X-ray apparatus and method for controlling the movement of an x-ray apparatus

An X-ray apparatus is provided. The Xpray apparatus has a C-arm on which an X-ray source and an X-ray detector can be mounted opposite one another, at least one actuator for positioning the C-arm relative to a patient table, and a control device for controlling the actuator. The X-ray apparatus includes at least one sensor which detects a deformation of the C-arm at a first position of the C-arm and converts it into an output signal, the deformation of the C-arm being influenceable by a force exerted by an operator and applied directly or indirectly at a second position on the C-arm. The control device influences the actuator as a function of the output signal of the sensor. A method for controlling the movement of an X-ray apparatus is also described.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German application No. 10 2011 005 492.8 filed Mar. 14, 2011, which is incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present invention relates to an X-ray apparatus having a C-arm on which an X-ray source and an X-ray detector can be mounted opposite one another, at least one actuator for positioning the C-arm relative to a patient table, and a control device for controlling the at least one actuator.

BACKGROUND OF INVENTION

Medical diagnostic and interventional systems in angiography, cardiology and neurology nowadays often use X-ray machines or X-ray devices as a basis for imaging. The X-ray machines are often equipped with a so-called C-arm. A C-arm mainly comprises an X-ray source and, disposed opposite it via a C-shaped, usually metal connecting structure, an X-ray detector. The C-arm can be stand- or ceiling-mounted. A plurality of movement axes, which can also be motorized, allow flexible positioning of the X-ray source and X-ray detector relative to an object under examination, e.g. a patient lying on the examination table. The positioning of the C-arm and of the components mounted thereon is also known as C-arm travel.

In the case of a diagnostic examination or a medical intervention, several people such as doctors, nurses or other medical technicians are often working in the examination room. By way of example,FIG. 1shows a setup in which a patient1016an examination table1015is being examined using an X-ray apparatus1001comprising a C-arm1002which is in turn mounted on a stand1014. A usual work sequence requires a structured division of responsibilities of the medical personnel involved and an expedient subdivision of the treatment room into different working areas of the respective personnel. For example, there tends to be a treating doctor1004at the side of the patient1016, a nurse1003next to the treating doctor1004, an anesthesiologist1005has his/her sphere of action at the head of the patient1016, an assistant nurse1006has the largest radius of action and operates in the working area1007. It is clear that during the examination the radii of action of the persons may change, possibly also overlap. In order to ensure a smooth treatment sequence, the position of the X-ray apparatus must be adapted to suit the changed conditions. In addition, different imaging positions require repositioning of the C-arm1002. In the case of a freely positionable medical X-ray system, it can happen that very complex movements are necessary for re-positioning which require well trained personnel to execute same quickly and precisely using normal controls such as joysticks or pushbuttons1010disposed on the examination table. In addition, the situation may arise that simple movements such as changing the height of an X-ray detector could be carried out by personnel who, however, are far away from the controls1010.

Hitherto, the position and attitude of the C-arm has been controlled by operator control modules mainly designed as joysticks and/or pushbuttons. The modules are mounted on the examination table or patient positioning table, on a trolley or in a separate room, the control room. In addition, there are fixed operator control switches with limited positioning capability which are mainly disposed on the X-ray detector. Additional C-arm system levers or handles with weight-compensated and smooth-acting mechanisms are used to extricate the patient in emergencies, e.g. in the event of system failure. In industrial applications, wireless operator control modules connected to the system via a radio link are known. The common feature of all these solutions is that an operator control is permanently mounted on the system at a particular location or must be carried by the user.

DE 10 2009 004 766 A1 discloses an X-ray device whose component parts are adjusted using a miniature model of the X-ray device, wherein manipulations of a model component part are translated into an adjustment of the corresponding component part. The disadvantage of this arrangement is that the miniature model is disposed at a central location and is usually operated only by one person.

SUMMARY OF INVENTION

The object of the invention is to propose an X-ray apparatus allowing decentralized and intuitive control of the movement of the X-ray apparatus. A method for controlling the movement of an X-ray apparatus shall also be specified.

This object is inventively achieved with an X-ray apparatus having the features set forth in the first independent claim and a method for controlling the movement of an X-ray apparatus having the features set forth in the second independent claim.

