Abstract:
A computed tomography system employs an optical communications link to reliably transmit high data rate data. The communications link comprises an optical emitter, an optical transmission line, a plurality of optical deflectors disposed randomly within the transmission line, and an optical receiver. The optical emitter is attached to the gantry of the computed tomography system and extends along the length of the gantry. The optical emitter generates a high data rate optical data signal, which travels along the optical transmission line in correspondence with data generated by detector array on the gantry. The plurality of optical deflectors causes portions the high data rate optical data signal to be internally reflected and subsequently refracted from the transmission line. The optical receiver disposed near the transmission line detects the portion of high data rate data refracted from the transmission line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to computerized tomography (CT) communication, and more specifically to an optical communication system employed in a CT system.  
           [0002]    CT systems typically employ a rotating frame or gantry to obtain multiple x-ray images, or views, at various rotational angles. Each set of images is identified as a “slice.” A patient or object is generally positioned in a central opening on the rotating frame, typically on a table. The table is axially movable within the central opening so that the patient may be positioned at various locations enabling respective slices to be obtained at multiple axial positions. Each of the slices obtained is then processed in a computer to produce enhanced images that are useful for diagnoses and inspection.  
           [0003]    The rotating frame typically includes an x-ray source, a detector array and electronics necessary to generate image data for each view. An electronics system, typically stationary, is employed to process raw image data. It is thus necessary to communicate image data between the rotating frame and the electronic system for image processing.  
           [0004]    The rate of data communication between the stationary electronic system and the gantry is important because the rate affects the speed at which the images can be processed. It is desirable to obtain image views as fast as possible to maximize image quality, reduce patient discomfort, and to maximize equipment utilization. In current CT systems, a single view typically comprises about 800 detector channels with a 16 bit representation for each individual detector channel and is typically repeated one thousand times per second, yielding a net data rate of about 13 million bits per second (Mbit/sec) for image data. Advanced CT systems capable of simultaneously constructing multiple image slices by employing four, eight, sixteen, or more times as many detector channels, will increase the required data rate to the hundreds of mega-bits per second range.  
           [0005]    Prior CT systems have employed brushes, slip rings, and radio frequency links for communicating the image data between the rotating frame and the stationary frame. CT systems utilizing brushes and slip rings for communications have generally suffered significant limitations in data transfer rates due to the substantial time required to propagate the signals around the circular slip rings. At the desired data rates, the electrical path length around the rings is an appreciable fraction of the data bit transfer period so that electromagnetic waves propagating around the rings in the opposite direction may arrive at the reception point at substantially different times within the bit transfer period causing signal interference.  
           [0006]    Additionally, radio frequency communication links, historically, have not been able to achieve the very high data transfer rates required of future CT systems at reasonable costs. Radio frequency links typically are more expensive to produce as the data rate increases because of the electromagnetic emissions requirements that must be met. As such, it is desirable to employ a CT communications link between the stationary electronics and rotating electronics that can operate with very high data rates without causing interference with other equipment.  
           [0007]    It is also desirable to provide a communication link between the stationary frame and the rotating frame that is immune to electromagnetic radiation interference such as is typically produced in a hospital environment by cellular telephones, defibrillating devices, surgical saws, and electrical noise produced by any given CT system.  
           [0008]    Current optical rotary links are expensive. One type uses lenses, mirrors, or many emitters and detectors to insure continuous optical communication at any gantry angle. Such systems require expensive alignment. Another type of rotary link uses an “optical brush” that contacts an optical transmission line with sufficient force to deform the line. At the deformity the high data rate optical data signal can enter (or exit) the line at such an angle as to be captured within the transmission line (total internal reflection) and then propagate, unimpeded, to a detector disposed at the end of the transmission line. This then provides a mechanism for coupling an externally generated high data rate optical data signal into the line at any point along the transmission line (at any gantry angle). The deformation point, however, moves along the transmission line as the gantry rotates and this process eventually causes the transmission line and coupler to wear, resulting in failure.  
           [0009]    Yet another type of rotary optical link uses a transmission line that is doped with a dye that becomes temporarily luminescent when irradiated with laser light. The luminescent radiation is from inside the optical transmission line and a portion of this optical data signal is at such an angle as to be captured within the transmission line. Existing dyes have a response that is too slow to support the desired high data rate. Furthermore, the dyes eventually degrade.  
           [0010]    Finally, another type of rotary link uses an optical transmission line, for example, a fiber that is heat treated to create many small fractures. Each fracture scatters high data rate optical data signal at such an angle as to be captured within the line. With this approach, the treated fiber is very small and brittle, and the coupling and propagation losses are high. In many of the above approaches, there is a dead spot or gantry angle where communication is not supported.  
