Week 1 - CT Basics

State what type of photon interaction is most common in CT

Compton Scattering

Explain photoelectric effect/absorption

Lower energy incident photons interact with a k-shell electron that completely absorbs its energy, causing the now photoelectron to be ejected from the atom.

State how the probability of PE occurrence changes with Z, E and p

• Proportional to Z³ (atomic number) - higher atomic number means greater binding energy of K-shell electron, increasing chances of complete absorption

• Inversely proportional to E³ (beam energy) - absorption only occurs at lower energies (<26keV)

• Linearly proportional to p (physical density) - more electrons means more for interaction to occur

Explain Compton scattering

An incident photon interacts with an outer-shell electron of an atom and transfers some of its energy. The outer shell electron is ejected from the atom and the x-ray scatters in a different direction lower energy (lower frequency and longer wavelength).

State how the probability of Compton scattering occurrence changes with Z, E and p

• Not dependent on Z (atomic number) - because all atoms have loosely-bound outer electrons

• Slowly decreases with E (beam energy) - not as rapidly as PE

• Linearly proportional to p (physical density) - more electrons means more likely for interactions to occur

Explain why monochromatic beams use keV units while polychromatic beams use kV units

Monochromatic beams contain photons with the a single energy value, therefore can be measured in keV. Whereas polychromatic beams contain photons with a range of energy values, therefore using kV indicates an average beam energy.

Explain attenuation

The removal of photons from a beam via photoelectric effect or Compton scattering as it passes through matter.

Explain linear attenuation coefficient

A value that represents the fraction of photons removed from a monoenergetic radiation beam after passing through a certain thickness of material.

State the attenuation formula

I_x = I₀ e^(mu)x

I_x = transmitted beam intensity

I₀ = initial beam intensity

e = 2.7183 (Eulers number)

mu = linear attenuation coefficient

x = absorption material thickness

Explain why the use of this attenuation formula is only an approximation in CT

In order for the formula to be accurate, the radiation beam must satisfy 2 conditions which are not present in CT in which the beam must be very narrow and monoenergetic.

State the approximate mu values for air and water and give units

• Air = 0/cm (no attenuation passing through air)

• Water = 2/cm

State the formula used to calculate CT number/Housefield Units

HU = (mu_x - m_water)_/ m_{water X 1000}

State the HU for water and explain

HU of water is 0 and therefore is the standard. Anything denser than water is >0 and anything less denser than water is <0.

Explain why HU units are preferred over mu values when differentiating between tissue types

There is a greater difference in values with HU units when comparing tissue types than with mu values, therefore HU make it easier to differentiate.