X-ray interaction 2- subject contrast, attenuation

Question 1

Subject contrast

Answer 1

Some of the x-ray photons are absorbed but some are transmitted. Bone tends to attenuate x-ray photons more than tissues. This gives us subject contrast.

Question 2

Factors affecting subject contrast: material thickness

Answer 2

If we increase the material we increase the probability of the probability of x-ray photons being absorbed by the thicker material, more likely to be more attenuation.

Question 3

Factors affecting subject contrast: Density

Answer 3

If we increase the density for a given energy between the material we increase the likeliness of absorption.

Question 4

Factors affecting subject contrast: Absorption spectrum

Answer 4

If we have an absorption spectrum we have to think about what our emission spectrum should be for our x-ray source to get good subject contrast.

If we arrange our spectrum then we will get a difference between the absorption of the bone and the soft tissue. Contrast between bone and soft tissue is what we will see. If we increase the mean x-ray photon beam energy and push up to higher energy, the difference between the absorption of the bone and soft tissues go down.

Question 5

What does Increasing the X-ray beam energy do to the absorption?

Answer 5

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 6

Object contrast

Answer 6

if we have region of interest in patient the object contrast difference in the density of the materials thickness and atomic number.

Question 7

Subject contrast function of xray beam energy is influenced by

Answer 7

Subject contrast function of xray beam energy , influenced by kv and the filtration applied to xray beam, function the material were trying to image density thickness and atomic number.

Question 8

Subject Contrast: Equation

Answer 8

𝐶=(𝐼𝐵−𝐼𝐴)/𝐼𝐵
It has values between 0 and 1, or is sometimes expressed as a percentage.

Question 9

What is Attenuation?

Answer 9

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

Question 10

Attenuation: x-ray photon scattering

Answer 10

X-ray photons that experience scattering are viewed as having been attenuated even though they are not absorbed

Question 11

Attenuation: Monochromatic Beam equation

Answer 11

𝑁=𝑁𝑜e^−𝜇𝑥
N - number of transmitted photons
No - number of incident photons
μ - linear attenuation coefficient (cm-1)
x - absorber thickness (cm)

Question 12

The extent to which a material attenuates the X-ray beam depends on a number of factors:

Answer 12

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

Question 13

Attenuation in bone, muscle lung

Answer 13

For the same thickness of material, bone has a higher attenuation than muscle, and muscle has a higher attenuation than lung tissue

Question 14

Linear Attenuation Coefficient equation

Answer 14

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

𝜇= 𝜇photo + 𝜇compton
In the diagnostic energy range, μ decreases with increasing energy except at absorption edges (e.g. K-edge)

Arises from 2 processes from 2 processes, photoelectric absorption and compton scattering.
Theyre the only 2 processes that are going to attenuate our x-ray beam.

Question 15

Linear Attenuation Coefficient: Example

Answer 15

At 40 keV, the linear attenuation coefficient for water is 0.24 cm-1
0.018 is from coherent scattering
0.18 is from Compton scattering
0.042 is from photoelectric attenuation.

Question 16

Linear attenuation coefficients: role

Answer 16

Provide information on the attenuation properties of a medium

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

Question 17

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

Answer 17

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 18

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

Answer 18

µwater > µice >water vapour

Question 19

Mass Attenuation Coefficient equation

Answer 19

µ/p = linear attenuation coefficient (µ)/ density of material (p)

This dependency can be normalized out by dividing the linear attenuation coefficient by the density of the material
The units of the mass attenuation coefficient are cm2/g

Question 20

Attenuation: Polychromatic Radiation equation

Answer 20

N(E) = N𝑜(E)e’-µ(E)x

N(E) - number of transmitted photons as a function of energy
No(E) - number of incident photons as a function of energy
μ - linear attenuation coefficient (cm-1) as a function of energy
x - absorber thickness (cm)

Question 21

X-ray photons are attenuated mainly via two processes: ?

Answer 21

Photoelectric effect
Compton scattering

Question 22

What does Photoelectric effect do?

Answer 22

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

Question 23

Compton Scattering key points

Answer 23

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 24

X-ray Imaging Factors (General Comments)

Answer 24

Low kV: low mean beam energy
High subject contrast – PE absorption dominates
High dose
May not achieve sufficient patient penetration
Increasing mAs may not be possible due to X-ray tube limitations
Insufficient X-ray photon fluence at the detector increases image noise

Question 25

X-ray Imaging Factors
High kV generally leads to:

Answer 25

Lower subject contrast due to higher levels of scatter and lower differential absorption between materials
Lower patient dose (less patient absorption)
May be able to reclaim lost contrast with digital image processing techniques

Question 26

1. For a given X-ray photon energy if the thickness of a material is increased then its absorption will
   a. decrease
   b. stay the same
   c. increase
   d. only increase near and absorption edge

