Patent Number: 056663960
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows diagrammatically an X-ray examination apparatus 1 in accordance with the invention. The X-ray source 2 emits an X-ray beam 15 for irradiating an object 16. Due to differences in X-ray absorption within the object 16, for example a patient to be radiologically examined, an X-ray image is formed on an X-ray sensitive surface 17 of the X-ray detector 3, which is arranged opposite the X-ray source. The X-ray detector 3 of the present embodiment is formed by an image intensifier pick-up chain which includes an X-ray image intensifier 18 for converting the X-ray image into an optical image on an exit window 19 and a video camera 23 for picking up the optical image. The entrance screen 20 acts as the X-ray sensitive surface of the X-ray image intensifier which converts X-rays into an electron beam which is imaged on the exit window by means of an electron optical system 21. The incident electrons generate the optical image on a phosphor layer 22 of the exit window 19. The video camera 23 is coupled to the X-ray image intensifier 18 by way of an optical coupling 24, for example a lens system or a fiber-optical coupling. The video camera 23 extracts an electronic image signal from the optical image, which signal is applied to a monitor 25 for the display of the image information in the X-ray image. The electronic image signal may also be applied to an image processing unit 26 for further processing. Between the X-ray source 2 and the object 16 there is arranged the X-ray filter 4 for local attenuation of the X-ray beam. The X-ray filter 4 comprises a large number of filter elements 5 in the form of capillary tubes whose X-ray absorptivity can be adjusted by application of an electric voltage, referred to hereinafter as adjusting voltage, to the inner side of the capillary tubes by means of the adjusting unit 7. The adhesion of the X-ray absorbing liquid to the inner side of the capillary tubes can be adjusted by means of an electric voltage to be applied to an electrically conductive layer (36) on the inner side of the capillary tubes (5). One end of the capillary tubes communicates with a reservoir 30 for an X-ray absorbing liquid. The capillary tubes are fried with a given quantity of X-ray absorbing liquid as a function of the electric voltage applied to the individual tubes. Because the capillary tubes extend approximately parallel to the X-ray beam, the X-ray absorptivity of the individual capillary tubes is dependent on the relative quantity of X-ray absorbing liquid in such a capillary tube. The electric adjusting voltage applied to the individual filter elements is adjusted by means of the adjusting unit 7, for example on the basis of brightness values in the X-ray image and/or the setting of the X-ray source 2; to this end, the adjusting unit is coupled to the output terminal 10 of the video camera and to the power supply 11 of the X-ray source 2. The construction of an X-ray filter 4 of this kind and the composition of the X-ray absorbing liquid are described in detail in the International Patent Application No. 1B95/00874). FIG. 2 is a side elevation of an X-ray filter 4 of the X-ray examination apparatus of FIG. 1. The Figure shows seven capillary tubes by way of example, but a practical embodiment of an X-ray filter 4 of an X-ray examination apparatus in accordance with the invention may comprise a large number of capillary tubes, for example 40,000 tubes in a 200.times.200 matrix arrangement. Each of the capillary tubes 5 communicates with the X-ray absorbing liquid 6 via an end 31. The inner side of the capillary tubes is covered by an electrically conductive layer 37, for example of gold or platinum which layer 37 is coupled to a voltage line 34 via a switching element 33. For application of the electric adjusting voltage to an electrically conductive layer 37 of a capillary tube, the relevant switching element 33 is closed while the voltage line 34 which thus electrically contacts the capillary tube has been adjusted to the desired electric adjusting voltage. The switching elements are driven by a control line 35. When brief voltage pulses having a length of a few tens of microseconds are used, adjusting voltages in a range of from 0 V to 400 V can be used. In this voltage range switches in the form of .alpha.-Si thin-film transistors can be used. Preferably, an adjusting voltage in the range of from 30 V to 100 V is used. Because the voltage pulses are so brief, the application of the adjusting voltage does not cause any, or hardly any, electrolysis of the lead salt solution used as the X-ray absorbing liquid. The X-ray absorptivity of the individual capillary tubes can be controlled on the basis of the period of time during which the electric adjusting voltage is applied to the capillary tubes. Each of the capillary tubes, notably the conductive layer 37 and the X-ray absorbing liquid in the capillary tube, constitutes a capacitor. During the filling of such a capillary tube with the X-ray absorbing liquid, the capacitance of said capacitor varies as a function of the level of the liquid in the capillary tube or, in other words, as a function of the relative filling of said capillary tube. The charging of the capacitor produces electric energy for filling the capillary tube with the X-ray absorbing liquid. The longer the electric adjusting voltage remains applied, the further the capacitor is charged and the more the tube is filled with the X-ray absorbing liquid. On the electrically conductive layer there is preferably provided a dielectric layer of a thickness which suffices to ensure that the electric capacitance of the capillary tubes remains low enough to enable fast response to the application of the electric voltage. In order to ensure that the contact angle