Abstract:
The invention provides a device for a computer tomography gantry ( 91 ) for transmitting data, wherein the gantry comprises a stationary part ( 92 ) and a rotary part ( 93 ). The device is adapted to transmit the data between the stationary part of the gantry ( 92 ) and the rotary part ( 93 ) of the gantry ( 91 ). The device comprises a hollow conductor ( 104, 204, 308 ) which is adapted to guide a first wave, a sender ( 102, 103 ) which is adapted to send the first wave inside the hollow conductor and a receiver ( 106 ) which is adapted to receive the first wave after a runtime inside the hollow conductor ( 104, 204, 308 ). A further aspect of the invention is a computer tomography gantry ( 91 ) comprising a device according to the inventive concept. Using the described device allows to transmit data between the rotary ( 93 ) and the stationary part ( 92 ), to measure a rotating speed of the rotary part ( 93 ) and to measure a position of the rotary part ( 93 ) with respect to the stationary part ( 92 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device for a computer tomography gantry for transmitting data and a computer tomography gantry comprising such a device. 
       BACKGROUND OF THE INVENTION 
       [0002]    In X-ray image acquisition using a computer tomography device with a rotatable gantry, a precise measurement of a gantry position and speed is very important for a correct image reconstruction. In former systems, the rotation position and speed is measured by counting marks on the gantry rotor. This method may be of low quality with respect to the precision. As a result thereof the image processing of the computer tomography system may suffer because of deficits in detecting a precise position of a rotary part of the gantry as well as a precise rotation speed of the rotary part of the gantry with respect to a stationary part of the gantry. 
         [0003]    Furthermore, it may be desired to provide a wide-band transmission of data between the rotary part of the gantry and the stationary part of the gantry. 
       SUMMARY OF THE INVENTION 
       [0004]    It may be desirable to provide an improved device for detecting the position and rotation speed of the rotary part of the gantry as well as an improved arrangement to transmit data between the rotary part of the gantry and the stationary part of the gantry in order to enable a wide-band transmission. 
         [0005]    According to a first aspect, the invention provides a device for a computer tomography gantry for transmitting data, wherein the gantry comprises a stationary part and a rotary part, 
         [0000]    wherein the device is adapted to transmit the data between the stationary part of the gantry and the rotary part of the gantry. Therein, the device comprises a hollow conductor which is adapted to guide a first wave, a sender which is adapted to send the first wave inside the hollow conductor, and a receiver which is adapted to receive the first wave after a runtime inside the hollow conductor. 
         [0006]    According to a second aspect, the invention provides a computer tomography (CT) gantry comprising a device according to the above first aspect. 
         [0007]    It may be seen as a gist of the present invention to use a hollow conductor for precise measurement of the position and/or the rotation speed of a rotary part e.g. of a gantry of a CT device. The hollow conductor can be provided either on the stationary part or on the rotary part of the gantry. A sender may emit an electromagnetic wave into the hollow conductor. The electromagnetic wave may travel along the hollow conductor and may finally be received by a receiver. Therein, the sender may be provided on one of the stationary and the rotary part of the gantry whereas the receiver may be provided on the other of the stationary and the rotary part of the gantry. There may be one or more senders and one or more receivers. A sender and a receiver may be combined in a transceiver. Measuring characteristics of the transmitted wave such as for example a time-of-flight from the sender to the receiver, interferences, beat-detection or a Doppler-shift may enable to precisely detect a current position of the receiver relative to the sender thereby allowing to precisely determine a relative position and rotation speed of the rotary part of the gantry with respect to the stationary part of the gantry. This may result in improving an image processing of the computer tomography system. 
         [0008]    Furthermore, as another essence of the invention, a broadband transmission of data may be provided with the help of the hollow conductor. Therein, for example control data may be provided from a control located at the stationary part to for example an X-ray tube located at the rotary part by a data transmission from the sender to the receiver. Therein, the data may be transmitted contactless, i.e. without any mechanical contact between the sender and the receiver. 
