X-ray photons can interact with matter through five basic interactions: coherent scattering, photoelectric effect, Compton scattering, pair production, and photodisintegration. At diagnostic energies, the most common interactions are coherent scattering, the photoelectric effect, and Compton scattering. The photoelectric effect produces high quality images but also results in higher patient radiation exposure. Compton scattering is the main source of scatter radiation in diagnostic radiology and can reduce image quality.

1. Interaction of X-Rays with
Matter.

2. Introduction
X-Ray photon interacts with the orbital
electron or proton(nucleus) of the atom . Its
either get absorbed or merely get scattered.
When photons are absorbed , they are
completely removed from the x-ray beam
and cease to exist.
If photons are scattered they no longer carry
useful information because there direction is
random. Only they adds noise to the system
or produce blackness on a film.

3. There are five basic ways that an X-Ray photon can interact with matter
i. Coherent Scattering
ii. Photoelectric Effect
iii. Compton Scattering
iv. Pair Production
v. Photodisintegration
Occurs at diagnosis energy
Coherent scattering(Classical)
Photoelectric effect (“The Good Guy”)
Compton Scattering(“The Bad Guy”)
The others two Pair production and Photodisintegration they occurs at much higher
energy then the diagnosis energy.

4. Coherent scattering
 Low energy(<10KeV) radiation encounters the electrons of an
atoms and sets them into vibration at the frequency of the
radiation. A vibrating electron because it is a charged particle
emits radiation.
 In this radiation undergoes only change in a direction without a
change in wavelength. Therefore also called as “Unmodified
Scattering” sometime.
 This is the only type of interaction between X-Rays and matter
that does not cause ionization, & no energy is transferred
 Its of two type
o Thompson scattering- A single electron involved in this
interaction
o Rayleigh interaction- Result from the cooperative
interaction with all the electrons of an atom.
 Less then 5% radiation undergo coherent scattering therefore
quantity is too small to be important in diagnostic radiology.
 Do not contribute significantly to the image, but does add a
small amount of dose to the patient.

5. Photoelectric Effect (“The Good Guy”)
 The incident photon of more then binding energy
ejects an electron from an orbit and it disappears
giving up all its energy to this electron.
This emitted electron or charged particle immediately
gets absorbed into space, because it have very little
penetrating power.
Now, the void is subsequently gets filled up by the
electrons from the adjacent shell by emitting a photon
of “Characteristic Radiation”.
 The photoelectric effect always yield three end
products
I. A Negative ion (the photoelectron)
II. A Positive ion (an atom deficient one electron)
III. Characteristic Radiation

6. Photoelectric effect depends on two factors-
 Energy of the Radiation- Reaction is more common with low energy photons or photon of
diagnostic range (20-120 KeV)
 Atomic number of the absorbers- reaction is common with absorbers with high atomic
numbers
Application to diagnostic radiology
 Advantage
 Produces radiographic image of excellent quality
a. Photoelectric effect does not produce scatter radiation
b. It enhances natural tissue contrast-contrast depends on difference in absorption
between adjacent tissues. Some tissues absorb more x-rays then other tissues(tissues
composed of different elements such as bone, muscles and soft tissues.)
 For E.g.- The Bone has a higher Z than soft tissue. Higher “Z” means more P.E, more P.E
means more absorbed photons, so less hitting the imaging receptor. Therefore, Bone
white on an X-Ray.
 Disadvantage
Patient exposure- All the energy of incident photon is absorbed by the patient. Patients
receive more radiation from photoelectric reactions than any other type of interaction.

7. Compton scattering (“The Bad Guy”
Definition
 The incident x-ray of higher energy interacts with an
outer shell electron and ejects it from the atom,
thereby ionizing the atom
 The ionizing electron is called a Compton electron
 The x-ray continues in a different direction with less
energy
 The energy of incident photon is distributed in two ways,
part of it goes to the recoil electron as kinetic energy, and
the rest is retained by deflected electron
 The reaction produces
• A Deflected Photon /Scattered Photon.
• A positive atom or Ionized atom.
• A negative electron or a recoil electron

8. Disadvantage
 Almost all the scatter radiation that we encounter in diagnostic radiology comes
from Compton scattering.
 At narrow angles of deflection, scattered photons retain almost all their original
energy and it reaches an x-ray film and producing fog.
 They are difficult to remove from x-ray film
 Can not be removed by filt ers because they are too energetic
 Can not be removed by grids because their angle of deflection is too small
 Compton is the major source of Occupation exposure.
 Probability of a Compton scattering does not depend of the Z of the atom-
because the energy of these outer shell electrons is low.
 Probability of Compton scatter is dependent on the density of the material-
more tightly packed atoms, means more electrons to crash into.

9. Pair production
A high energy photon interacts with the nucleus of an atom, the
photon disappears, and its energy is converted into matter in the
form of two particles as electron and positron.
Photodisintegration
In photodisintegration, part of the nucleus of an atom is ejected
by a high energy photon. The ejected portion may be a neutron,
a proton, an alpha particle, or a cluster of particles
Pair production and photodisintegration , do not occur in the
diagnostic energy range.
o Pair production does not occur with photon energies less than
1.02MeV
o Photodisintegration does not occur with energies less than 7MeV.