The basic concept of the invention is an X-ray apparatus having a C-arm on which an X-ray source and an X-ray detector can be mounted opposite one another, at least one actuator for positioning the C-arm relative to a patient table, and a control device for controlling the actuator. The X-ray apparatus comprises at least one sensor which detects a deformation of the C-arm at a first position of the C-arm and converts it into an output signal, said deformation of the C-arm being influenceable by a force exerted by an operator and directly or indirectly acting at a second position on the C-arm. The control device influences the actuator as a function of the sensor output signal.

The X-ray apparatus therefore comprises known components such as a C-arm, comprising an X-ray source and an X-ray detector, at least one actuator which, controlled by a control device, can move the C-arm to a desired position, said desired position relating to a patient positioning arrangement, e.g. in the form of an examination table on which a patient lies. The controlling of the actuator via the control device is now influenced such that an operator, e.g. a doctor, a nurse or a medical technician, applies a force at a point on the C-arm, i.e. the operator pushes against the C-arm at this location, causing the latter to bend or deform slightly not only at the point of application of the force. At usually another location on the C-arm, the deformation of the C-arm is measured using a sensor. The control device then controls the actuator taking into account the sensor output signal. The force can be exerted on the C-arm directly by the operator, i.e. the operator pushes against a point on the C-arm. However, the force can also be applied indirectly to the C-arm if, for example, the operator pushes against the X-ray detector, the X-ray source, or against another X-ray apparatus component fixedly connected to the C-arm.

The essential feature of the invention is that the positioning of the X-ray apparatus is no longer carried out centrally by an operator control device, but is initiated by measuring a deformation caused by pressure at one of a plurality of possible points on the C-arm.

A large part of the C-arm structure therefore becomes a kind of control, i.e. the limitation that the C-arm can only be controlled from one location, in some cases only by one person, no longer exists. By installing a small number of sensors, the implementation costs and complexity can be reduced compared to a solution achieving a comparable effect. Lastly, the operator control concept is intuitive, so that expensive training of the operating personnel, such as the training that would be required using e.g. pushbuttons or joysticks, is no longer necessary.

The control device can be an electronic computer. The actuator comprises, for example, electric motors, possibly with gears, hydraulic or pneumatic servo drives or a group of same. Actuators allow positioning of the C-arm, with translatory, rotatory or mixed translatory-rotatory motions being possible. The sensor(s) can be of the resistive, piezoelectric, optical, inductive or capacitive type.

The sensor is preferably implemented as a strain gage, in particular as a single-axis strain gage or multi-axis strain gage.

Strain gages are measuring devices for detecting tensile or compressive deformations. The most frequently used measuring effect is the change in the electrical resistance of a metallic material or of a semiconductor material when it deforms. Other types are capacitive, piezoelectric, photoelastic or mechanical strain gages. Metallic strain gages, which are often implemented as sensing grids made of resistance wire 3 to 8 μm thick on a thin plastic substrate, change their resistance even in the event of small deformations and are therefore best suited as strain sensors. Depending on the measurable direction of effect of stress states—single-axis, dual-axis, three-axis or spatial stress states—a distinction is drawn between single-axis and multi-axis strain gages. Other usable types are detailed e.g. in Karl Hoffmann, “An Introduction to Measurements using Strain Gages”, published by Hottinger Baldwin Messtechnik GmbH, Darmstadt, 1989.

Another advantageous embodiment of the invention provides that the influencing of the actuator by the control device brings about a change in the position of the C-arm, the direction of the change including a component in the direction of the force exerted by the operator.

In other words, the C-arm is moved by the actuator, e.g. a motor, in a direction in which the operator attempts to push the C-arm. As the direction of motion of the actuator is usually predefined by an axis, or rather a translatory travel possibility, it may be sufficient if the direction of motion coincides with a force direction component. As a result, the operator will have the impression that his/her applied force would move the C-arm essentially in the direction of the force. This constitutes a very intuitive method for motor-assisted C-arm travel. At the same time, the method improves safety, as the change in position is aligned such that the C-arm evades the force and the movement comes to a halt if the operator ceases to apply force to the C-arm. Safety is further improved in that, in the event of a collision or if the C-arm comes up against an obstacle, the C-arm deforms counter to the direction of travel and therefore also counter to the direction of the force applied by the operator, thereby slowing down or stopping the movement of the C-arm.

