X-RAY EXAMINATION APPARATUS INCLUDING A COLLIMATOR

Abstract: An X-ray examination apparatus includes an X-ray source (1), an X-ray detector (4) and a collimator (5) which is arranged therebetween. The collimator is provided with a plurality of collimator elements (6) which can be individually adjusted to an X-ray transparent state or an X-ray intercepting state. The individual collimator elements can be adjusted to, for example the X-ray intercepting state by filling the elements with an X-ray absorbing liquid (8).

Description: X-ray examination apparatus including a collimator. The invention relates to an X-ray examination apparatus which includes an X-ray source for emitting an X-ray beam, an examination space and a collimator which is arranged between the X-ray source and the examination space in order to intercept a part of the X-ray beam, which collimator is provided with separate collimator elements. An X-ray examination apparatus of this kind is generally used for radiological examination of an object. Such an object is, for example a patient to be medically examined. The X-ray source is arranged to irradiate the patient, positioned in the examination space, by means of the X-ray beam. An X-ray detector detects X-rays having traversed the patient. Thus, an X-ray image is formed on the X-ray detector and an image signal is derived from the X-ray image by the X-ray detector. The collimator shields a part of the patient from the X-ray beam. The collimator of such an X-ray examination apparatus ensures that, wherever possible only given parts of the object to be examined, for example of a patient to be examined, are exposed to the X-rays. Unnecessary exposure of the patient to X-rays is thus counteracted. Such an X-ray examination apparatius is known from the German patent DE 30 30 332. The known X-ray examination apparatus includes a collimator having collimator elements in the form of metal strips. The metal strips are movable. In order to intercept a part of the X-ray beam, some of the metal strips are slid into the X-ray beam from the circumference of the cross-section of the X-ray beam. As the metal strips are mechanically moved into or out of the X-ray beam, a rather long period of time is required to change the setting of the known collimator. Because the metal strips are moved into the beam from the circumference of the X-ray beam, there are restrictions as regards the shape of the cross-section of the X-ray beam transmitted by the collimator. The known X-ray apparatus notably is not very well capable of irradiating a part of an object having a complex shape and at the same time suitably shielding the object from X-rays outside said part. It is an object of the invention to provide an X-ray examination apparatus which includes a collimator which, in comparison with the known apparatus, offers more possibilities for forming a limited X-ray beam having a cross-section which accurately corresponds to a region to be irradiated. This object is achieved by means of an X-ray examination apparatus according to the invention which is characterized in that separate collimator elements can be switched between an X-ray transparent state and an X-ray intercepting state. The collimator elements of the X-ray examination apparatus have a fixed position in the region traversed by the X-ray beam. The collimator elements are preferably arranged in a two-dimensional array. The collimator elements can be individually and independently adjusted to an X-ray transparent state or an X-ray intercepting state. In the X- ray transparent state, the relevant collimator element absorbs hardly any or no X-rays. In the X-ray intercepting state the relevant collimator element absorbs so much X-rays that it transmits hardly any or no X-rays. Consequently, practically arbitrary parts of the X-ray beam can be intercepted. As the collimator includes more, and in that case possibly smaller, collimator elements, there are more possibilities for intercepting arbitrary parts of the X-ray beam. A prefeπed embodiment of an X-ray examination apparatus according to the invention is characterized in that separate collimator elements can be switched between said states by controlling a quantity of X-ray absorbing material, notably an X-ray absorbing liquid, in said collimator elements. As a result of the use of an X-ray absorbing liquid, separate collimator elements can be switched between the X-ray intercepting state and the X-ray transparent state by making the X-ray absorbing liquid flow into and out of the relevant collimator elements, respectively. The filling with the X-ray absorbing liquid can be controlled by means of an electric voltage applied between the X-ray absorbing liquid and the wall of the individual collimator elements. Except for the X-ray absorbing liquid, the collimator of the X-ray examination apparatus according to the invention does not include moving parts and its mechanical construction is much simpler, more compact and lighter in weight than that of the known collimator. A further preferred embodiment of an X-ray examination apparatus according to the invention is characterized in that separate collimator elements can be switched between said states on the basis of image information in an X-ray image of an object to be examined. The X-ray image contains image information concerning the patient region to be irradiated. On the basis of said image information the appropriate collimator elements can be switched to the X-ray intercepting state or the X-ray transparent state, so that exactly the relevant region can be exposed to the X-rays as well as possible. Image information can be used which has been derived from the X-ray image formed during a separate test exposure or, when a series of X-ray images are formed, image information from a previous X-ray image can be used. However, if a sufficiently long period of time elapses to form an X-ray image, the collimator can be readjusted while the patient is being irradiated for the X-ray image. A further preferred embodiment of an X-ray examination apparatus according to the invention is characterized in that separate collimator elements can be switched between said states on the basis of mutual positions of the X-ray source