EP0013241B1 - Radiological intensifier tube with video output and radiological network provided with such a tube - Google Patents

Description

The invention relates to a radiological image intensifier tube, and to the radiology chain which includes such an intensifier.

In radiological image intensifier tubes (IIR tubes) the incident X-rays are converted into light in a luminescent screen, then into photo-electrons in a photocathode. These photo-electrons are accelerated by electronic optics and focused on a luminescent powder giving a bright image of the density of the flux of the indicative X photons. For a televised operation, this output image is taken up by an optic which transforms the image on the photosensitive target of a shooting tube, a vidicon for example, where it creates a distribution of charges which are read by an electron beam. then giving the video signal.

It is desirable to remove the IIR-vidicon coupling optics because of its weight and size, its lack of brightness, and generally the additional faults that it introduces into the chain.

A first solution of the prior art was an IIR-vidicon coupling by optical fibers. The light output screen of the IIR is put in contact with a wafer of optical fibers as well as the target of the shooting tube, the two wafers then being coupled together (see French patent n ° 7,423,277 ).

However, optical fibers have defects which prove to be serious for radiological use. A defect in one of the individual fibers forming the whole of the wafer results in a black spot or zone; moreover, the design of the fiber mosaic is apparent on the image.

A second solution of the prior art consists in eliminating the output screen of the intensifier and the coupling optics and in sending the photoelectrons directly to a target of the vidicon, sensitive to the impact of the electrons - the all being placed in the same enclosure - such as a diode mosaic target. This gives a very high X-ray video signal gain.

Unfortunately, it is necessary to minimize the quantum noise of the rays by using very high X doses. In addition, the dimensions of the input field of the intensifier impose high voltages on the electronic optics, which gives high energy to the photoelectrons arriving on the target, and therefore, a very large electronic gain in the target. . Because of the high dose X and the high target gain, it is necessary, in order to avoid the electric saturation thereof, to make provisions to reduce the target gain. It is also necessary to provide, in this kind of tubes, the possibility of a variable gain of the target between 1 and 50 for example, to function, depending on the uses, either in writing or in scanning.

One solution for this consisted in the prior art of depositing on a diode mosaic target, on the side of the photoelectron arrival, one or more barrier layers, metallic, thick, for example of 1 μm of aluminum, absorbing part of the energy of the electrons (see French patent application No. 7,705,031). This layer, although decreasing the gain, introduced a considerable multiplication noise due to the fact that the energy loss of the photoelectrons in the barrier layer is a statistical phenomenon which presents significant fluctuations.

The object of the invention is another solution not having these drawbacks.

• - figure 1: une vue schématique d'ensemble d'un tube intensificateur d'image radiologique de l'invention;
• - figures 2a, 2b: des vues en coupe schématique comparées des cibles d'un tube intensificateur d'image radiologique de l'invention et de l'art antérieur respectivement;
• - figure 3: un diagramme de chaîne de radiologie utilisant un tube intensificateur d'image de l'invention.

The invention will be better understood by referring to the description which follows and to the attached figures which represent:

• - Figure 1: a schematic overview of a radiological image intensifier tube of the invention;
• - Figures 2a, 2b: schematic sectional views compared of the targets of an X-ray image intensifier tube of the invention and the prior art respectively;
• - Figure 3: a radiology chain diagram using an image intensifier tube of the invention.

The radiological image intensifier tube with video output according to the invention comprises, in the same envelope maintained under vacuum, a luminescent input screen in contact with a photocathode which converts X-rays into photo-electrons, as in an intensifier. known image. These photo-electrons are focused by electronic optics and accelerated towards a layer of luminescent powder, after having passed through a metallic layer causing them to lose part of their energy. This luminescent layer is, according to the invention, deposited on the rear face of a photo-sensitive target previously covered with a semi-transparent layer causing an additional attenuation of the signal produced in response to the incident photoelectron bombardment. The light photons emitted by the luminescent layer, and not absorbed in the semi-transparent layer, create carriers in the target, which create at the level of the scanned face of the target a charge distribution which is read by the electron beam ; all the signals read constitute the video signal.

Figure 1 shows in schematic section, such a tube, and Figure 2 the structure of the target compared to that of a target used in the prior art.

The tube of FIG. 1 has two sections 1 and II, image and analysis sections respectively. In the first of these two sections, on the left in the figure, are produced the photo-electrons directed from the entry screen of the tube towards the target which constitutes the entry face of the second section, a vidicon target for example. This target is scanned by an electron beam from the other end of this section on the right of the figure. The image section 1 comprises, in order, from left to right of the drawing, an input screen 10, composed, according to known art, of a scintillator11 and a photocathode 12, and exposed to incident X-radiation (left arrows) crossing the object to be observed 30.

In operation, a beam of electrons or photo-electrons, coming from photocathode 12, is focused and accelerated towards the exit face of this first section occupied by the target of the second section. This target bears the mark 16 and the various focusing electrodes the mark 14; the electron beam is represented by the beam of straight lines in dashed lines. The second section of the tube further comprises means for producing an electron brush, symbolized by the arrow, and means ensuring, in operation, the point-by-point scanning of the target by the latter; this scanning uses a deflection device carrying the mark 20; the cathode and all the electrodes of the barrel bear the mark 18. All of the two sections are kept under vacuum in the envelope 24. In operation, the acceleration of the photoelectrons is ensured by a DC voltage source shown at 22. Finally, the assembly is placed in the protective envelope 25. The video signal is taken from the electron beam circuit, under the conditions known in this matter and not shown.