between the X-ray absorbing liquid and the inner side of the capillary tubes varies, as a function of the applied electric voltage, in a range of values which includes the contact angle value 90.degree., for example a coating layer having suitable hydrophilic/hydrophobic properties is provided on the dielectric layer. Use is preferably made of metal capillary tubes whose inner side is covered by successively the dielectric layer and the coating layer. The electric voltage can then be applied to the metal of the tubes. The manufacture of an embodiment of this kind is easier than providing glass capillary tubes with a metal coating. When a teflon layer is used as the dielectric layer covering the inner side of a metal tube, a separate coating layer can be dispensed with. FIG. 3 is a plan view of an X-ray filter 4 of the X-ray examination apparatus shown in FIG. 1. An X-ray filter 4 comprising 16 capillary tubes in a 4.times.4 matrix arrangement is shown by way of example; however, in practice the X-ray filter 4 may comprise a much larger number of capillary tubes, for example 200.times.200 tubes. Each of the capillary tubes is coupled, by way of the electrically conductive layer 37, to the drain contact 40 of a field effect transistor 33 which acts as a switching element and whose source contact 41 is coupled to a voltage line. For each row of capillary tubes there is provided a control line 35 which is coupled to the gate contacts of the field effect transistors in the relevant row in order to control the field effect transistors in this row. The control line 35 of the relevant row is energized by an electric control voltage pulse in order to apply an adjusting voltage to the electrically conductive inner side of the capillary tubes in the row, so that the field effect transistors in the relevant row are electrically turned on during the control voltage pulse. The adjusting unit 7 comprises a voltage generator 27 for applying an electric voltage to the timer unit 8 which applies the control voltage pulses having the desired duration to the individual control lines of the rows of capillary tubes. While the relevant field effect transistors are turned on, i.e. the switching elements are closed, the electric adjusting voltage of the relevant control lines 34 is applied to the capillary tubes. The periods of time during which the electric adjusting voltage is applied to individual capillary tubes in a row can be differentiated by application of the electric adjusting voltage to the respective voltage lines 34 of individual columns for different periods of time. To this end, the adjusting unit 7 comprises a column driver 36 which controls a period during which the electric adjusting voltage generated by the voltage generator 27 is applied to the individual voltage lines. The electric adjusting voltage is applied to a contact 43 via a switch 42. Each of the voltage lines 34 is coupled to a respective switching element, for example a transistor 44, by way of the contact 43. When the transistor 44 of the voltage line 34 is turned on by energizing the gate contact of the relevant transistor by means of a gate voltage, the adjusting voltage is applied to the voltage line. The gate contacts of the transistors 44 are coupled, via a bus 45, to the voltage generator 27 which supplies the gate voltage. The period of time during which the individual voltage lines are energized by the adjusting voltage is controlled by way of the period during which the gate voltages are applied to the gate contacts of the individual transistors 44. A large effective surface area with adhesion to the X-ray absorbing liquid is realized by providing filter elements with a plurality of capillary tubes. The quantities of X-ray absorbing liquid in capillary tubes of one and the same filter element, which may be coupled to one and the same transistor in their control line, of course, cannot be separately controlled. FIGS. 4 and 5 show diagrammatically, for two different ways of adjusting the X-ray Filter 4, the variation of control voltage pulses applied to the X-ray filter 4. As is shown in FIG. 4, first a control voltage pulse V.sub.1 of duration .tau..sub.1 is applied to the control line of the first row of capillary tubes; subsequently, control voltage pulses V.sub.2,V.sub.3 and V.sub.4 of a duration .tau..sub.2, .tau..sub.3 and .tau..sub.4, respectively, are applied to control lines of the second, the third and the fourth row of capillary tubes, respectively. The capillary tubes in the respective rows are thus successively filled with the X-ray absorbing liquid to a level which is dependent on the period of time during which the relevant voltage line is excited in the period in which a control voltage is supplied. The periods .tau..sub.i (i=1, 2, 3 . . . ) amount to approximately one millisecond, so that a few tenths of a second are required to adjust an X-ray filter 4 comprising a few hundred rows of capillary tubes; the adjusting time t.sub.f of the X-ray filter 4 thus mounts to a few tenths of a second. FIG. 4 also shows the X-ray absorptivity of capillary tubes in the respective rows .alpha..sub.x as a function of time. The X-ray absorptivity is related directly to the relative quantity of liquid in the capillary tubes. When the control voltage pulse V.sub.1 is applied to the first row, the capillary tubes become filled with the X-ray absorbing liquid and the X-ray absorptivity increases because the capillary tube is electrically charged. Filling takes place with some delay relative to the control voltage pulse, because some time is required for application of the electric charge (to charge the capacitance) and for the subsequent inflow of the X-ray absorbing liquid. Ultimately, the X-ray absorptivity in the first row reaches the value .alpha..sub.1, being the maximum value of the