IL95401A - Slit radiography apparatus - Google Patents

Description

Slit radiography apparatus Abstract A slit radiography apparatus is provided with an absorption device which operates in conjunction with a slit diaphragm. The absorption device comprises electrically controllable piezoelectric tongues. A control device provides electrical control signals for the tongues. A damping device is present for each tongue to receive the respective control signals for the tongues. It comprises for each tongue an EMF measuring circuit and provides, during operation, an output signal which represents the counter-EMF generated by the relative tongues. The relative output signals are combined with the control signals for the tongue concerned and act to counteract any uncontrolled vibrations of the tongues.

Title; Slit radiography apparatus The invention relates to a slit radiography-apparatus , provided with an absorption device which operates in conjunction with a slit diaphragm and which comprises electrically controllable piezo-electric tongues, and also with a control device which provides electrical control signals for the tongues.

Such an apparatus is disclosed, for example, by the Dutch Patent Application 8400845. The known apparatus comprises an X-ray source which is capable of scanning a patient or object to be examined with a flat fan-shaped X-ray beam via a slit-type diaphragm. In order to obtain an equalized X-ray image, there is provided near the slit-type diaphragm an absorption device which comprises a number of adjacently situated absorption elements which can be inserted to a greater or lesser degree in the X-ray beam under the control of electrical control signals .

Each absorption element is capable of affecting a sector of the fan-shaped X-ray beam. The control signals are obtained by means of a detector which is provided behind the patient or the object and which measures the amount of radiation transmitted per sector of the X-ra beam and provides appropriate electrical control signals.

As absorption elements, use may advantageously be made of tongues of piezoelectric material which are clamped at one end and whose other end can be swivelled into the X-ray beam under the control of the above-mentioned electrical control signals. Such tongues already absorb X-ray radiation to a certain degree themselves, but they can be provided at "the" free ends with special elements which absorb X-ray radiation.

The tongues may be simple tongues of piezoelectric material which can be brought to a curved configuration by means of an electrical control voltage applied between the top and bottom face. The tongues may also be - 2 - so-called bimorphous elements which are composed of two strips of piezoelectric material laid on top of each other. The electrical control voltage is then between the outside faces (top and bottom face) which are connected to each other and the common central face .

It is important that the tongues react rapidly and accurately to the electrical control signals. It has been found that the tongues may commence resonance vibration, as a result of which the tongues become uncontrollable and may even break. Although such uncontrolled vibrations can be prevented by filtering the control signals concerned, the use of a filter suitable for this purpose also results in a relatively sluggish control of the tongues.

There is therefore a need for a facility for damping the movement of piezoelectric tongues of a slit radiography apparatus in an effective way, with a rapid reaction of the tongues to the control signals being maintained.

For this purpose, according to the invention, an apparatus of the type described is characterized by a damping device which receives the control signals for the tongues and which comprises for each tongue an E F measuring circuit which interacts with it and which provides, during operation, an output signal which represents the counter-EMF generated by a tongue, which output signal is combined with the control signal for the tongue concerned.

It is pointed out that European Patent Applica-tion 0,155,065, describes various i methods for damping tongue-type absorption elements or tongue-type carriers of absorption elements with the aid of mechanical means or with the aid of eddy currents generated in metal vanes attached to the tongues. However, these known techniques are less suitable for tongues of piezoelectric material, which is relatively brittle, and require, in addition, an appreciable amount of extra space in the absorption device .

The invention will be described below in more detail with reference to the attached drawing of an exemplary embodiment.

Fig. 1 shows diagrammatically in side elevation an example of a slit radiography apparatus provided with an absorption device; Fig. 2 shows diagrammatically in side elevation an example of a piezoelectric tongue provided with an absorption element; Fig. 3 shows by way of example, in a block diagram, how the piezoelectric tongues can be controlled according to the invention; and Fig. 4 shows an exemplary embodiment of a part of Fig. 3.

Fig. 1 shows diagrammatically in side elevation an example of a (known) slit radiography apparatus provided with an absorption device. The apparatus shown comprises an X-ray source 1 which is capable of scanning a patient 4 or an object to be examined in the direction, indicated by arrows 5, transversely to the plane of the f n-shaped beam 3 via a slit-type diaphragm 2. An X-ray detector 6, which may comprise, for example, an X-ray film cassette or a concomitantly moving oblong X-ray image intensifier tube, picks up the radiation transmitted through the patient and effects the formation of the desired X-ray shadow image. The scanning in accordance with the arrows 5 takes place because, during operation, the X-ray source swivels about an axis, as indicated by an arrow 7, extending transversely to the plane of the drawing, preferably through the X-ray focus of the X-ray source.

In order to obtain an equalized X-ray picture, use is made of an absorption device 8 which is capable of modulating the X-ray beam per sector. The absorption device comprises adjacently situated piezoelectric tongues, also known as piezoceramic tongues because the tongues are manufactured from ceramic material having piezoelectric properties . An example of such a tongue is shown in Fig. 2. Each tongue is capable of affecting a particular sector of the X-ray beam 3. For this purpose - - the tongues are clamped at one end. As a result of supplying suitable electrical signals to a tongue it bends, with the result that the free end penetrates the X-ray beam. The necessary control signals are provided by a detector 9 which is situated, in this example, between the patient and the X-ray detector and which picks up the radiation transmitted through the patient at every instant and provides per sector of the X-ray beam electrical signals which are supplied via a processing circuit 10 to the corresponding tongues. As shown, the detector may be an oblong radiation detector which moves synchronously with the scanning movement of the X-ray beam but it can also be a two-dimensional stationary detector.

