Method and structure for optimizing radiographic quality by controlling X-ray tube voltage, current focal spot size and exposure time

Abstract

A structure and method for providing optimum recording of X-ray images without need for experimenting to learn optimum X-ray tube voltage, current, anode focal spot size and exposure time. The method and structure of this invention sample the radiation passed through an object to be radiographed during a short portion of the total exposure time and adjust voltage, current and focal spot size so that the radiation delivered by the X-ray tube during the remainder of the exposure time will produce optimum contrast between structures within the object being radiographed, also optimum sharpness of the image and optimum darkening of a film, xerographic picture, fluoroscopic image, or other recording medium. The method and structure of this invention account for variations in absorption coefficient between one object to be radiographed and the next. This invention is particularly useful for medical applications, and in the medical field, particularly important in mammography.

I claim: 
     
       1. An X-ray apparatus comprising an X-ray tube having a cathode which emits electrons and an anode on which said electrons impinge and which is a source of X-rays, said tube having an operating voltage which is the voltage drop between said anode and said cathode, a tube current which is the current between said anode and said cathode, and a focal spot size which is the area of said anode on which said electrons impinge, and in which said voltage, said current .[.and said current.]. and said focal spot size have initial settings;   a collimator having an opening through which said X-rays leaving said source in the direction of said opening may pass;   means for holding tissue to be examined in position for being irradiated;   means for showing an image of X-rays which have passed through said tissue;   at least one sensor which generates sensor signals for detecting radiation passed through said tissue and through said means for showing an image, and for calibrating said X-ray apparatus; and   means for receiving said sensor signals from said at least one sensor and based on said sensor signals sending control signals which control said operating voltage, said tube current, and said focal spot size.   
     
     
       2. An X-ray apparatus as in claim 1 in which said at least one sensor is two sensors, a first sensor located to detect radiation passed through said tissue and a second sensor located to detect radiation not passed through said tissue for calibrating said X-ray apparatus, and in which said means for receiving is a microprocessor which determines a tissue attenuation coefficient using said sensor signals from said first and second sensors. 
     
     
       3. An X-ray apparatus as in claim 1 in which said means for receiving comprises: a microprocessor which determines tissue attenuation coefficient and based on said coefficient sends signals which cause said operating voltage, said tube current, and said focal spot size to increase, decrease, or remain unchanged.   
     
     
       4. An X-ray apparatus as in claim 2 in which said microprocessor comprises means for determining maximum radiation dosage, midpoint dosage in said tissue, and average dosage of glandular tissue accumulated during an exposure, and displaying said radiation dosages on a display panel. 
     
     
       5. An X-ray apparatus as in claim 1 in which said at least one sensor is one sensor located to detect radiation passed through said tissue, and usable when said tissue is not present for calibrating said X-ray apparatus, and in which said means for receiving includes an integrator, a calibration signal source, and a comparator, and in which said comparator serves to cause said operating voltage to increase when the output of said integrator is less than the output of said calibration signal source by a first selected amount;   cause said operating voltage to decrease when the output of said integrator is more than the output of said calibration signal source by a second selected amount;   cause both said operating voltage and said focal spot size to increase when the output of said integrator is less than the output of said calibration signal source by a third selected amount larger than said first selected amount; and   cause both said operating voltage and said focal spot size to decrease when the output of said integrator is more than the output of said calibration signal source by a fourth selected amount larger than said second selected amount.   
     
     
       6. An X-ray apparatus as in claim 5 in which said means for receiving further comprises: an adaptor/interface which receives signals from said comparator and based on said signals from said comparator sends signals to a controller of said X-ray tube causing said controller to increase, decrease or not change said voltage, tube current, and focal spot size of said X-ray tube.   
     
     
       7. An X-ray apparatus as in claim 1 in which said at least one sensor is one sensor located to detect radiation passed through said tissue, and used when said tissue is not present for calibrating said X-ray apparatus, and in which said means for receiving is a microprocessor which calculates a calibration coefficient using calibration signals from said one sensor taken while operating said X-ray apparatus when said tissue is not present, and   calculates a tissue absorption coefficient using said calibration coefficient and exposure signals taken from said one sensor while operating said X-ray apparatus when said tissue is present.   
     
