INTERACTION OF XRAYS WITH MATTER

1

• INTERACTION OF X-RAYS WITH MATTER
• Introduction
• When x-ray photons pass through matter, three
  scenarios are possible (Fig. 1).
• A photon can be deflected from its its path and
  be scattered.
• A photon can pass straight trough and be
  transmitted.
• A photon can lose all its energy to an atom and
  be absorbed.
• The reduction in the number of photons (or beam
  intensity) as radiation passes through matter is
  called attenuation.
• Attenuation is caused by the processes of
  absorption and scattering.
• Attenuation depends on
• The thickness of the material the greater the
  thickness, the greater the attenuation.
• The density of the material the greater the
  density, the greater the attenuation.
• The energy of the x-ray photons the greater the
  energy, the lower the attenuation.
• There are five basic interactions of x-rays with
  matter
• Coherent scattering.
• Compton scattering.
• Photoelectric absorption.
• Pair production, and
• Photo disintegration.

2

• Coherent Scattering (Fig. 2)
• Occurs mostly for x-rays with energies less than
  10 keV.
• A photon interacts with an electron of an atom,
  giving it energy and causing it to vibrate.
• The excited electron immediately emits this
  energy in the form of a secondary photon.
• The secondary photon has the same energy as the
  incident photon but travels in a slightly
  different direction.
• The result is a scattered photon.
• Coherent scattering is also known as classical or
  elastic scattering.
• Only a few percent of x-rays in the diagnostic
  range undergo coherent scattering, which
  contributes somewhat to film fog.
• Compton Scattering (Fig. 3)
• Occurs when an x-ray photon interacts with a
  loosely held outer-shell electron and ejects it,
  thus ionising the atom.
• The ejected electron is called a Compton or
  recoil electron.
• The x-ray photon continues in a different
  direction (is scattered) with reduced energy.
• From the law of conservation of energy
• Energy before the interaction energy after the
  interaction or
• Ei Es Eb Ek, where
• Ei energy of incident photon
• Es energy of scattered photon

3

• Eb binding energy of electron
• Ek kinetic energy of electron.
• Compton scattering is described by the following
  relationship
• or Dl 2.4 x 10-12 (1 cos q) where
• Dl is the change in wavelength (in metres)
  between the incident and the scattered photons.
• q is the angle of scatter.
• Recall, for photons E hf or E hc/l.
• The incident photon energy is essentially divided
  between the recoil electron and the scattered
  photon because Eb is small.
• The energy of the scattered photon depends on the
  energy of the incident photon and the angle of
  scatter.
• The higher the initial energy of the incident
  photon, the greater the energy of the scattered
  photon.
• The scattered photon can be scattered at any
  angle between 0 and 180.
• At 0, no energy is transferred because the
  photon proceeds in its original direction.
• At 180, maximum energy is transferred to the
  recoil electron and the scattered photon has
  minimum energy.
• X-rays scattered in the direction of the incident
  photon are called backscatter radiation.

4

• Most photons will scatter in the forward
  direction, especially as photon energy increases
  (Fig. 4).
• The probability of a given photon undergoing
  Compton scattering is directly proportional to
  the electron density and inversely proportional
  to photon energy, ie,
• Since the electron densities are similar for all
  materials, the probability for Compton scatter is
  similar for all materials.
• As photon energy increases, the probability of
  Compton scatter decreases (Fig. 5).
• Compton scatter photons create a radiation hazard
  both for the patient and the radiographer.
• They also are the main contributor to film fog.
• Photoelectric Absorption (Fig. 6)
• Occurs when an x-ray photon interacts with an
  inner-shell electron.
• The photon gives up all its energy to the
  electron (and thus disappears) and the electron
  (called a photoelectron) is ejected from the atom
  thus ionising the atom.
• From the law of conservation of energy
• Ei Eb Ek
• The kinetic energy of the electron is therefore
  equal to the difference between the energy of the
  incident photon and the binding energy of the
  electron Ek Ei Eb

5

• Clearly, photoelectric absorption can occur only
  if the incident photon has an energy equal to or
  greater than the binding energy of the electron.
• The binding energies of K-shell electrons for
  most atoms found in the body are low (Table 1).
• Thus, most of the energy of the incident photon
  is transformed into kinetic energy of the
  photoelectron.
• Photoelectrons are absorbed within 1 to 2 mm in
  soft tissue and contribute to the patient dose.
• The removal of an inner-shell electron in
  photoelectric absorption leaves a vacancy in the
  inner shell.
• The vacancy is instantly filled by an electron
  from a higher-energy shell.
• In the process, the electron loses energy which
  is emitted in the form of a characteristic photon
  known as secondary radiation.
• This process is identical to the process that
  produces characteristic radiation in the target
  but the photon energies are generally low.
• The probability of a given photon undergoing
  photoelectric absorption is directly proportional
  to atomic number cubed and inversely proportional
  to photon energy cubed, ie,
• Fig. 7 and Table 2.

6

• Pair Production (Fig. 8)
• Occurs when a high energy photon (E 1.02 MeV)
  interacts with the electric field of a nucleus
  and disappears.
• The energy of the photon is used to create an
  electron and a positron.
• The positron is the electrons antiparticle,
  identical to the electron in all respects but
  with a positive electric charge.
• The positron loses energy until eventually it
  meets a free electron and in the reverse process
  called pair annihilation they annihilate each
  other.
• In this process the mass of both particles is
  completely converted into energy in the form of
  two 0.51 MeV photons.
• Pair production does not occur in the diagnostic
  x-ray range.
• Photodisintegration (Fig. 9)
• Occurs when a very high energy photon (E gt 10
  MeV) is absorbed by a nucleus and the nucleus
  emits a proton, neutron or other nuclear
  fragment.
• Photodisintegration does not occur in the
  diagnostic x-ray range.

7

• Differential Absorption
• Only Compton scatter and photoelectric absorption
  are of importance in diagnostic radiography.
• An x-ray image results from the difference
  between the x-rays absorbed photoelectrically in
  the patient and the x-rays transmitted to the
  film.
• This is called differential absorption.
• The Compton scattered x-rays contribute no useful
  information to the image, they only contribute to
  film fog.
• At low energies, most x-ray interactions with
  tissue are photoelectric.
• At high energies, Compton scattering predominates
  (Fig. 10).
• Of course, as photon energy increases, the
  probability of any interaction occurring
  decreases.
• As kVp is increased, more x-rays get to the film
  and therefore lower x-ray quantity (lower mAs) is
  required.