BASIC INTERACTIONS BETWEEN X RAYS AND MATTER, MMED Physics, Lesson 4

BASIC INTERACTIONS BETWEEN X RAYS AND MATTER

X-ray photons may interact either with orbital electrons or with the nucleus of atoms. In the diagnostic energy range, the reactions are always with orbital electrons.

If the electron disrupted is an electron used to bind atoms together to form molecules, then the molecular structure of the tissue may be disrupted or altered. An atom will stop the same number of incident photons in a solid, liquid or gaseous state – it does not matter if oxygen is in ice, water or steam – neither does it matter if oxygen is in air or bound to hydrogen in water.

There are 5 basic ways that an x-ray photon can interact with matter: (LIST THE 5 WAYS)

1.       Coherent Scattering

2.       Photoelectric effect

3.       Compton Scattering

4.       Pair Production

5.       Photodisintegration

Absorbed – photons completely removed from the x-ray beam and crease to exist

Scattered – photons are deflected into a random course, and no longer carry useful information.

Noise – scattered photons carry no useful information and add noise to the system. It destroys image quality. Noise is also referred to as “film fog”. Noise Covers valid information with distracting or obscuring “garbage.”

v  COHERENT SCATTERING

Coherent scattering occurs when interactions undergo a change in direction without a change in wavelength.  Both can be described in terms of the wave-particle interaction or “classic scattering”

1.       Thomson scattering – photons interact with a single electron.

2.       Rayleigh scattering – photons interact with all the electrons of an atom.

When low energy radiation encounters an atom there is absorption of radiation, vibration of the atom, and emission of radiation as the atom returns to its undisturbed state.

·         Change in direction.

·         No change in energy, frequency or wavelength. 

·         There is NO IONISATION.  

·         Contributes to scatter as film fog.

·         Contributes less than 5% of all radiation, not significant but does contribute, in a small way, to “film fog” throughout the diagnostic energy range.

v  PHOTOELECTRIC EFFECT

K-shell electrons are at a lower energy level than electrons in the L shell. The outer electrons are “free”, and the inner electrons are in energy debt and energy debt is greatest when close to the nucleus in an element with a high atomic number. (Sounds like my bank balance).

The energy produced when electrons drop from a higher energy shell (L-shell) to a lower energy shell (K-shell) is characteristic for each element, and the radiation produced is called “characteristic radiation.”

Three end products of photoelectric effect:

1.       Characteristic radiation;

2.       A negative ion (the photoelectron – that is quickly absorbed and has obviously poor penetrating power); and

3.       A positive ion (an remaining atom that is deficient of one electron)

v  Probability of Occurrence of Photoelectric Interaction Probability – Three rules

1.       The Incident photon must have sufficient energy to overcome the binding energy of the electron

2.       A photoelectric reaction is most likely to occur when photon energy and electron binding energy are nearly the same

Ø  Low energy event

Ø  Photoelectric effect is proportionate to  1/(energy) ³ 

3.       The tighter an electron is bound in its orbit, the more likely it is to be involved in a photoelectric reaction

Ø  Photoelectric effect ~ (Atomic Number) ³

Photoelectric effect is most likely to occur with low energy photons and elements with high atomic numbers provided that the photons have sufficient energy to overcome the forces binding the electrons in their shells.

v  Characteristic Radiation

The method used is different from the high speed electrons used – here we are using the energy of an incident photon to eject the K-shell electron.

Iodine and Barium emit enough characteristic radiation to leave the patient and fog and reach the x-ray film screen – so called “secondary radiation”.

Applications to Diagnostic Radiology

Photoelectric effect enhances natural tissue contrast = good quality

Photoelectric effect does not produce scatter radiation.

Photoelectric interactions deposit most beam energy that ends up in tissue

¨       always use highest kVp technique consistent with imaging contrast requirements

Contrast is greatest when the difference in tissue absorption between adjacent tissues is greatest. Photoelectric effect ~ (Atomic Number) ³ – It magnifies the difference in tissues composed of different elements, such as bone and soft tissue.

Patient exposure is undesirable – patients receive more exposure from the photoelectric reactions than from any other type of interaction. All the energy from the incident photon is absorbed by the patient in a photoelectric absorption.

v  COMPTON SCATTERING

Virtually  all scatter radiation in diagnostic radiology comes from Compton Scattering.

A high energy incident photon strikes an outer shell free electron, ejecting it from its orbit. The photon, retaining part of its original energy, is deflected and travels in a new direction leaving an ion pair – a positive atom and a negative “recoil” electron. (ionization)

Two factors determine the amount of energy retained by the photon:

1)      Its initial energy

The angle of deflection off the recoil electron

Photons have momentum and the higher the energy of the photon the more difficult they are to deflect.

Ø  Change in wavelength Δλ = 0.024 (1 – cos Θ)

Ø  Δλ = change in wavelength Ǻ

Ø  Θ  = angle of photon deflection

But we don’t think in wavelength by keV

Ø  keV = 12.4 / λ

Ø  Δλ = change in wavelength Ǻ

Ø  keV  = energy of photon

Really very little energy is transferred to the recoil electron. At narrow angles of deflection, scattered photons retain most of their original energy. They have an excellent chance of reaching the x-ray film and producing fog:

Ø  as they can’t be removed by filters – they are too energetic; and

Ø  can’t be removed by grids because the angle of deflection is too small

Compton Scattering is a major Safety hazard – as it retains most of its initial energy. Both for the patient and the operator – especially during fluoroscopic examination.

v  Probability of Occurrence

Compton scattering is independent of the atomic number – as all elements contain the same number of electrons per gram, regardless of their atomic number. It is dependent on the energy of the radiation and the density of the absorber. The number of reactions diminish as the energy diminishes.

v  PAIR PRODUCTION AND PHOTODISINTERGRATION

Don’t occur in the diagnostic energy range.

Pair production – the high energy incident photon interacts with the nucleus and disappears – and its energy is converted into matter in the form of two particles, a positron and an electron – both with energies of 0.51 MeV – (Minimum energy required is 0.51 x 2 = 1.02 MeV).

v  Photodisintegration – part of the nucleus is ejected by high energy photon. The ejected part may be a neutron, a proton or an alpha particle, or a cluster of particles. Enough energy to overcome the nuclear binding energy is required. Typically 7 to 15 MeV. Threshold above diagnostic energies – does not occur in diagnostic radiology.

v  RELATIVE FREQUENCY OF BASIC INTERACTIONS