Patent Publication Number: US-6215850-B1

Title: X-ray beam control for an imaging system

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to x-ray tubes used in imaging systems and more particularly, to a switching unit to control the duration and magnitude of x-ray beams transmitted from an x-ray tube. 
     In at least one known imaging system configuration, an x-ray source projects an x-ray beam. The x-ray beam passes through an object being imaged and after being attenuated by the object, impinges upon a radiation detector. The intensity of the attenuated beam radiation received at the detector is dependent upon the attenuation of the x-ray beam by the object. The detector produces an electrical signal that is a measurement of the beam attenuation. A plurality of attenuation measurements are acquired to produce an image of the object. 
     The x-ray source, sometimes referred to as an x-ray tube, typically includes an evacuated glass x-ray envelope containing an anode, a control grid and a cathode. X-rays are produced by applying a high voltage across the anode and cathode and accelerating electrons from the cathode against a focal spot on the anode by applying a high voltage to the x-ray tube control grid. 
     At least one known imaging system includes a costly grid control power supply as a means of turning on and off the control grid voltage for controlling x-rays from the x-ray source. 
     It would be desirable to provide a switching unit, or circuit, which adjusts the signals applied to the x-ray source so that the magnitude and duration of the x-ray beams emitted from the x-ray tube are altered. It would also be desirable to provide a switching unit which includes any number of modular switching elements which may be combined to provide incremental control of the tube signals as required by the application while minimizing cost of the switching unit. Additionally, it would also be desirable to provide such a unit which utilizes a beam or beams of light to control the switching elements to provide isolation from the high voltage tube signals. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other objects may be attained, in one embodiment, by a switching unit for altering the signals supplied to an x-ray tube to control the duration and magnitude of an x-ray beam emitted from the x-ray tube. More specifically, and in one embodiment, the switching unit controls a grid voltage of the x-ray tube so that the x-ray dosage to the patient is altered. 
     More particularly, and in an exemplary embodiment, the switching unit includes any number of switch elements for altering a grid bias voltage supplied to the x-ray tube, an insulating support structure for securing the modular switch elements together, and an electrostatic shield for eliminating corona discharge from the switch elements. Each switch element utilizes a beam of light excitation signal to alternate between two different modes, or states, of operation. These states of operation are sometimes referred to herein as the conduction state and the steady state. In the conduction state, if an excitation signal is received by the switch element, a switch element voltage drop across the element becomes approximately zero and a maximum signal is applied to the x-ray tube so that a maximum number of x-rays are emitted from the x-ray source. The steady state refers to the condition when an excitation signal is not received by a switch element. In the steady state, a voltage drop is generated by the switch element so that the signal applied to the x-ray tube is decreased by an amount determined by a voltage drop element. 
     In operation, the duration and magnitude of the x-ray beam emitted from the x-ray tube is altered by configuring each switch element in a steady state or conduction state. Specifically, by transmitting a light excitation signal to selected switch elements, the grid bias voltage supplied to the x-ray tube is altered. More specifically, by transitioning individual switch elements between the steady state and conduction state, the magnitude of the x-ray beams emitted from the x-ray tube may be incrementally altered. Particularly, and in one embodiment, the grid bias voltage is incrementally reduced so that the magnitude of the emitted x-ray beam is incrementally reduced. 
     In one embodiment, as a result of the modular configuration of the switching elements, the desired incremental change in the grid control voltage may be determined by combining a selected number of selected voltage drop configuration switching elements. More specifically, a switching unit is fabricated by combining any number of a switching elements, each having a specific voltage drop, in order to reduce cost and provide the proper incremental grid voltage change. 
     The above described switching unit controls x-ray tube signals so that the magnitude and duration of the x-ray beams emitted from the x-ray tube are altered. In addition, the switching unit includes a selectable number of switching elements to incrementally control the signals of the x-ray tube as required by the application while reducing cost of the switching unit. Further, the switching unit provides isolation from the x-ray tube high voltage signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic diagram of an exemplary imaging system. 
     FIG. 2 is a block diagram of the imaging system of FIG.  1 . 
     FIG. 3 is a circuit schematic diagram of a switching unit in accordance with one embodiment of the present switching unit. 
     FIG. 4 is a circuit schematic diagram of a switching unit in accordance with one embodiment of the present invention. 
     FIG. 5 illustrates the physical configuration of switching elements of FIG.  4 . 
