Fluoroscopic Image Intensifier Image Production

Minification Gain Example Minification Gain = Input screen diameter2 Occurs as a result of the same number of electrons produced at the large input screen being compressed into an area of the smaller output screen Minification Gain = Input screen diameter2 Output screen diameter2 ^ is the symbol for squared if using MS Excel Typical output screen diameter is 1” (2.5cm) What is the minification gain if the input screen diameter is 12” and the output screen diameter is 1”? Example Minification Gain = 122 12 = 144 1 = 144

Flux Gain Measurement of increase in light photons due to conversion efficiency of the output screen Flux gain deals only with the gain accomplished by the electron to light conversion at the output screen This will cause a decrease in image quality exactly like an intensifying screen causing decreased resolution in diagnostic images due to the penumbral effect of individual phosphor crystals If the output phosphor produces 50 light photons for each electron that strikes it, the flux gain would be 50. Example

Total Brightness Gain = Minification gain x Flux gain Measurement of the multiplication of the light photons at the output phosphor compared to the incident radiation at the input phosphor Total Brightness Gain = Minification gain x Flux gain What is the total brightness gain from an II tube with a minification gain of 36 and a flux gain of 60? Example Total Brightness Gain = 36 x 60 = 2,160

Another Brightness Gain Measure Conversion factor = cd/m2 or (cd/m2/mR/sec) mR/sec Ratio of light intensity of output phosphor to input exposure rate Conversion factor = Intensity of output phosphor / mR / sec Candelas (cd) is a measurement of output phosphor light intensity (which is a unit of luminous intensity) Typical values of modern image intensifier systems are cd/m2 mR/sec 80-250 This represents a gain of 8,000 – 25,000 times

Brightness Gain Deterioration As much as 10% a year due to aging of input and output phosphors (Diagnostic intensifying screens also age in the same manner) Brightness gain can be evaluated by monitoring radiation dose required to obtain diagnostic image using a standardized fluoroscopic phantom When required dose increases to a significant amount, the image intensifier tube should be replaced

Magnification Increased voltage to electrostatic lenses results in increased electron deflection Causes focal point to move or shift toward input screen Results in reduced primary field of view being magnified prior to the striking the output screen Creates “analog electronic magnification” so to speak Electron flow of reduced primary field of view being magnified via a focal point shift prior to striking the output screen

input screen image cannot be viewed by output screen Magnification Tubes These tubes often termed: Multi- Dual- Triple- or Quad-field intensifiers Magnification image intensifiers are capable of 1.5 – 4.0 levels of magnification Resolution can be increased from ~ 4 lp/mm to 6 lp/mm When operating in magnification mode, electron image created at outer edge or periphery of input screen image cannot be viewed by output screen Therefore, primary photon beam should be collimated to useful viewing field of intensifier when operating in a magnification mode Minimal magnification used for most studies

Magnification Factor Example Magnification = Diameter of image intensifier input screen Diameter of input screen utilized during magnification Image intensification tubes described according to diameter of area of input screen utilized Magnification is in direct proportion to ratio of input screen to output screen diameters A 23/15cm dual-focus tube has a 9” (23cm) input screen when operating normally and uses a 6” (15cm) area when operating in a magnified mode. What is the magnification factor? Example Magnification = 9” / 6” = 1.5 magnification factor

Magnification Effects Minification gain reduced due to fewer photoelectrons being incident at the output phosphor as a result of magnification (Results in a dimmer image) To maintain brightness, tube mA must be increased to produce more x-rays Results in more x-rays per unit area present at input phosphor and, consequently, increased patient dose Magnification modes will result in increased: Spatial Resolution Contrast Patient Dose Rate

Please close this PowerPoint presentation, and continue the lesson. What’s Next? Please close this PowerPoint presentation, and continue the lesson. Presented by Based on: Principles of Radiographic Imaging, 4th Ed. By: R. Carlton & A. Adler Radiologic Science for Technologists, 8th Ed. By: S. Bushong Syllabus on Fluoroscopy Radiation Protection, 6th Rev. By: Radiologic Health Branch – Certification Unit