     
       8. An X-ray apparatus as in claim 7 in which said microprocessor further comprises a table of optimum operating voltage and optimum focal spot size and current as a function of tissue absorption coefficient, and said microprocessor sends signals causing said X-ray tube to operate at said optimum operating voltage and said optimum focal spot size and current. 
     
     
       9. An X-ray apparatus as in claim 7 in which said microprocessor further controls exposure time. 
     
     
       10. An X-ray apparatus as in claim 7 further comprising a thickness sensor for sensing thickness of said tissue and in which said microprocessor receives signals from said thickness sensor and uses said signals from said thickness sensor to calculate said optimum voltage, current and focal spot size. 
     
     
       11. An X-ray apparatus as in claim 1 in which said at least one sensor is at least one scintillation counter. 
     
     
       12. An X-ray apparatus as in claim 1 in which said at least one sensor is at least one semiconductor. 
     
     
       13. An X-ray apparatus as in claim 12 in which said semiconductor is a photodiode. 
     
     
       14. An X-ray apparatus as in claim 1 in which said tissue is breast tissue and said image is a mammogram. 
     
     
       15. An X-ray apparatus as in claim 1 in which said initial settings are average settings for the type of tissue being exposed. 
     
     
       16. An X-ray apparatus as in claim 1 where said initial settings are optimal settings of the previous exposure. 
     
     
       17. An X-ray apparatus as in claim 1 in which said means for showing said image is photosensitive film. 
     
     
       18. An X-ray apparatus as in claim 17 in which said photosensitive film is adjacent to a film screen sensitive to X-ray photons. 
     
     
       19. An X-ray apparatus as in claim 1 in which said means for showing said image is a fluoroscope. 
     
     
       20. An X-ray apparatus as in claim 1 in which said means for showing said image in an electrostatically printed page. 
     
     
       21. An X-ray apparatus as in claim 1 in which said means for showing said image is a recording on a digital recording means. 
     
     
       22. An X-ray apparatus as in claim 1 in which said means for showing an image of X-rays which have passed through said tissue includes a grid device, a film screen cassette, a breast tray, and an X-ray film. 
     
     
       23. An X-ray apparatus as in claim 1 further comprising a front panel on which are displayed said optimum values of current, voltage, a focal spot size. 
     
     
       24. A method for optimizing an X-ray image from an X-ray apparatus comprising an X-ray tube having a cathode which emits electrons and an anode on which said electrons impinge and which is a source of X-rays, said tube having an operating voltage which is the voltage drop between said anode and said cathode, a tube current which is the current between said anode and said cathode, and a focal spot size which is the area of said anode on which said electrons impinge, and in which said voltage, said current and said focal spot size have initial settings;   a collimator having an opening through which said X-rays leaving said source in the direction of said opening may pass;   means for holding tissue to be examined in position for being irradiated;   means for showing an image of X-rays which have passed through said tissue;   at least one sensor which generates sensor signals for detecting radiation passed through said tissue and through said means for showing an image, and for calibrating said X-ray apparatus; and   means for receiving said sensor signals from said at least one sensor and based on said sensor signals sending control signals which control said operating voltage, said tube current, and said focal spot size; comprising the steps of:   positioning said tissue;   determining thickness of said tissue and providing said tissue thickness to said microprocssor;   operating said X-ray apparatus at said initial settings;   in a sampling time small in comparison to an expected total exposure time calculating optimum voltage, current and focal spot size settings for the tissue being exposed;   at the end of said small sampling time changing voltage, current and focal spot size settings to said optimum voltage, current and focal spot size settings for the tissue being exposed; and   continuing to operate said X-ray apparatus at said optimal settings until an optimum visualization of said means for showing an image is achieved.   
     
     
       25. A method as in claim 24 where said tissue comprises breast tissue, said means for holding comprises breast compression plates, and said determining tissue thickness comprises determining separation between said breast compression plates. 
     
     
       26. A method as in claim 24 further comprising the step of calibrating said sensors positioning said tissue. 
     
     
       27. A method as in claim 26 further comprising additional calibration in the form of providing information from a second sensor during exposure. 
     
