Post on 16-Sep-2018
Lesson Objectives
At the end of the lesson, student should able to:
Define attenuation
Explain interactions between x-rays and matter in
the following :
a) coherent (elastic) scatter
b) photoelectric absorption
c) Compton (inelastic) scatter
d) pair production
Interaction of x-ray photons with matter
When a beam of x-rays passes through matter, its
intensity is reduced: attenuated
x-rays are attenuated by either absorption or
scatter
Some x-ray photons are transmitted
Interaction of x-ray photons with matter
Attenuation increases with:
A. physical density
B. atomic number
C. electron density
Attenuation decreases with beam energy
Inverse Square Law
Reduction in intensity of a diverging x-ray beam is
also due to beam geometry the inverse square law:
The intensity of a beam of x-rays, from a point source, is
inversely proportional to the square of the distance
from the source, in a vacuum
example
If the output of an x-ray tube is measured as
0.2 mGy at 100 cm, what will be the
approximate output at 3 m?
Absorption
Some x-ray photons are absorbed in matter: their
energy is transferred and the photon disappears
The energy deposited per unit mass of matter is
called absorbed dose
1 joule per kg = 1 Gray (Gy)
The energy deposited as absorbed dose causes
chemical changes due to ionisation which may
result in biological effects
Scattering
Some photons are scattered:
deflected from their original
path
At high photon energies,
scatter is in the forward
direction and contributes to
absorbed dose
At low photon energies,
scatter occurs in all
directions and may
contribute to loss of image
quality by increasing the
overall density of the film
Scattered x-ray photons
contribute to exposure of
staff, patient and public
Exponential Attenuation
When a beam of x-rays passes through matter, its
intensity is reduced i.e. attenuated, due to
absorption and scatter
The quantity of x-rays removed depends on the x-
ray photon energy and the density of the matter
Interaction Processes
coherent (elastic) scatter
photoelectric absorption
Compton (inelastic) scatter
pair production
Coherent (elastic) scatter
This type of attenuation process
occurs when the energy of
photons in the beam is small
compared with electron binding
energies in the atoms of the
attenuating medium.
This type of attenuation process
therefore occurs with low
energy radiation and is
sometimes referred to as
"elastic" scattering
This process is not important at
the X-ray energies normally used
in radiography.
Photoelectric absorption
This process occurs when the energy of the incoming photon in
the X-ray beam is equal or not much greater than the electron
binding energy
The photon transfers all it’s energy to an electron
The electron is ejected from the atom (photoelectron)
True absorption has taken place
This process leaves an electron vacancy, which is filled by electron
transition from a higher energy level
There is also the emission of characteristic radiation
The photon energy of characteristic radiation is generally low,
because the elements interacted with in the body have low
proton numbers and low binding energies.
Compton scatter
If an X-ray photon has an energy very much
greater than the binding energy of the electron
with which it interacts, Compton scattering can
take place.
As the photon energy is so great compared with
the electrons binding energy, the electron recoils
from the collision and is ejected at speed from the
atom.
The amount of energy transferred to the electron
increases with the angle of scatter
Pair production
This type of absorption process can only take place
when the incoming photon energy exceeds 1.02 MeV
As the photon passes the nucleus of an atom in the
attenuating material, it experiences the strong electric
forces around the nucleus caused by the positive
charges on the protons there.
As a result, the photon undergoes a dramatic change
of state - its energy is transformed into matter in the
form of two minute particles, hence the given term
pair production.
Summary: Interaction of x-ray photons
with matter
When a beam of x-rays passes through
matter, its intensity is reduced: attenuated
x-rays are attenuated by either absorption or
scatter
attenuation increases with density of matter,
atomic number and electron density
interaction process depends on x-ray photon
energy