Answer 26

increase

Question 27

2. For a given X-ray photon energy if the density of a material is increased then its absorption will
   a. decrease
   b. stay the same
   c. increase
   d. only increase near and absorption edge

Answer 27

increase

Question 28

3. The X-ray absorption of a given material will
   a. always decrease as the incident X-ray photon energy is increased
   b. always increase as the incident X-ray photon energy is increased
   c. always decrease as the incident X-ray photon energy is increased except at absorption edges (such as the k-edge)
   d. remain the same as incident X-ray photon energy is increased

Answer 28

c. always decrease as the incident X-ray photon energy is increased except at absorption edges (such as the k-edge

Question 29

4. Which one of the following does subject contrast not depend?
   a. object density
   b. object thickness
   c. incident X-ray energy
   d. detector type
   e. X-ray beam filtration

Answer 29

detector type

Question 30

5. In radiography attenuation usually refers to
   a. a reduction in the energy of an X-ray beam due to absorption or scattering by a medium through which it passes
   b. a reduction in the intensity of an X-ray beam due to absorption or scattering by a medium through which it passes
   c. a reduction in the number of electrons in an X-ray beam due to absorption or scattering a medium through which it passes
   d. a reduction in the intensity of an X-ray beam due to a change in the temperature of the medium through which it passes

Answer 30

b. a reduction in the intensity of an X-ray beam due to absorption or scattering by a medium through which it passes

Question 31

a. 10 %
b. 25 %
c. 75 %
d. 33 %

Answer 31

25 %

Question 32

7. A 1 cm thickness of material is found to attenuate a monoenergetic X-ray beam by 10 %. If the intensity of the X-ray beam was 1000 photons/mm2 which one of the following values is closest to the intensity that would be expected to be transmitted through a thickness of 3 cm?
   a. 500 photons/mm2
   b. 700 photons/mm2
   c. 800 photons/mm2
   d. 900 photons/mm2

Answer 32

700 photons/mm2

Question 33

8. The attenuation of monoenergetic X-ray beam intensity is described by which of the following mathematical relationships
   I=Io e^μx
   I=Io 10^-μx
   I=Io ux
   I=Io e^-μx

Answer 33

I=Io e^-μx

Question 34

9. The units of the linear attenuation coefficient can be expressed as
   a. cm2
   b. cm-2
   c. cm-1
   d. mm-3

Answer 34

cm-1

Question 35

10. The linear attenuation coefficient is _________ to the ¬¬________ of the material. Match the correct pair of terms.
    a. inversely proportional, volume
    b. inversely proportional, mass
    c. proportional, density
    d. proportional, temperature

Answer 35

proportional, density

Question 36

11. Dividing the linear attenuation coefficient of a material by its density will give its
    a. volume attenuation coefficient
    b. mass attenuation coefficient
    c. electron density coefficient
    d. atomic number coefficient

Answer 36

mass attenuation coefficient

Question 37

12. The units of the mass attenuation coefficient are
    a. cm2/g
    b. cm3/g
    c. cm2/g2
    d. cm-2/g

Answer 37

cm2/g

Question 38

13. The dominant X-ray photon interaction process for iodine in the diagnostic energy range is
    a. Rayleigh scattering
    b. Compton scattering
    c. Photoelectric absorption
    d. Pair production

Answer 38

Photoelectric absorption

Question 39

14. A monochromatic 100 keV X-ray beam passes through a 10 cm thickness of water. Compared to a polychromatic X-ray sources generated at 100 kVp which of the following statements is true.
    a. The 100 keV beam will be attenuated more than the 100 kVp beam
    b. The 100 keV beam will be attenuated less than the 100 kVp beam
    c. The 100 keV beam will be attenuated the same as the 100 kVp beam
    d. None of these is true

Answer 39

The 100 keV beam will be attenuated less than the 100 kVp beam

Question 40

15. Which one of the following will not reduce the effect of Compton scattering on image contrast
    a. narrow collimation of the X-ray beam
    b. employing a anti-scatter grid
    c. decreasing the kV
    d. increasing the mAs

Answer 40

increasing the mAs

Question 41

16. Employing a high kV for a radiographic exposure generally does not result in which one of the following
    a. lower subject contrast
    b. increased scatter
    c. increase spatial resolution
    d. lower patient dose

Answer 41

increase spatial resolution

Question 42

17. Over the range of photon energies employed in radiographic imaging Compton scattering.
    a. increases rapidly
    b. is approximately constant
    c. decreases rapidly
    d. decreases apart from at k-edges

Answer 42

is approximately constant

Question 43

18. Compton scattering
    a. improves image contrast
    b. degrades image contrast
    c. does not affect image contrast
    d. only improves image contrast of dense objects

Answer 43

degrades image contrast