Abstract:
A non-contacting rotary joint for transmitting an input signal guided within an input optical waveguide has an electrical transmission line and a probe. The transmission line has an even number N≧4 of equal length transmission line segments. The optical waveguide is connected to an optical distribution network which has an optical power splitter for splitting the input signal into N or N/2 individual signals of equal optical power. These signals are forwarded to opto-electrical converters to generate electrical signals for driving the electrical transmission line. The lengths of the optical transmission lines are adjusted so that the propagation time of the optical signal from the end of the input optical waveguide to the end of each individual optical transmission line is approximately the same.

Description:
PRIORITY CLAIM 
     This application claims priority to pending German Application No. DE 10 2010 000 525.8 filed on Feb. 23, 2010, which is fully incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a data transmission system for transmitting data between a rotating part and a stationary part, in particular between the rotating part and the stationary part of a computer tomograph, and also to a computer tomograph having a corresponding transmission system. 
     2. Description of the Relevant Art 
     Rotatable units such as radar devices or computer tomographs often require transmitting electrical signals or energy in either direction between a rotating and a stationary part. For this, usually a conductor structure for conducting electrical signals to be transmitted is provided in a primary unit, and a suitable tap or probe in a secondary unit. These transmission systems are known as rotary joints and are categorized into contacting and non-contacting rotary joints. The contacting rotary joints mostly have a comparatively solid conductor in the primary unit and a brush like a wire or a carbon brush in the secondary unit. The brush is in electrical and mechanical contact with the conductor. The contacting rotary joints are also known as sliprings. Non-contacting rotary joints generally have a conductor designed as a transmission line which may be a printed circuit board in the primary unit and which is generally designed to generate low far-field radiation while conducting the signal to be transmitted. In the secondary unit there is a comparatively small probe which capacitively picks up near field signals from the conductor of the primary unit. 
     Non-contacting rotary joints are often used in computer tomography (CT) scanners. The transmission line being part of the rotating part of the gantry is often attached to an outer surface of said rotating part. In most cases it has a diameter of approximately 1.5 meters. This results in an overall length of the transmission line of about 4.5 meters. Up to date, CT scanners have to transmit data at a rate of more than 5 GB per second, up to 30 GB per second. In the future, even higher data rates are expected. It is obvious that the transmission line has to carry signals at high frequency in the Gigahertz frequency range. The transmission line itself is designed similar to a strip line and may have a specific pattern for filtering signals. It must be highly symmetric to avoid far field radiation which leads to enhanced electromagnetic radiation of the scanner, which is not desired. Furthermore, the transmission line must have low losses to lead the high frequency signals without significant attenuation to maintain a good signal/noise ratio over the full length of the transmission line and therefore independently of the rotational position of the CT scanner gantry. 
     U.S. Pat. No. 5,579,357 discloses a transmission line which is split into two equal parts. Each part receives the signals to be transmitted by a phase splitter at one end and is terminated at the other end. Here, still each section of the transmission line has a length of more than 2 meters which is still difficult to manufacture. 
     Such transmission lines may be manufactured by using enhanced glass fiber reinforced epoxy printed circuit boards. Modern printed circuit board technologies using substrates based on PTFE (polytetrafluorethylene) or ceramic materials are only available for small RF printed circuit boards and cannot be used here. 
     SUMMARY OF THE INVENTION 
     The problem to be solved by the invention is to make a rotary joint with a diameter suitable for CT scanners, which has a primary side with a transmission line which can be manufactured by using radio frequency printed circuit board manufacturing materials and methods. A further problem to be solved is to enhance the data rate of the rotary joint. Furthermore manufacturing of the rotary joint shall be simplified and costs shall be reduced. 
     A further problem to be solved is to allow the use of standard printed circuit board technologies instead of RF printed circuit boards while maintaining low electromagnetic interference and good signal transmission quality. 
     Another aspect of the invention relates to a CT scanner with a rotary joint. 
     Solutions of the problems are described in the independent claims. The dependent claims relate to further improvements of the invention. 
     According to the invention, a rotary joint for transmitting electrical signals or energy from a primary part to a secondary part, both parts being rotatable against each other, receives a signal to be transmitted by an optical line. This optical line may be a single-mode fiber or a multi-mode fiber, for example a plastic optical fiber. Due to the high transmission rates, a single-mode fiber is preferred. Attached to this fiber is a splitter dividing the optical signal into N or N/2 equal signals, where N is an even number and N≧4. Preferably N is 4, 6 or 8. For the case, N is selected as N=8, the transmission line is divided into N=8 equal sections and the optical splitter divides the optical input signal into 4 or 8 equal signals. The individual optical signals from the splitter are converted into electrical signals by opto-electrical converters. For the case the splitter divides the optical signal into N signals, one opto-electrical converter is provided for each segment. If the splitter divides the optical signal into N/2 signals, then one opto-electrical converter is provided for each pair of segments. To adapt the signals from the opto-electrical converter to the characteristics of the transmission line segments, an additional driver or amplifier may be provided. All segments have the same length. Each segment has two conductors which are fed by a differential signal. Any two segments neighbored to each other are fed by electrical signals such that they conduct the signals propagating into opposite directions. The segments have a feeding point at one end which is connected to an electrical differential signal source, and a termination which may consist of terminating resistors at the other end. 
