RAD 350 Chapter 9 X-ray Interaction with Matter
Of the five primary interactions of x-rays with matter, ONLY Photoelectric and Compton are of imaging importance!
Coherent scattering (also called Raleigh, Thompson, classical and excitation radiation) – An incident photon with an energy of 10 kEv or less interacts with an atom – actually enters the atom and may cause the atom to be “excited” BUT NO ENERGY TRANSFER TAKES PLACE!
The photon exits the atom with the SAME amount of energy, but may exit in a different direction from which it entered.
Compton scattering – the incident photon interacts with outer shell electron and EJECTS the electron from the atom (ionized the atom). The ejected electron is called a “Compton electron” or “Secondary electron” and is free to do what ANY electron can do. The incident photon continues to exist, BUT in a different direction and MINUS the energy it expended to
REMOVE the orbital electron from it’s shell AND it’s “on board” kinetic energy. The ANGLE of scatter of the incident photon is sometimes dwelled upon, BUT do NOT “spin your wheels” figuring the problems or energy changes, sines or cosines, etc. for this class!
The probability of a Compton interaction is inversely proportional to energy and
INDEPENDENT of atomic number of the atom.
Compton interactions REDUCE radiographic contrast and are contributing scatter radiation!
Photoelectric effect (interaction) – The incident photon interacts with an orbital electron
(usually an inner, more tightly bound binding energy electron) and ejects the electron. The incident photon CEASES TO EXIST (its energy was totally consumed to overcome the binding energy of the electron it ejected. The ejected electron is called a photoelectron.
P.E. increases with increasing ATOMIC NUMBER (Z number) of the absorbing material
(directly proportional to the 3 rd power of the atomic number of the absorbing material’s atomic number – THIS IS NOT A MAJOR ITEM I would worry about!). The incident photon energy MUST be at or just slightly higher energy than that of the ORBITAL ELECTRON it will eject for P.E. to occur!
SIMPLY STATED: AS X-RAY PHOTON ENERGY INCREASES, THE LIKELIHOOD OF P.E.
DEACREACES AND THE LIKIHOOD OF COMPTON INCREASES

~~~~~~~~~~Also remember both Compton and P.E. REMOVR orbital electrons (ionize the atom) and set the stage for Characteristic radiation to also be emitted from the ionized atoms.
PAIR PRODUCTION: This occurs at over 1.02 MeV, this is way beyond diagnostic energy ranges – but still must be understood. An “incident photon” with a MINIMUM energy of
1.02 MeV interacts with the “strong nuclear, electronically charged nuclear forces of the nucleus” and has all it’s energy converted into MATTER – TWO electrons, each with .51 MeV appear – one with a negative charge and one with a positive charge (called a POSITRON).
Should the incident photon have more than 1.02 MeV, the excess is evenly distributed among the two electrons produced.
PHOTDISTINTIGRATION: This is also way above the diagnostic imaging energy range BUT is significant to us as it has such tremendous force.
An incident photon with an energy over 10 MeV is absorbed by the NUCLEUS of an atom.
The nucleus is raised to an :EXCITED STATE” AND IMMEDIATELY emits a nucleon or other
“nuclear fragment” from the nucleus.
Now know that both Compton and P.E. are the PRIMARY interaction we will deal with.
What do they interact with? The patient’s body, which is made up of many different body atoms with VARIED ATOMIC MASS DENSITIES which will ALL attenuate/absorb the polychromatic (many x-ray photon energies) in DIFFERING WAYS – thus DIFFERENTIAL
ABSORPTION!!!
Go back to George’s basic hand vs a CXR – hand is exposed with a low kVp – thus a low differential absorption (we only see bone and blackness of the soft tissue, etc.) and we see black and white (hi contrast – short scale).
A CXR uses high kVp and the chest has MANY different ATOMIC MASS DENSITIES and produces a LONGER SCALE CONTRAST (low contrast), with MANY SHADES OF GRAY due the many different atomic mass densities which give MORE DIFFERENTIAL ABSORPTION.
P.E. are the PRIMARY interaction that produce the image and COMPTON scattered photons reduce contrast and may be thought of as “FOG”

The photons leaving the patient are termed “remnant” radiation and are comprised of only
TWO types of radiation – NON-Interacting photons (those that pass right thru the pt. with
NO interaction/absorption) and those that are partially absorbed (Compton) with lesser energy scatter radiation reaching the image plate/receptor.
With this said – remember the photons that are totally absorbed create a ‘void” on the image, such as the white/clear shade like bone which does contribute to the image -= BUT – this void is NOT part of the “remnant” beam.