Tech, Positioning, Special Imaging Flashcards
What part of the x-ray machine PRODUCES x-ray?
The tube comprised of a glass envelope, exit window, cathode (- small and large filaments with focusing cup) and anode (+ target plate slanted 12o)
What is the housing of the tube made of?
Lead-lined steel or aluminum and contains oil
When to use small vs large filaments?
Small: smaller targets, longer exposure time, more penumbra
Large: larger objects, shorter exposure time, less penumbra
Characteristic radiation aka Hard x-rays:
10-15% of beam and doesn’t contribute to formation of image
Bremsstrahlung radiation aka Braking radiation AKA soft x-rays:
85-90% of x-ray beam and creates image
Deflected x-rays create?
Filmfog
Compton effect:
interacts with matter causing ionizing radiation which may damage tissues with lots of H+ (fat)
How does a grid help with the Compton effect?
Reduces scatter radiation that reaches the film
Photoelectric effect:
X-rays completely absorbed by patient and has no effect on film
Radiographic noise:
Unwanted radiographic densities that obstruct viewing of structural details on film
Artifact:
Caused by objects in or on patient like necklaces, rubber bands, implanted medical devices or caused by damage to film emulsion
Three types of mottle:
Quantum: variation in density of uniformly exposed film from random spatial distribution of x-ray quanta absorbed in screen
Structure
Film graininess
Ways to reduce scatter radiation:
Reduce beam size, increasing kVp, decreasing field size (collimation, grid, air-gap),
Thermionic emissions:
Boiling off of electrons from heated filament, current is mA increasing current current increase temperature increases number of electrons generated
Rectifier:
Converts AC to DC voltage measured in kVp and affects speed of electrons
Port filters:
made of aluminum and decrease amount of radiation patient is exposed to and decrease the amount of tissue damage
Collimator:
Device made of two layers of metal laid at right angles narrows field size to reduce pt exposure dose, scatter and improves quality of film
Lead shields
Prevent x-rays from passing through organs like gonads, thyroid, etc
Increasing the mA brings what filament into play? Decreasing?
Large filament
Small filament - sharper image
Good and best grid ratios?
12: 1
6: 1
Put in order: grid, film, pt
Pt, grid, film
Intensifying screen:
Made of small phosphorus crystals, two of them on either side of film, crystals fluoresce and expose film, large crystals increase light and small ones increase detail
Silver halide crystals:
embedded in emulsion layer of gelatin in film plastic which is sensitive to light emitted by screens and x-rays
Six processing steps of film:
- Pre-wet - allow chemicals to penetrate emulsion uniformly
- Develop: in solution for 5 seconds so silver halide crystals reduce to black metallic silver raising temperature 1o will decrease developing time by 20%
- Stop bath: water + acetic acid for 30s to stop action of developer
- Fix: solution for 10 minutes, stops development, hardens film, sodium thiosulfite and reclaim silver from old films
- Wash: removes developer and fixer, prevent brown film
- Dry
Density:
Overall blackness of film
Directly proportional to kVp and mA (1o)
Contrast:
Differences in shades of gray in film and indirectly related to kVp (low contrast high kVP, high contrat, low kVp)
Detail:
How clear structures are visualized
How would you increase detail?
Increae FFD, decrease OFD or kVp
FFD of 40” vs 72”
40: more radiation, penumbra, magnification and less detail
72: less radiation, penumbra, magnification, and more detail
Inverse square law:
radiation intensity is inversely related to square of FFD
50/15 rule:
50% decrease in mAs is equivalent to a 15% increase in kVp
16:20% rule:
Decrease kVp by 16% you have to double mAs or it will halve the film density
If you increase kVp 20% you have to decrease mAs by half or you will double the film density
Increasing kVp will:
increase film exposure, scatter radiation, fog and decrease contrast
Increasing mAs will:
increase film exposure and darken film
Anode heel effect:
Cathode should be closer to larger or fatter part of the object