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Transmission
X-ray photons that pass through the patient unchanged
Absorption
X-ray photons that transfer their energy to the patient
The absorption of the X-ray radiation by a material is proportional
to the degree of X-ray attenuation and is dependent on the energy of
the X-ray radiation and the following material parameters:
Thickness; Density; Atomic number
Scatter
Radiation that, during its passage through a substance, has been
changed in direction. It may also have been modified by a decrease
in energy
Attenuation,
The process by which radiation loses power as it travels through
matter and interacts with it. Attenuation of x-rays in solids takes
place by several different mechanisms, some due to absorption, and
others due to the scattering of the beam.
HVT
That thickness of a specified material (usually a metal) which
reduces the exposure rate to one-half its initial value.
Intensity
Relative number of x-ray photons in the x-ray beam
Quality
Quality is a measurement of the penetrating power of the X-Ray
photons. The quality of the beam increases as the proportion of high
energy photons increases.
Factors affect the
quality and intensity of the beam
kV - the greater the potential difference across the tube, the
faster the electrons move and the higher the energy of the X-Ray
photons. Thus the quality and intensity are increased
mA- the higher the tube current, the greater
the intensities of all the photon energies, and the intensity of the
beam is increased
time - the longer the exposure, the greater the
time during which X-Rays are produced, and the greater the beam
intensity. The time (seconds) and mA are generally considered
together as the composite factor mAs
distance - increasing distance from the source
of radiation results in a decrease in the intensity of the beam,
according to the inverse square law. Hence doubling the distance
from the tube head will result in a beam of one quarter its original
intensity.
Example values of
m linear
coefficient of attenuation
|
Material |
Density
g cm-3 |
Electron density
x 1023 g-1 |
Effective atomic number |
Photon energy |
m
cm-1 |
|
Water |
1.0 |
3.343 |
7.5 |
100 keV |
0.17 |
|
|
|
|
|
10 MeV |
0.03 |
|
Bone |
1.65 |
3.19 |
12.3 |
100 keV |
0.3 |
|
|
|
|
|
10 MeV |
0.04 |
|
Lead |
11.35 |
2.38 |
82 |
100 keV |
62 |
|
|
|
|
|
10 MeV |
4.3 |
For example, for a 100 keV x-ray
beam, 1 cm of water will attenuate 17% (0.17) of the x-ray photons
in the beam.
The attenuation or
absorption, usually defined as the linear absorption coefficient,
µ,
is defined for a narrow well-collimated,
monochromatic x-ray beam. The linear absorption coefficient is the
sum of contributions of the following:
-
Thomson scattering (R)
(also known as Rayleigh, coherent, or classical scattering) occurs
when the x-ray photon interacts with the whole atom so that the
photon is scattered with no change in internal energy to the
scattering atom, nor to the x-ray photon.
-
Photoelectric (PE)
absorption of x-rays occurs when the x-ray photon is
absorbed resulting in the ejection of electrons from the atom,
resulting in the ionization of the atom. Subsequently, the ionized
atom returns to the neutral state with the emission of an x-ray
characteristic of the atom.
-
Compton Scattering (C)
(also known as incoherent scattering) occurs when the incident
x-ray photon ejects an electron from an atom and an x-ray photon
of lower energy is scattered from the atom.
-
Pair Production (PP)
can occur when the x-ray photon energy is greater than 1.02 MeV,
when an electron and positron are created with the annihilation of
the x-ray photon (absorption).
-
Photodisintegration (PD)
is the process by which the x-ray photon is captured by the
nucleus of the atom with the ejection of a particle from the
nucleus when all the energy of the x-ray is given to the nucleus (absorption).
This process may be neglected for the energies of x-rays used in
radiography.
There are three main processes that may occur resulting in
exponential attenuation of x-ray energy:
- Photoelectric absorption
- Compton (inelastic scatter)
- Pair production
If we compare the probability of each of these processes in water
at different x-ray photon energies, we would see something like
this:
|
X-ray photon energy |
Photoelectric absorption |
Compton scatter |
Pair production |
|
10 keV |
95% |
5% |
0 |
|
25 keV (Mammography) |
50% |
50% |
0 |
|
60 keV
(Diagnostic) |
7% |
93% |
0 |
|
150 keV |
0 |
100% |
0 |
|
4 MeV |
0 |
94% |
6% |
|
10 MeV
(Therapy) |
0 |
77% |
23% |
|
24 MeV |
0 |
50% |
50% |
Photoelectric
absorption
Photoelectric (PE) absorption of x-rays occurs
when the x-ray photon is absorbed resulting in the ejection of
electrons from the inner shell of the atom, resulting in the
ionization of the atom. Subsequently, the ionized atom returns to
the neutral state with the emission of an x-ray characteristic of
the atom.
This subsequent emission of lower energy
photons is generally absorbed and does not contribute to (or hinder)
the image making process. Photoelectron absorption is the dominant
process for x-ray absorption up to energies of about 500 KeV.
- Photoelectron absorption is also dominant
for atoms of high atomic numbers.
- Photoelectric Effect is dependent on Z3
(Atomic number z)
Photoelectric absorption is a process of
total absorption
Note that an ion results when the photoelectron
leaves the atom.