     
       28. A method for optimizing an X-ray image from an X-ray apparatus as in claim 24 wherein said at least one sensor is one sensor and said step of calculating optimum voltage, current and focal spot size for the tissue being exposed comprises: before said step of positioning said tissue, operating said X-ray apparatus at said initial settings, and determining current through said one sensor when said tissue to be examined is not in position for being irradiated;   determining thickness of said tissue to be examined in position for being irradiated;   determining current through said one sensor when said tissue to be examined is being irradiated;   calculating a tissue absorption coefficient as ##EQU9## where μ is absorption coefficient of said tissue,   i cal  is said current through said one sensor when said tissue is not in position for being irradiated,   i exp  is said current through said one sensor when said tissue is being irradiated,   x is thickness of said tissue to be examined in position for being irradiated,   D cal  is distance from source to sensor when said tissue is not in position for being irradiated, and   D exp  is distance from source to sensor when said tissue is being irradiated; and   determining said optimum voltage, current, and focal spot size as a function of said absorption coefficient and said initial settings of voltage, current, and focal spot size.   
     
     
       29. A method for optimizing an X-ray image from an X-ray apparatus as in claim 28 wherein said means for receiving said sensor signals comprises a microprocessor and said step of determining said optimum voltage, current, and focal spot size as a function of said absorption coefficient and said initial settings of voltage, current and focal spot size comprises reading said optimum voltage, current, and focal spot size from a table stored in said microprocessor. 
     
     
       30. A method for optimizing an X-ray image from an X-ray apparatus as in claim .[.22.]. .Iadd.24 .Iaddend.wherein said at least one sensor is two sensors, a first sensor located in the path of X-rays which have passed through said tissue and through said means for showing an image, and a second sensor located in the path of X-rays which have not passed through said tissue, and said step of calculating optimum voltage, current and focal spot size settings for the tissue being exposed comprises: before positioning said tissue but after locating said means for producing an image, said first sensor, and said second sensor.[...]..Iadd.; .Iaddend.   determining a normalization factor N between said first and second sensors by operating said X-ray apparatus and measuring current through said first sensor iN 1 , current through said second sensor iN 2 , distance from said X-ray source to said first sensor DN 1 , and distance from said X-ray source to said second sensor DN 2 , and calculating N=iN 1  DN 1   2  /iN 2  DN 2   2  ;   then before a particular exposure of said tissue,   determining distance D 1  from said source of X-rays to said first sensor;   determining distance D 2  from said source of X-rays to said second sensor;   after said positioning said tissue, said determining said thickness x, and said operating said X-ray apparatus at said initial settings in said sampling time small compared to said expected total exposure time, determining current through said first sensor i 1 , and current through said second sensor i 2  ;   calculating a tissue absorption coefficient as ##EQU10## and determining said optimum voltage, current, and focal spot size as a function of said absorption coefficient and said initial settings of voltage, current, and focal spot size.   
     
     
       31. A method for optimizing an X-ray image from an X-ray apparatus as in claim 30 wherein said means for receiving said sensor signals comprises a microprocessor and said step of determining said optimum voltage, current, and focal spot size as a function of said absorption coefficient and said initial settings of voltage, current and focal spot size comprises reading said optimum voltage, current, and focal spot size from a table stored in said microprocessor. 
     
     
       32. A method for optimizing an X-ray image as in claim 24 in which said step of calculating optimum voltage produces a voltage calculation to the nearest 1 kV. 
     
     
       33. A method for optimizing an X-ray image as in claim 24 comprising the further step of displaying said optimum voltage, current, and focal spot size on a panel. 
     
     
       34. An X-ray apparatus comprising an X-ray tube having a cathode which emits electrons and an anode on which said electrons impinge and which is a source of X-rays, said tube having an operating voltage which is the voltage drop between said anode and said cathode, a tube current which is the current between said anode and said cathode, and a focal spot size which is the area of said anode on which said electrons impinge, and in which said voltage, said current and said focal spot size having initial settings;   a collimator having an opening through which said X-rays leaving said source in the direction of said opening may pass;   means for holding an object to be examined in position for being irradiated;   means for showing an image of X-rays which have passed through said object;   at least one sensor which generates sensor signals for detecting radiation passed through said object and through said means for showing an image, and for calibrating said X-ray apparatus; and   means for receiving said sensor signals from said at least one sensor and based on said sensor signals sending control signals which control said operating voltage, said tube current, and said focal spot size. .Iadd.   
     
