays, referenced 1, which can be, for example, an X-ray generator associated with an X-tube. This radiology system also includes a screen for converting X-rays into light visible, referenced 2, and digital means for detecting and displaying the pixelated image supplied

by the conversion screen.

  These detection and display means consist of an optical system 3, such as coupling optics, associated with one (or more) CCD camera (s), referenced (s) 4 in FIG. 1. This CCD camera is connected to a display screen on which the digital image obtained by the radiology system is displayed, the display screen not being represented

  in the figure for simplification.

  In other words, the detection and display means consist of a pixelated system for reading the light image supplied by the conversion screen. In other words, this system converts the light emitted by the conversion screen 2 into electrical charges which are, in turn, converted into digital values so as to create a set of pixels producing an image. This pixelated proofreading system can be carried out by means of a reading matrix of the same size as the conversion screen 2, or else by an assembly of matrices which together cover the surface of the screen. This matrix can be, for example, a reading matrix with photodiodes. The pixelated proofreading system can also be carried out by means of one or more reading units each comprising reading matrices of dimensions smaller than the conversion screen. In this case, an optical coupling is made between the conversion screen and the reading matrices. This coupling can be done by an optical or

  by a fiber optic reducer.

  FIG. 2 schematically represents the conversion screen, referenced 2 in FIG. 1. This conversion screen comprises a rigid substrate, or support plate, referenced 20, and having a first face 20a and a second face 20b. The second face 20b is covered with a layer of sodium iodide CsI which has the property of converting X-rays

in visible light.

  According to the invention, the support plate 20 is made of a carbide. This carbide can be a boron carbide B4C. Preferably, for reasons of

  cost, this carbide can be a silicon carbide SiC.

  Silicon carbide also has the advantage of being extremely rigid and moderately attenuating. The support plate can therefore be thin, which allows low attenuation during the transmission of X photons, resulting in better light efficiency of the screen and a reduced dose of X-rays to be emitted to the patient or the object for

  acquire a good quality image.

  Furthermore, silicon carbide is a material transparent to X-rays, but opaque to visible light, which avoids the problems of internal reflection of the substrate. The conversion screen has

  thus good optical properties.

  In addition, silicon carbide resists high temperatures, which allows easy deposition of layer 22 of sodium iodide. Producing a conversion screen from such a carbide substrate and a layer of sodium iodide is therefore relatively easy to implement, which makes it possible to produce large conversion screens. All of these remarks relate to silicon carbide SiC; But

  they are also true for boron carbide B4C.

  According to the invention, the conversion screen 2 is placed so that the face 20a of the support plate 20 is facing X-rays and that the layer 22 of sodium iodide is facing the optical system and the camera. CCD. The plate 20 thus receives, by its first face 20a, the X photons which it transmits to the layer of sodium iodide 22; this, upon receipt of an X photon, converts it into several

  light photons, denoted LV in Figure 2.

  Preferably, the layer of sodium iodide 22 is covered with a waterproof film 26, transparent to visible light rays emitted by the layer of sodium iodide and whose role is to protect said layer 22 from moisture. According to one embodiment of the invention, a layer 24 of visible light reflecting material is placed between the support plate and the layer of sodium iodide 22 so as to increase the light efficiency of the converter screen 2. This layer 24 of visible light reflecting material can be, for example, an aluminum film. The choice of the thickness of the layer of sodium iodide 22 depends on two criteria, namely: - the detection efficiency of the screen, which increases with the thickness of sodium iodide; and - the resolution of the screen which decreases in

  depending on the thickness of sodium iodide.

  The interesting thickness ranges vary according to the radiological applications. For example, for a high resolution X-ray of the bones or for an X-ray, the thickness of the layer of sodium iodide varies between 150 μm and 600 μm, or even

700 Pm.

  The thickness of the silicon carbide substrate is determined so that the deflection of the screen (the deflection being the deformation of the substrate due to gravity) is compatible with the depth of

  field of the optical image acquisition system.

  For an application to digital medical radiology, the conversion screen can have a size of 450 mm x 450 mm; the dimensions of the silicon carbide support can, for example, be 450 mm x 450 mm x 1 mm thick, this support being covered with a deposit of sodium iodide with a thickness of 450 pn on a minimum surface of

440 mm x 440 mm.

l1

Claims (6)

  1. Screen converting X-rays into light photons comprising a rigid support plate (20), covered on one side with a layer of sodium iodide (CsI), characterized in that the support plate is produced in a carbide, transparent to X-rays and in that it has a first face (20a) capable of receiving and transmitting X-rays and a second face (20b), covered with a layer of sodium iodide (22), ensuring conversion of X-rays transmitted by   the support plate in visible light photons (LV).   2. Screen according to claim 1, characterized in that the sodium iodide layer is covered with a waterproof film (26), transparent to visible light rays.   3. Screen according to claim 2, characterized in that the support plate is in silicon carbide (SIC).   4. Screen according to claim 2, characterized in that the support plate is in boron carbide (B4C). 5. Screen according to any one of   Claims 1 to 4, characterized in that it comprises   a layer of visible light reflecting material (24), located between the support plate (20) and the layer of sodium iodide (22).   6. Digital radiology system comprising: - means (1) for emitting X-rays towards an object or a body; - means (2) for receiving and converting X-rays transmitted by the object or the body; and - digital means (3, 4) for detecting and displaying the image determined by the reception and conversion means,   characterized in that said reception and conversion means consist of a converter screen according to any one of claims 1 to 5.