In an advantageous further development, the rate of change in position of the C-arm is a function of the size of the deformation of the C-arm, the size of the deformation of the C-arm being detected by means of the sensor. In particular, a positive correlation exists between the rate of change in position of the C-arm and the detected size of the deformation of the C-arm.

This means that the travel speed depends on the C-arm deformation detected by the sensor and that when a large deformation is detected, the C-arm travels a higher speed than when a smaller deformation is detected. This can be based e.g. on a linear relationship or on a deformation/velocity pair assignment in the manner of a look-up table.

Depending on the geometric ratios of the C-arm and the placement of the sensor, different relationships can exist between the size of the deformation and the magnitude of the force exerted by the operator. For example, a linear relationship can exist, i.e. the deformation of the C-arm is proportional to the force exerted. However, the deformation can also depend on the point at which the force is applied, e.g. a lever action takes effect.

This feature makes the travel even more intuitive, as it provides an easily comprehensible relationship between size of force and travel velocity.

The sensor is advantageously placed on the C-arm or rather incorporated in the C-arm such that, in the event of deformation of the C-arm, the change in the output signal of the sensor is greater than for other possible placements on the C-arm or possible incorporations in the C-arm.

Depending on the geometric ratios of the C-arm, when a force is applied externally, the deformation will be of varying size at different points of the C-arm, i.e. the sensor will measure different deformation sizes depending on its position on the C-arm. In order to obtain a large sensor output signal, it is advisable to determine the C-arm position which exhibits a large or rather the maximum deformation in the event of a particular application of force. This position can be determined by a series of measurements in which the sensor signal is measured at a plurality of positions. Another possibility is to determine, by structural analysis of the C-arm, positions which exhibit a greater deformation than other positions when a directional force is applied. Such a structural analysis can be performed by mathematical calculation or by computer-assisted analysis, e.g. based on a finite element method (FEM). In the structural analysis, which can result in a mechanical structural model of the C-arm, geometric aspects and material properties of the C-arm must particularly take into account. Sensor placement relates to both the position of the sensor and its orientation, as both parameters affect the sensor signal for a given direction of action. The sensor can be mounted on the C-arm, e.g. by adhesive bonding or other attachment method or attachment means, or it can be incorporated in the material of the C-arm. A criterion for selecting the sensor position can be not only high sensor sensitivity to deformations but also selectivity in respect of the direction of action. For example, it is desirable that deformation of the C-arm in a direction in which the C-arm cannot travel also does not produce an output signal. This becomes increasingly important the more movement axes an X-ray machine possesses. It is also advisable to place a plurality of single-axis sensors or at least one multi-axis sensor such that the direction of an acting force can be reconstructed. For example, a plurality of single-axis strain gages can be positioned such that they are sensitive in differing degrees to applications of force from different directions, and the direction of force can therefore be reconstructed by linearly combining the individual sensor signals. In addition, general requirements such as good accessibility, simple attachment possibilities, etc. play a role in sensor positioning.

The deformation of the C-arm is detected at least essentially by two sensors, the difference between the sensor output signals being fed to the control device.

The use of two sensors whose output signals are subtracted from one another generally has the advantage, depending on the positioning of the sensors, that the difference signal is less prone to interference due to e.g. voltage, temperature, moisture, sensor material variations or aging effects or that the difference signal has a better level, i.e. signal-to-noise ratio. When a first sensor and a second sensor, termed a reference sensor, is used, the reference sensor can be placed such that a deformation of the C-arm does not significantly affect the output signal of the reference sensor. However, the output signal of the reference sensor changes in the same way as that of the first sensor in the event of a change in the operating conditions, such as temperature or humidity or operating voltage. By taking the difference, the sensor signals caused by variations in the operating conditions are compensated, so that only the sensor signal change due to the deformation of the C-arm undergoes further processing. Another possibility is to place the sensors such that the deformation of the C-arm in the case of the reference sensor changes inversely to that in the case of the first sensor, e.g. a placement of the sensors such that the deformation of the C-arm causes the first sensor to lengthen and the reference sensor to shorten. The difference between the two sensor signals is again compensated for operating condition fluctuations and has an amplitude twice as large as that of an individual sensor.

In another advantageous embodiment, the sensors of the X-ray apparatus are disposed in opposite branches of a bridge circuit and the output voltage of the bridge circuit is fed to the control device.

