EP0125962B1 - X-ray image intensifier and its use in computed x-ray image processing - Google Patents

Description

The present invention relates to an intensifier of radiological images. It also relates to the application of this intensifier to a digital radiology system.

Radiological image intensifiers or I.I.R. are well known in the prior art. For example, see the article published in the technical journal THOMSON-CSF, volume 8, number 4, December 1976, entitled “Image intensification in medical and industrial radiology •.

• - un écran luminescent d'entrée assurant la conversion des rayons X incidents en photons lumineux ;
• - une photocathode en contact optique avec l'écran luminescent qui assure la conversion des photons lumineux en photo-électrons ;
• - une optique électronique assurant la focalisa- tion des trajectoires électroniques et le gain en énergie des photo-électrons ;
• - un écran d'observation assurant la conversion des photoélectrons en photons lumineux.

Radiological image intensifiers convert an X-ray image formed into an image that can be seen on a screen. They include:

• - a luminescent input screen ensuring the conversion of incident X-rays into light photons;
• - a photocathode in optical contact with the luminescent screen which converts light photons into photo-electrons;
• - electronic optics ensuring the focusing of the electronic trajectories and the energy gain of the photoelectrons;
• - an observation screen ensuring the conversion of photoelectrons into light photons.

The present invention relates more particularly to luminescent entry screens for I.I.R.

Currently, these screens are generally produced by vacuum deposition, on a concave substrate, of a luminescent material with a high atomic number, such as cesium iodide.

Known screens have either more often a greater thickness of luminescent material on the edges than in the center, or a substantially constant thickness, but rather greater on the edges than in the center.

Figure 1 shows a sectional view of a luminescent screen 1 whose thickness h _b on the edges is greater than the thickness h _c in the center. The dotted curves a and b in FIG. 2 show that, for known screens, the variation in thickness, from the center towards the edges, of the layer of luminescent material as a percentage of its thickness at the center of the screen is either increasing - curve a - is substantially horizontal but rather increasing - curve b -.

We will explain below the different elements of the problem that the Applicant proposes to solve.

The Applicant wishes to use I.I.R. for systems such as digital radiology systems in which the same image must be taken several times, using different energies of X-rays. The various images thus obtained are converted to digital and processed on a computer, for example by weighted subtraction, which ultimately gives an image where certain organs stand out in relation to others.

The I.I.R. known are ill-suited to this use for the following reasons.

We have seen that in I.I.R. known the thickness of the luminescent screen is greater at the edges than in the center. This increases the number of X-rays absorbed on the edges of the screen which corrects the weakness of sensitivity which generally exists at the edges of the field of observation. This weakness of sensitivity is due, in particular, to the geometric divergence of the X-rays used to form the image, to the cushioned distortion of the electronic optics of the I.I.R. ... etc.

The difference in thickness between the center and the edges of the luminescent layer of the I.I.R. results in a difference in X-ray absorption. As the energy of X-rays increases, their probability of absorption decreases faster in the center than at the edges, because the edges are thicker than the center, and the sensitivity of the edges increases. compared to that of the center.

The Applicant has concluded that the I.I.R. known for a given X-ray energy, a substantially uniform edge-center sensitivity, but that when the X-ray energy varies, the sensitivity of the I.I.R. in the center of the screen and their sensitivity on the edges evolve very differently. The I.I.R. known therefore are not very suitable for digital radiology systems.

The present invention proposes to solve the problem of designing a luminescent IIR screen, usable in particular in a digital radiology system, and the sensitivity of which on all points of the screen varies substantially in the same way, when the x-ray energy varies.

The present invention relates to a radiological image intensifier, comprising a curved luminescent screen, having a concave face facing the interior of the intensifier, ensuring the conversion of incident X-rays into light photons, this intensifier being used in a system of digital radiology in which the same image is taken several times using different X-ray energies, characterized in that the thickness of the screen is smaller at the edges of the screen than in the center and in that the thickness of the screen h is substantially linked to its thickness at the center h _c by the relation: h = h _c · cos 6, with 6 = a + β, α and p being respectively the angles at which the points of X-ray impacts on the screen are seen from the center of curvature of the screen and from the X-ray source.

