GB2212039A - Apparatus for high energy irradiation imaging - Google Patents

Abstract

In a radiation therapy machine, an irradiation image of a patient is provided by the high energy source using a conversion screen 16 to provide an optical image which is relayed via a mirror and lens to a video camera. The conversion screen 16 comprises a metal plate 25 generating Compton electrons and a transparent, clear scintillation monocrystal wafer 28, e.g. of NaI(TI), whose input and output faces 29, 30 are optically worked and providing an optical specular reflecting surface 26 coupled optically 27 to the input surface of the crystal 28 to increase the optical output without degrading the image resolution. The reflector 26 can be a silvered or polished aluminium layer on the surface of the Compton electron heavy metal plate 25 which may be stainless steel or copper. <IMAGE>

Description

DESCRIPTION APPARATUS FOR HIGH ENERGY IRRADIATION IMAGING This invention relates to apparatus for providing an irradiation image of an object under irradiation by radiation having an energy greater than 1MV, including means for sensing the high energy radiation after passing through the object in order to monitor the position of the object relative to the irradiation beam, said means including a conversion screen for converting the incident radiation into optical radiation, and optical means including video camera means for converting the two-dimensional optical image thereby produced into electrical signals representative of the two dimensional optical image.

When selectively irradiating an object with a beam of high energy, i.e. penetrating, radiation it is often necessary to monitor the position of the irradiating beam relative to certain reference regions such as variations in the density or thickness of, or the presence of cavities in the object in the radiation path. One application in which it is important that the actual irradiation path is accurately located is in the treatment of tumours by radiotherapy.

Much effort has been expended on providing accurate diagnostic images of a patient, for example by radiography, computed tomography (CT) and nuclear magnetic imaging (NMI). Further effort has been employed in the subsequent pre-treatment use of fluoroscopy in marking a patient and then positioning the patient on a radiotherapy machine relative to the adjusted treatment beam so that, theoretically, an optimal treatment dose can be applied to the tumour and a minimal dose is applied to surrounding healthy tissue and vital organs.

It has been found in practice, however, that in spite of careful positioning together with the applications of local restraints to immobilise that part of the body of the patient under treatment, positioning errors often occur resulting in insufficient irradiation of the tumour and avoidable damage to heathy tissue.

It has therefore become the practice to monitor the relative positions of the treatment beam and of the patient by radiography using the treatment beam itself. However the use- of portal verification films with the high energy X-radiation used in radiotherapy machines of from 1 to 50 MV, results generally in poor image quality which is frequently insufficient to reveal essential detail. Furthermore, the film takes time to process and does not, of course, reveal any displacement of the patient that may occur after exposing the film and before applying treatment.

A difficulty in forming a radiographic image with high-energy penetrating radiation is that differences in the density of different constituents in the body cause smaller differences in attenuation than is the case for X-radiation of lower energy employed in diagnostic fluoroscopy. Thus processing film to increase contrast has been attempted in order to make detail slightly more visible, but the improvement achieved has been very limited.

In a paper by H. Meertens et al., Phys. Med. Biol. 1985, Vol.

30, No. 4, pages 313 to 321, a detector in the form of an ionisation chamber with a matrix of electrodes, is described which forms accurate spot measurements of radiation intensity over a two dimensional region. Each spot measurement can be digitised and can form a 2D image field of values which can then be processed digitally in order to enhancce required features of the image. In a paper by J. Leong in Phys. Med. Biol., 1986, Vol. 31, No. 9 pages 985-992, a similar picture enhancing process is disclosed applied to video images taken from an E-2 fluorescent screen. Figure 4 of European patent application 0-187-066 discloses radiotherapy apparatus of the kind hereinbefore specified and employing as the conversion screen, a fluorescent screen formed by fluorescent powder held in a transparent organic binding agent.

A disadvantage of the form of fluorescent screen employed hitherto, for example in EP A-0-187-066, is that the suspension of the fluorescent powder agent causes light to be scattered, i.e.

the screen has a milky or diffusing appearance. With high energy radiation it is necessary to provide as long a path as possible through the fluorescent medium in order to obtain sufficient light output above noise. This means that image detail will become blurred by scattering of light. A similar problem arises in the use of scintillator plates employed for gamma cameras since these are made with surfaces which scatter light, i.e. by roughening and often with a diffusing powderised layer because a gamma camera operates on the principle of locating single optical events by means of an array of photomultiplier tubes. Thus the position of an event is measured by the relative light intensities sensed' by the photomultipliers from a given event. Thus prior forms of conversion screen tend to give poor resolution of detail.

It is an object of the present invention to provide improved apparatus of the kind specified which can provide an optimum combination of image resolution and optical conversion sensitivity.

