Abstract:
A rotation monitoring system ( 70 ) detects the rotational speed of an anode ( 10 ) of an x-ray tube during use. The system ( 70 ) includes a detector ( 72 ), which detects a pulse of secondary x-rays generated by the interaction of a stream (C) of electrons with a known defect ( 83 ) on a surface ( 84 ) of the anode. The detector may be position inside or outside a vacuum envelope ( 14 ) of the x-ray tube. The stream of electrons is supplied by a secondary source ( 80 ), separate from a main source ( 18 ) of electrons used to generate the primary or working x-ray beam (B) of the x-ray tube. A single pulse is detected with each rotation of the anode, providing a simple method of calculation of the anode rotation speed. Preferably, a feed back loop is used to correct the rotational speed of the anode so that overheating of the anode is avoided and the useful life of the x-ray tube is extended.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates the medical diagnostic arts. It finds particular application in connection with monitoring of the speed of rotation of a rotating anode in an x-ray source, and will be described in conjunction therewith. It should be appreciated, however, that the invention is also applicable to the measurement of the rotation speed of other rotating bodies. 
     X-ray sources, such as those utilized in the field of medicine for the imaging of subjects, frequently employ a rotating anode, which is bombarded by a beam of electrons from a thermionic filament cathode. A heating current, commonly of the order of 2 to 5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, of about 100 to 200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards the anode. The beam of electrons is directed to a focal track on an inclined, annular surface or target area of the anode. X-radiation radiates in response to the impingement of the electrons on the target area. 
     The acceleration of electrons causes a tube or anode current of about 500-600 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot. The temperature of the anode can be as high as about 1,400° C. In high energy tubes, therefore, the anode rotates at high speeds during x-ray generation to spread the heat energy over a large area and to inhibit the target area from overheating. The cathode and the envelope remain stationary. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the target area that was heated by the electron beam has substantially cooled before returning to be heated by the electron beam. 
     The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the glass envelope, and a rotor with an armature and a bearing shaft, within the envelope. The armature and/or bearing shaft is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the armature which cause the armature and hence the target area of the anode to rotate. 
     For maximum useful life of the X-ray source, it is important to maintain the rotational speed of the anode at, or close to, a predetermined value. If the anode rotation speed drops too low, thermal damage to the target area can result. High anode rotation speeds, on the other hand, result in the stator motor operating more than is needed, and can lead to thermal damage. Whenever the motor is running, heat is generated and is transferred to the x-ray tube housing. It is also undesirable for the source to be operated at the rotation speed of mechanical resonance of the anode and the rotor. Additionally, on start-up, it is preferable to delay application of the power to the cathode for generation of electrons until the anode has reached a minimum rotation speed. Accordingly, it is important to be able to measure the speed of rotation of the anode and to be able to make adjustments, if needed, in response to the detected speed. 
     Various detectors have been developed to ensure that the anode is rotating at its design operation speed. In one design, bearing shaft rotation is detected. For example, an optical feed-through with a fiber optic source is used to detect the movement of an optically readable timing marker fitted to the bearing shaft of the rotor. Devices which measure bearing shaft angle rotation, however, typically involve the installation of an optical, mechanical, or electrically responsive device along the shaft itself, which, in the case of an x-ray source, invades the housing of the source in order to install such a detection device. 
     In another design, the power to the stator is shut off momentarily, and the back EMF generated by the spinning rotor is measured across the stator. This results in a drop in rotation speed each time the speed is measured. 
     Devices have been developed which make use of naturally occurring defects in the target area to determine rotation speed. However, these employ complex analytical equipment to compensate for the irregularities of the defects and their uneven spacing on the target. 
     Lasers have been used as an indirect measurement of the rotational speed. An externally generated laser beam is reflected off the target and used to measure the temperature. The temperature of the target area is dependent on the rotation speed, and thus the measured temperature gives an indirect indication of speed. However, this method does not facilitate correction of the rotation speed. The anode takes a finite time to cool or heat up when the speed is increased or decreased, and thus over-correction may occur. 