         [0009]    Typically, the hollow conductor, also referred to as waveguide, may be realized with lateral surfaces which incorporate a volume. Usually, this volume is air-filled. The included electromagnetic waves will be reflected by the lateral surfaces in order to keep them in the hollow conductor. One of the lateral surfaces may be slotted along its length. This slot may render the possibility for extending a sender, a receiver and/or an antenna into the hollow conductor. 
         [0010]    Further embodiments are incorporated in the dependent claims. 
         [0011]    According to the present invention it is provided a device, wherein the hollow conductor is arranged at one of the stationary part and the rotary part of the gantry, wherein the gantry comprises an inner bore, wherein the hollow conductor is arranged around the inner bore of the gantry. 
         [0012]    In this situation a sender or a receiver is arranged at the rotary part of the gantry and the hollow conductor is arranged at the stationary part of the gantry. The hollow conductor may be a tubular ring having a slit along its circumference and being arranged concentrically with the gantry. The hollow conductor comprises at least one further sender, receiver, antenna, e.g. extending from the rotary part. The opposite arrangement is also possible, i.e. that the hollow conductor is arranged at the rotary part of the gantry. 
         [0013]    According to an exemplary embodiment it is provided a device wherein the device is adapted to transmit a second wave inside the hollow conductor, wherein the first wave runs clockwise and the second wave runs counterclockwise, wherein the device is adapted to measure the runtimes of the first wave and the second wave. 
         [0014]    Herein, the runtimes refer to the duration the first and the second wave need to come from the sender to the receiver. These runtimes and, in particular, differences between the runtime of the first wave and the runtime of the second wave may be used to determine the position and/or the rotational speed of the rotary part with respect to the stationary part of the gantry. 
         [0015]    According to the present invention it is provided a device wherein the device is adapted to calculate or determine the distance between the receiver and the sender by processing the runtimes of the first wave and the second wave. 
         [0016]    Therein, the distance between the receiver and the sender may refer to a distance along the travel path of the first and second waves. In other words, as the waves travel along the hollow conductor which itself is mounted to one of the stationary and the rotary part of the gantry, the determined distance between the receiver and the sender may indicate a relative position between the rotary part and the stationary part. This relative position may also indicate an angular position of the rotary part with respect to the stationary part. 
         [0017]    According to an exemplary embodiment it is provided an X-ray device, wherein the device is adapted to repeat the calculation or determination of the distance, wherein the device is adapted to calculate the rotation speed of the rotary part of the gantry. 
         [0018]    In other words, the distance between the sender and the receiver is repeatedly determined at different points in time and from the changes of the determined distances and the time between the respective determinations the rotation speed of the rotary part of the gantry may be determined. 
         [0019]    The measurement according to this concept may lead to precise results of the position and the rotation speed of the rotary part of the gantry. These precise results may have a positive impact on the whole image processing of the computer tomography system. 
         [0020]    According to an exemplary embodiment it is provided an X-ray device, wherein the first wave comprises data, wherein the device is adapted to receive the first wave after a runtime inside the hollow conductor, wherein the device is adapted to extract the data from the first wave after a runtime of the first wave in the hollow conductor. 
         [0021]    The data transmission with the help of the hollow conductor renders the possibility for a broadband transmission of data. Therein, data may be transmitted from the stationary part to the rotary part of the gantry or vice versa without the need for mechanical or electrical connection between the rotary part and the stationary part. Independent of the current rotational position and speed of the rotary part, data can be transmitted with a high data transmission rate of for example 8 bits/2 μs from the sender arranged at one of the stationary and the rotary part to the receiver arranged at the other of the stationary and the rotary part of the gantry. Thereby, additional to a wireless power transfer between the stationary and the rotary part of the gantry, an extra data link can be provided to control for example the voltage supplied to an X-ray tube arranged on the rotary part of the gantry. 
         [0022]    According to an exemplary embodiment it is provided an X-ray device, wherein the hollow conductor is adapted to dampen the first wave, wherein the hollow conductor is adapted to diminish the amplitude of the first wave after one circulation or revolution around the hollow conductor considerably. 
         [0023]    The damping of the first wave traveling within the hollow conductor should be such that undesired reflections or interferences of waves running around the circular hollow conductor several times are essentially suppressed. For example, the amplitude of a wave having traveled once around the hollow conductor should be clearly distinguishable from the amplitude of a wave having traveled twice around the hollow conductor. 