A bridge circuit, H-circuit, H-bridge or specifically a Wheatstone bridge is a device for measuring small changes in resistance. Particularly when using sensors, it is employed as a measuring transducer. It is known from the prior art and described, for example, in U. Tietze and Ch. Schenk, “Halbleiter-Schaltungstechnik” (Electronic Circuits), 13th edition, Springer-Verlag, Berlin, Heidelberg, 2010, p. 1056 et seq. The bridge circuit is generally comprised of three resistors, also known as bridge resistors, at least one bridge resistor being represented by the resistive sensor. The resistors are connected to form a closed ring or rather a quadrilateral, wherein the voltage source is in one diagonal and the voltage difference, known as the diagonal voltage, bridge transverse voltage or bridge voltage, is measured in the other diagonal. With bridge circuits, a distinction is drawn between quarter bridges in which one bridge resistor is variable, half bridges in which two bridge resistors are variable, and full bridges in which four bridge resistors are variable. The non-variable bridge resistors are usually calibrated precision resistors having resistance values of the order of magnitude of the sensor resistors. When at least two sensor resistors are used, they are preferably arranged such that, in the event of C-arm deformation, the resistances change inversely, and similar effects, caused e.g. by temperature, voltage or humidity fluctuations, compensate one another.

Another advantageous embodiment of the invention provides that the control device for controlling the actuator takes into account the output signal of the sensor and an expected value for the output signal of the sensor, wherein the expected value for the output signal of the sensor can be calculated by means of a structural model of the C-arm and/or of the X-ray source and/or of the X-ray detector.

As a result of the mechanical flexibility of modern C-arm systems, small deformations or deflections of the C-arm occur even during normal travel processes, i.e. processes without e.g. additional force being applied by an operator or obstacles. As these deformations are also detected by the sensors, the control device of the X-ray apparatus cannot initially discriminate between these inherent deformations and a deformation caused by an externally applied force. With the aid of a structural model incorporating the geometrical aspects and material properties of the C-arm and associated components of the X-ray apparatus, an expected value for the output signal of the sensor without externally applied force can be calculated. The absolute value and the direction of the externally applied force can be inferred from the comparison, e.g. the difference between the expected value for the output signal of the sensor and the measured sensor output value. The structural model can comprise an analytical mathematical model whose parameters are provided by specifications, e.g. from material tables, or derived from measurements. The structural model parameters can represent physical features of the C-arm system, such as e.g. the weight of the X-ray detector or the module of elasticity of the C-arm. Also conceivable, however, are structural models whose parameters, so-called fitting parameters, are not directly assignable to physical features, and whose values are adjusted by series of measurements and optimization methods. For rotatory movement possibilities there often exists, to a first approximation, a sinusoidal relationship between angle of rotation and deformation. Other movements may also have linear, square-law and other relationships.

The calculation of the expected value for the output signal of the sensor advantageously takes into account a model of the mechanical structure of the C-arm and/or of the X-ray source and/or of the X-ray detector, and/or the material properties of the components of the C-arm and/or of the X-ray source and/or of the X-ray detector, and/or the position of the C-arm and/or of the X-ray source and/or of the X-ray detector, and/or the movement of the C-arm and/or of the X-ray source and/or of the X-ray detector, and/or the acceleration of the C-arm and/or of the X-ray source and/or of the X-ray detector.

The mechanical structure of the C-arm, the material properties of the components of the C-arm, the position and orientation of the C-arm, the movement of the C-arm and the acceleration of the C-arm affect the deformation of the C-arm, also independently of an external force. For example, if the C-arm is accelerated during a movement, the C-arm will bend because of the inertia of the accelerated mass. For an unambiguous interpretation of the sensor signal, this deformation is taken into account. In addition to the modeling of the C-arm, preferably components connected to the C-arm, e.g. the X-ray detector, the X-ray source, handles and other attachments, are also incorporated in the model, as these generally also contribute to the statics and dynamics of the X-ray system. It is generally the case that the quality of the expected value increases the more accurately the system and the dynamics of the X-ray apparatus are modeled. A structural model of the X-ray apparatus taking the dynamics into account can also be termed a dynamic model of the X-ray apparatus. In contrast to a force exerted by an operator, gravity, accelerational and inertial forces can be termed regular system forces.

