X-ray interaction 2- subject contrast, attenuation Flashcards
Subject contrast
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.
Factors affecting subject contrast: material thickness
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.
Factors affecting subject contrast: Density
If we increase the density for a given energy between the material we increase the likeliness of absorption.
Factors affecting subject contrast: Absorption spectrum
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.
What does Increasing the X-ray beam energy do to the absorption?
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
Object contrast
if we have region of interest in patient the object contrast difference in the density of the materials thickness and atomic number.
Subject contrast function of xray beam energy is influenced by
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.
Subject Contrast: Equation
𝐶=(𝐼𝐵−𝐼𝐴)/𝐼𝐵
It has values between 0 and 1, or is sometimes expressed as a percentage.
What is Attenuation?
A reduction in the intensity of an X-ray beam as it passes through a medium due to absorption or to scattering
Attenuation: x-ray photon scattering
X-ray photons that experience scattering are viewed as having been attenuated even though they are not absorbed
Attenuation: Monochromatic Beam equation
𝑁=𝑁𝑜e^−𝜇𝑥
N - number of transmitted photons
No - number of incident photons
μ - linear attenuation coefficient (cm-1)
x - absorber thickness (cm)
The extent to which a material attenuates the X-ray beam depends on a number of factors:
Thickness & density of material
Type of material
X-ray photon energy
Attenuation in bone, muscle lung
For the same thickness of material, bone has a higher attenuation than muscle, and muscle has a higher attenuation than lung tissue
Linear Attenuation Coefficient equation
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.
Linear Attenuation Coefficient: Example
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.
Linear attenuation coefficients: role
Provide information on the attenuation properties of a medium
Provide a measure of the effectiveness of a medium as an attenuator
Critical factors affecting attenuation of X-ray photons by matter are:
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)
The linear attenuation coefficient is proportional to the density of the material
µwater > µice >water vapour
Mass Attenuation Coefficient equation
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
Attenuation: Polychromatic Radiation equation
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)
X-ray photons are attenuated mainly via two processes: ?
Photoelectric effect
Compton scattering
What does Photoelectric effect do?
Dominates at low photon energy
Produces excellent radiographic contrast
Responsible for majority of patient dose
Compton Scattering key points
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)
X-ray Imaging Factors (General Comments)
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