     
       35.  An x-ray apparatus comprising: means for generating x-rays, said means for generating having an operating anode to cathode voltage;   means for showing an image of x-rays which have passed through an object being examined;   means for determining thickness of said object;   at least one sensor for detecting radiation passed through said object and through said means for showing an image, said at least one sensor generating sensor signals in response to said radiation; and   means for receiving said sensor signals from said at least one sensor and receiving said thickness from said means for determining thickness, and based on said sensor signals and said thickness sending control signals which control said operating anode to cathode voltage. .Iaddend. .Iadd.   
     
     
       36.  An x-ray apparatus as in claim 35 in which said control signals further control exposure time. .Iaddend. .Iadd.37. An x-ray apparatus as in claim 35 in which an operating current flows between said cathode and said anode, and said control signals further control said operating current. .Iaddend. .Iadd.38. An x-ray apparatus as in claim 35 in which electrons impinge on said anode at a focal spot having a focal spot size; and said control signals further control said focal spot size. .Iaddend. .Iadd.39. An x-ray apparatus as in claim 35 in which said at least one sensor is one sensor. .Iaddend. .Iadd.40. An x-ray apparatus as in claim 35 in which said at least one sensor is two sensors, a first sensor located to detect radiation passed through said object being examined and a second sensor located to detect radiation not passed through said object. .Iaddend. .Iadd.41. An x-ray apparatus as in claim 35 in which said means for receiving comprises: a microprocessor which determines a value related to attenuation coefficient of said object and based on said value sends signals which cause said operating voltage to increase, decrease, or remain unchanged. .Iaddend. .Iadd.42. An x-ray apparatus as in claim 41 in which said object comprises tissue and said microprocessor comprises means for determining maximum radiation dosage, mid-point dosage to said tissue, and average dosage to glandular tissue accumulated during an exposure, and displaying said radiation dosages on a display panel. .Iaddend. .Iadd.43. An x-ray apparatus as in claim 41 in which said object comprises tissue, said apparatus further comprising a thickness sensor for sensing thickness of said tissue and in which said microprocessor receives signals from said thickness sensor and uses said signals from said thickness sensor to determine said optimum voltage. .Iaddend. .Iadd.44. An x-ray apparatus as in claim 35 in which said object comprises tissue and said means for receiving comprises a microprocessor which determines a value related to density of said tissue and based on said value sends signals which cause said operating voltage to increase, decrease or remain unchanged. .Iaddend. .Iadd.45. An x-ray apparatus as in claim 35 in which said at least one sensor is at least one scintillation counter. .Iaddend. .Iadd.46. An x-ray apparatus as in claim 35 in which said at least one sensor is at least one semiconductor. .Iaddend. .Iadd.47. An x-ray apparatus as in claim 46 in which said semiconductor is a photodiode. .Iaddend. .Iadd.48. An x-ray apparatus as in claim 35 wherein said apparatus is a mammography device and in which said object is breast tissue and said image is a mammogram. .Iaddend. .Iadd.49. An x-ray apparatus as in claim 35 in which said means for showing said image is photosensitive film. .Iaddend. .Iadd.50. An x-ray apparatus as in claim 49 in which said photosensitive film is adjacent to a film screen sensitive to x-ray photons. .Iaddend. .Iadd.51. An x-ray apparatus as in claim 35 in which said means for showing said image is a fluoroscope. .Iaddend. .Iadd.52. An x-ray apparatus as in claim 35 in which said means for showing said image is an electrostatically printed page. .Iaddend. .Iadd.53. An x-ray apparatus as in claim 35 in which said means for showing said image is a recording on a digital recording means. .Iaddend. .Iadd.54. An x-ray apparatus as in claim 48 in which said means for showing an image of x-rays which have passed through said tissue includes a grid device, a film screen cassette, a breast tray, and an x-ray film. .Iaddend. .Iadd.55. An x-ray apparatus as in claim 35 further comprising a front panel on which is displayed said optimum voltage. .Iaddend.   
     