Another basic concept of the invention relates to a method for controlling the movement of an X-ray apparatus having a C-arm on which an X-ray source and an X-ray detector can be mounted opposite one another, at least one actuator for positioning the C-arm relative to a patient table, and a control device for controlling the at least one actuator. At least one sensor detects a deformation of the C-arm at a first position of the C-arm and translates the deformation into an output signal, said deformation of the C-arm being influenceable by a force applied by an operator and directly or indirectly exerted at a second position on the C-arm. The control device influences the actuator as a function of the output signal of the sensor.

Particularly advantageously, the method for controlling the movement of an X-ray apparatus comprises the following steps:a) Detecting the measured value for the deformation of the C-arm at the first position of the C-arm using a measuring element comprising the at least one sensor, in particular at least one strain gage, wherein the deformation of the C-arm is influenceable by the force applied by the operator and exerted directly or indirectly at second position on the C-arm;b) Calculating an expected value for the measured value of the deformation of the C-arm at the first position of the said expected value being determined using a structural model or a dynamic model which takes into account the mechanical structure of the C-arm and/or of the X-ray source and/or of the X-ray detector, and/or the material properties of the components of the and/or of the X-ray source and/or of the X-ray detector, and/or the current position of the C-arm and/or of the X-ray source and/or of the X-ray detector, and/or the current movement of the and/or of the X-ray source and/or of the X-ray detector, and/or the current acceleration of the C-arm and/or of the X-ray source and/or of the X-ray detector;c) Taking the difference between the measured value for the deformation of the C-arm at the first position of the C-arm and the expected value for the measured value of the deformation of the at the first position of the C-arm;d) Calculating a control signal using a controller which is in particular a component part of the control device, such that an absolute-value increase in the difference is counteracted;e) Supplying the control signal for a final control element which is in particular a component part of the actuator;f) Repeating steps a) to e) until an abort criterion is fulfilled, in particular by actuation of the switch.

The examples described in greater detail below represent preferred embodiments of the present invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 2shows an exemplary embodiment of the X-ray apparatus1according to the invention. It comprises a C-arm2on which an X-ray source3and an X-ray detector4are mounted opposite one another. The C-arm2is connected to a stand14. The C-arm2can be moved in the z-direction, i.e. in the vertical direction9, by a first actuator5, e.g. an electric motor. The C-arm2can be rotated about an axis parallel to the y-axis, i.e. about a horizontal axis10, by a second actuator6, e.g. an electric motor with gearbox. The movement of the C-arm2changes the position of the X-ray source3and of the X-ray detector4relative to a patient table15, here an examination table, on which an object under examination16, here a patient, is lying. Mounted on the C-arm2are a first sensor11and a second sensor12which are implemented as strain gages. If an operator, e.g. a doctor, applies a first force7in the z-direction to the upper region of the C-arm2, i.e. pushes against the C-arm in the z-direction, the upper part of the C-arm2will deform slightly in the z-direction. The deformation produces a compressive effect at the position of the first sensor11. The compression is detected by the first sensor11and converted into an e.g. electrical output signal. The output signal is forwarded to a control device13, e.g. an electronic computer, which influences the first actuator5such that the C-arm2is moved in the direction19, i.e. in the direction of the z-axis. If the operator applies a second force8to the upper region of the C-arm2in the x-direction, i.e. pushes against the C-arm in the x-direction, the upper part of the C-arm2will bend slightly in the x-direction. The bending produces an extension effect at the position of the second sensor12. The extension is detected by the second sensor12and converted into an output signal. The output signal is forwarded to the control device13which influences the second actuator6such that the C-arm2executes a rotational movement20, i.e. a movement about an axis parallel to the y-axis. An operator control unit21, which is implemented e.g. as a switch, enables the movement to be halted. This illustrates the intuitive motion control of the X-ray apparatus2, which requires no specialized knowledge: lateral pressure8on the C-arm2causes the C-arm2to rotate in the direction20of the pressure until the desired position is reached and the operator ends the pressure8. The position of the point of pressure is not limited to the position indicated by the reference character8, but can vary across a range.

FIG. 3shows a side view of another example of part of the X-ray apparatus. The C-arm2again comprises the X-ray source3, the X-ray detector4, the actuator5and the sensor11. Pressure7on the C-arm2in the upper region of the C-arm in the z-direction causes the upper part of the C-arm to bend and assume the shape22, the X-ray detector4being displaced by the distance17. The bending of the C-arm is detected by the sensor11. A control device (not shown) connected to the sensor11receives the sensor signal and controls the actuator5such that the C-arm2is caused to travel in the direction19, i.e. in the z-direction.