X-ray absorptivity that can be reached in the first row; lower values can be adjusted by applying the adjusting voltage to relevant columns for a period of time which is shorter than the duration of the control voltage pulse. After the voltage pulse V.sub.1, the second and subsequent rows receive successive control voltage pulses V.sub.2, V.sub.3, V.sub.4, having durations .tau..sub.2, .tau..sub.3, .tau..sub.4, respectively, so that in the second and subsequent rows maximum X-ray absorptivities .alpha..sub.2, .alpha..sub.3, .alpha..sub.4 can be reached. The X-ray absorptivities of filter elements in the rows are adjusted to different values by way of the period of time during which the voltage lines of the individual columns are energized. Because of the inertia of the inflow of the liquid, the durations of the control voltage pulses in this embodiment cannot be substantially shorter than a few milliseconds; however, the major advantage of this method of adjustment resides in the simplicity of the switching procedure which can be carried out by means of a simple timer unit. Because the adjusting time is shorter than one second, the filter setting, as it is controlled on the basis of the electronic image signal, follows movements in or of the object which have a duration of more than approximately one second. Such movements may be, for example movements of the patient or be caused by respiration, cardiac action or peristaltic movements of the patient. A particularly advantageous method of adjusting the X-ray filter 4 will be described in detail with reference to FIG. 5. According to this method all rows of the X-ray filter 4 are activated a number of times (n) in succession by control voltage pulses. A setting involving three repeats (n=3) will be described with reference to the Figure. During the first activation first a control voltage pulse V.sub.1.sup.1 of duration .tau..sub.1.sup.1 is applied to the control line of the first row; furthermore, control voltage pulses V.sup.1.sub.2, V.sup.1.sub.3, V.sub.4.sup.1, having a duration .tau..sub.2.sup.1, .tau..sub.3.sup.1, .tau..sub.4.sup.1, respectively, are applied to the second and subsequent rows. The control voltage pulses are successively applied to the respective rows, so that a control voltage pulse is applied to a row always after termination of a control voltage pulse for the preceding row. During this first activation period capillary tubes in the first and then in the second and subsequent rows become filled with the X-ray absorbing liquid, at least in as far and for as long as the relevant voltage lines carry an adjusting voltage. The periods .tau..sub.i.sup.j amount to approximately one pulse period t.sub.p =t.sub.f /Nn, where N denotes the number of rows. t.sub.p =25 .mu.s for N=200, n=20 and t.sub.f =0.1 s. Subsequently, during a second activation period control voltage pulses V.sup.2.sub.1, V.sup.2.sub.2, V.sup.2 .sub.3, V.sup.2.sub.4 having durations .tau..sup.2.sub.1, .tau..sup.2.sub.2, .tau..sup.2.sub.3, .tau..sup.2.sub.4, are applied to respective rows so that the filling of the capillary tubes continues. Finally, during the third activation period control voltage pulses V.sup.3.sub.1, V.sup.3.sub.2, V.sup.3.sub.3, V.sup.3.sub.4, having durations .tau..sub.1.sup.3, .tau..sub.2.sup.3, .tau..sub.3.sup.3, .tau..sub.4.sup.3, are applied. Because the control pulses are applied, the capillary tubes are filled with the X-ray absorbing liquid in a phased fashion and the X-ray absorptivity also increases in a phased fashion; the X-ray absorptivity remains approximately constant between the successive control voltage pulses. After termination of the control voltage pulse V.sup.j.sub.i, in the i.sup.th row an X-ray absorptivity .alpha..sub.i.sup.j is reached and the next control voltage pulse V.sub.i .sup.j+1 increases the X-ray absorptivity to .alpha..sub.1.sup.j+1 until ultimately, after the control voltage pulse V.sup.3 .sub.i,the value .alpha..sub.i is reached. The capillary tubes in the k.sup.th row are thus filled with a quantity of X-ray absorbing liquid which is controlled on the basis of the overall duration t.sub.k =.tau..sub.k.sup.1 +.tau..sub.k.sup.2 +.tau..sub.k.sup.2 +. . . +.tau..sub.k.sup.n of the control voltage pulses applied to the k.sup.th row. Because the capillary tubes in different rows are filled partly simultaneously, the adjusting time is reduced and, because the electric charges are delivered in fractions, the durations of the control voltage pulses can be reduced as the number of sampling periods is taken to be larger. A further advantage consists in that more time is available for the filling of the capillary tubes in the rows which are filled last. Furthermore, in comparison with the adjustment of the X-ray filter 4 of FIG. 4, a smaller time difference exists between the filling of the capillary tubes in the first rows and those in the last rows. The adjustment of the X-ray filter has been explained with reference to the FIGS. 4 and 5 for an X-ray filter comprising only four rows of capillary tubes and involving only three activation repeats by means of control voltage pulses. Evidently, to those skilled in the art it will be obvious that the method in accordance with the invention can be used equally well for an X-ray filter with a large number of rows, for example hundreds of rows, and with a large number of, for example from some tens to some hundreds of repeated activation periods. In FIG. 3 each capillary tube is coupled to a control line via a respective transistor; it is alternatively possible to couple a plurality of capillary tubes together to a control line via one transistor. In a contemporary X-ray examination apparatus the functions of the adjusting unit can also be executed by a suitably programmed computer or by a microprocessor designed for this purpose.