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
Scatter and contrast
⬇️scatter = ⬆️ contrast
Anatomic part
-amount of contrast achieved is highly influenced by the anatomy being imaged
🔹thickness
🔹composition
-main factors of absorption characteristics
Thickness of part
-as the thickness increases, the amount of scatter increases (resulting in decreased contrast)
-if kVp is increased, it will only add to the amount of scatter
🔹this will further degrade the quality of the film
🔹mAs would be the adjustment of choice
-compression devices and positioning can help
-decreasing the thickness will decrease patient dose, and increase contrast
Composition
-anatomy that has similar tissue types
🔹LOWER subject contrast as compared with
-anatomy with a wide range of tissue composition
🔹HIGH subject contrast
-tissues with higher atomic #’s absorb more radiation than those with a lower atomic #
Contrast media
-used to image anatomy with low subject contrast (soft tissue structures most commonly)
-PE interactions vary with atomic #
🔹with larger differences in atomic #, subject contrast will increase
-changes absorption characteristics of tissue
🔹increase/decrease attenuation of beam
-POSITIVE CONTRAST AGENT
🔹high atomic #
🔹absorbs more x-rays than surrounding tissues
🔹barium, iodine
-NEGATIVE CONTRAST AGENT
🔹low atomic #
🔹decreased attenuation: more transmission
🔹air
Digital imaging
-digital IRs are MORE sensitive to scatter radiation than film-screen receptors
Takeaways
- kVp is the primary controlling factor of radiographic CONTRAST
- change in kVp results in change in ratio of PE: Compton
- high kVp settings result in long contrast scales (low contrast)
- low kVp settings result in short contrast scales (high contrast)
- subject contrast is influenced by thickness and composition of the body part being imaged