FIG. 4shows a front view of the other example of part of the X-ray apparatus. The C-arm2again comprises the X-ray source3and the X-ray detector4. The actuator6can rotate the C-arm about the y-axis, the sensor12is placed and oriented on the C-arm2such that it can detect bending of the C-arm2in the direction of the x-axis. Pressure8on the C-arm2in the upper region of the C-arm in the x-direction causes the upper part of the C-arm to bend, the X-ray detector4being displaced by the distance18and assuming the position32. The bending of the C-arm is detected by the sensor12. The control device (not marked) is connected to the sensor12, receives the sensor signal and controls the actuator6such that the C-arm2is rotated in the direction20.

By way of example,FIG. 5is a front view of a section of the C-arm12showing the position and orientation of two sensors implemented as a single-axis strain gage. The Cartesian coordinate system fromFIG. 3andFIG. 4applies. A force acting in the x-direction and deforming the C-arm2in the x-direction is detected almost exclusively by the first strain gage11, i.e. bending of the C-arm2in the x-direction produces a resistance change in the first strain gage11, whereas the resistance value of the second strain gage12does not change significantly.

FIG. 6shows a side view of the section of the C-arm2known fromFIG. 5with the two sensors which are implemented as a single-axis strain gage. The Cartesian coordinate system fromFIG. 3andFIG. 4again applies. A force acting in the z-direction and deforming the C-arm2in the z-direction is detected almost exclusively by the second strain gage12, i.e. bending of the C-arm2in the z-direction produces a resistance change in the second strain gage12, whereas the resistance value of the first strain gage11does not change significantly.

FIG. 7shows another example of a C-arm2with X-ray source3, X-ray detector4, actuator6and sensor12. The C-arm2is rotated through a deflection angle θ31from the neutral θ=0° position. A Cartesian coordinate system with x-, y- and z-axis is associated with the C-arm2. In addition to gravity28, an external force8is applied to the X-ray detector4. The sensor12is implemented as a resistive strain gage system which can detect deformations of the C-arm2in the x-direction and in the z-direction. To measure deformation in the z-direction, a two-part strain gage sensor is used whose sensing surface is placed such that, in the event of deformation of the C-arm in the z-direction, the first part is extended whereas the second part is compressed, or the first part is compressed whereas the second part is extended.

A possible electronic circuit40for evaluating the x-direction-sensitive sensor12is shown inFIG. 8. The circuit40is a half bridge circuit operated at e.g. 3 to 15 VDC, characterized by an upper operating voltage41and a lower operating voltage42. The two measuring resistors44disposed diagonally in the bridge circuit represent the resistors of the x-direction-sensitive sensor12, their resistance values varying as a function of any deformation of the C-arm2fromFIG. 7in the x-direction. The resistors45of the electronic circuit40are implemented as calibrated precision resistors whose resistance values are in the order of the nominal resistance values of the measuring resistors44, e.g. 120Ω). The series-connected resistors44and45in each case form voltage dividers whose voltages at the midpoint nodes vary with the resistance value of the measuring resistors44. Due to the arrangement of the resistors44and45, the two midpoint node voltages behave in an inverse manner. The size of the deformation and the type of deformation, i.e. extension or compression, of the strain gage and hence the deformation of the C-arm2can be inferred from the voltage43which is measured between the potentials of the midpoint nodes and is termed the bridge output voltage.