           [0011]    Accordingly, there is a need in the art for an improved communications link for CT x-ray machines.  
         SUMMARY OF THE INVENTION  
         [0012]    A computed tomography system employs an optical communications link to reliably transmit high data rate data. The communications link comprises an optical emitter, an optical transmission line comprising at least two sections, a plurality of optical deflectors disposed randomly within the transmission line, and an optical receiver. The optical emitter is attached to the gantry of the computed tomography system and extends along the length of the gantry. The optical emitter generates a high data rate optical data signal, which travels along the optical transmission line in correspondence with data generated by detector array on the gantry. The plurality of optical deflectors causes portions the high data rate optical data signal to be internally reflected and subsequently refracted from the transmission line. The optical receiver disposed near the transmission line detects the portion of high data rate data refracted from the transmission line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a conventional computed tomography system having a gantry and stationary electronics;  
         [0014]    [0014]FIG. 2 is a functional block diagram of one embodiment of a communications link of the present invention coupled to a gantry of a computed tomography system;  
         [0015]    [0015]FIG. 3 is a block diagram illustration of one embodiment of a communications link employing translucent bubbles to generate a scattered data signal;  
         [0016]    [0016]FIG. 4 is a block diagram illustration of one embodiment of a communications link employing a transmission line with a rough inner cladding surface;  
         [0017]    [0017]FIG. 5 is an illustration of a transmission line having a rough portion of the outer surface area;  
         [0018]    [0018]FIG. 6 is a block diagram illustration of a lens focused optical detector of the present invention;  
         [0019]    [0019]FIG. 7 is a block diagram illustration of a multi-segment communications link; and  
         [0020]    [0020]FIG. 8 is a block diagram illustration of a multi-segment bi-directional communications link. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    A computed tomography (CT) system  50 , typically employs a CT base  2 , a source of imaging energy  13 , a detector array  14 , an annular rotating gantry  15  having an outer circumference  16 , and a stationary electronics system  30  to obtain multiple x-ray images of a patient or object, as shown in FIG. 1.  
         [0022]    Detector array  14  comprises a plurality of detectors, for example, several thousand detectors, which detectors generate x-ray data that is utilized to simultaneously construct multiple image slices. Detector array  14  is typically mechanically coupled to gantry  15  and rotates therewith. In one embodiment, gantry  15  is about four (4) feet in diameter and rotates at about 2 revolutions per second.  
         [0023]    A patient or object is generally positioned in or near a central aperture  11  of gantry  15  on a table that is axially movable along base  2 , enabling respective x-ray slices to be obtained at multiple axial positions. The x-ray slices are processed at stationary electronics systems  30  to produce enhanced images for diagnoses or inspection.  
         [0024]    In accordance with the instant invention, an optical communications link  100  is employed in a CT system  98  to transmit detector array data from gantry  15  to stationary electronics  30 , as shown in FIG. 2.  
         [0025]    In one embodiment of the present invention optical communications link  100  comprises an optical transmission line  120  that is disposed about the circumference of gantry  15 , an optical emitter  150  coupled to a first end of optical transmission line  120 , and a stationary optical detector  180  disposed adjacent to transmission line  120 .  
         [0026]    Optical emitter  150 , for example, a light emitting diode or a laser diode, is modulated with binary data  156 , generated by detector array  14 , and produces a high data rate optical data signal  240 , which high rate signal  240  is transmitted through transmission line  120 .  
         [0027]    Stationary optical detector  180  is positioned adjacent transmission line  120  to detect a portion of high rate signal  240  refracted outside of transmission line  120 , which portion is termed a refracted signal  230 .  
         [0028]    Optical transmission line  120  typically comprises two, generally, semi-circular segments, each optically coupled at a first end with optical emitter  150  for receiving high rate signal  240  and at a second end with an optical absorber  130 , which optical absorber  130  minimizes internal reflections of high rate signal  240 .  
         [0029]    In one embodiment, transmission line  120  is made of glass, plastic, or other translucent polymeric material. Transmission line  120  further comprises internal light scatter(s)  220 , for example bubbles, to redirect portions of high rate signal  240 , as shown in FIG. 3. Internal light scatters  220  redirect portions of high rate signal  240  to an outer surface  226  of transmission line  120  such that a portion of high rate signal  240  escapes transmission line  120 . The escaping refracted signal  230  is then detected by optical detector  180 . The non-escaping portions of high rate signal  240  propagate through transmission line  120  and are absorbed by optical absorber  130 .  
