Contrast Flashcards
Radiographic contrast
- the difference in OD between adjacent structures; or variation of OD on an image
- result of differential absorption
- function: to make anatomy more visible
Balance
If an entire image was made up of only: -direct transmission 🔹extremely high contrast image 🔹very sharp -indirect transmission 🔹no image, only grey 🔹image solely made up of scatter -combo will give us moderate contrast
Levels of contrast: high contrast
- aka short-scale contrast
- few densities, but great differences between them
- ‘black and white’
Levels of contrast: low contrast
- aka long-scale
- large number of densities, but small differences between them
- ‘lots of grey’
Radiographic contrast
- is the product of image receptor contrast and subject contrast
- simplest, most common procedure is to: standardize the IR contrast and make appropriate adjustments to our subject contrast
Composition and density
Composition of anatomy
-important factor to consider when choosing technical factors
-ideal contrast maximizes the amount of information visible for diagnosis
Radiographic density
-if too high or too low, cannot assess radiographic contrast
-must be acceptable level
Controlling factor (of contrast)
- kVp is considered to be the controlling factor for contrast
- controls: quality or penetrability of the beam
- for adequate contrast, the part must be sufficiently penetrated by the beam
kVp relationships
⬆️ kVp⬆️ comptons (scatter)
⬆️kVp ⬆️ fog
⬆️kVp ⬇️ interactions
⬆️kVp ⬇️ contrast
KVp
- high kVp means less absorption, more transmission, fewer density differences (also known as low contrast)
- low kVp means more absorption, less transmission, greater density differences (also known as high contrast)
Factors that affect scatter reaching our IR
- grids
- collimation
- OID
Grids
- absorb scatter that leaves the patient before it reaches the IR
- placed BETWEEN the patient and IR
- less scatter on the image, means increased contrast
- the higher the efficiency (ratio), the greater the effect on contrast
Grids- K factor
-contrast improvement factor
-grid usage will at minimum, double the contrast
K= contrast with grid/ contrast without a grid
Collimation
-variation in the size of the x-ray field changes the amount of tissue being irradiated
-more collimation
🔹less tissue, less scatter
-less collimation
🔹more tissue irradiated, more scatter
-‘more matter, more scatter’
OID
- air gap technique
- gap prevents scatter from reaching IR
- increased OID also DECREASES density (compensation with mAs may be needed)
- not as effective with high kVp settings
Filtration
- filters out the low energy photons, resulting in a higher percentage of high energy photons (more penetrating beam)
- with increased x-ray energy, scatter is increased, and contrast is decreased