FIG. 9shows a typical graphical analysis of the electrical properties of the circuit40fromFIG. 8. The graph shows the bridge output voltage43in volts (vertical axis46) versus the deflection angle θ31in degrees. The continuous line47corresponds to the bridge output voltage42of the x-direction-sensitive sensor12when no external force8is applied. The sinusoidal shape of the bridge output voltage42is explained by the proportion of gravity28fromFIG. 7acting in the x-direction. For a deflection angle θ=0° or θ=180°, the C-arm2is not deformed in the x-direction. The two measuring resistors44therefore have the same resistance value as the other resistors45and the bridge output voltage43is zero. For other deflections, the C-arm2deforms slightly due to gravity28, causing the bridge output voltage43to assume non-zero values. Depending on the direction of the deformation, the bridge output voltage43is greater or less than zero. For a deflection angle θ=30°, in the absence of an externally applied force a bridge output voltage48greater than zero is produced. If the external force8is applied to the C-arm2, a bridge output voltage49is produced which is less than the bridge output voltage48. This can be explained qualitatively in that, due to gravity28, the C-arm2is subjected to a deformation component in the negative x-direction which is partly compensated again by the effect of the force8, as the force8has a component which deforms the C-arm2in the direction of the positive x-axis. If the deflection angle θ and the response of the bridge output voltage47over the deflection angle θ without externally applied force are known, the external force8can be inferred by comparison with the measured bridge output voltage49. The output voltage47as a function of the deflection angle θ can be interpreted as a target curve which is compared with the measured actual values49. Any deviation of the actual value49from the target value48is fed to a control device, e.g. an electronic computer, which then controls the actuator6fromFIG. 7using a control algorithm, for example.

FIG. 10shows an embodiment of a circuit50for evaluating the z-direction-sensitive sensor12fromFIG. 7. The circuit50is implemented as a full-bridge circuit operated with a DC voltage, characterized by an upper operating voltage51and a lower operating voltage52. The two first measuring resistors54disposed diagonally in the bridge circuit represent the resistors of the first section of the z-direction-sensitive sensor12, whose resistance values change as a function of a z-direction deformation of the C-arm2fromFIG. 7. The second measuring resistors55of the electronic circuit50represent the resistors of the second section of the z-direction-sensitive sensor12, whose resistance values likewise change as a function of a z-direction deformation of the C-arm2. However, the way the sensor2is positioned causes the resistance values of the measuring resistors54and55to change in an inverse manner, since a deformation of the C-arm2in the z-direction produces an extending or compressing effect in the first section of the sensor12, with the opposite effects occurring in the second section of the sensor12. The series connected measuring resistors54and55each constitute voltage dividers whose voltages at the midpoint nodes vary with the resistance values of the measuring resistors54and55. In addition, due to the arrangement of the resistors54and55, the two midpoint node voltages also behave in an inverse manner in the event of deformation of the C-arm. The size of the deformation and type of deformation, i.e. extension or compression in relation to the z-axis, of the strain gage and therefore the deformation of the C-arm2can be inferred from the voltage53measured between the midpoint node potentials and referred to as the bridge output voltage.

InFIG. 11shows a typical graphical analysis of the electrical properties of the circuit50fromFIG. 10. The graph shows the bridge output voltage53in volts (vertical axis56) versus the deflection angle θ31in degrees. The continuous line57corresponds to the bridge output voltage42of the z-direction-sensitive sensor12when no external force8is applied. The sinusoidal shape of the bridge output voltage52is explained in that, for a deflection angle θ=0° or θ=180°, the C-arm2is deformed maximally in the z-direction, so that the two measuring resistors54and55have maximally different resistance values and the bridge output voltage53therefore assumes maximum values. For other deflections, the C-arm2deforms less due to gravity28, causing the bridge output voltage53to assume values between the maxima. For a deflection angle θ=90° or θ=270°, the bridge output voltage53is ideally zero. For a deflection angle θ=30°, in the absence of an externally applied force a bridge output voltage58is produced. If the external force8is applied to the C-arm2, a bridge output voltage59is produced which is greater than the bridge output voltage58. This can be explained qualitatively in that, due to gravity28, the C-arm2is subjected to a deformation component in the negative z-direction which is amplified still further by the effect of the force8, as the force8has a component which likewise deforms the C-arm2in the direction of the negative z-axis. If the deflection angle θ and the response of the bridge output voltage57over the deflection angle θ without externally applied force are known, the external force8can be inferred by comparison with the measured bridge output voltage59. The output voltage57as a function of the deflection angle θ can again be interpreted as a target curve which is compared with the measured actual values59. Any deviation of the actual value59from the target value58is fed to a control device, e.g. an electronic computer which then controls the actuator6fromFIG. 7using a control algorithm, for example.