Figure 2a shows a schematic section of the target 1'6 of the tube of the invention, compared to that used in tubes of the same type of the prior art (Figure 2b), and their incorporation into the intensifier tube. We see in these figures the input screen 10 (11, 12) subjected to incident X-radiation (wavy arrow) and the target 16 of the previous figure, between which the electrons of charge e- are accelerated. The target of the invention, FIG. 2a, comprises superimposed, on the target proper 4, on the side opposite to that read by the electron beam (bottom arrow), three layers which consist respectively of a metallic barrier layer 1, a luminescent screen 2 and a semi-transparent layer 3, unlike the targets of the known art (FIG. 2b), which only comprise, in contact with the target 4 proper, the metallic layer 1.

In the latter, the electrons are braked by the barrier layer 1, so as to approach the target with an energy sufficiently reduced compared to their acceleration energy to avoid the drawbacks reported. This layer is, for example, made of aluminum and has a thickness of 1 micrometer.

In the target of the invention braking of the electrons is exerted, as in the previous case, by the metal layer 1; this braking leaves them enough energy to excite the underlying luminescent layer 2 which emits photons towards the semi-transparent layer 3; the metal layer 1, also made of aluminum for example, in this case has a thickness less than that of the prior art, of the order of 5000 angstroms. The photons emitted by part 2 of the target are absorbed by the semi-transparent layer 3 in a proportion which depends on its thickness and its nature. The semi-transparent material used is for example chromium, deposited on the target proper 4 over a thickness of approximately 500 angstroms; the luminescent layer is made of a cathodo-luminescent material such as calcium tungstate, Ca W0 ₄ , with a thickness of '5000 angstroms also, or zinc sulfide, ZnS.

Thus, in the invention, the reduction in gain takes place at two levels; first by braking, as in the prior art, at the metal barrier layer 1, and then at the semi-transparent layer 3, by photon absorption. This arrangement therefore makes it possible to use two parameters to reduce the gain X-ray video signal and adjust its value between the desired limits.

In particular, it makes it possible to reduce this gain by acting on a high number of particles, which, all things being equal, reduces noise. It makes it possible to adjust the gain, by acting on the acceleration voltage of the electrons in particular.

In addition, a sufficiently high acceleration voltage makes it possible to confer on the photoelectrons sufficient energy for them to pass through both the metal barrier layer 1, the luminescent layer and the semi-transparent layer, and that they reach the target itself 4 and directly excite it, with a gain high enough to allow fluorography observations at low dose of incident X-rays. These possibilities constitute advantages of the invention compared to the prior art.

The gain breaks down as follows: each incident photon X creates P photo-electrons (approximately '150 to fix ideas), which photo-electrons each create G photons in the luminescent layer 2 of the target of the tube of the invention, that the semi-transparent layer 3 partially absorbs to allow only the fraction T to pass through; each of these photons creates a carrier in the target proper 4; the number of free carriers in the target per incident photon X is therefore finally TGP. This gain is reduced to the last two factors GP in the case of a target of the known art comprising only the barrier layer 1, by admitting that each incident electron creates in the target a number G of carriers.

The input screen 10 of the tubes of the invention is of the type used in the art for the formation of radiological images, namely a two-layer screen, one of cesium iodide ICs, by example with a thickness of 100 to 200 micrometers, and the other in a photo-emissive material, such as potassium sodium antimonide, Sb Na ₂ K, with a thickness of the order of 500 angstroms.

It was admitted that the target proper 4, read by the electron brush, was a semiconductor target constituted by a mosaic of diodes formed in a semiconductor substrate, as shown in the drawings, where these diodes bear the mark 42 and the substrate mark 40. More generally, this target can be, within the limits of the invention, any photo-sensitive target, read by an electron beam, of the prior art.

The tubes of the invention are used in radiology channels, in particular in fluoroscopy, for direct viewing on a television screen, or in fluorography for viewing with memory. The chain diagram of this kind is given in FIG. 3 where the whole of the tube carries the mark 100. The mark 102 designates the display screen terminating the chain in the first case and the marks 104 and 106 the memory tube and the display screen, in the second case. The signals are taken directly at the outlet of the tube in the target scanning circuit, under known conditions.

Claims (3)

1. Radiological image amplifier tube having a video output and joining, within the same vacuum enclosure and on both sides of a target, means transforming the incident X ray image into a photo-electron beam directed towards said target and producing an impact thereat causing an electric image of the incident image to be formed in the target, and a barrier layer located on the target on the side thereof exposed to the photo-electrons, on the one hand, and, on the other hand, means for reading the electric signals building the thus formed image, characterized in that the barrier layer (1) is adjacent a layer of a luminescent material (2) producing a luminous image corresponding to the incident photo-electronic image and to which the actual target (4) is sensitive, and in that a further attenuation of the signal produced in response to the incident photo-electronic bombardement is obtained by interposition of a semi-transparent layer (3) between the layer of luminescent material and the actual target.

2. Radiological image amplifier tube in accordance with claim 1, characterized in that the actual target is a diode array target formed in a semi-conductor substrate.

3. Radiology set comprising a radiological image amplifier tube (100) and a screen (102, 106) supplied by the reading signals of this tube for visualization of the incident image, characterized in that the amplifier tube is a tube in accordance with claims 1 or 2.

Images