Lesson 2 - Differential Absorption

Question 1

What is Differential Absorption

Answer 1

An x-ray image results from the difference between those x-rays absorbed photoelectrically in the patient and those transmitted to the image receptor. Absorption due to photoelectric interaction results in light areas (eg. bone) - radiopaque anatomical structures. Those x-trays transmitted through patient result in dark areas (anatomical structures that are radioluscent). This difference in x-ray attenuation is called differential absorption

Question 2

Differential absorption __________ as the kVp is reduced

Answer 2

Increases. But results in increased patient dose

Question 3

Differences between Density (g/cm3) and Radiographic Density

Answer 3

Density - physical density, patient or body part. Eg) bone is more dense then soft tissue. Bone absorbs more radiation

Radiographic Density - degree of blackening on an x-ray film. The more radiation that hits the IR, the denser (or darker) the image - for film screen.
Radiograph (Film)
Lighter area (under bone) = less dense
darker area (lung tissue) = more dense

Question 4

Radiographic Density (RD):
Increase mAs________
Increase kVp__________

Answer 4

mAs: more x-ray photons, area is more dense radiographically (darker)

kVp: increase energy, also increases quantity, same thing happens (darker - more dense)

Question 5

Soft tissue is _______ dense radiographically because it is _______ dense physically

Answer 5

MORE dense radiographically because it is LESS dense physically

Question 6

To image small differences in soft tissue, one must use _________ kVp to get maximum differential absorption

Answer 6

LOW kVp

Question 7

Define Attenuation

Answer 7

Any process that decreases the intensity of the beam. Absorption, scatter (deflection).
If the beam is attenuated, it has been reduced in intensity.

Question 8

Primary Radiation that has been attenuated is:

Answer 8

Remnant Radiation

Question 9

Define Absorption

Answer 9

Transfer of energy to matter. If radiation is absorbed it no longer exists - it does not hit or expose the IR

Question 10

Factors Affecting Absorption (form of attenuation)

Answer 10

1 - Atomic Number (Z) of absorber: Increase Z, increase absorption
2 - Physical density (g/cm3): Increase physical density, increase absorption
3- Thickness: Increase thickness, increase absorption
4-kVp (penetrating ability): Increase kVp, DECREASE absorption.
*mAs does not affect absorption

Question 11

Lung Tissue Vs Soft Tissue - what absorbs least amount of radiation

Answer 11

They have the same Z, but lung tissue has a lot of air so it has less physical density therefore will absorb the least.

Question 12

Factors that affect attenuation:

Answer 12

Z, Physical Density and Thickness

Question 13

Density in body structures
Soft Tissue:
Bone:

Answer 13

Soft Tissue: not as dense physically - MOST dense radiographically. Image is DARKER (lower atomic number)

Bone: Most dense physically - LEAST dense radiographically, more absorption - better image. (higher atomic number)

Question 14

Differential absorption - what is it?

Due to differences in:

Answer 14

Two areas of an anatomical part absorbing differently. Due to differences in Z, density, thickness.

Question 15

Subject contrast

Answer 15

Tissue contrast, object contrast, radiation contrast.
A measurement of intensity of the remnant beam caused by differential absorption - result of attenuation by the patient. How much differential absorption is occurring.

Question 16

The greater the differential absorption taking place the greater the _______

Answer 16

subject contrast

Question 17

Radiographic Contrast (Film Screen)
What does it depend on?

Answer 17

Differences in density on the radiograph

RC depends on: subject contrast, film contrast, fog and scatter

Question 18

High Vs. Low Contrast

Answer 18

mostly black & whites, few shades of gray. Differences in RD are large. It’s a “High contrast image”

Low: many shades of gray - differences in RD are small, it’s a “Low contrast image”

Question 19

kVp always affects the ________ contrast. In film it also affects the _________ contrast

Answer 19

SUBJECT contrast

In film it also affects the RADIOGRAPHIC contrast. In digital we can adjust on our own so kVp not a factor

Question 20

Contrast Scale

High Contrast
Low Contrast

Answer 20

Radiographic image containing various shades of gray - gray scale imaging.

HIgh Contrast = short scale of contrast
Low contrast - long scale of contrast

Question 21

More shades of gray we can see, the _________ the scale of contrast.
Least shades of gray, the _________ the scale

Answer 21

More shads of gray the longer the scale of contrast

Less shades of gray, the shorter the scale of contrast