According to the invention, it is sought to make the luminescent input screen identical in all points for the X-rays, that is to say that it is sought to have a constant "apparent screen thickness for all incident X-rays. It is necessary to reduce the thickness of the edges of the screen relative to the center so that the length of the path in the luminescent material is substantially the same for all X-rays regardless of their angle of incidence on the screen. So when the energy of the X-rays varies, the length of the path in the incident material being the same for all the X-rays, the sensitivity at all points of the screen varies substantially in the same way.

It can therefore be seen that the screen according to the invention is completely different from the known screens. It can be considered that, given the known screens, there was a technical prejudice dissuading those skilled in the art from designing luminescent screens of greater thickness in the center than at the edges. Calculation and experience have shown the advantage of these screens according to the invention.

• - les figures 1 et 3, des vues en coupe d'un écran luminescent d'I.I.R. selon l'art antérieur et selon un mode de réalisation de l'invention ;
• - la figure 2, des courbes montrant différents profils de variation de l'épaisseur de la couche de matériau luminescent du centre vers les bords de l'écran ;
• - les figures 4 et 5, des schémas expliquant le fonctionnement de l'écran selon l'invention.

Other objects, characteristics and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• - Figures 1 and 3, sectional views of an IIR luminescent screen according to the prior art and according to an embodiment of the invention;
• - Figure 2, curves showing different profiles of variation of the thickness of the layer of luminescent material from the center to the edges of the screen;
• - Figures 4 and 5, diagrams explaining the operation of the screen according to the invention.

In the different figures, the same references designate the same elements, but, for reasons of clarity, the dimensions and proportions of the various elements are not observed.

Figure 1 has been described in the introduction to the description. The same is true of curves a and b in Figure 2.

In FIG. 2, the curve c, in solid line, shows the variations, in percentage, of the quantity Ae = (h - h _c ) / h _c when one moves from the center towards the edges of a luminescent screen according to one embodiment of the invention, with h the thickness of the screen at any point on the screen and h _c the thickness at the center of the screen.

The curve c is decreasing. On the edges of the image field, Δe is substantially equal to - 20%. We define the edge of the image control as follows. The projection of a screen, such as that shown in Figure 1, on a surface gives a circle of radius r. The edge of the image control consists of a ring of width r / 10 or r / 16 approximately which occupies the periphery of this circle.

Figure 3 is a sectional view of an embodiment of a screen according to the invention whose thickness h _b on the edges is smaller than the thickness at the center h _c . The variation in screen thickness is radial.

Luminescent screens are generally produced by vacuum deposition, on a concave substrate, which is required for the proper functioning of electronic optics, of a luminescent material with a high atomic number such as cesium iodide. This substrate can either be the entrance window of the I.I.R., or an attached part inside the I.I.R.

To absorb as much X-rays as possible, the thickness of the luminescent material layer must be as large as possible, but this is to the detriment of the resolution. A compromise must be found. When using cesium iodide deposited under vacuum, this compromise is currently between 200 and 500 micrometers thick.

To manufacture a luminescent screen whose thickness is thinner at the edges than at the center, one is led to modify the geometrical conditions of evaporation which are usually used to manufacture screens whose thickness is greater at the edges than in the center.

The invention will be explained with reference to Figures 4 and 5.

In Figure 4, there is shown schematically an intensifier of radiological images 2. The luminescent screen 1 is located on the right side of the I.I.R. This screen receives the impact of X-rays produced by a source 3 placed on the axis 00 'of the I.I.R. at a distance F.

The luminescent screen is concave. It is assumed that in the example of FIG. 4 this screen is constituted by a spherical cap with a radius of curvature R. The curvature of the screen can take various other forms; you can use concave luminescent screens, hyperbolic, parabolic ... etc. The screen therefore consists of a quasi-spherical cap. The screen arrow can take various values which are involved in the characteristics of electronic optics.