         [0030]    High data rate optical data signal  240  comprises x-ray data generated by detector array  14 . The x-ray data comprises both image and control information. In the present example, the x-ray data corresponds with image data and communication protocol data generated by CT system  50 . The x-ray data may, for example, be pulse width modulated or frequency modulated. The data rate of the x-ray data may be in the Giga-Hertz range, and is typically from about 10 mega-Hertz to about 10 Giga-Hertz. The information content of optical data signal  240  is also present in refracted optical data signal  230 , as such, the information content of optical data signal  240  is coupled to optical detector  180  during the communication process of the present invention.  
         [0031]    In another embodiment, transmission line  120  is constructed using ⅛ inch diameter plastic rod with an index of refraction of about 1.5. During manufacture, the plastic rod may be drawn through an orifice where small air bubbles are injected before the rod completely solidifies. The air flow is adjusted to entrain approximately 10 bubbles per inch of line length, with the bubbles being approximately 0.01 inches in diameter. The density and size of bubbles  220  is adjusted so that, for each 1 inch section of transmission line  120 , approximately 5% of the power in the optical data signal  240  is refracted by bubbles  220  to become refracted optical data signal  230 . This yields an exponential decay of signal power along the line, with approximately 50% of the power remaining at the end of a 12-inch line section.  
         [0032]    A further exemplary embodiment of the present invention is illustrated in FIG. 4. In this embodiment transmission line  120  comprises a hollow tube, or alternatively, translucent waveguide material  210 . An inner surface  222  of the tube is etched to give it a roughness for scattering high data rate optical data signal  240 . Waveguide material  210  has a dielectric constant that is different from the dielectric constant of the material having inner surface  222 . Optical data signal  240  propagates within waveguide core material  210 . Optical data signal  240  scatters after colliding with rough inner surface  222 . Waveguide core material  210  additionally comprises, for example, clear plastic, a gas, a clear liquid, or the like.  
         [0033]    In another embodiment, transmission line  120  comprises a waveguide core material  210  having a coarse portion of an outer surface and smooth portions of an outer surface. The coarse portions of outer surface provide a breach of the total internal reflection condition to redirect portions of high rate signal  240  to escape transmission line  120 , as shown in FIG. 5.  
         [0034]    As an example, a 0.25-inch diameter plastic rod is used as the radiating transmission line section. To break the TIR condition and allow some radiation, a single rough stripe is established along the length of the line section. The surface roughness along the stripe may be approximately 1 mil root-mean square (surface height variation). The stripe width is approximately 0.125 inches, or equivalently span approximately 1 radian of the circumference.  
         [0035]    In a further embodiment of the instant invention, a control signal  231  is directed into transmission line  120  at course portion(s)  128  to enable bi-directional communication.  
         [0036]    In another alternative embodiment, transmission line  120  comprises a reflective outer surface, as shown in FIG. 6. Transmission line is encapsulated with a reflective cladding  226 , for example, aluminum. An aperture  228  is disposed along the axial length of transmission line  120 . Aperture  228  may alternatively comprise a plurality of intermittently spaced narrow slits.  
         [0037]    Optical detector  180  utilizes a lens  178  and a filter  179  to collect refracted signals  230  from transmission line  120 . Aperture  228  is sized to enable a portion of high rate signal  240  to escape from transmission line  120  so as to be detected by optical detector  180 . The width of aperture  228  is in the range between about {fraction (1/10)} to about {fraction (1/100)} of the circumference of transmission line  120 .  
         [0038]    In accordance with another embodiment of the instant invention, a sub-divided transmission line  120  is shown in FIG. 7. Subdivision of transmission line  120  provides shorter lengths for each transmission line section. This reduces the worst case delay dispersion in each transmission line section. The data rate of high rate signal  240  can be, correspondingly, increased as the delay dispersion of high rate signal  240  is reduced. To allow reliable data communication, the delay dispersion in refracted optical data signal  230  at any detector position should be such that the eye pattern is substantially open, as is well known by one skilled in the art. For example, for a data rate of 1.0 Gigabit/second and binary (on-off) signaling, approximately 90% of refracted optical data signal  230  reaching the detector should have a time dispersion of less than ±0.125 nano-seconds (nsec). In order to reduce delay dispersion in transmission line  120 , transmission line  120  is subdivided into a plurality of subsections.  
