Fluoroscopic Unit Thomas Edison 1896

Fluoroscopy Dynamic imaging Spot films 3-5 mA kVp and mA determine brightness level Automatically adjusted during fluoro by automatic brightness control (ABC)

Image Intensifier Electronic vacuum tube Converts the remnant beam to light, then to electrons, then back to light Remnant beam-light electrons light light intensity in the process Parts Input Phosphor Photocathode Electrostatic focusing lenses Accelerating anode Output phosphor

Input Phosphor Made of Cesium iodide Absorbs the remnant x-ray photon energy and emits light in response Side toward the tube Side toward the patient Bonded to the curved surface of the tube

Photocathode Made of cesium and antimony compounds These metals emit electrons in response to light stimulus in a process called Photoemission Bonded directly to the input phosphor Curved so that all of the electrons emitted travel the same distance to the output phosphor

Electrostatic Focusing Lens Negatively charged plates along the length of the image intensifier tube Repel the electron stream, focusing it on the small output phosphor

Accelerating Anode Sets the electron stream in motion at a constant velocity Maintains constant potential of 25kV

Output Phosphor Silver-activated zinc cadmium sulfide Absorbs electrons and emits light in response End result An increase in image intensity and brightness

Magnification Mode Focal point is shifted farther from output phosphor Voltage to electrostatic focusing lenses is increased. The increase tightens the diameter of electron stream and focal point is shifted Increase in pt dose Spatial Resolution is improved Smallest structure that may be detected Measure in line pairs/mm Magnification If fewer electrons are incident on the output phosphor , the output intensity decreases What does the ABC do to compensate? (increase radiation) What does this do to pt dose? (increase)

Magnification 9-inch Input” button, you’ll see that an image of the figure appears smaller and inverted near the output phosphor – this simulates a normal mode in fluoroscopy. When you click on the “6-inch Input” button, the input phosphor size is reduced, the output phosphor size remains the same and the image is magnified.

Intensification Principles Brightness gain(Conversion Factor) Ability of image intensifier tube to convert x-ray energy into light energy & increase the brightness of the image Minification gain x flux gain Flux gain Ratio of #light photons at the output phosphor to the # of light photons emitted in the input phosphor Represents the tubes conversion efficiency Conversion factor Luminance at output phosphor input exposure rate Appears brighter because the same # of electrons are being concentrated on a smaller surface area What do you think happens to the brightness as the tube ages? (deteriorates) How do you compensate for the decrease in brightness? (use more radiation) What happnes to pt dose? (increase)

image appears brighter Minification gain – the brightness gain caused from a decrease in size from the input to the output phosphor image appears brighter Input phosphor daimeter2 / output phosphor diameter2

Automatic Brightness Control (ABC) Adjusts and maintains the overall image density and contrast during fluoroscopy Determined by the brightness control setting that the radiographer sets Contrast and density By automatically adjusting kVp and mA or both

Distortion Misrepresentation of true size or shape of an object Caused by the curved shape of the photcathode Inaccurate control or focusing of electrons released at photocathode Pincushion appearance Vignetting-loss of brightness around the edge of the fluoroscopic image caused by the curve of the photocathode Loss of brightness

Viewing Systems The signal is sent to a television monitor for viewing through the Vidicon tube Diode tube in a glass envelope Connected to the output phosphor Digital fluoroscopy Computerized images from output phosphor No need for overhead/spot films

Camera Tube

Cathode Cathode Electron gun Provides continuous stream of electrons Control grid Forms the electron stream into a beam

Anode Anode Face plate Signal plate Target