FIG. 12schematically illustrates an exemplary embodiment for the sequence of force-assisted travel of a C-arm2. The C-arm2comprises an X-ray source3and an X-ray detector4. An external force8is applied by an operator (not shown) to an X-ray detector4which is thereby caused to move to a second position32while at the same time slightly distorting the C-arm2. A sensor system implemented as a strain gage12and a reference strain gage52detects the deformation of the C-arm2and, in a step61, transmits the signal representing the difference between the output signal of the strain gage12and the output signal of the reference strain gage52to the control device13. The control device13comprising, for example, an electronic computer calculates an expected value for the measured value of the deformation of the C-arm2at the position of the sensor system with the aid of a structural model or a dynamic model which takes into account the mechanical structure of the C-arm2, the material properties of the components of the C-arm2, the current position of the C-arm2, the current movement of the C-arm2and the current acceleration of the C-arm2. From the comparison of the expected value for the measured value of the deformation of the C-arm2with the actual measured value of the deformation, e.g. by taking the difference, the direction and the absolute value of the external force acting on the C-arm2at the position of the sensor or at another position on the C-arm2is calculated. From the information concerning the direction and absolute value of the external force, the control device13calculates the control signal for an actuator6such that an absolute value increase in the difference is counteracted, i.e. the structural model or the dynamic model also includes actuator modeling which goes into a control algorithm. The preferably electronic control signal is communicated to the actuator6in a step62. In a step63, the actuator6, which is implemented e.g. as a motor, translates the electronic control signal into a mechanical movement or rather a mechanical torque, causing the C-arm to travel in a direction20which comprises at least one directional component of the external force. The entire sequence is preferably repeated continuously or rather in rapid succession, e.g. fifty times per second, so that the travel process follows the movement requirements of the operator without noticeable delay. The process ends when, for example, an off-switch is actuated.

FIG. 13shows by way of example possible coordinate systems and motion vectors of a C-arm2comprising an X-ray source3and an X-ray detector4. To describe the position of the C-arm2in space, a local coordinate system is preferably used which is attached to a central point of the C-arm2, the so-called tool center point, TCP, and moves with the C-arm2, relative to a fixed measuring system. The local coordinate system attached to the C-arm2comprises the coordinate axes TCPx91, TCPy92and TCPz93, the reference system comprising the coordinate axes TCPφ94, TCPφ95and TCPγ96. The C-arm2can execute translatory movements in the direction of the axes of the coordinate system98or more specifically linear combinations thereof or also translatory movements in the directions of the movement arrows99. The C-arm2also has a movement component97whereby the so-called source-image distance, i.e. the distance between X-ray source3and X-ray detector4, can be varied.

Finally,FIG. 14schematically illustrates an exemplary embodiment of a method70for controlling the movement of an X-ray apparatus. Movement control should also be understood as meaning travel involving a closed chain of effects, i.e. strictly speaking closed-loop movement control. The system to be influenced, the controlled system, is the C-arm71, the influencing quantity81is an external force applied thereto, defined by a point of application, a direction of force and an absolute value of the force. As a result of the external force applied and the flexible C-arm71, the C-arm71will deform not only at the point at which the external force is applied. A measuring element72, implemented as a strain gage sensor, translates the physical measured variable ‘material extension’82, measured at a location which is generally different from the point of application of the external force, into an electrical measurement signal83. Using a structural model73or a dynamic model of the C-arm71, an expected value85for the electrical measured value of the deformation of the C-arm71at the measuring position of the sensor is calculated. The structural model73takes into account the mechanical structure of the C-arm71, material properties of the components of the C-arm71and the reference variables84, such as the current position of the C-arm71, the current movement of the C-arm71and the current acceleration of the C-arm71. Likewise incorporated in the structural model73are the mechanical structure, material properties and the dynamics of the components connected to the C-arm. A difference element74takes the difference86between the measured value83for the deformation of the C-arm71at the measuring position and the expected value85for the measured value of the deformation of the C-arm71at the measuring position. The difference86is a measure of the extent to which the external force deforms the C-arm in71in addition to the deformation of the C-arm71due to gravity and inertial forces. A controller75, which is preferably implemented as a control algorithm in an electronic computer, calculates from the difference86a controlled variable which is preferably fed as an electrical control signal87to a final control element76comprising an actuator, e.g. a motor. The final control element76acts on the C-arm71as a function of the control signal87with a force88or an angular momentum, thereby enabling the position of the C-arm71to change. The control algorithm is designed such that a change in the position of the C-arm71counteracts an absolute value increase in the difference86. The process is repeated continuously or in quick succession until an abort criterion, which can be in particular the actuation of an off-switch, is fulfilled.