Fig. 2 shows diagrammatically in side elevation a piezoceramic tongue 11 of the bimorphous type composed of two strips 12, 13 of piezoelectric material mounted on each other. The strip shown is clamped in a carrier 14 near an end and, in this example, is provided with an absorption element 15 at the free end. Broken lines indicate a possible working position. The tongue is controlled by a control voltage Ve which is applied during operation between two terminals 16, 17, of which one is connected to the central face 18 between the two strips 12, 13 and the other to the two outside faces 19, 20 of the strips.

Fig. 3 shows, by way of example, a block diagram of a control circuit for the piezoceramic tongues .

The invention is based on the insight that a piezoelectric tongue generates, during operation, a counter-E F which is dependent on the curvature of the tongue. It should be possible to derive from this ounter-EMF a damping signal for the tongue concerned which is capable of preventing tongue resonance phenomena .

According to the invention, the counter-EMF of each tongue is continuously measured and an electrical signal derived from the instantaneous counter-EMF is subtracted from the control signal provided for the tongue concerned by the detector 9 after a possible preprocessing. All this is shown diagrammatically in Fig. 3 . During operation, the control signal provided by the detector 9 , optionally after a preprocessing, is present at the input 31 of the control circuit 30 shown. Said signal is fed via a feedback point 32 to an amplifier 33 whose output is fed to a EMF measuring circuit 34 still o be described in more detail. The EMF measuring circuit 34 is coupled to a piezelectric tongue which is diagrammatically indicated by the block 35 . In addition, the EMF measuring circuit forms, in a way still to be described in detail, a signal which is dependent on the instantaneous counter-EMF of the tongue concerned and which is fed with a negative sign to the feedback point 32 via a feedback amplifier 36 .

The counter-EMF is dependent on the movement of the tongue, with the result that the control circuit 30 forms a so-called motional feedback system.

According to the invention, a feedback signal representing the counter-EMF of a tongue during operation can be obtained with the aid of a circuit of the type shown, for example, in Fig. 4 .

Fig. 4 shows a bridge circuit having four branches 40 to 43 inclusive. Branch 41 comprises two connecting points 44 , 45 which are connected during operation to the tongue 46 to be controlled. The branches 42 and 43 both comprise a resistor R which may be, for example, 10 kn in a practical situation.

Assuming a component Q which exhibits a behaviour corresponding to the behaviour of a stationary tongue is connected in the branch 40 , the bridge can be used to measure the counter-EMF of the tongue 46 . For this purpose, the output signal provided by the amplifier 33 is fed between the vertices 47 (junction point of the branches 40 and 41 ) and 48 (junction point of the branches 42 and 43 ) . Said signal also controls the tongue 46 . A feedback signal which is a measure of the counter-EMF of the tongue 46 is produced between the other vertices 49 and 50 of the bridge circuit. Said signal is fed to a - 6 - difference amplifier which corresponds to the feedback amplifier 36 of Fig. 3 or can be conceived as incorporated therein.

The component Q may be a capacitor because piezo-ceramic tongues are capacitive to a first approximation. Preferably, however, a piezoceramic tongue similar to the tongue to be controlled is used as component Q. The tongue used as component Q is, however, fully clamped, with the result that it cannot yield a counter-EMF generated by movement. The signal present between the vertices 49 and 50 is then due entirely to the counter-EMF of the tongue 46. Since, as has been pointed out, the tongues are capacitive and the other branches 42, 43 of the bridge in the example shown contain resistors R, the output signal of the bridge is proportional to the differentiated input signal of the bridge. The feedback signal is therefore proportional to the velocity of movement of the tongue 46. The movement of a tongue can be controlled very well with such a feedback signal, while a rapid response of the tongue to control signals is nevertheless guaranteed.

It is pointed out that, after the above disclosure, various modifications are obvious to the person skilled in the art. Thus, for example, the resistors R in the bridge circuit can be replaced by capacitors. In that case, the output signal of the bridge is not proportional to the velocity of movement but to the time integral thereof, namely deviation of the free tongue end. This and similar modifications are considered to fall within the scope of the invention. - 7 -

Claims (6)

1. Slit radiography apparatus, provided with an absorption device which operates in conjunction with a slit diaphragm and which comprises electrically controllable piezo-electric tongues, and also with a control device which provides electrical control signals for the tongues, characterized by a damping device which receives the control signals for the tongues and which comprises for each tongue an EMF measuring circuit which interacts with it and which, provides, during operation, an output signal which represents the counter-EMF generated by a tongue, which output signal is combined with the control signal for the tongue concerned.

2. Apparatus according to Claim 1, characterized in that the damping device for each tongue comprises a control circuit having an input for the control signal for the tongue concerned, and a feedback point to which the output signal from the EMF measuring circuit is fed with negative sign via a feedback amplifier, and having an amplifier which feeds the output signal from the feedback point to the input of the EMF measuring circuit, and in that the measuring circuit also provides the damped control signal for the tongue.

3. Apparatus according to Claim 1 or 2, characterized in that the EMF measuring circuit comprises a bridge circuit having two parallel sets, connected between two input terminals, of two bridge branches each connected in series via an output junction point, the one set of bridge branches having a first bridge branch which comprises a component representing the electrical behaviour of a piezoelectric tongue and a second bridge branch which contains a resistor and the second set of bridge branches comprising a third bridge branch which contains the tongue to be controlled, and a fourth branch which contains a resistor.

4. Apparatus according to Claim 3 , characterized in that the component representing the electrical behaviour of a piezoelectric tongue is a capacitor.

5. Apparatus according to Claim 3, characterized in - 8 - that the component representing the electrical behaviour of a piezoelectric tongue is a stationary clamped piezoelectric tongue.

6. Apparatus according to one of Claims 3 to 5 inclusive, characterized in that the second and fourth bridge branches contain a capacitor instead of a resistor.