To achieve this apparatus of the kind specified is characterised in that the conversion screen comprises in the following order along the path of radiation from the source, a metal screen for conversion of high energy radiation into electrons by the Compton process, an optical specular reflecting surface, and a scintillation plate formed from a clear transparent material and having optically polished major surfaces to form respectively an input surface for the radiation and associated Compton electrons and an output surface for optical radiation, the reflecting surface being optically coupled to the input surface of the scintillation plate.

The scintillation plate can be an inorganic monocrystalline wafer such as Nal or CsI and can be suitably doped with an appropriate dopant such as TI or Na. In order to provide as great a sensitivity as possible without reducing the resolution too far, the thickness of the scintillator plate lies preferably in the range 5mm to 15mm. The metal screen is suitably made of heavy metal and can be of stainless steel or of copper. The optical specular reflecting surface can preferably be formed on the surface of the metal screen facing the input surface of the scintillation plate and can comprise a specular layer of silver or aluminium and the optical coupling between the reflecting layer and the input surface of the scintillation plate can be formed by a silicone optical coupling medium.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, of which: Figure 1 illustrates a radiotherapy machine provided with apparatus in accordance with the invention, and Figure 2 illustrates a conversion screen in accordance with the invention.

Figure 1 diagrammatically illustrates radiotherapy apparatus for irradiating a predetermined region of a patient 8 with a beam of high energy radiation such as x-rays. The patient 8 is supported on an adjustable table 1. A gantry 2, rotatable through substantially 360 degrees about a horizontal axis 3, supports an electron source 4, a linear accelerator 5 which accelerates the electrons to a selectable energy typically in the range 4-25MEV, a beam deflection system 6 which deflects the electrons through an angle greater than 90 degrees so that the beam is directed normally towards the axis 3, and a head 7 which includes means for providing a radiotherapy beam of high-energy X-rays produced by causing the electron beam, after deflection at 6, to strike a suitable x-ray target. The linear accelerator 5 and the beam deflection system 6 are further arranged to bring the electron beam substantially to a point focus 10 which forms the effective point source of the high energy x-ray beam generated by the x-ray target located at the point 10.

The normal radial distance from the source 10 to the horizontal axis 3, i.e. to the isocentre, is 100 cms.

Also mounted on the gantry 2 is apparatus 15 for providing an irradiation image of the patient 8 using the treatment beam generated by the source 10 and limited by a collimator, suitably a multileaf collimator arrangement, housed in the head 7. The imaging apparatus 15 comprises a conversion screen 16 for converting the incident radiation from the source 10 after passing through the patient 8, into optical radiation in the form of a two dimensional radiographic image, and optical means comprising a mirror 21, a wide aperture lens 17 and a video camera 18, suitably a charge coupled (CCD) solid state silicon camera. The two-dimensional optical image received by the camera 18 is fed.as a sequence of preferably digital pixel point values to a storage and processing means 19 for storing complete images and applying suitable known picture enhancement and processing routines.Thus the means 19 can comprise a suitable digital store e.g. disk drive or semiconductor memory and associated computer suitably programmed. The images thereby produced can then be displayed on a monitor 20 and, if desired, recorded on tape or disc for future reference. The mirror 21 is necessary so that the camera 18 is not subject to incident high energy radiation which would generate undesired signals therein, and screening, suitably lead, is provided around the camera to attenuate scattered radiation.

Thus far the apparatus is similar to that disclosed in Figure 4 of EP A-0-187-066 hereinbefore referred to.

In accordance with the invention the conversion screen 16 comprises in the following order along the path of radiation from the source 10 and with reference to Figure 2 in which the thickness dimension, shown vertically, is exaggerated and not to scale, firstly a heavy metal screen 25. This causes incident high energy X-radiation to generate energetic electrons by the process of Compton scattering and the electrons thereby generated are more effective than the high energy radiation in causing a scintillator to emit photons of light. A suitable material for the screen 25 is a sheet of stainless steel. The number of electrons emitted for a given input radiation flux increases with the thickness of the sheet 25 and this will correspondingly increase the optical image brightness.However as the thickness increases so does the lateral scattering of the incident ray of radiation and this reduces the image definition. A reasonable compromise is to employ a stainless steel sheet 25 having a thickness in the range 0.5mm to 2mm and in one example a thickness of 1.5mm was used. An alternative metal for the sheet 25 is copper which can give an improved yield of Compton electrons.

Next in the radiation path, there is provided an optical specular reflecting surface 26, optically coupled by a coupling layer 27, suitably of a silicone optical medium, to the input surface 29 of a scintillation plate 28.

The scintillation plate 28 is formed from a clear transparent material, preferably an inorganic monocrystalline wafer of, in the present example NaI doped, with thallium. Other suitable scintillator crystals can be used, for example CsI (TI) which provides a better optical spectral match to the CCD camera and therefore greater sensitivity but has the disadvantage that it is too brittle to be formed easily into a large wafer and tends to crack easily.