     The present invention provides a new and improved apparatus and method of monitoring the speed of rotation of an anode, which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a detection system for detecting the rotational speed of an anode of an x-ray tube is provided. The x-ray tube includes a first source of electrons which are accelerated at a target area of an anode to generate a primary x-ray beam. The detection system includes a second source of electrons which are accelerated at the anode to generate a second x-ray beam. A defect on the anode periodically changes an x-ray distribution of the second x-ray beam at least along a detection direction. An x-ray detector detects an intensity of the second x-ray beam along the detection direction. 
     In accordance with another aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an evacuated envelope and an anode rotatably mounted in the evacuated envelope. The anode has a circular primary target area around a periphery of the anode and an inner circular track of smaller radius than the primary target area. The anode has a construction along the inner track that alters a distribution of generated x-rays. A first cathode cup is mounted within the evacuated envelope for generating electrons that are accelerated into the primary target area to generate a primary x-ray beam. A second cathode cup is mounted within the evacuated envelope for generating electrons that are accelerated at the inner track to generate a secondary x-ray beam. An x-ray distribution of the secondary beam changes each time the accelerated electrons strike the construction. An x-ray detector is positioned to monitor the changes in the secondary beam distribution as the electrons strike the construction. A motor rotates the anode. 
     In accordance with another aspect of the present invention, a method for determining rotational speed of a rotating anode of an x-ray source is provided. The x-ray source includes a first source of electrons which are directed at a rotatable anode to generate a primary x-ray beam. The method includes providing the anode with a defect in a surface thereof, rotating the anode, and, while the anode is rotating, directing electrons at the anode from a second source of electrons to generate a secondary beam of x-rays. The intensity of the secondary beam of x-rays along a detection direction changes as the defect interacts with the electrons from the second source of electrons. the method further includes determining a rotation speed of the anode from a frequency at which the intensity of the secondary beam of x-rays changes in response to the interaction of the electrons from the second source with the defect. 
     One advantage of the present invention is that the speed of a rotating x-ray anode is measured. 
     Another advantage of the present invention is that it enables correction of the rotation speed of the anode in response to the detected rotation speed. 
     Another advantage of the present invention is that it enables an x-ray tube to be operated at optimum efficiency for a longer useful life. 
     Another advantage of the present invention is that it enables measurement of anode rotation speed and generation of x-rays to be carried out simultaneously. 
     Another advantage of the present invention is that it avoids the use of complex analytical equipment for determining anode rotation speed. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following data ed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a schematic sectional view of a rotating anode tube according to the present invention; 
     FIG. 2 is a schematic plot of intensity of the x-ray beam received by the detector with time for a defect which directs the x-ray beam toward the detector at times P 1  and P 2 ; 
     FIG. 3 is a schematic plot of intensity of the x-ray beam received by the detector with time for a defect which directs the x-ray beam away from the detector at times P 1  and P 2 ; and 
     FIG. 4 is a schematic sectional view of a rotating anode tube according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a rotating anode x-ray tube of the type used in medical diagnostic systems for providing a beam of x-ray radiation is shown. The tube includes a rotating anode  10  which is disposed in an evacuated chamber  12 , defined typically by a glass envelope  14 . The anode  10  is disk-shaped and beveled adjacent its annular peripheral edge to define a target area  16 . A cathode assembly  18  supplies and focuses an electron beam A which strikes the anode target area  16 . The cathode assembly includes an axially extending housing  20 , mounted to one end of the glass envelope  14 . The cathode assembly  18  also includes a source of electrons  21 , such as a thermionic filament mounted in a cathode cup  22 , off center in the chamber  12 , which directs the beam A of electrons at the target area  16 . Filament leads  26  lead in through the glass envelope  14  and into the housing  20  of the cathode assembly to supply an electrical current. When the electron beam A strikes the rotating anode, a portion of the beam is converted to x-rays which are emitted from the anode target area  16  and a beam B of the x-rays passes out of the x-ray tube through the envelope  14  and a window  28  of a surrounding cooling oil enclosure or housing  30 . It is this beam B of x-rays which serves the medical and diagnostic functions of the x-ray tube. 