     The optical power splitter may be one splitter or it may be divided into a plurality of cascaded splitters. 
     To avoid signal distortion when a probe passes from one section to another section, the signals at the joint between neighbored sections of the transmission line must have a maximum allowable time difference which may be less than 10% of a bit period of the digital signal to be transmitted. It may also be acceptable if the time difference is less than 20%, 30%, 40% or even 50% of a bit period. Anyway it is preferred to have the time difference below 10% of a bit period, preferably below 5%, 2% or 1%. To achieve this, the optical length of the optical path between a given position of the single light wave guide from the optical transmitter and each of the optical receivers must be approximately the same. It must be selected or manufactured such that the maximum length difference between the paths to individual optical receivers results in propagation time differences less than the maximum allowed time difference between neighbored transmission segments. The lengths are preferably determined by measuring the path length from the first input of the optical splitter to the input of each opto-electrical converter. Of course the path length within the optical splitters must be included. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings. 
         FIG. 1  shows the primary side of an inventive rotary joint in detail. 
         FIG. 2  shows a simplified embodiment. 
         FIG. 3  shows a modified coupling scheme. 
         FIG. 4  shows the optical distribution network. 
         FIG. 5  shows a different embodiment of the optical distribution network. 
         FIG. 6  schematically shows in a general form a computer tomograph. 
         FIG. 7  schematically shows a data transmission system. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A rotary joint  3 , such as shown in  FIG. 7 , in accordance with an embodiment of the invention has an electrical transmission line  6   a - c  which is disposed on or integrated into a primary part, which may be the rotating part  1  preferably along the circumference. The transmission line has at least one line, preferably two differentially driven lines for conducting electro-magnetic waves, which is mounted preferably along at least one circular segment or a circular track on the primary part. 
     A probe (receiving coupler)  7  is disposed on or integrated into a secondary part to be opposite to the primary part, i.e. on the stationary part  2 . It is designed to couple signals from the transmission line  6   a - c . The signals from the probe may be converted to optical signals which may be guided within an optical waveguide, preferably a single mode fiber by means of an electro-optical converter. The signals from the probe  7  are conducted to a receiving unit  23 , such as shown in  FIGS. 2 and 3 . This receiving unit amplifies the signals from the receiving coupler arrangement, and conditions the signals, if necessary. 
     The transmission line is divided into a number N of individual segments  20   a ,  20   b ,  20   c ,  20   d . The ends of neighbored segments preferably are in close proximity with each other to avoid gaps in between causing signal loss. The transmission line segments are mounted on a dielectric base  23 , which preferably is a RF printed circuit board, preferably comprising of ceramic or polytetrafluorethylene materials. There may be one common dielectric base for all segments, but it is preferred to have individual dielectric basis for each individual segment or for a pair of neighbored segments. At least one and preferably two neighbored segments are fed by a driver  24   a ,  24   b ,  24   c ,  24   d . A driver is a circuit for adapting the signals at the input to be fed into the transmission line. Therefore, it adapts a signal in its amplitude and power level to the given impedance of the transmission line. It furthermore may do filtering to adapt to the frequency characteristic of the transmission line. Each driver receives electrical signals from an opto-electrical converter  25 . In another embodiment, one opto-electrical converter supplies signals to two drivers, the drivers feeding neighbored segments of the transmission line. 
     The individual opto-electrical converters are fed by an optical distribution network  30  comprising light wave guides and optical power splitters. In most CT scanners, data from the data acquisition system attached to the X-ray detector is transmitted by optical signals, transferred via single-mode or multi-mode optical fibers. Such an optical fiber  28  is connected to at least one optical power splitter  27  splitting the optical signals into a plurality of equal output signals. These output signals may be further split into further sets of equal output signals. The optical distribution network  30  converts one single input signal from input line  28  into a plurality of equal signals in lines  26   a ,  26   b ,  26   c ,  26   d . It is essential that the optical paths through the optical distribution network starting from the input to which line  28  is connected to the input of each individual opto-electrical converter  25   a ,  25   b ,  25   c ,  25   d  are equal. 