     FIG. 6 illustrates the physical configuration of a switching unit in accordance with one embodiment of the pesent invention. 
     FIG. 7 is circuit schematic diagram of a switch unit in accordance with an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic diagram of an exemplary embodiment of an x-ray imaging system  10  including an x-ray tube, or source  14 , an x-ray detector  18 , and an x-ray controller  20 . Generally, by supplying the appropriate signals from controller  20  to tube  14 , an x-ray beam  22  is radiated from tube  14  toward detector  18 . In one embodiment, an object  23 , for example a patient, is interposed between x-ray tube  14  and detector  18 . System  10  generates an image of object  23  by determining the intensity of x-ray beam  22  at detector  18  in a manner known in the art. Particularly and referring to FIG. 2, x-ray beam  22  is radiated toward object  23  by supplying a high voltage, typically up to 150,000 volts, to an anode  24  with respect to a cathode  26  of tube  14 . In one embodiment, a large negative control voltage, or bias voltage, signal is supplied to a control grid  28  of tube  14 . Adjustment of the duration and magnitude of the grid bias voltage signal alters, or adjusts, the duration and magnitude of x-ray beam  22 . As a result of different imaging requirements, the duration and magnitude of x-ray beam  22  is altered so that the x-ray dosage received by object  23  is determined by the signal applied to control grid  28 . For example, in order to improve the quality of the image of a patient&#39;s vascular system, the control grid signal supplied to tube  14  is altered so that the radiated x-ray energy coincides with a particular portion of a patient&#39;s heart pumping cycle. 
     Referring again to FIG.  2  and in one embodiment, controller  20  includes a power source means, or power supply  30  and a switching unit, or circuit  32  to alter the signals supplied to source  14 . Power supply  30  is coupled to x-ray tube  14  and switching unit or means,  32  to supply signals to tube, or x-ray emitting means,  14  and unit  32 . More particularly, voltage and current signals from supply  30  are supplied to anode  24  and cathode  26  of tube  14 . A high voltage signal is also supplied from supply  30  to switching unit  32 . Utilizing control signals  34  supplied to switching unit  32 , for example, signals from a control panel source or computer (not shown), switching unit  32  alters the signals supplied to tube  14 . More specifically, by altering signals  34 , the signal supplied to control grid  28  of tube  14  is altered so that the speed at which the electrons travel from anode  24  to cathode  26  is modified, therefore, altering the magnitude and duration of x-ray beams  22  emitted from tube  14 . 
     In one embodiment and referring to FIG. 3 and 4, switching unit  32  includes at least one switching element  40  to alter the control grid voltage signal supplied to control grid  28 . More specifically as shown in FIG. 3, unit  32  includes a single element  40  and as shown in FIG. 4, unit  32  includes six elements  40 . Each switching element  40  includes a receiver  60  which is configured to detect an excitation, or control signal  34 . For example, receiver  60  includes at least one photo-optic device  70 , i.e. a opto-coupler or photodiode, for receiving a light, or illumination excitation signal  34  in order to provide isolation from the high voltage signals present within switching unit  32 . Each element  40  also includes a diode  72 , a transistor  74 , a capacitor  76 , a field effect transistor (FET)  78 , and a voltage drop element, or means for generating a voltage drop  80 . 
     Voltage drop element  80  may, for example, be a zener diode which generates a selected voltage drop. Voltage drop element  80  may, in alternative embodiments is a spark gap or any other suitable device to regulate or control the voltage across FET  78 . Each voltage drop element  80  is selected to generate an appropriate voltage drop to provide incremental change to the control voltage as required by the specific application. For example, in order to control the emission of x-ray beam  22  as required, the voltage drop value of a drop element  80  of a first element  40  is 1000 volts, the voltage drop of a drop element  80  is 1000 of a second element  40 , the voltage drop value of a drop element  80  of a third element  32  is 1000 volts, the voltage drop value of a drop element  80  of a fourth element  40  is 1000 volts, the voltage drop value of a drop element  80  of a fifth element  40  is 1000 volts, and the voltage drop value of a drop element  80  of a sixth element  40  is 1000 volts. 
