XR interaction 2

Question 1

what happens if we increase the XR beam energy regarding subject contrast and absorption

Answer 1

Increasing the X-ray beam energy decreases the difference between the absorption of different materials like bone and soft tissue resulting in lower subject contrast
BUT
Reduces the overall absorption of the X-ray beam resulting in lower patient dose

Question 2

what is object contrast

Answer 2

efers to differences in density, thickness and atomic number in materials.

Question 3

what is subject contrast

Answer 3

he density difference between neighbouring regions on a plain radiograph

Question 4

factors affecting subject contrast

Answer 4

Density
-Higher Density: Tissues with higher density (e.g., bone) absorb more X-rays, appearing lighter on the radiograph.
-Lower Density: Tissues with lower density (e.g., fat, air) absorb fewer X-rays, appearing darker.

Thickness
-Thicker Parts: Thicker areas absorb more X-rays, resulting in higher subject contrast.
-Thinner Parts: Thinner areas absorb fewer X-rays, reducing subject contrast.

Atomic number (Z):
-Higher Atomic Number: Materials with higher atomic numbers (e.g., calcium) absorb more X-rays due to increased photoelectric absorption, enhancing contrast.
-Lower Atomic Number: Soft tissues, with lower atomic numbers, absorb fewer X-rays, resulting in less contrast

Question 5

define attenuation

Answer 5

A reduction in the intensity of an X-ray beam as it passes through a medium due to absorption or to scattering

Question 6

what is a monochromatic beam

Answer 6

A monochromatic beam refers to an X-ray or other radiation beam that consists of photons all having the same energy (or wavelength). This is in contrast to a polychromatic beam, which contains photons of a range of energies.

Question 7

what is the equation for a monochromatic beam

Answer 7

𝑁=𝑁_𝑜 𝑒^(−𝜇𝑥)

N - number of transmitted photons
No - number of incident photons
μ - linear attenuation coefficient (cm-1)
x - absorber thickness (cm)

Question 8

The extent to which a material attenuates the X-ray beam depends on what factors?

Answer 8

Thickness & density of material
Type of material
X-ray photon energy

Question 9

linear attenuation coefficient

Answer 9

The linear attenuation coefficient is the sum of individual linear attenuation coefficients for each type of interaction process

-Provide information on the attenuation properties of a medium

-Provide a measure of the effectiveness of a medium as an attenuator

The linear attenuation coefficient is proportional to the density of the material

Question 10

Critical factors affecting attenuation of X-ray photons by matter are:

Answer 10

Thickness of material (t) – the thicker the more attenuation

Linear attenuation coefficient (µ) influenced by:
-Atomic number (Z) – higher Z more attenuating
-Density (ρ) – higher density more attenuating
-Photon energy (E) – higher photon energy less attenuating (except in the region of absorption edges)

Question 11

what is the photoelectric effect

Answer 11

Dominates at low photon energy
Produces excellent radiographic contrast
Responsible for majority of patient dose

Question 12

compton scattering key points

Answer 12

Dominates at high X-ray photon energy
Degrades radiographic contrast if incident on X-ray detector
Can be reduced by:
Collimation of the X-ray beam
Decreasing kV (at expense of patient dose)
Using a grid (at the expense of patient dose)
Using an air gap between the exit of the patient and the entrance of the detector (at the expense of spatial resolution)
Compression of the anatomy (if possible)

Question 13

what is polychromatic radiation

Answer 13

Contains photons of multiple energies. The attenuation of a polychromatic beam is more complex because lower energy photons are attenuated more rapidly, leading to a change in the beam’s energy spectrum as it passes through matter.