         [0024]    According to another exemplary embodiment it is provided an X-ray device, wherein the hollow conductor comprises a material, which is adapted to diminish the amplitude of the first wave after one circulation around the hollow conductor considerably. In other words, the characteristics of a material forming the hollow conductor may be such that a wave traveling within the hollow conductor is considerably dampened thereby diminishing its amplitude. For example, the material of the hollow conductor may have a sufficiently low electrical conductivity. 
         [0025]    According to another exemplary embodiment it is provided an X-ray device, wherein the hollow conductor is at least partly coated in the interior zone. 
         [0026]    For example, the hollow conductor may comprise a highly electrically conducting base material which is coated with a material having low electrical conductivity and/or having a high damping factor. Alternatively, the base material of the hollow conductor may be electrically non-conducting and may be coated with a material of low electrical conductivity. 
         [0027]    According to an exemplary embodiment it is provided an X-ray device, wherein the coating of the hollow conductor is homogeneous. This means that at least a part of the coating may be of similar, especially uniform, chemical composition. 
         [0028]    The damping can be achieved by using such a material for the construction of the hollow constructor, which is characterised by damping waves. Another possibilty is the coating of a part of the interior zone of the hollow conductor. It is also possible to coat the whole area of the interior zone. Usually the coating is homogeneous. But it is also possible that the coating differs along the hollow conductor. 
         [0029]    According to an exemplary embodiment it is provided an X-ray device, wherein the hollow conductor is adapted to diminish the amplitude of the first wave with a first frequency after one circulation around the hollow conductor considerably, wherein the hollow conductor is adapted not to diminish the amplitude of a second wave with a second frequency after one circulation around the hollow conductor considerably, wherein the first frequency is different to the second frequency. 
         [0030]    According to another exemplary embodiment it is provided an X-ray device, wherein a side surface of the hollow conductor is closed. 
         [0031]    According to an exemplary embodiment it is provided an X-ray device, wherein the hollow conductor is ring-shaped. 
         [0032]    According to another exemplary embodiment it is provided an X-ray device, wherein the hollow conductor is a slotted hollow conductor also referred to as slotted wave-guide. 
         [0033]    According to another exemplary embodiment, the device is adapted to determine a relative position between the stationary part and the rotary part based on a measurement of the first wave running from the sender to the receiver. 
         [0034]    For example, the receiver is arranged at the stationary part of the gantry whereas the sender is arranged on the rotary part of the gantry. A rotation angle in between the receiver and the sender may be described as time-dependent angle α(t). A variation in this angle α(t) may introduce a small variation in a delay of data transmitted between the sender and the receiver. As the electromagnetic wave travels in the hollow conductor at the speed of light, i.e. at approximately 3×10 8  m/s, and because the circumference of a typical gantry is for example 3.8 m, a maximum delay may be approximately 6.3 ns when the sender and the receiver are 180° apart. Measuring this delay may reveal a rotational angle between the sender and the receiver. This may be interpreted as the well-known time-of-flight principle. State-of-the-art electronics should be able to measure the delay with picosecond resolution, resulting in an angular resolution of typically 0.03° (1 ps/6.3 ns×180°. This may be about three times better than the resolution of a resolver that is conventionally being employed for measuring rotational angles. Instead of using a plain time-of-flight principle, one could also use interference, beat-detection or the Doppler-shift to measure the angular position or speed of the parts of the gantry with respect to each other. 
         [0035]    According to another exemplary embodiment, the sender and the receiver are both provided at the same one of the stationary part and the rotary part. The sender and the receiver may be provided as one single component referred to as transceiver. A reflector is provided at the other of the stationary part and the rotary part. Then, the device may be adapted to determine a relative position between the stationary part and the rotary part based on a measurement of the first wave running from the sender to the reflector and back to the receiver. 
