This document discusses fluoroscopy, including conventional fluoroscopy units and modern fluoroscopic units. It describes the key components of a fluoroscopic unit, including the image intensifier, vidicon camera, and TV monitor. It also discusses factors that influence fluoroscopic image quality such as radiation dose rates, image resolution both vertically and horizontally, and techniques to reduce image noise like frame averaging.
12. Intensifier Performance
Conversion factor is the ratio of output phosphor
image luminance (candelas/m2) to x-ray exposure
rate entering the image intensifie (mR/second).
Very difficult to measure: access to output phosphor
No absolute performance criteria
13. Intensifier Brightness Gain (BG)
BG = Minification Gain x Flux Gain
Minification gain (MG): The ratio of the squares of
the input and output phosphor diameters. This
corresponds to “concentrating” the light into a
smaller area, thus increasing brightness
MG = (Input Diamter )2 / (Output Diameter)2
14. Intensifier Brightness Gain (con’t)
Flux Gain (FG): Produced by accelerating the
photoelectrons across a high voltage (>20 keV), thus
allowing each electron to produce many more light
photons in the output phosphor than was required
to eject them from the photcathode.
Summary: Combining minification and flux gains:
15. Intensifier Brightness Gain (con’t)
Example:
Input Phosphor Diameter = 9”
Output Phosphor Diamter = 1”
Flux Gain = 75
BG = FG x MG = 75 x (9/1)2 = 6075
Typical values: a few thousand to >10,000 for
modern image intensifiers
16. Fluoroscopic Noise (Quantum
Mottle)
Fluoroscopic image noise can only be reduced
by using more x-ray photons to produce
image. Can possibly accomplish in 3 ways:
Increase radiation dose (bad for patient dose)
Frame-averaging:
– forms image using a longer effective acquis time
– Can cause image lag (but modern methods good)
Improve Absorption Efficiency of the input
phosphor
29. TV RESOLUTION-Vertical
Conventional TV: 525 TV lines to represent
entire image. Example: 9” intensifier (9” FOV)
1) 9” = 229 mm
2) 525 TV lines/229 mm = 2.3 lines/mm
3) Need 2 TV lines per test pattern line-pair
4) (2.3 lines/mm) /2 lines/line-pair = 1.15 lp/mm
Actual resolution less because test pattern bars
don’t line up with TV lines. Effective resolution
obtained by applying a Kell Factor of 0.7.
Example: 1.15 x 0.7 Kell Factor = 0.8 lp/mm
30. TV RESOLUTION-Horizontal
Along a TV line, resolution is limited by how fast
the camera electronic signal and monitor’s electron
beam intensity can change from minimum to
maximum. This is bandwidth. For similar horiz
and vertical resolution, need 525 changes (262 full
cycles) per line. Example (at 30 frames/second):
262 cycles/line x 525 lines/frame x 30 frames/second
= 4.2 million cycles/second or 4.2 Megahertz (MHz)