4️⃣ Photon Interactions with Matter

Three possibilities for an x-ray photon in the patient

1) Transmitted (passes through with no interaction), 2) Attenuated by absorption, 3) Scattered (changes direction, may or may not be absorbed later).

Attenuation – definition

The progressive decrease in the intensity of the x-ray beam as it passes through matter due to absorption and scatter.

Main causes of attenuation

Photoelectric absorption and Compton scatter interactions within the patient.

Differential attenuation (subject contrast)

Differences in the amount of attenuation between different tissues based on their thickness, density, and atomic number; this creates the subject contrast that forms the image.

Coherent (classical, Thomson, unmodified) scatter

A low-energy interaction in which the incident photon interacts with the whole atom, is deflected in a new direction, and leaves with the same energy; no ionization occurs and it contributes little to image formation at diagnostic energies.

Photoelectric interaction – conditions

Most likely to occur when photon energy is equal to or slightly greater than the inner-shell electron binding energy in the tissue.

Photoelectric interaction – what happens?

The incident photon interacts with an inner-shell electron, gives up all its energy, ejects the electron (photoelectron), and is completely absorbed; the atom is ionized and characteristic (secondary) photons may be emitted.

Secondary radiation from PE vs tube characteristic

Secondary radiation in the patient comes from inner-shell vacancies being filled after photoelectric absorption, whereas characteristic radiation in the tube comes from target atoms in the anode when struck by projectile electrons.

Compton interaction – conditions

Most likely with moderate to higher energy photons in the diagnostic range interacting with loosely bound outer-shell electrons.

Compton interaction – what happens?

The incident photon gives up part of its energy to an outer-shell electron, ejecting it (Compton or recoil electron), and is scattered in a new direction with reduced energy; the atom is ionized.

Why Compton scatter is important in radiography

It is the main source of scatter reaching the image receptor (reducing contrast) and the main source of occupational dose to the radiographer.

Pair production – summary

An interaction that occurs only at photon energies of 1.02 MeV or higher; the photon interacts with the nuclear field, disappears, and its energy is converted into an electron–positron pair; the positron later annihilates with an electron, producing two 0.511 MeV photons emitted 180° apart.

Photodisintegration – summary

An interaction that occurs at photon energies above about 10 MeV in which the photon is absorbed by the nucleus, causing it to emit a nuclear fragment and changing the atomic mass and atomic number; not part of normal diagnostic imaging.

Diagnostic-range interactions we care most about

In routine radiography, the most important interactions are photoelectric absorption (contrast and patient dose) and Compton scatter (fog and occupational dose); coherent scatter and the high-energy processes are of minor or no practical importance.