         [0036]    In this specific embodiment, it may not be necessary to actually transmit data from the sender to the receiver as both, sender and receiver, are arranged at the same part of the gantry. Instead, a simple signal can be emitted by the sender in a direction towards the reflector. At the reflector, this signal is reflected and travels back to the receiver. Using such arrangement with a reflector, the distance of the travel path of the wave is doubled thereby reducing the requirements for the time resolution of a detection circuit. In other words, doubling the distance traveled by the electromagnetic waves may result in halving the required resolution. 
         [0037]    According to another exemplary embodiment, the device comprises two receivers and an electromagnetic shield is positioned between the two receivers. A first receiver is adapted to detect a first wave running from the sender to the first receiver in a clockwise direction and a second receiver is adapted to detect a second wave running from the sender to the second receiver in a counter-clockwise direction. 
         [0038]    In other words, the bi-directional propagation of waves through a circular hollow conductor or waveguide, i.e. propagation in both the clockwise (CW) and the counter-clockwise (CCW) direction, may be used to determine the relative position of the sender and the receiver. While it may be difficult for a single receiver to distinguish between receiving the clockwise traveling wave and receiving the counter-clockwise traveling wave, it may be advantageous to use two receivers in between which there is an electromagnetic shield. The electromagnetic shield may be adapted to block or at least to significantly dampen the transmission of an impacting electromagnetic wave. In this way, there may be a dedicated receiver for each electromagnetic wave, clockwise and counter-clockwise, making it easier to discern the waves for an electronic circuitry. 
         [0039]    It should be noted that the above features may also be combined. The combination of the above features may also lead to synergetic effects, even if not explicitly described in detail. 
         [0040]    These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    Exemplary embodiments of the present invention will be described in the following with reference to the following drawings. 
           [0042]      FIG. 1  shows a side view onto a hollow conductor for an X-ray device and a diagram of intensities, 
           [0043]      FIG. 2  shows a side view onto a hollow conductor for an X-ray device, 
           [0044]      FIG. 3  shows a cross section of a hollow conductor arranged at a transformer for an X-ray device, 
           [0045]      FIG. 4  shows a side view onto a specific hollow conductor for an X-ray device, 
           [0046]      FIG. 5  shows a side view on another specific hollow conductor for an X-ray device; 
           [0047]      FIG. 6  shows a circuitry for measuring an angular position in a computer tomography gantry, 
           [0048]      FIG. 7  shows a perspective view of a computer tomography gantry. 
       
    
    
       [0049]    The drawings in the figures are only schematically and not to scale. Similar components are indicated with similar reference signs throughout the figures. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0050]    In a rotation system, e.g. a CT system gantry, round, closed and slotted hollow conductors (waveguides) can be used for low latency or response-time free data transfer between rotating and non rotating side. A pulse-patterned wave with a defined frequency can be sent by the sender. A receiver measures this signal, either as two single pulses or as one pulse with a very high amplitude. Depending on the position of the sender of the gantry these alternating kinds of pulses can occur. A high precision rotation speed measurement is possible either by intensity measurement of the received pulses or by a detuning of sender and receiver frequency and measuring a interference pulse. 
         [0051]      FIG. 1  shows a hollow conductor (waveguide)  104 , wherein two different positions of the same sender (position  1 , position  2 )  102 ,  103  are depicted. Starting from the sender with the position  1   103  a first wave runs clockwise to the receiver  106  and a second wave runs counterclockwise to the receiver  106 . With the processing of the runtimes of the first wave and the second wave, which are detected by the receiver  106  it is possible to measure the position  1   103  of the sender. The diagram of the intensities shows a situation  108  in which the sender is opposed to the receiver  106  (180 degrees, position  2 ,  102 ). The dotted line  110  shows the situation that the sender  102  is arranged directly opposed to the receiver  106 . This situation results in the intensity  108 , detected by the receiver  106 . The dotted line  110  passes the center of the hollow conductor  104 . The intensities  107  and  109  show the situation which is detected by the receiver  106  when the sender is in a position  1   103 . 