Consider in FIG. 4 the impact on the screen of the X-rays emitted by the source 3 which arrive on the screen at a point P located at a distance B from the axis 00 '.

A and β denote the angles at which the point of impact P on the screen is viewed respectively from the center C of the sphere whose screen is a cap and from the source 3 of X-rays.

FIG. 5 is an enlargement of the region of the screen comprising the point of impact P.

Denote by d the path in the luminescent material of the X-rays crossing the screen obliquely at the point P.

According to the invention, this path d must be equal to the thickness h _c of the screen at its center, on the axis 00 ', which corresponds to the path in the luminescent material of the X-rays moving along the axis 00 '.

• d = h_c = hp/cos 9, avec hp, l'épaisseur de l'écran au point P et 9 = α + β.

The following equality must therefore be checked:

• d = h _c = hp / cos 9, with hp, the thickness of the screen at point P and 9 = α + β.

We therefore deduce that the thickness hp of the screen at point P equals h _c · cos 9 and is therefore less than the thickness h _c at the center of the screen.

• a = Arc sin (B/R) et β = Arc tg (B/F), avec B la distance entre l'axe de l'I.I.R et le point d'impact sur l'écran, F la distance entre l'écran et la source de rayons X et R le rayon de courbure de l'écran au point d'impact.

In conclusion, - whether it is a concave screen in the form of a spherical cap or a concave screen of any shape - so that the path of the X-rays in the luminescent material of the screen has substantially the same length whatever the point of impact of the X-rays on the screen, the thickness of the screen h, at all its points, must be linked to its thickness at the center h _c . by the relation: h = h _c · cos 6, with 6 = a + p, a and β being respectively the angles under which the points of impact of X-rays on the screen are seen from the center of curvature of the concave screen and from the X-ray source. These angles are expressed as follows:

• a = Arc sin (B / R) and β = Arc tg (B / F), with B the distance between the axis of the IIR and the point of impact on the screen, F the distance between the screen and the source of X-rays and R the radius of curvature of the screen at the point of impact.

The following numerical values are given by way of example for the example of FIGS. 4 and 5: B = 100 mm, the point P defined by this distance B is located on the edges of the screen, at approximately 1/10 ° from the edge of the image control.

• a = Arc sin (B/R) et β = Arc tg (B/F), ce qui donne a = 30°, β = 8° et 6 = 38°.

We calculate the angles a and p:

• a = Arc sin (B / R) and β = Arc tg (B / F), which gives a = 30 °, β = 8 ° and 6 = 38 °.

• hp = h_c · cos θ = h_c x cos 38° = 0,79 x h_c.

So we get:

• hp = h _c · cos θ = h _c x cos 38 ° = 0.79 xh _c .

The magnitude Δe = (h - h _c ) / h _c = -1 + cos θ is therefore equal to - 0.21. This means that the thickness of the luminescent layer is approximately 21% thinner at the edges, i.e. at 1/16 ^th or 1/10 ^th of the edge of the image field, than at the center of the screen.

It should be noted that the desired result is approached satisfactorily by manufacturing a screen whose thickness at the edges, at 1/10 ^th or approximately 1/16 ^th of the edge of the image field, is approximately 15 to 25% less than the thickness at the center of the screen, depending on the shape of the curvature of the screen and the value of the arrow. This means that the relation h = h _c . cos 6 is not necessarily applied to all points of the screen in a rigorous manner and that satisfactory results are obtained by applying this relation to the edges of the screen, for example to approximately 1/10 ^th or 1 / 16 ^e from the edge of the image field and applying it only approximately on the rest of the screen.

Curve c in FIG. 2 can therefore have various shapes, while remaining decreasing from the center towards the edges. It can be noted that satisfactory results are obtained with a curve in which Δe varies as the square of the distance from the center.