     
        .Iadd.     A method for optimizing an x-ray image from an x-ray apparatus comprising the steps of: positioning an object to be exposed;   determining thickness of said object;   operating said x-ray apparatus at an initial anode to cathode voltage;   after a sampling time small in comparison to an expected total exposure time making a determination related to an optimum anode to cathode voltage for said object based on said thickness and on radiation passed through said object at said initial anode to cathode voltage, and changing said initial anode to cathode voltage to said optimum anode to cathode voltage; and   continuing to operate said x-ray apparatus at said optimum anode to cathode voltage until an optimum visualization of said means for showing an image   
     
     
        is achieved. .Iaddend. .Iadd.57.  A method for optimizing an x-ray image as in claim 56 in which said initial anode to cathode voltage is changed to said optimum anode to cathode voltage directly after said sampling time. .Iaddend. .Iadd.58. A method for optimizing an x-ray image as in claim 56 further comprising the steps of: operating said x-ray apparatus at an initial current;   after said sampling time determining an optimum current; and   changing said initial current to said optimum current. .Iaddend. .Iadd.59. A method for optimizing an x-ray image as in claim 56, further comprising the steps of:   operating said x-ray apparatus at an initial focal spot size;   after said sampling time determining an optimum focal spot size; and   changing said initial focal spot size to said optimum focal spot size.   
     
     
        .Iaddend. .Iadd.60.  A method for optimizing an x-ray image as in claim 56 further comprising the step of calibrating a sensor before positioning said object. .Iaddend. .Iadd.61. A method for optimizing an x-ray image as in claim 60 further comprising additional calibration in the form of providing information from a second sensor while exposing said object. .Iaddend. .Iadd.62. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said initial anode to cathode voltage is an average anode to cathode voltage for the type of object being exposed. .Iaddend. .Iadd.63. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said initial anode to cathode voltage is an optimal anode to cathode voltage of a previous exposure. .Iaddend. .Iadd.64. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said step of making a determination related to an optimum anode to cathode voltage comprises: determining the integral of voltage provided by a first sensor which detects radiation from said x-ray apparatus passing through said object;   determining the integral of voltage provided by a second sensor which detects radiation from said x-ray apparatus not passing through said object;   determining the ratio of said integrals; and   determining from a table an attenuation factor corresponding to said ratio. .Iaddend. .Iadd.65. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said step of positioning an object comprises positioning tissue. .Iaddend. .Iadd.66. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said step of making a determination related to an optimum anode to cathode voltage comprises:   determining the integral of voltage provided by said sensor which detects radiation passing through said object from said x-ray apparatus being operated at said initial anode to cathode voltage; and   determining from a table an attenuation factor corresponding to said integral. .Iaddend. .Iadd.67. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said object comprises tissue and said step of making a determination related to an optimum anode to cathode voltage comprises:   determining the integral of voltage provided by said sensor which detects radiation passing through said tissue from said x-ray apparatus being operated at said initial anode to cathode voltage; and   determining a tissue density corresponding to said integral. .Iaddend. .Iadd.68. A method for optimizing an x-ray image from an x-ray apparatus as in claim 56 in which said object comprises tissue and said step of making a determination related to an optimum anode to cathode voltage comprises:   determining the integral of voltage provided by a first sensor which detects radiation passing through said tissue from said x-ray apparatus being operated at said initial anode to cathode voltage;   determining the integral of voltage provided by a second sensor which detects radiation from said x-ray apparatus not passing through said tissue;   determining the ratio of said integrals; and   determining a tissue density corresponding to said ratio. .Iaddend.   
     
     
        .Iadd.  .  An x-ray apparatus comprising: means for generating x-rays, said means for generating having an operating anode to cathode voltage;   means for compressing tissue;   means for showing an image of x-rays which have passed through said tissue;   at least one sensor for detecting radiation passed through said tissue and through said means for showing an image, said at least one sensor generating sensor signals in response to said radiation; and   means for receiving said sensor signals from said at least one sensor and based on said sensor signals sending control signals which control said   
     