         [0052]    In  FIG. 1  a waveguide ring  104  with one sender  102 ,  103  and one receiver  106  is shown. A signal, which is coded in a microwave and which is sent, runs along both sides of the waveguide ring. Two pulses  107 ,  109  can be measured by the receiver. If sender and receiver are at opposite positions (180°, both pulses have the same runtime and only one resulting pulse  108  (addition of both pulses) is measured. The time between one pulse and the maximum distance of both pulses gives an indication of the rotation speed. More easily the time between two “one-pulse-is-measured” situations can be measured. That&#39;s the time for one rotation. 
         [0053]    Advantageously, the rotation speed can be measured without additional mechanical components (like slits or counting marks). According to the inventive concept a higher precision measurement of the position and the rotation speed of the gantry is possible than with former systems by the use of signal-run time or interference patterns. 
         [0054]      FIG. 2  shows a hollow conductor  204 , and the rotary part of a gantry  205 . It is also depicted a data sender/receiver  203  with processing units  202  and  201 . It is also depicted a sender/receiver  208  with means for processing of data  207 ,  206 . 
         [0055]    The  FIG. 2  shows the rotary part of a gantry  205  with a hollow conductor  204  in which a wave can be guided. This renders the possibility to transmit data inside the hollow conductor between the rotary part of the gantry to the stationary part of the gantry without a mechanical connection between the rotary part of the gantry and the stationary part of the gantry. It is possible to arrange a sender or a receiver at a stationary part of the gantry and to arrange a sender or receiver at the rotary part of the gantry. Therefore, data can be transmitted in both directions, data can be transmitted from the stationary part to the rotary part of the gantry and data can be transmitted from the rotary part to the stationary part of the gantry. It is especially advantageous to arrange periodically i.e. equidistant, or not periodically dampings or an absorption structure in the interior zone of the hollow conductor  204 . It is also possible to arrange deficits in a dielectricum or slots in the hollow conductor in order to dampen the wave, circulating in the hollow conductor. 
         [0056]    A unit  206  supplies an antenna  208  with data, wherein the sender transmits a wave in the hollow conductor  204 . The hollow conductor dampens the wave in such a way, that the wave is considerably dampened after a first circulation in the hollow conductor  204 . This leads to the effect, that because of the reduced amplitude of the wave after a circulation it is possible to filter waves after one or several circulations in the hollow conductor  204 . The receiver  203  detects the wave. Further, the data which is encoded in the wave can be decoded by processing units  202 ,  201 . Therefore, this device offers the possibility to transmit data between the rotary part of a gantry at a stationary part of the gantry without a mechanical connection between the rotary part of the gantry and the stationary part of the gantry. This device renders also the possibility to transmit data without interruption between the rotary part of the gantry and the stationary part of the gantry. 
         [0057]    The hollow conductor  204  is typically ring-shaped and at least one lateral surface of the hollow conductor  204  is closed. Advantageously the hollow conductor  204  is slotted and filled with air. The hollow conductor  204  comprises at least one sender and one receiver, wherein it is also possible that the hollow conductor  204  comprises two units, which comprise both functions of a sender and a receiver. Due to this fact, data can be transmitted from the stationary part to the rotary part of the gantry and vice versa. It is also possible to arrange several senders or several receivers in the hollow conductor  204 , wherein the units can fulfil both functions of a sender and a receiver. Advantageously the hollow conductor dampens the waves which were sent by the sender, in order to dampen the waves after a first circulation. This can be realized for example by materials of the hollow conductor, which conduct badly. A further example to arrive at a dampening hollow conductor  204  is the use of coating the interior zone of the hollow conductor  204 . 
         [0058]      FIG. 3  shows a cross-section of a transformer which may be used, on the one hand, to supply power from a stationary part  305  to a rotary part  303  of a gantry, and on which, on the other hand, provisions are made for data submission from the stationary part to the rotary part via a slotted hollow conductor  308 . It is depicted the primary side of the transformer  305  with a E-shaped core and the secondary side of the transformer  303 , which is the rotating part of the transformer. It is also depicted windings  304 ,  311 ,  312 ,  310  of the transformer. The secondary part of the transformer (rotating part)  303  is rotating around the center line  302 . It is arranged a unit for sending or receiving or of both functions of sending and receiving  307  at the secondary side of the transformer  303 . With the help of the unit  307  it is possible to transmit data inside the hollow conductor  308 . The unit  307  enables also the possibility to detect the position of the secondary side of the transformer as well as the speed of the rotation of the secondary side of the transformer  303 . The hollow conductor  308  is filled with air. The hollow conductor  308  is slotted in order to enable the unit  307  to extend into the hollow conductor  308 . The hollow conductor  308  comprises five lateral surfaces  309 . two of the five lateral surfaces  309  are bent in order to enable the sender/receiver/antenna  307  to extend into the hollow conductor  308 . 
