Image Quality Flashcards
Minification Gain
- describes what occurs within the II when a large input screen produces electrons, then the electrons must trace to the small output screen
- the same number of electrons must be compressed to the smaller area which increases brightness or intensity of image itself
- there is a gain in brightness because the signal is concentrated
minification gain = input screen diameter² / output screen diameter²
Flux Gain
- ratio of number of light photons at the output phosphor to the number of xray photons at the input phosphor (conversion of electron energy into light energy)
- how many output light photon do you get for every xray photon that strikes the input phosphor
flux gain = # of output light photons / # of input xray photons
Total Brightness Gain
- measurement of the increase in image intensity achieved by the II (bight ness with II vs without II)
total brightness gain = flux gain x minification gain
How does an over exposed fluoro image look? Underexposed?
- over exposed: white, washed out
- under exposed: black, dark image
Automatic Brightness Control (ABC)
- maintains the image density and contrast at the image monitor by adjusting the radiation out put of the xray tube
- maintains constant brightness by adjusting technique (mA, kV, pulse width)
- increase and decrease patient dose
- helps control flare
Flare
occurs when moving from thick to thin body part, image becomes white and washed out
Geometric Magnification
- changes by adjusting SID, SOD, and OID
- decreased magnification = decrease in OID (patient is closer to II)
Automatic Gain Control
- similar to ABC, also adjusts image brightness but is part of the video signal rather than adjusting technical factors
- no change to patient dose
Electronic Magnification
- occurs within the II when tech engages mag mode
- in the II the focal spot moves further from the output screen which creates magnification
- this process utilized less xray photons = reduced minification gain
- an increase in mA must be made to maintain brightness (increases patient dose)
- spacial resolution increase because peripheral edges not visualized
Spacial Resolution
- fluoro: 2-5 lp/mm
- gen rad: 10 lp.mm
- maintaining higher spacial resolution would require prohibited amount of dose
- smaller pixel pitch = better spacial resolution
- spacial resolution increases when mag mode engaged
- lower OID = higher spacial resolution
- under table decrease spacial resolution, over table tube increases spacial resolution
Veiling Glare
- arises from incoming light/xray photons that stray from the normal image path and reach the output phosphor
- aluminum filter present but not 100% accurate
- decreases contrast resolution
Normal Mode vs Mag Mode
normal mode:
- large FOV
- minification gain higher
- lower spacial resolution
- image distortion higher
- lower radiation dose
mag mode:
- smaller FOV
- lower minification gain
- higher spacial resolution
- image distortion lower
- higher radiation dose
Size Distortion
- caused by change sin OID
- distortion will appear worse than with mag mode but thins just because is bigger and easier to see
Shape Distortion
- primarily caused by II design
- makes up 8-10% of imaging area
- pincushion and vignetting
Pincushion
- only in II
- occurs as electrons travel from a curved surface like the input screen to a flat screen like the out put screen
- the electrons from the peripheral edges flare outwards due to repulsion from other electrons or attraction to electrostatic lens