and an object to be examined, and/or of the object and an X-ray detector, and/or of the X-ray source and the X-ray detector. The mutual position and orientation of X-ray source, patient and X-ray detector determine, to a substantial extent, the cross-section of the X-ray beam transmitted by the collimator which is suitable for selectively irradiating a region of the patient. The collimator can be accurately adjusted on the basis of said mutual positions and orientation. A further preferred embodiment of an X-ray examination apparatus according to the invention is characterized in that the X-ray examination apparatus includes a selection unit for selecting predetermined collimator settings. It has been found that the type of radiological examination determines the desired collimator setting to a substantial degree. For example, for radiology of given parts of the anatomy, for example given organs such as the heart, the intestinal tract or lungs, there is little difference between individual patients in respect of the desired collimator setting. Various collimator settings can thus be pre-programmed for the various types of radiological examinations. The selection unit enables selection of the desired collimator setting, so that the desired cross-sectional shape of the transmitted X-ray beam is very simply achieved. A further preferred embodiment of an X-ray examination apparatus according to the invention is characterized in that the collimator can be adjusted on the basis of an effective surface area of the X-ray detector. When an X-ray detector in the form of an image intensifier pick-up chain is used, the effective surface area is dependent on the setting of the X-ray image intensifier. When the collimator is adjusted on the basis of the setting of the X-ray image intensifier, it is achieved that the region transmitting X-rays corresponds as well as possible to the effective surface area of the X-ray detector. The setting of the collimator is automatic so it does not require the attention of the operator of the X-ray examination apparatus, for example the radiologist. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows diagrammatically an X-ray examination apparatus in which the invention is used, Fig. 2 is a diagrammatic cross-sectional view of a collimator of the X-ray examination apparatus shown in Fig. 1, and Fig. 3 is a diagrammatic plan view of a collimator of the X-ray examination apparatus shown in Fig. 1. Fig. 1 shows diagrammatically an X-ray examination apparatus in which the invention is used. The X-ray source 1 emits an X-ray beam 2 in order to irradiate an object 3, for example a patient to be examined, by means of X-rays. An X-ray image is formed on an X-ray-sensitive surface 15 of the X-ray detector 4 due to local differences in X-ray absorption within the patient. The X-ray detector 4 derives an image signal from the X-ray image. In order to visualize the image information in the X-ray image, the image signal, for example an electronic video signal, is applied to a monitor 16 on which the image information is displayed. The image signal may also be applied to an image processing unit 17 for further processing. The X-ray detector 4 of the present embodiment consists of an image intensifier pick-up chain which includes an X-ray image intensifier 18 and a television camera 19. The X-ray-sensitive surface 15 is formed by a conversion layer 15 of the entrance screen 20 of the X-ray image intensifier. Incident X-rays are converted into an electron beam 26 by the entrance screen. The entrance screen includes the conversion layer and a photocathode. The conversion layer converts X-rays into blue or ultraviolet light whereto the photocathode 14 is sensitive. The photocathode emits the electron beam 26 to a phosphor layer 21 on an exit window 22. The photocathode is imaged on the phosphor layer 21 by means of an electron-optical system 23 which includes the photocathode, a hollow anode 25 and alignment electrodes 24. The incident electron beam generates an optical image in the phosphor layer. The optical image is converted into an image signal by the television camera 19. The television camera 19 is coupled to the exit window 22 by way of an optical coupling 27. The optical coupling 27 includes, for example a lens system or a fiber optical coupling. In order to ensure that unnecessary exposure of the patient to be examined to the X-rays is minimized, the X-ray examination apparatus includes a collimator 5 which is arranged between the X-ray source and the patient to be examined. The collimator can be accommodated within the housing of the X-ray source, but the drawing shows the collimator arranged outside the X-ray source for the sake of clarity. The collimator 5 intercepts a part of the X-ray beam in order to ensure that only a relevant part of the patient is exposed to the transmitted X-ray beam 30. The collimator includes a large number of collimator elements 6 which can be adjusted to an X-ray transparent state or an X-ray intercepting state. The part of the X-ray beam 2 which is incident on X-ray intercepting collimator elements is substantially completely blocked and the part of the X-ray beam which is incident on X-ray transmitting collimator elements is transmitted substantially without attenuation. The cross- section of the transmitted beam 30 can be controlled as desired by adjusting appropriate groups of collimator elements 6 to the X-ray transparent state or the X-ray intercepting state. This cross-section may have a complex shape. Not only a cross-section having a contour of complex shape can be matched, but also a cross-section of the transmitted beam can be realized which contains islands which do not transmit X-rays. Notably, the cross section can be controlled so as to be topologically multiply connected. Actually, it is only the dimension of the collimator elements determining the smallest possible details of the cross-section that still imposes a limitation in respect of the cross-sections that can be achieved. Fig. 2 is a diagrammatic cross-sectional view of a collimator of the X-ray examination apparatus shown in Fig. 1. Seven collimator elements 6 of the collimator 5 are shown by way of example. In practice the collimator may