       FIG. 1  shows the primary side of an inventive rotary joint in detail. The individual transmission line segments  20   a - 20   h , each comprising of two parallel differentially driven transmission lines are circularly arranged. Each transmission line segment has the same length and has a feeding point  21   a - 21   h  and a termination  22   a - 22   h  at the opposite end. The electrical signals are conducted in each line segment from the feeding point to the termination. The termination absorbs the signal conducted in the line segment and avoids a reflected signal to propagate backwards through the line segment. The transmission line segments are attached to dielectric base or carrier segments  23 ,  23   b ,  23   c ,  23   d . Here, exemplarily two neighbored feeding points  21   a ,  21   b ;  21   c ,  21   d ;  21   e ,  21   f ;  21   g ,  21   h  are connected together to drivers  24   a ,  24   b ,  24   c ,  24   d . Each driver is connected to an opto-electrical converter  25   a ,  25   b ,  25   c ,  25   d . These opto-electrical converters are fed by optical transmission lines  26   a ,  26   b ,  26   c ,  26   d  from optical power splitters  27   a ,  27   b ,  27   c . A first optical power splitter  27   c  receives an input from input optical transmission line  28  from the data source which may be a data acquisition system of a CT scanner. The signal is divided into two equal signals, from which a first one is fed into the second optical power splitter  27   a , and the other one is fed into the third optical power splitter  27   b . Said second and third optical power splitters split the input signals received from first optical power splitter  27   c  into equal output signals coupled over optical transmission lines  26   a ,  26   b ,  26   c ,  26   d  to the opto-electrical converters  25   a ,  25   h ,  25   c ,  25   d . For ensuring the equal optical delay and therefore the equal optical length between the end of fiber  28  and the inputs of the opto-electrical converters the optical transmission lines  26   a ,  26   b ,  26   c ,  26   d all have the same length. Therefore in this figure the optical transmission lines  26   a  and  26   d  are wound up like coils. 
       FIG. 2  shows a simplified embodiment where only 4 transmission line segments  20   a ,  20   b ,  20   c ,  20   d  are provided. Accordingly, there are only two opto-electrical converters and two drivers attached thereto. There is only one optical power splitter  27  dividing the input signal from input line  28  into two equal signals distributed over the optical transmission lines  26   a  and  26   b.    
       FIG. 3  shows a modified coupling scheme of the transmission line segments to the drivers. First, the optical input signal from input line  28  is divided by optical power splitter  27  into two equal signals distributed through optical transmission lines  26   a  and  26   b  to corresponding opto-electrical converters  25   a  and  25   b . These feed the drivers  24   a  and  24   b  with electrical input signals. Both drivers are connected via interface lines  31   a - 31   d  to the individual lines of each line segment. First driver  24   a  is connected to line segment  20   a  via interface lines  31   a  and  31   b . Driver  24   b  is connected to transmission line segment  20   b  through interface lines  31   c  and  31   d . Alternatively two drivers  24   a  and  24   b  fed by one common opto-electrical converter. In another alternative embodiment only one driver  24   a  may be connected to both line segments  20   a  and  20   b.    
       FIG. 4  shows the optical distribution network  30  as explained before. This optical distribution network has three cascaded optical 3 dB splitters  27   a ,  27   b ,  27   c.    
       FIG. 5  shows a different embodiment of the optical distribution network. This optical network uses a single four channel 6 dB power splitter, dividing signal from input line  28  into four signals at optical transmission lines  26   a - 26   d.    
       FIG. 6  shows a rotary joint on an example of a computer tomograph. The computer tomograph (CT scanner) consists of two main mechanical parts. A stationary part  2  serves as a base and support of the entire instrument, in which the rotating part  1  rotates. A patient  104  is positioned on a berth  107  in the opening of the rotating part. An X-ray tube  101  and also a detector  103  disposed opposite thereto are provided for scanning the patient by means of X-rays  102 . The X-ray tube  101  and the detector  103  are disposed to be rotatable on the rotating part  1 . A rotary joint  3  serves for electrical connection between the rotating part  1  and the stationary part  2 . With this, on the one hand the high electrical power for feeding the X-ray tube  101  is transmitted in the direction of the rotating part  1 , and simultaneously the raw data of the image are transmitted in the opposite direction. Parallel to this, communication of control information in both directions is provided. An evaluation and control unit  105  serves for operating the computer tomograph and also for displaying generated images. Communication with the computer tomograph is effected via a bidirectional link  105 . 
       FIG. 7  shows in simplified form an example of an arrangement of a data transmission system. The data from a data source  4  (detector  103  with subsequent signal processing or DAS) on the rotating part  1  are conditioned with a first transmission means  8  and relayed to the transmission line which here is illustrated by way of example as comprising three parts  6   a ,  6   b ,  6   c , each having two segments. This transmission line arrangement now conducts the high-frequency signals. These are probed by the probe  7 . A probe which is fixedly connected with the stationary frame is illustrated by way of example. The signals intercepted by this probe  7  are relayed to a first receiving means  9  for conditioning. Output signals from the latter are then conducted to a data sink  5 . 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide optical rotary joints and computer tomography scanners. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and de-scribed herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.