The major surfaces 29, 30 of the scintillation plate 28 are optically polished to form respectively an input surface 29 for the radiation and associated Compton electrons and an output surface 30 for optical radiation. The thickness of the scintillation plate is selected in order to balance the requirement of a large optical output for a given incident radiation flux i.e. requiring a thick crystal, and that of good resolution implying as little lateral scattering as possible, i.e. requiring a thin crystal. In the present example a monocrystalline wafer of NaI(Tl) having a thickness of 8mm was employed, however, a thickness in the range 5mm to 15mm can be employed with reasonable results.

The optical specular reflecting surface 26 is preferably formed on the surface of the metal screen 25 which is adjacent the scintillation plate'28. In the present example the reflecting surface 26 is a specular layer of silvering applied to the stainless steel sheet 25. Alternatively a copper sheet 25 can be employed on which a polished aluminium layer 26 is used to form the reflecting surface. The function of the reflecting surface 26 is to reflect light from a scintillation occurring in the scintillation plate 28 and which would travel away from the output surface 30, back towards the output surface 30 and hence the camera 18 from an image position which, at least near the centre of the field of view, would effectively correspond to that of the forwardly emitted photons.Thus apparatus in accordance with the invention can provide a light output for a given input flux which can be 80 per cent greater than when no reflecting surface is employed, without significantly degrading the resolution at least in the centre of the image field.

The output surface 30 of the scintillation plate 28 is optically coupled, suitably by a layer of silicone optical medium 31, to a protective pyrex glass plate 32 with optically worked major surfaces. The whole conversion screen assembly is housed for protection and mounting purposes -in an aluminium housing 33.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of radiotherapy systems and high energy radiation imaging devices and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (17)

CLAIM(S)

1. Apparatus for providing an irradiation image of an object under irradiation by radiation having an energy greater than 1MV, including means for sensing the high energy radiation after passing through the object in order to monitor the position of the object relative to the irradiation beam, said means including a conversion screen for converting the incident radiation into optical radiation, and optical means including video camera means for converting the two-dimensional optical image thereby produced into electrical signals representative of the two dimensional optical image, characterised in that the conversion screen comprises in the following order along the path of radiation from the source, a metal screen for conversion of high energy radiation into electrons by the Compton process, an optical specular reflecting surface, and a scintillation plate formed from a clear transparent material and having optically polished major surfaces to form respectively an input surface for the radiation and associated Compton electrons and an output surface for optical radiation, the reflecting surface being optically coupled to the input surface of the scintillation plate.

2. Apparatus as claimed in Claim 1, characterised in that the scintillation plate is a suitably doped inorganic monocrystalline wafer.

3. Apparatus as claimed in Claim 2, characterised in that the inorganic monocrystalline wafer comprises sodium iodide.

4. Apparatus as claimed in Claim 2, characterised in that the inorganic monocrystalline wafer comprises caesium iodide.

5. Apparatus as claimed in any one of Claims 2 to 4, characterised in that the doping agent is thallium.

6. Apparatus as claimed in any one of the preceding claims, characterised in that the scintillator plate has a thickness in the range Smm to 15mm.

7. Apparatus as claimed in any one of the preceding claims, characterised in that the metal screen comprises a stainless steel sheet.

8. Apparatus as claimed in Claim 7, characterised in that the stainless steel sheet has a thickness lying in the range lmm to 2mm.

9. Apparatus as claimed in any one of Claims 1 to 6, characterised in that the metal screen comprises a copper sheet.

10. Apparatus as claimed in any one of the preceding claims, characterised in that the optical specular reflecting surface is formed on the surface of the metal screen adjacent the scintillation plate.

11. Apparatus as claimed in any one of the preceding claims, characterised in that the optical specular reflecting surface comprises a silver layer.

12. Apparatus as claimed in any one of Claims 1 to 10, characterised in that the optical specular reflecting surface comprises a layer of aluminium.

13. Apparatus for providing an irradiation image of an object under irradiation by radiation having an energy greater than 1MV, substantially as herein described with reference to the accompanying drawings.

14. A radiotherapy machine including a source of high energy radiation, a collimator, and apparatus as claimed in any one of the preceding claims.

15. A radiotherapy machine as claimed in Claim 14, characterised in that the apparatus as claimed in any one of Claims 1 to 13 is mounted on a gantry which also supports the high energy source and collimator.

16. A radiotherapy machine as claimed in Claim 14 or Claim 15 and provided with image processing apparatus including computing means for enhancing the image obtained.

17. A radiotherapy machine substantially as herein described with reference to the accompanying drawings.