     
        operating anode to cathode voltage. .Iaddend. .Iadd.70.  An x-ray apparatus as in claim 69 further comprising means for controlling anode to cathode current. .Iaddend. .Iadd.71. An x-ray apparatus as in claim 69 further comprising means for controlling focal spot size. .Iaddend. .Iadd.72. An x-ray apparatus as in claim 69 further comprising means for controlling exposure time. .Iaddend. .Iadd.73. A method for optimizing an x-ray image from an x-ray apparatus comprising the steps of: positioning and compressing tissue to be exposed;   operating said x-ray apparatus at an initial anode to cathode voltage;   after a sampling time small in comparison to an expected total exposure time, making a determination related to an optimum anode to cathode voltage for said tissue based on radiation passed through said tissue at said initial anode to cathode voltage, and changing said initial anode to cathode voltage to said optimum anode to cathode voltage; and   continuing to operate said x-ray apparatus at said optimum anode to cathode voltage until an optimum visualization of means for showing an image is achieved. .Iaddend. .Iadd.74. A method for optimizing an x-ray image as in claim 73 further comprising:   after said sampling time making a determination related to an optimum anode to cathode current for said tissue, and changing an initial anode to cathode current to said optimum anode to cathode current; and   continuing to operate said x-ray apparatus at said optimum anode to cathode current until an optimum visualization of said means for showing an image is achieved. .Iaddend. .Iadd.75. A method for optimizing an x-ray image as in claim 73 further comprising:   after said sampling time making a determination related to an optimum focal spot size for said tissue, and changing an initial focal spot size to said optimum focal spot size; and   continuing to operate said x-ray apparatus at said optimum focal spot size until an optimum visualization of said means for showing an image is achieved. .Iaddend. .Iadd.76. An x-ray apparatus comprising:   means for generating x-rays, said means for generating having an operating anode to cathode voltage;   means for showing an image of x-rays which have passed through an object being examined;   a first sensor for detecting radiation passed through said object and through said means for showing an image, and a second sensor for detecting radiation not passed through said object, said sensors generating sensor signals in response to said radiation; and   means for receiving said sensor signals from said sensors and based on said sensor signals sending control signals which control said operating anode to cathode voltage. .Iaddend. .Iadd.77. An x-ray apparatus as in claim 76 further comprising means for controlling anode to cathode current. .Iaddend. .Iadd.78. An x-ray apparatus as in claim 76 further comprising means for controlling focal spot size. .Iaddend. .Iadd.79. An x-ray apparatus as in claim 76 further comprising means for controlling exposure time. .Iaddend. .Iadd.80. A method for optimizing an x-ray image from an x-ray apparatus comprising the steps of:   positioning an object to be exposed;   operating said x-ray apparatus at an initial anode to cathode voltage;   after a sampling time small in comparison to an expected total exposure time, making a determination related to an optimum anode to cathode voltage for said object based on radiation passed to a first sensor through said object and on radiation passed to a second sensor not through said object at said initial anode to cathode voltage, and changing said initial anode to cathode voltage to said optimum anode to cathode voltage; and   continuing to operate said x-ray apparatus at said optimum anode to cathode voltage until an optimum visualization of means for showing an image is   
     
     
        achieved. .Iaddend. .Iadd.81.  A method for optimizing an x-ray image as in claim 80 further comprising: after said sampling time making a determination related to an optimum anode to cathode current for said object, and changing an initial anode to cathode current to said optimum anode to cathode current; and   continuing to operate said x-ray apparatus at said optimum anode to cathode current until an optimum visualization of said means for showing an image is achieved. .Iaddend. .Iadd.82. A method for optimizing an x-ray image as in claim 80 further comprising:   after said sampling time making a determination related to an optimum focal spot size for said object, and changing an initial focal spot size to said optimum focal spot size; and   continuing to operate said x-ray apparatus at said optimum focal spot size until an optimum visualization of said means for showing an image is achieved. .Iaddend. .Iadd.83. A method for optimizing an x-ray image from an x-ray apparatus comprising the steps of:   positioning an object to be exposed; operating said x-ray apparatus at an initial anode to cathode voltage;   after a sampling time small in comparison to an expected total exposure time making a determination related to an optimum anode to cathode voltage for said object, comprising: determining the integral of voltage provided by a sensor which detects radiation from said x-ray apparatus passing through said object, and   determining from a table a value related to attenuation factor corresponding to said integral;     changing said initial anode to cathode voltage to said optimum anode to cathode voltage; and   continuing to operate said x-ray apparatus at said optimum anode to cathode voltage until an optimum visualization of said means for showing an image is achieved. .Iaddend.