Lesson 2 - Differential Absorption Flashcards
What is Differential Absorption
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
Differential absorption __________ as the kVp is reduced
Increases. But results in increased patient dose
Differences between Density (g/cm3) and Radiographic Density
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
Radiographic Density (RD):
Increase mAs________
Increase kVp__________
mAs: more x-ray photons, area is more dense radiographically (darker)
kVp: increase energy, also increases quantity, same thing happens (darker - more dense)
Soft tissue is _______ dense radiographically because it is _______ dense physically
MORE dense radiographically because it is LESS dense physically
To image small differences in soft tissue, one must use _________ kVp to get maximum differential absorption
LOW kVp
Define Attenuation
Any process that decreases the intensity of the beam. Absorption, scatter (deflection).
If the beam is attenuated, it has been reduced in intensity.
Primary Radiation that has been attenuated is:
Remnant Radiation
Define Absorption
Transfer of energy to matter. If radiation is absorbed it no longer exists - it does not hit or expose the IR
Factors Affecting Absorption (form of attenuation)
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
Lung Tissue Vs Soft Tissue - what absorbs least amount of radiation
They have the same Z, but lung tissue has a lot of air so it has less physical density therefore will absorb the least.
Factors that affect attenuation:
Z, Physical Density and Thickness
Density in body structures
Soft Tissue:
Bone:
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)
Differential absorption - what is it?
Due to differences in:
Two areas of an anatomical part absorbing differently. Due to differences in Z, density, thickness.
Subject contrast
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.
The greater the differential absorption taking place the greater the _______
subject contrast
Radiographic Contrast (Film Screen) What does it depend on?
Differences in density on the radiograph
RC depends on: subject contrast, film contrast, fog and scatter
High Vs. Low Contrast
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”
kVp always affects the ________ contrast. In film it also affects the _________ contrast
SUBJECT contrast
In film it also affects the RADIOGRAPHIC contrast. In digital we can adjust on our own so kVp not a factor
Contrast Scale
High Contrast
Low Contrast
Radiographic image containing various shades of gray - gray scale imaging.
HIgh Contrast = short scale of contrast
Low contrast - long scale of contrast
More shades of gray we can see, the _________ the scale of contrast.
Least shades of gray, the _________ the scale
More shads of gray the longer the scale of contrast
Less shades of gray, the shorter the scale of contrast