     The cathode assembly  18  includes an arm  32  which extends radially between the housing  20  and the cathode cup  22  to position the cup adjacent the target area  16 . 
     An induction motor  40  rotates the anode  10 . specifically, the induction motor includes a stator  42  having driving coils  44 , which are positioned outside the glass envelope  14 , and a rotor  48 , within the envelope, which is connected to the anode  10 . The rotor includes an outer, cylindrical armature or sleeve portion  52  and an inner bearing member or shaft  54 , which is centrally aligned within the armature. The armature  52  is connected to the anode by a neck  60  of molybdenum, or other suitable material. When the motor is energized, the driving coils  44  induce magnetic fields in the armature, which cause the armature to rotate relative to the stationary bearing member. Other types of rotors are also contemplated. 
     A rotation monitoring system  70  detects the rotational speed of the anode  10  as it rotates, preferably in revolutions per minute (rpm). The system  70  includes an x-ray pulse detector  72 , which is positioned within the chamber  12 . FIG. 1 shows the x-ray pulse detector secured by a bracket  74  to the exterior of the housing  20  of the cathode assembly, although other locations are also contemplated. The detector  72  comprises a scintillation material, such as sodium iodide, for the detection of x-rays that are received by the detector. The x-ray detector  72  is preferably situated on the opposite side of the x-ray tube (i.e. generally 180° C.) from the cathode cup  22 , so that the detector is shielded by the cathode assembly and receives little or no x-rays from the portion of the target area  16  adjacent the cathode cup at any given time. 
     A calibrating filament  80  is built into the housing  20  of the x-ray tube also approximately 180° C. from the cathode cup  22 , although other locations in the evacuated chamber are also contemplated. Leads  81  lead in through the glass envelope to the housing  20  to supply an electrical current to the calibrating filament  80 . The calibrating filament generates a small cloud of electrons C, which are focused by a surrounding cup  82 . The electrons are attracted by the voltage applied between the cathode and the anode into a stream of electrons of much lower energy than the stream A produced by the cathode cup  22 , but sufficient to generate a small, low power x-ray beam D when it impinges on the anode  10 . The calibrating filament is positioned and focused such that the stream of electrons strike a known defect  83  on the anode, such as a groove, as the defect passes by the calibrating filament. The positioning of the filament  80  is thus preferably such that the center of the calibrating filament is located on the same bolt circle arc as the known defect  83  in the anode. 
     The known defect  83  can be a hole or pit in the anode surface  84  which faces the cathode assembly  18 , or a surface depression, surface prominence, groove, or the like, i.e., anything that will deflect the radiation beam to or from a predetermined direction. Preferably, the defect is positioned away from the target area  16  of the anode. For example, the defect in FIG. 1 is positioned closer to the center of the anode than the target area in a central portion  86  of the anode surface. It is also contemplated that the defect may be positioned on a surface of the anode which faces away from the cathode cup  22 , such as on a rear surface of the anode. The filament  80  and detector  72  would also be positioned rearward of the anode, to direct electrons and receive x-rays, accordingly. 
     During operation of the anode, the calibrating filament  80  is activated and emits a stream of electrons C that impinge on the anode surface  84 , creating low energy x-rays, which have a first distribution including a ray D directed generally in a first direction. When the known defect  83  moves directly below the filament electron beam, the distribution changes and the x-ray beam D created by the electron beam is momentarily deflected in another, second direction. 