Two subsequent points should also be noted:
Firstly, the photoelectron can cause ionisations along its
track,
Secondly, X-ray emission can occur when the vacancy left by the
photoelectron is filled by an electron from an outer shell of the
atom.
There are a number of rules which govern the probability of a
photoelectric event:
The incident photon must have sufficient energy to overcome the
binding energy of the electron.
Once the threshold imposed by the binding energy has been exceeded,
then the interaction probability is at a maximum.
The probability of an interaction is greatest if the electron is
deeply bound. That is, the larger the atomic number, Z, the greater
is the probability of a photoelectric process
Compton Scattering
Compton Scattering, also known as incoherent
scattering or inelastic scattering, occurs when the incident
x-ray photon ejects an outer shell electron from an atom and an
x-ray photon of lower energy is scattered from the atom.
Relativistic energy and momentum are conserved in this process and
the scattered x-ray photon has less energy and therefore greater
wavelength than the incident x-ray photon. Compton Scattering is
important for low atomic number specimens. At energies of 100 keV --
10 MeV the absorption of radiation is mainly due to the Compton
effect.
The scattered x-ray photon has an energy which
is dependent on its angle of emission and on the incident photon
energy.
The probability of a Compton event depends on
the number of electrons in an absorber, which depends on the density
of the absorber and the number of electrons per unit mass. Now with
the exception of hydrogen, all elements contain approximately the
same number of electrons per unit mass. Therefore the number of
Compton reactions is independent of atomic number. However, for
tissues of biological interest, the probability of an interaction
does decrease slowly with increasing photon energy above about 50
keV.
Compton scatter is an attenuation process of partial absorption and
partial scatter
Pair Production (PP) is of particular
importance when high-energy photons pass through materials of a high
atomic number. Energy: > 1.02 MeV
When the energy of
the incident photon is greater than
1022 keV,
the photon may be absorbed through the process of Pair Production.
When such a photon passes near the nucleus of the atom it
experiences the strong field of the nucleus and may be absorbed with
the creation of a positive and negative electron pair. This is an
example of conversion of energy to mass as espoused by Albert
Einstein. No electronic charge is created since the positron and
electron are equal and oppositely charged. Ignoring the tiny amount
of energy given to the recoiling nucleus we may write:
E = 2mc2 +
E+ + E-
where:
-
m: electron rest mass,
-
c: the speed of light,
-
E+: kinetic
energies of the positron, and
-
E-: kinetic
energy of the electron.
The total energy
given to the electron-positron pair can be divided randomly although
there is a slightly greater probability that the positron will carry
off more energy than the electron because it experiences the
repulsive Coulomb force of the nucleus' positive charge.
The most important
feature to note is that the process is not possible unless the
photon energy is greater than the rest mass energy of the
electron-positron pair, i.e.
2 x 511 keV = 1022 keV.
The fate of the
positron has an important bearing on the ultimate decay products.
In travelling through matter, the positron excites and ionises
atoms, just as an electron does, until it is finally brought to
rest. Then it combines or annihilates with a free electron
with the production of two 511 keV photons. In order to conserve
momentum and energy the two photons move essentially at an angle of
180o to each other.
A forth process called Coherent Scattering
occurs mainly at low energies and large values of Z and is typically
a just small proportion of the total number of interaction
Here a gamma-ray or X-ray photon undergoes an interaction where it
changes its direction without loss of energy. In the idealistic
situation of the interaction being between a photon and a single
free electron the process is called Thomson Scattering.
This should be contrasted with the real world
situation where photons are scattered by bound electrons. The
electrons are set vibrating by the oscillating electromagnetic field
associated with the photon. Subsequently, a photon of radiation is
emitted with the same wavelength as the incident radiation leaving
the atom in its original undisturbed state. The waves from
electrons within the atom combine with each other to form the
scattered wave. The scattering is a cooperative phenomenon and the
process is called Coherent Scattering. There is no net ionisation
in the process, a property which distinguishes coherent scattering
from other photon interactions.
A fifth process Nuclear Photodisintegration
At extremely high energies ( > 8 MeV), a photon
may interact directly with the nucleus of an atom and eject a
neutron, proton or on rare occasions even an alpha particle.
Summary
- Photoelectric (PE)
absorption of x-rays occurs when the x-ray photon is absorbed
resulting in the ejection of electrons from the atom, resulting in
the ionization of the atom. Subsequently, the ionized atom returns
to the neutral state with the emission of an x-ray characteristic
of the atom.
- Compton Scattering
(C) (also known as incoherent
scattering) occurs when the incident x-ray photon ejects an
electron from an atom and an x-ray photon of lower energy is
scattered from the atom.
- Pair Production
(PP) can occur when the x-ray
photon energy is greater than 1.02 MeV, when an electron and
positron are created with the annihilation of the x-ray photon (absorption).
-
Photodisintegration (PD)
is the process by which the x-ray photon is captured by the
nucleus of the atom with the ejection of a particle from the
nucleus when all the energy of the x-ray is given to the nucleus (absorption).
This process may be neglected for the energies of x-rays used in
diagnostic radiography.
- Thomson scattering
(R) (also known as
Rayleigh, coherent, classical, elastic
scattering) occurs when the x-ray photon interacts with the whole
atom so that the photon is scattered with no change in internal
energy to the scattering atom, nor to the x-ray photon.
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