comprise a very large number of collimator elements (for example 10,000 or 40,000) in a matrix array. The collimator elements are constructed as capillary tubes 6, one of end of which communicates with a reservoir 35 containing a highly X-ray absorptive liquid 8, preferably being solution of a lead salt or a uranium salt in water. The collimator elements are provided with a metal layer 36 and are preferably formed by metal capillary tubes. On the inner side of such a capillary tube the metal layer is covered by a hydrophobic coating 37. The capillary tubes are arranged approximately parallel to the propagation direction of the X-ray beam 2. In order to switch individual collimator elements 6 to the X-ray intercepting state, the relevant collimator elements are filled substantially completely with the X-ray absorbing liquid 8. When a capillary tube is filled with the highly X-ray absorptive liquid over a length of one or a few cm, the incident X-rays practically will not pass the capillary tubes but will be absorbed substantially completely therein. This means that such a capillary tube filled with the highly X-ray absorptive liquid intercepts X-rays. Individual capillary tubes can be filled with the X- ray absorbing liquid by applying an electric voltage to the metal layer of the relevant capillary tube. The collimator includes an adjusting unit 7 for applying the electric voltages to the individual collimator elements so as to adjust the collimator elements to the X-ray intercepting or the X-ray transparent state. The adjusting unit 7 includes a selection unit 9 for selecting pre-programmed settings of the collimator. The electric voltage is the electric potential difference between the relevant metal layer and the X-ray absorbing liquid. Preferably, an electric voltage of between 30 V and 400 V is used. The hydrophobicity of the coating 37 is reduced on the basis of the electric voltage so that the relevant capillary tube is filled with the X-ray absorbing liquid. When the electric voltage is switched off, the coating 37 becomes hydrophobic again and the capillary tube is drained, so that it becomes X-ray transparent. The coating 37 is preferably impermeable to the X-ray absorbing liquid in order to avoid electrolysis of the X-ray absorbing liquid; however, between the metal layer and the coating 37 there may also be provided an electrically insulating layer 34 which is impermeable to the X-ray absorbing liquid. The electrically insulating layer is, for example a silicon-nitride (Si 3 N 4 ) layer. Polymer layers of PET, polystyrene or PTFE can notably used for the coating. Individual capillary tubes 6 are electrically connected, by way of their metal layer 36, to a drain contact 41 of a thin-film transistor 40 which acts as a switch. The source contact 42 of the relevant thin-film transistor is electrically connected to a voltage lead 43 via which the electric voltage to be applied is supplied. The transistor 40 is turned on by a control voltage which is applied to the gate contact 44. The control voltage is supplied via a control lead 45 which is coupled to the gate contact of the relevant transistor 40. The control system formed by the voltage leads, control leads and switches is preferably constructed using α-Si technology. Fig. 3 is a diagrammatic plan view of a collimator of the X-ray examination apparatus shown in Fig. 1. The collimator elements are arranged in the form of a matrix. By way of example, only a small number of 4x4 collimator elements 6 is shown for the sake of simplicity of the Figure. In practice a square matrix of 100x100 or 200x200 collimator elements is preferably accommodated in the collimator. A hexagonal structure with, for example 100x115 or 200x230 collimator elements can also be used. Individual columns of the matrix are provided with a voltage lead 43 and per row 47 of the matrix there is provided a control lead. The control voltages are applied to the relevant gate contacts 38 of the transistors in the relevant row by way of a row driver circuit 46. The control voltages are generated by a voltage source 48 which is coupled to the row driver circuit. A column driver 39 applies the electric voltages to the relevant voltage leads of the individual columns. The electric voltages are also produced by the voltage source.

Claims: CLAIMS: 1. An X-ray examination apparatus which includes an X-ray source (1) for emitting an X-ray beam (2), an examination space, and a collimator (5) which is arranged between the X-ray source (1) and the examination space in order to intercept a part of the X-ray beam (2), which collimator (5) is provided with separate collimator elements (6), characterized in that separate collimator elements (6) can be switched between an X-ray transparent state and an X-ray intercepting state. 2. An X-ray examination apparatus as claimed in Claim 1, characterized in that separate collimator elements can be switched between said states by controlling a quantity of X-ray absorbing material, notably an X-ray absorbing liquid, in said collimator elements. 3. An X-ray examination apparatus as claimed in Claim 1 or 2, characterized in that separate collimator elements can be switched between said states on the basis of image information in an X-ray image of an object to be examined. 4. An X-ray examination apparatus as claimed in any one of the preceding Claims, characterized in that separate collimator elements can be switched between said states on the basis of mutual positions of the X-ray source and an object to be examined, and/or of the object and an X-ray detector, and/or of the X-ray source and the X-ray detector. 5. An X-ray examination apparatus as claimed in any one of the preceding Claims, characterized in that the X-ray examination apparatus includes a selection unit for selecting predetermined collimator settings. 6. An X-ray examination apparatus as claimed in any one of the preceding Claims, characterized in that the collimator can be adjusted on the basis of an effective surface area of the X- ray detector.

Inventors: LINDERS PETRUS WILHELMUS JOHAN, STEGEHUIS HERMAN, LINDERS, PETRUS, WILHELMUS, JOHANNES, STEGEHUIS, HERMAN
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: A61B6
IPC codes: A61B6, G21K1