     In one embodiment, the defect increases the radiation along ray D toward the pulse detector  72 , as shown in FIGS. 1 and 2. FIG. 2 shows a schematic plot of x-ray intensity with time for this embodiment. P 1  represents a first pulse corresponding to the interaction of the electron beam C with the defect  83 . As the known defect moves past the electron stream, the x-ray pulse directed at the pulse detector is redirected back to its original condition (i.e., the first direction). Each time the defect passes the electron stream created by the calibrating filament  80 , it sends an x-ray pulse towards the detector  72 , thereby indicating the start of another revolution of the anode as indicated by P 2 . Thus, each revolution of the anode is accompanied by a single pulse P n . 
     In an alternative embodiment, illustrated graphically in FIG. 3, the detector  72  receives the x-ray beam until the defect  83  deflects the beam away from the detector, in a short pulse P 1 . In either embodiment, the detector  72  registers a change in the strength of the x-ray beam each time the defect passes by the filament  80 , i.e., with each revolution of the anode. The time for one rotation is the time between P 1 , and P 2 . 
     Other embodiments are also contemplated, in which the strength of the beam detected by the detector is merely changed as the defect passes by, without complete absence of signal. Similarly, multiple pulses can be generated per revolution by multiple markings on the anode. 
     The pulse detector  72  signals a measurement system  90 , such as a computer control system, which includes electronic circuitry that counts the pulses over time, measures duration between pulses, or measures the frequency of the pulse train and converts the signals to revolutions per minute or other indicator of rotational speed. The speed of the anode is thus monitored without the need to shut off the power to the motor  40 , and consequent momentary braking of the anode rotation during the monitoring process. 
     The defect  83  is preferably intentionally formed, rather than being a naturally occurring defect, and is configured such that the defect deflects the beam of x-rays D with sufficient accuracy and intensity along a preselected angle θ (see FIG. 1) to provide a large x-ray pulse. In this way, the computer control system  90  is able to differentiate a single, large pulse P n  of x-rays with each rotation of the anode  10  (or a single large absence P n  of x-rays in the case of the embodiment of FIG.  3 ). The single pulse is thus distinct from any other changes in the intensity resulting from naturally occurring defects in the anode surface. This avoids the need for providing complex filtering systems or compensating systems in the control system to filter out or compensate for the minor variations in x-ray intensity resulting from natural defects. The computer control system thus registers a single pulse Pn for each rotation of the anode, rather than a plurality of small pulses, resulting from interactions with naturally occurring defects on the anode surface. 
     The information about rotation speed is preferably used in a feedback loop, to adjust the rotation speed of the anode, by supplying more or less current to the driving coils  44 . Specifically, the control system  90  signals a power supply  92 , which delivers the current to the induction motor stator  42 . The control system may include a look-up table  94  which indicates what adjustments are necessary in the power supplied to the motor in order to achieve a desired anode rotation speed. For example, the control system may instruct the motor to increase the pulse width of frequency of the current supplied to the motor if the rotation speed is too low, i.e., below a predetermined minimum speed. The control system reduces the power supplied, or even initiates regenerative braking for a short period of time, if the rotation speed is too high, i.e., above a predetermined maximum speed. 
     Preferably, the control system  90  keeps a record of the measurements made over time. The information may be stored by the control system until accessed by an inspection engineer, and/or printed out periodically for review by the x-ray tube operator. The information can be used to determine x-ray tube performance over time (tube loading and optimization). Scanner electronics can also monitor RV/RW conditions of the rotating anode. The information enables a determination of when the change-out time for the x-ray tube is near and provides an inspection engineer with a record of real time anode performance over the life of the tube. The information also may be used to determine previously undetected customer misuse. 
     Detection of the rotation speed of the anode can be carried out while the first source  18  of x-rays is on or off, and may be carried out continuously or intermittently. 
     With reference to FIG. 4, in an alternative embodiment, an x-ray tube is similar in most respects to the x-ray tube of FIG.  1 . Like parts are numbered with the same numerals. A detection system  170  is similar to the detection system  70  of FIG. 1, except in that the pulse detector  172  is positioned outside the x-ray tube. The detected x-rays D pass directly through the envelope  14  and an appropriately positioned window  174  in the cooling oil enclosure  30  to the detector  172 . 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.