Contrast Flashcards
- makes details visible
- a psychovisual perception
- the variation in density levels that makes detail visible
contrast
- caused by attenuation and differential absorption
- variation, changes or differentiation of adjacent densities on the radiograph
- produced by photoelectric effect
contrast
its primary function is to make detail visible
contrast
differentiations between densities or details be readily made
satisfactory contrast
___ controls differential absorption of the x-ray beam by the body because of its control of x-ray beam energy
kVp controls differential absorption of the x-ray beam by the body because of its control of x-ray beam energy
- inherent in the screen-film combination
- influenced somewhat by processing of the film
image receptor contrast
- affected by film (IR), film type & direct/IS exposure
- affected by range of ODs and film processing technique
image receptor contrast
film-screen images have _______ contrasts than direct film exposure images
film-screen images have HIGHER contrasts than direct film exposure images
controlled by anatomy and pathology; affects contrast through variations of Z, tissue thickness, and tissue density
subject contrast
most important factor in controlling subject contrast
kVp
subject contrast
↓ kVp = _ contrast
subject contrast
↓ kVp = ↑ contrast
photoelectric effect varies in proportion to the cube of Z
effective atomic number
enhances contrast by enhancing photoelectric absorption
contrast media
objects that coincide with the x-ray beam have high subject contrast
object shape
caused by the shape of the structure
absorption blur
- The difference in density/brightness between adjacent values are great.
- The transition from white to black areas are fast and abrupt.
- Can be achieve with the use of low kVp.
- Is usually pleasing to the eye, but does not give better diagnostic value.
high contrast
high contrast
↑ contrast = _ gray tones / short-scale contrast
high contrast
↑ contrast = ↓ gray tones / short-scale contrast
low contrast
↓ contrast = _ gray tones / long-scale contrast
low contrast
↓ contrast = ↑ gray tones / long-scale contrast
- The differences in density/brightness between adjacent values are small
- Transition from black to white areas are slow and there are many shades of gray that represent varying tissues in the body
- Can be achieve with the use of high kVp.
low contrast
means the range of optical densities from the lightest to the blackest part of the radiograph
scale of contrast
there is great differences between available densities
high contrast
low kVp
short scale contrast
there is little differences between available densities
many shades of gray
low contrast
high kVp
long scale contrast
____ is the only factor that can control contrast of the radiograph
kVp is the only factor that can control contrast of the radiograph
controls the differential absorption of the x-ray beam in the body
kVp
OID
↑ OID (15 to 25 cm) = air gap technique = _ scatter cleanup (equivalent to 8:1 grid ratio) = _ contrast
↑ OID (15 to 25 cm) = air gap technique = ↑ scatter cleanup (equivalent to 8:1 grid ratio) = ↑ contrast
air gap technique disadvantage
_ magnification, _ SID, _ mAs to maintain density
air gap technique disadvantage
↑ magnification, ↑ SID,↑ mAs to maintain density
Grid
↓ scatter reaching the image receptor = _ scatter fog = _ gray tones = _ contrast
↓ scatter reaching the image receptor = ↓ scatter fog = ↓ gray tones = ↑ contrast
limits are being irradiated
beam restriction
beam restriction
↓ field size = _ number of Compton interactions
= _ scatter = _ patent dose = _ gray tones = ↑ contrast
↓ field size = ↓ number of Compton interactions
= ↓ scatter = ↓ patent dose = ↓ gray tones = ↑ contrast
as filtration is ↑, beam becomes ______ (average photon striking the patient has shorter wavelength)
as filtration is ↑, beam becomes HARDER (average photon striking the patient has shorter wavelength)
↑ filtration = harder photons = _ Compton scatter = _ contrast
↑ filtration = harder photons = ↑ Compton scatter = ↓ contrast
air-gap technique = _ contrast (owing to air filtration)
air-gap technique = ↓ contrast (owing to air filtration)
aka subject contrast
anatomy and pathology
cause differential absorption of x-ray photons, which results in contrast
anatomy and pathology
anatomy and pathology
↑ atomic number = _ contrast
(owing to photoelectric effect)
↑ tissue density = _ contrast
(owing to Compton scattering)
↑ tissue thickness = _ contrast
(owing to Compton scattering)
use of a contrast agent = _ contrast
(owing to photoelectric effect)
↑ atomic number = ↑ contrast
(owing to photoelectric effect)
↑ tissue density = ↓ contrast
(owing to Compton scattering)
↑ tissue thickness = ↓ contrast
(owing to Compton scattering)
use of a contrast agent = ↑ contrast
(owing to photoelectric effect)
↑ processing time = ↑ chemical fog = _ contrast
↑ processing time = ↑ chemical fog = ↓ contrast
↑ chemical temperature = ↑ chemical fog = _ contrast
↑ chemical temperature = ↑ chemical fog = ↓ contrast
In digital images, _______ can be enhanced through post-processing and is mostly affected by the pixel bit depth available in the system.
In digital images, CONTRAST can be enhanced through post-processing and is mostly affected by the pixel bit depth available in the system.
number of bits available for each pixel
pixel bit depth
