Fluoro Physics

Fluoroscopy

• Basically many shitty radiographs with smaller radiation dose played together

• ABC

  • automatic brightness control, with the detector, under the grid

• Differences from radiography

  • smaller focal spot (0.3-1.2 mm)

    • much less heating because use in pulse mode rather than a continuous beam of radiation

  • Pulsed beam (7.5-15 pps or fps)

  • Low tube current

    • <1 to 10 mA (low mA)

    • This is why the image is shitty

    • About 1%% of the dose as a normal XR

• Image intensifier (same as detector for fluoro)

  • CsI

  • Takes XR in —> hits CsI —> puts out light —> hits a cathode —> turns into low energy electrons —> pass through a current which makes electrons move faster through the intensifier —-> hits output phospher —> electrons converted back to light —> light goes to a camera

    • cannot go from original light because it is too faint and would have to give deadly amounts of radiation to get same dose as if you were to just amplify it as here

    • How is light amplified

      • flux gain

        • moves along electric field to make it gain energy

        • multiplies it about 100x

        • Flux gain ~50

        • Basically if you were to point a flashlight at a wall it would vaguely light up the wall but if you start walking closer to the wall it will have a smaller area that is brighter even though the flashlight is still putting out the same amount of power

  • Takes small amount of light and amplifies it about 5000x

• Display monitor

  • Basically image has too much data to put on screen at once so came up with ways to get around that

  • <30 Hz interlaced

    • Put image up and then update every other line

    • lowest frame rate that avoids flicker

  • <60 Hz progressive

• Collimation

  • Lower dose to patient

  • Less scatter

  • Improves image quality

• Temporal filter (recursive frame average)

  • Taking a few frames that were already taken and averaging them together

  • Decreases noise

  • Improves signal to noise ratio by the square root of the number of frames

    • If recursive filtering averages 4 frames the signal to noise ratio increases by 2X

  • Worsens temporal resolution (worsened motion artifact)

  • Decreasing recursive filtering improves temporal resolution and vice versa

    • Want high temporal resolution = means less motion blur

• Monitor

  • refreshes images every 33 ms

  • at 30 image will be shown once on screen before next refresh

  • at 15 pps then monitor will refresh every 2 images you take

    • each acquired frame shown twice on the screen

      • if it was 7.5 then 4x and so on

• Contrast media

  • want 75 kV when able

• Barium meal

  • 110 kV (higher than iodine level), allows you to see through bowel a bit, not just whited out

    • This is above the k-edge on purpose

• Acquisition (exposure) Photospot

  • lower noise than spot frame image

  • larger focal spot and smaller pixel size vs spot

  • 10 spot images = 1 min fluoro

• Cine

  • 8-15x fluoro dose

  • larger focal spot