         [0059]    The device according to the inventive concept is adapted to be able to be used without a mechanical connection between the rotary part of the gantry and the stationary part of the gantry. Due to that fact the hollow conductor (waveguide) can be regarded as maintenance-free. 
         [0060]    When building a waveguide link system, several data slots (at different frequencies) are available. One frequency (data slot) of the slotted waveguide has to be used for the rotation-speed-pulse-pattern. The receiver is able to detect this pattern and can measure the runtime. To analyze/evaluate/interpret the rotation-speed-pulse-pattern various methods are possible. Either a Fourrier transformation of the pulse-run-time can be implemented in a processor and gives a frequency spectrum which contains a peak. The height depends on the rotation speed (constructive interference). The time between two maxima is the time of one gantry revolution. Another possible evaluation is the analysis of the runtime of both signals. A Kalman filter model can be used to predict the runtime. If the predicted runtime matches with the measured runtime, a given gantry speed is achieved. 
         [0061]      FIG. 4  shows a specific embodiment of a rotary part of a gantry  405  with a hollow conductor  404  arranged thereon. A transceiver  403  serving as both a receiver and a sender is provided at a stationary part of the gantry and is connected to processing units  401 ,  402 . The hollow conductor  404  is provided with a reflector  410  which is adapted for reflecting electromagnetic waves traveling within the hollow conductor. The reflector  410  rotates together with the rotating part of the gantry  405 . 
         [0062]    Electromagnetic waves emitted by the transceiver  403  travel in a clockwise direction indicated by the arrow  411  towards the reflector  410 . There, the waves are reflected and travel back in a counter-clockwise direction, indicated by the arrow  412 , towards the transceiver  403  where they are detected. Using a time-of-flight principle involves such reflection of the electromagnetic waves to enable collocation of the sender and the receiver in a transceiver  403 . This doubles the distance traveled by the electromagnetic wave, halving the required resolution of the processing unit  401 ,  402  and allowing using the same timing signal for both the sender and the receiver. Thereby, the need for synchronization of two separate clocks may be eliminated. 
         [0063]    A circular waveguide or hollow conductor  404  will usually guide the electromagnetic wave emitted by the transceiver  403  in two directions, i.e. both clockwise and counter-clockwise. Although not specifically indicated in  FIG. 4 , the electromagnetic wave emitted by the transceiver  403  may also first travel in a counter-clockwise direction before being reflected at the reflector  410  and traveling back in a clockwise direction towards the transceiver  403 . 
         [0064]    Depending on the relative positions of the transceiver  403  and the reflector  410 , one wave will arrive earlier than the other. The difference in time of arrival is the only measurement that is needed. In the embodiment shown in  FIG. 4 , only the transceiver  403  needs an accurate clock. In the embodiment shown in  FIG. 2 , only the receiver  208  needs such accurate clock. This clock does not need to provide an absolute time nor does it need to be synchronized but only needs to measure a difference in time between the arrival of a wave originally emitted in a clockwise direction and the time of arrival of a wave originally emitted in a counter-clockwise direction. 
         [0065]      FIG. 5  shows another specific embodiment of a rotary part of the gantry  505  provided with a hollow conductor  504 . Two separate receivers  508 ,  509  are provided on the rotary part and connected to processing units  506 ,  507 . An electromagnetic shield  513  is provided between the receivers  508 ,  509 . The electromagnetic shield  513  is adapted and arranged in the hollow conductor  504  such that electromagnetic waves coming from a sender  503  cannot be transmitted through the shield  513 . Accordingly, as shown in  FIG. 5 , an electromagnetic wave traveling in a counter-clockwise direction is received by the receiver  509  whereas an electromagnetic wave traveling in the clockwise direction is received with the receiver  508 . This makes it easier to discern the wave with the circuitry  506 ,  507 . 
