RT 209 - image intensified fluoroscopy

• fluoroscopy 


  to aid the radiologist in dynamic studies of the human body
• fluoroscopy 


  "real time" imaging
• 1896 


  thomas edison invented the fluoroscope made of calcium tungstate screen
• 1941 


  william chamberlain studies on the poor illumination from fluoroscopic screen
• 1950 


  development of image intensifier tube
• illumination 


  measured in units of lamberts (L) and millilamberts (mL)
• radiographs are viewed under illumination level of 10 to 1000 mL
• • rods and cones
  • photopic and scotopic vision
  • visual acuity 


  human vision: (3)
• • image intensifier
  • recording system
  • viewing system 


  fluoroscopic chain: (3)
• image intensifier 


  electronic vacuum tube that converts the remnant beam to light then to electrons, then back to light, increasing the light intensity in the process
• image intensifier 


  complex electronic device that receives the remnant x-ray beam, converts it into light and increases the light intensity
• image intensifier 


  • it brightened the image significantly
  • allows for a means to indirectly viewing the fluoroscopic image
• • input phosphor
  • photocathode
  • accelerating anode
  • output phosphor
  • electrostatic focusing lenses 


  basic parts of an image intensifier: (5)
• input phosphor 


  • made of cesium iodide (CsI)
  • converts energy to visible light
  • CsI crystals are tightly packed as 100 to 200 micrometer layer
• photocathode 


  • composed of cesium and antimony compounds
  • emits electrons when stimulated by light (photoemission)
• accelerating anode 


  maintains constant potential of approximately 25 kV
• output phosphor 


  • electrons interact to produce light
  • usually made of silver activated zinc-cadmium sulfide
• electrostatic focusing lens 


  • located along the length of image intensifier tube
  • not really lenses, but are negatively charged plates along the length of the image intensifier tube
• image intensifier tube 


  approximately 50 cm in length and 15 to 58 cm in diameter
• image intensifier tube 


  high energy electrons that interact with the output phosphor each results in substantially more light photon than was necessary to cause their release at the photocathode
• flux gain 


  the ratio of the number of light photons at the output phosphor to the number of x-rays at the input phosphor
• flux gain 


  represents the tube's conversion efficiency
• minification gain 


  an expression of the degree to which the image is minified from the input phosphor to output phosphor
• minification gain 


  the ratio of the square of the diameter of the input phosphor to the square of the diameter of the output phosphor
• minification gain 


  this characteristic makes the image brighter because the same number of electrons is being concentrated on a smaller surface area
• • output phosphor size is fairly standard at 2.5 or 5 cm
  • input phosphor size varies from 10 to 35 cm
  • brightness gain of most image intensifiers is 5000 to 20,000
• brightness gain 


  is an expression of the ability of an image intensifier tube to convert x-ray energy into light energy and increase the brightness of the image in the process
• brightness gain 


  the ability of the image intensifier tube to increase the illumination level of the image
• brightness gain formula

  brightness gain = minification gain x flux gain
• conversion factor 


  is an expression of the luminance at the output phosphor divided by the input exposure rate
• conversion factor 


  recommended by the ICRU to quantify the increase in brightness created by image intensifier
• exposure rate and age of the image intensifier 


  as an image intensifier ages, the exposure rate to the patient increases to maintain brightness
• automatic brightness control (ABC) 


  a function of the fluoroscopic unit that maintains the overall appearance of the fluoroscopic image by automatically adjusting the kVp, mA or both
• magnification mode or multi-field mode 


  the voltage to the electrostatic focusing lenses is increased
• magnification mode or multi-field mode 


  the increase tightens the diameter of the electron stream and the focal point is shifted father from the output phosphor
• magnification mode 


  the effect is that only those electrons from the center area of the input phosphor and contribute to the image, giving the appearance of magnification
• magnification mode 


  the degree of magnification (MF) may be found by dividing the full-size input diameter by the selected input diameter
• magnification mode formula

  MF = 30/15 = 2 x magnification
• magnification 


  improves the fluoroscopist's ability to see small structures (spatial resolution) but at the price of increasing patient dose
• fluoroscopic systems - 4 to 6 Lp/mm