         [0039]    One embodiment is illustrated in FIG. 2 in which transmission line  120  is subdivided into two generally equal subsections  120   a ,  120   b . Subsections  120   a ,  120   b  are each optically coupled at a first end to a splitter  127  and at a second end to optical absorber  130 . An additional section  120   c  may be utilized to optically couple high rate signal  240  to splitter  127 .  
         [0040]    In communications link  100 , as gantry  15  is rotated and detector  180  moves away from splitter  127  along transmission line  120 , the data communication delay increases until optical detector  180  is disposed adjacent optical absorber  130 . As rotation continues, the delay then decreases until detector is again disposed adjacent to splitter  127 .  
         [0041]    In an alternative embodiment, illustrated in FIG. 7, transmission line  120  is subdivided into four subsections ( 117 ,  119 ,  121 , and  123 , respectively), so as to reduce the delay dispersion in transmission line  120 .  
         [0042]    It is preferable that high rate signal  240  arrive at any two ends of transmission line  120  adjacent to a given gap  128  at substantially the same time, less than about 0.125 nsec. For example, when communicating with binary signaling at 1.0 Gigabits/second, the optical data signal  240  at any two adjacent ends of transmission line  120  should be time aligned within about ±0.125 nsec. That is, the leading edge of a single on-off transition at the output of the laser diode should arrive at the two sides of a respective gap  128  within ±0.125 nsec. For transmission line material that has an index of refraction of about 1.5, the propagation speed is about 1.5 nsec/foot and the two optical paths lengths to an associated gap should be within about 1 inch of each other in order to achieve a time alignment of ±0.125 nsec.  
         [0043]    In operation, high rate signal  240  is generated by optical emitter  150  and traverses along four paths of transmission line  120 . In a first path, high rate signal  240  is split by a first splitter  129  and travels through a section  115   b , is split by a second splitter  127  and travels through section  117  to an optical absorber  130 . In a second path, high rate signal  240  is split by first splitter  129  and travels through a section  115   a , is split by a third splitter  125  and travels through section  121  to optical absorber  130 . In a third path, high rate signal  240  is split by first splitter  129  and travels through section  115   b  to second splitter  127  and travels through section  119  to optical absorber  130 . In a fourth path, high rate signal  240  is split by first splitter  129 , and travels through section  115   a  to third splitter  125 , and travels though a section  123  to optical absorber  130 . In this embodiment the length of each path is substantially equal. Each path length is selected such that high rate signal  240  arrives at each optical absorber  130  at substantially the same time, typically about ±0.125 nsec apart.  
         [0044]    Accordingly, in this embodiment, two portions of a respective high rate signal  240  arrive at respective optical absorbers  130 , adjacent to a respective gap  128 , at substantially the same time. Further, a refracted signal  181  radiating from a first side of gap  128  is also in time alignment with a refracted signal  127  radiated from a second side of gap  128 .  
         [0045]    Consequently, detector  180  receives refracted signals  181 ,  187  at substantially the same time. Accordingly, in this embodiment, inter-symbol interference is minimized when detector  180  is disposed adjacent to gap  128 , and “dead spots” are minimized in transmission line  120 . As used herein, inter-symbol interference is the interference between time adjacent symbols transmitted on a communication channel. The signal, voltage, current, etc. associated with the symbol transmitted at one time must decay sufficiently so as not to have a substantial residual during the transmission time for the following symbol. The residual level, relative to the desired symbol level, is a measure of the inter-symbol interference. As used herein, the phrase “dead spot” is defined as a portion of transmission line in which optical detector  180  cannot detect a refracted signal.  
         [0046]    In a further exemplary embodiment of the present invention an additional optical emitter  151  is employed in communications link  300  to enable bi-directional communications, as is illustrated in FIG. 8. Optical emitter  151  is disposed sufficiently close to transmission line  120  to couple data control optical signal  231  into transmission line  120 . Emitter  151  is thus adapted to enjoy relative movement with respect to transmission line  120  along a path  182 . A second optical detector  180   a  is also employed and is coupled to transmission line  120  so as to detect control data signal  231 . Control data signal  231  traverses section  121 , passes through splitter  125 , traverses through section  115   a , passes through splitter  129  and a splitter  131 . Control data signal  231  is then focused by lens  178   a , filtered by filter  179   a , and detected by detector  180   a . Data may thus be coupled from optical emitter  150  to detector  180  and, additionally, control data signal  231  may be coupled from emitter  151  to detector  180   a . It is to be understood that control signal  231  may also comprise data.  
         [0047]    It will be apparent to those skilled in the art that, while the invention has been illustrated and described herein in accordance with the patent statutes, modifications and changes may be made in the disclosed embodiments without departing from the true spirit and scope of the invention. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.