         [0066]    When simply sending a pulse or a train of pulses, the required resolution is in the picosecond range. If however continuous sinusoidal waves are being used, one can apply beat detection, phase-shift measurement or may be even Doppler-shift measurement. These commonly used methods may relax the constraints on the timing circuitry. 
         [0067]    As schematically indicated by the circuitry shown in  FIG. 6 , when using a plain pulse, an accurate timing discriminator (TDC) is required. For example, a rectangular pulse may be emitted by a sender  603  on a stationary part of a gantry. The wave signal may be detected by a receiver  608  arranged at the rotary part of the gantry. A timing discriminator being part of a processing unit  607  may accurately detect the rising slope of a pulse applied to its input port. Two pulses corresponding to a wave transmission in a clockwise direction and a wave transmission in a counter-clockwise direction may be detected shortly after each other and the difference in their time-of-arrival may be used to calculate the angular position α(t). 
         [0068]    In an alternative embodiment, a carrier signal that is being used to establish the data link between the stationary and the rotary part of a gantry is also used to calculate the difference in the time-of-arrival. An interference of the two opposite waves (CW), (CCW) contains information on the angular position of the sender/receiver. This information can be extracted accurately through interpolation and using well-known interferometric principles. Ideally, in such case, (a) the electromagnetic wave is absorbed at the antenna for example by an additional absorber inserted between two receiving antennas to prevent the wave from making many revolutions which could otherwise disturb the measurement, or (b) the circumference measures a multiple of the wavelength of the carrier wave, creating a standing wave. 
         [0069]      FIG. 7  shows an exemplary embodiment of a computer tomography gantry  91  arrangement. The gantry  91  comprises a stationary part  92  connected to a high frequency power source  98  and a rotary part  93  adapted to rotate relative to the stationary part  92 . An X-ray source  94  and an X-ray detector  95  are attached to the rotary part  93  at opposing locations such as to be rotatable around a patient positioned on a table  97 . The X-ray detector  95  and the X-ray source  94  are connected to a control and analysing unit  99  adapted to control the X-ray detector  95  and the X-ray source and to evaluate the detection results of the X-ray detector  95 . 
         [0070]    It has to be mentioned that the wording sender can be replaced by the wording transmitter. 
         [0071]    It should be noted that the term ‘comprising’ does not exclude other elements or steps and the ‘a’ or ‘an’ does not exclude a plurality. Also elements described in association with the different embodiments may be combined. 
         [0072]    It should be noted that the reference signs in the claims shall not be construed as limiting the scope of the claims. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           91  Computer tomography gantry 
           92  Stationary part of the gantry 
           93  Rotary part of the gantry 
           94  X-ray source 
           95  X-ray detector 
           97  Table 
           98  High frequency power source 
           99  Control and analysing unit 
           101  Distance 
           102  Sender 
           103  Sender 
           104  Hollow conductor 
           105  Distance 
           106  Receiver 
           107  Intensity 
           108  Intensity 
           109  Intensity 
           201  Processing unit 
           202  Processing unit 
           203  Receiver/sender 
           204  Hollow conductor 
           205  Rotary part of a transformer 
           206  Processing unit 
           207  Processing unit 
           208  Receiver/sender 
           301  Core 
           302  center line 
           303  Rotary part of a transformer 
           304  Winding 
           305  Stationary part of a transformer 
           306  Core
         307  Sender/Receiver   
     
           308  Hollow conductor 
           309  Lateral surface of a hollow conductor 
           310  Winding 
           311  Winding 
           312  Winding 
           401  Processing unit 
           402  Processing unit 
           403  Transceiver 
           404  Hollow conductor 
           405  Gantry 
           410  Reflector 
           411  Clockwise wave 
           412  Counter-clockwise wave 
           501  Processing unit 
           502  Processing unit 
           503  Sender 
           504  Hollow conductor 
           505  Gantry 
           506  Processing unit 
           507  Processing unit 
           508  Receiver 
           509  Receiver 
           513  Electromagnetic shield