     More specifically and in one embodiment of each switching element  40 , receiver  60  includes photodiodes  62 ,  64 , and  66  for receiving signal one or more of excitation signals  34 . Anode of photodiode  64  is connected to cathode of photodiode  62  and anode of photodiode  66  is connected to cathode of photodiode  64 . Anode of diode  72  and the base of transistor  74  are connected to receiver  70 , specifically anode of photodiode  62 . The junction of cathode of diode  72  and emitter of transistor  74  is connected to capacitor  76  and the gate of FET  78 . The junction of receiver  70 , specifically cathode of photodiode  66 , the collector of transistor  74 , capacitor  76 , the source of FET  78  and a first end of voltage drop element  80  is connected to the junction of cathode  26  and power supply  30 , for example to a −KV signal. The junction of a second end of voltage drop element  80  and the drain of FET  78  is connected to control grid  28  of source  14 . A second lead of cathode  26  is connected to power supply  30 . Anode  24  of tube  14  is connected to power supply  30 , for example to a +KV signal. 
     Each element  40  has two different modes, or states of operation. These states of operation are referred to herein as the steady state and the conduction state. The steady state refers to that state of element  40  when the excitation signal  34  is not being supplied to element  40 . In steady state, therefore, receiver  60  is not enabled and no current flows through receiver  60 . Consequently, the voltage applied to the base of transistor  74  decreases to zero. As a result, current flows from emitter to collector of transistor  74  discharging the voltage across capacitor  76  to approximately zero. By discharging capacitor  76 , the voltage applied to the gate of FET  78  is zero and current through the source and drain of FET  78  is stopped. Therefore, in the steady state, a voltage drop across element  40  is approximately equal to the voltage drop of element  80 . 
     In the conduction state, at least one excitation signal  34  is applied to receiver  60  so that transistor  74  transitions to a non-conducting state which causes the voltage to develop sufficiently across capacitor  76 . As a result, FET  78  transitions to a conducting mode, and current flows from the source to the drain of FET  78  so that the voltage drop across element  80  is approximately equal to zero. As a result, the voltage drop across element  40  is approximately equal to zero. 
     For example, in one embodiment where unit  32  includes a single switching element  40  having a 1,000 voltage drop element  80 , in the steady state, the voltage signal supplied to control grid  28  from unit  32  is the voltage signal supplied from power supply  30  to unit  32  less the voltage drop across element  80 , i.e, 1,000 volts. If, in one embodiment, the output of power supply  30  is −20,000 volts, in the steady state mode approximately −19,000 volts is supplied to control grid  28  and a voltage drop of approximately 1,000 volts exists across drop element  80 . In the conduction state, the voltage drop across element  80  is approximately zero and the current flows through FET  78  so that approximately −20,000 volts is supplied to control grid  28 . 
     In the embodiment shown in FIG. 4, switching unit  32  utilizes a plurality of switch elements  40  and excitation signals  34  so that the total voltage drop across switch unit  32  is altered to change the duration and magnitude of the x-ray beams emitted from tube  14 . Specifically, each switch element may be placed in the steady state or conduction mode so that the total voltage drop varies the according to the combined value of drop elements  80 . 
     More specifically, the desired voltage and incremental voltage step size to be supplied to tube  14  is altered by the selection of the voltage drop of each drop element  80  and the number elements  40  to meet the requirements of imaging system  10 . Specifically, each switch element includes a selected voltage drop element  80 . In one embodiment, unit  32  is configured so that tube  14  is transitioned between emitting x-ray beams  22  and preventing x-ray beams  22  from being emitted by simultaneously transitioning each switch element between the steady and conduction states. As a result of transitioning between these two states, the time period, or duration, of emitting x-ray beams  22  is controlled. 
     In addition, the magnitude of the x-ray beams transmitted by tube  14  is altered by placing less than all of switch elements  40  in the conduction mode. Specifically, the voltage and current applied to tube  14  is altered by placing at least one, but less than all, of switch element  40  in the conduction state. As a result, each switch element  40  placed in the steady state mode will generate a voltage drop so that the voltage signal supplied to control grid  28  is reduced to less than the voltage supplied from power supply  30  to unit  32 . 