Whether applied at all points of the screen or only on its edges, the relation h = h _c · cos 6 involves the distance F between the screen and the X-ray source. We can choose an average value for this distance F which is generally between 700 and 1500 mm. In this range of variation of F, the value of cos 6 depends only very slightly on the value of F.

In the screens according to the invention, the thickness of which is smaller at the edges than at the center, for a given X-ray energy, the sensitivity of the edges may be lower than that of the center if nothing is done to remediate.

• - on peut modifier sur les bords le dopage du matériau luminescent ;
• - on peut accroître sur les bords, ou diminuer au centre, le couplage optique de la photocathode avec l'écran, par exemple en modifiant l'état de surface de la couche luminescente ou/et en modifiant l'état du support sur lequel cette couche est déposée ;
• - on peut jouer sur les caractéristiques des électrodes faisant partie de l'optique électronique de l'I.I.R. pour diminuer la distorsion en coussin ;
• - on peut modifier la texture de la couche luminescente pour que le rendement de conversion des rayons X en lumière, soit plus important sur les bords qu'au centre de l'écran.

It is often preferable to compensate for this lack of sensitivity of the edges by modifying the construction parameters of the luminescent screens, according to one or more of the methods described below, the list of which is not exhaustive:

• - the doping of the luminescent material can be modified at the edges;
• - one can increase on the edges, or decrease in the center, the optical coupling of the photocathode with the screen, for example by modifying the surface state of the luminescent layer or / and by modifying the state of the support on which this layer is deposited;
• - one can play on the characteristics of the electrodes forming part of the electronic optics of the IIR to reduce the cushion distortion;
• - the texture of the luminescent layer can be modified so that the conversion efficiency of X-rays into light is greater at the edges than at the center of the screen.

The screens according to the invention are particularly suitable for use in digital radiology systems using a computer to obtain a radiological image, for example by weighted subtraction of images obtained with different energies of X-rays. X-rays are used, the average energy varies from approximately 20 to 30 KeV to 100 KeV. The screens according to the invention can however be used in systems other than digital radiology systems, such as for example conventional radiology systems.

Claims (6)

1. An X-ray image intensifier including an in- curved luminescent screen (1) having a concave surface turned to the inside of the intensifier and performing the conversion of the incident X-rays into light photons, this intensifier being used in a digital X-ray system in which several times the same picture is taken by using different X-ray energies, characterized in that the thickness of the screen is lower at the periphery (h_b) of the screen than in the center (he) and that the thickness h of the screen is substantially related to the thickness at the center he through the formula h = h_c · cos 8 with 6 = a + β and α and β being respectively the angles at which the impact points of the X-rays on the screen (1) are seen from the screen center of curvature (C) and from the X-ray source (3) respectively.

2. An intensifier according to claim 1, characterized in that the thickness (h_b) at the periphery of the screen (1) is about 15 to 25 % lower than the thickness (h_c) at the screen center, depending on the shape of the screen curvature and the deflection value.

3. An intensifier according to claim 1, characterized in that the formula h = h_c · cos 6 is applied at any point of the screen.

4. An intensifier according to claim 1, characterized in that the formula h = h_c · cos 6 is applied essentially at the periphery of the screen.

5. An intensifier according to one of claims 1, 3 or 4, characterized in that the formula h = h_c · cos 8 is calculated by taking a mean value in the range of distance (F) variation between the screen and the X-ray source (3).

6. An intensifier according to one of claims 1 to 5, characterized in that the loss of sensitivity at the periphery of the screen is compensated by modifying one or more of the following parameters of luminescent screens, which list is not exhaustive :

- the doping rate of the luminescent material ;

- the optical coupling of the photocathode of the intensifier with the screen, by acting upon the surface state of the luminescent layer or/and upon the state of the support on which this layer is deposited ;

- the characteristics of the electrodes, belonging to the electronic lens of the intensifier, in order to reduce the pincushion distorsion.

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