Contrast

Question 1

Radiographic contrast

Answer 1

• the difference in OD between adjacent structures; or variation of OD on an image
• result of differential absorption
• function: to make anatomy more visible

Question 2

Balance

Answer 2

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

Question 3

Levels of contrast: high contrast

Answer 3

• aka short-scale contrast
• few densities, but great differences between them
• ‘black and white’

Question 4

Levels of contrast: low contrast

Answer 4

• aka long-scale
• large number of densities, but small differences between them
• ‘lots of grey’

Question 5

Radiographic contrast

Answer 5

• 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

Question 6

Composition and density

Answer 6

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

Question 7

Controlling factor (of contrast)

Answer 7

• 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

Question 8

kVp relationships

Answer 8

⬆️ kVp⬆️ comptons (scatter)
⬆️kVp ⬆️ fog
⬆️kVp ⬇️ interactions
⬆️kVp ⬇️ contrast

Question 9

KVp

Answer 9

• 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)

Question 10

Factors that affect scatter reaching our IR

Answer 10

• grids
• collimation
• OID

Question 11

Grids

Answer 11

• 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

Question 12

Grids- K factor

Answer 12

-contrast improvement factor
-grid usage will at minimum, double the contrast
K= contrast with grid/ contrast without a grid

Question 13

Collimation

Answer 13

-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’

Question 14

OID

Answer 14

• 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

Question 15

Filtration

Answer 15

• 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

Question 16

Scatter and contrast

Answer 16

⬇️scatter = ⬆️ contrast

Question 17

Anatomic part

Answer 17

-amount of contrast achieved is highly influenced by the anatomy being imaged
🔹thickness
🔹composition
-main factors of absorption characteristics

Question 18

Thickness of part

Answer 18

-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

Question 19

Composition

Answer 19

-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 #

Question 20

Contrast media

Answer 20

-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

Question 21

Digital imaging

Answer 21

-digital IRs are MORE sensitive to scatter radiation than film-screen receptors

Question 22

Takeaways

Answer 22

• 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