     For example, unit  32  may be configured so that the voltage drop across unit  32  is selectable between 0 and 3,875 volts in 125 volt increments. In one embodiment, three switch elements  40  each have a voltage drop element  80  of 1000 volts, one element  40  has a voltage drop element  80  of 500 volts, one element  40  has a voltage drop element  80  of 250 volts and one element  40  has a voltage drop element  80  of 125 volts. By transmitting individual excitation signals  34  to specific selected elements  40  the voltage drop of unit  32  is altered. Specifically, by transmitting an excitation signal to two switch elements  40 , having drop elements of 2,000 volts, placing these elements  40  in the conduction state, a voltage drop of 1,875 volts (1,000+500+250+125 or 3,875−2,000) is generated across unit  32 . As a result, the voltage signal applied to control grid  28  is the voltage signal supplied to cathode  26  from power supply  30  minus the 1,875 voltage drop across unit  32 . In addition to combining any number of switch elements  40 , each of switch element  40  may include a voltage drop element  80  of any size. For example, an inventory of standard switch elements  40  having different standard voltage drop elements, i.e., 1,000 volts, 500 volts, 250 volts, 125 volts, may be fabricated. By combining the proper number of each element  40 , the specific requirements of an application may be achieved. 
     More specifically and as shown in FIG. 5, elements  40 , in one embodiment, are configured to interconnect with each other so that additional elements may be quickly and easily added or removed to achieve the desired total voltage drop and voltage drop increment size of unit  32 . Specifically, modular switch elements  40  are coupled together utilizing intermodule connectors  100 . The voltage and current signals are transmitted from unit  32  to tube  14  utilizing an external high voltage cable (not shown in FIG. 5) coupled to switch elements  40 . 
     In one embodiment, excitation signals  34  are supplied to unit  32  utilizing signal connectors  102 . In one embodiment, each signal connector  102  includes an electrical connection and an opto-coupling device (not shown). Each opto-coupling device converts a respective electrical excitation signal  34  to a light excitation signal which is transmitted to receiver  70 . In alternative embodiments, connectors  102  are optical ports for receiving a light signal  34 . For example, signal connectors  102  may be a lens, light pipe, or fiber optic cable. 
     In one embodiment shown in FIG. 6, switch unit  32  includes an insulating support structure  110  and is coupled to power supply  30  utilizing a high voltage cable  112 . Structure  110  includes an electrostatic shield  114  which is coupled to ground potential to eliminate corona discharge from switch elements  40 . High voltage cable  112  includes a connector  116  that is coupled to unit  32 . Specifically, connector  116  couples to intermodule connector  100 . 
     Unit  32  is fabricating by selecting the appropriate quantity of switch elements each having the desired voltage drop element based on the voltage and current signals to be applied to tube  14 . Specifically, the total voltage drop and incremental voltage drop size are utilized to determine the quantity of switch elements and the particular voltage drop element  80  for each switch element. The selected switch elements are coupled together utilizing the intermodule connectors  100  and then secured to insulating support structure  110 . High voltage cable  112  is then coupled to switch elements  40  via connector  116 . 
     In operation, after determining the desired configuration of the x-ray beams to be emitted from tube  14 , the proper excitation signals  34  are transmitted to unit  32 . In one embodiment, excitations signals  34  are timed so that the x-ray beams are emitted from tube  14  only when image data, or information, is being collected by system  10 . After the data has been collected, excitation signals  34  are transitioned so that the excitation signals  34  are not transmitted to unit  32 . Consequently, the x-ray beams are not emitted from tube  14 . Utilizing unit  32 , the x-ray beams are emitted only when needed and turned off when the x-ray beams are not being used to generate image data. As a result, the x-ray dosage received by patient  24  is reduced. Additionally, the magnitude of the x-ray beams emitted from tube  14  may be altered by selectively transmitting individual excitation signals  34  to unit  32  as described above. 
     In another alternative embodiment, shown in FIG. 7, unit  200  alters the duration and magnitude of the x-ray beams by altering the voltage and current signals applied to cathode  66  of tube  14 . Unit  200  is identical to unit  32  as described above, except the duration and magnitude of x-ray beams emitted from tube  14  are altered by modifying the voltage and current applied to cathode  26 . Specifically, by applying different excitation signals  34  to unit  200 , the voltage drop across unit  200  is altered so that the voltage and current signal applied to cathode  26  is altered. 
     The above described switching unit controls x-ray tube signals so that the magnitude and duration of the x-ray beams emitted from the x-ray tube are altered. In addition, the switching unit includes a selectable number of switching elements to incrementally control the signals of the x-ray tube as required by the application while reducing cost of the switching unit. Further, the switching unit provides isolation from the x-ray tube high voltage signals. 
     From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. For example, although the described switch unit includes one or more switch elements, the switch unit may also be configured to include one switch element having multiple voltage drop elements so that the duration and magnitude of the x-ray beams may be altered. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.