Download Interaction of x-ray photons with matter

Interaction of x-ray photons
(and gamma ray photons)
with matter
Interaction of x-ray photons with matter
When a beam of x-ray photons passes through
matter, its intensity (energy – or number of x-ray
photons - flowing per second) is reduced:
the beam has been attenuated, photon energy has
been removed from the beam
photon energy is attenuated by either being
absorbed by the matter or scattered out of the beam
Any photon energy not attenuated, is transmitted.
Attenuation and transmission of
x-ray photon energy
Electrical energy
In the x-ray tube for
diagnostic imaging
or linear accelerator for
radiotherapy treatment
converted into
x-ray photon energy
absorbed energy:
radiation dose
transmitted x-ray energy
at the x-ray target
converted into
in the patient
scattered x-ray energy
Absorption
some or all of the x-ray photon energy may be
absorbed when energy is transferred to matter
The energy deposited per unit mass of matter is
called absorbed dose
Units: joule per kg = 1 gray (Gy)
the energy deposited as absorbed dose causes
ionisation and subsequent chemical changes
that may result in biological effects
CT scan: differential absorption in tissues using
kilovoltage x-ray photons
RT treatment plan: absorbed dose distribution using
megavoltage x-ray photons
Scatter
Some x-ray photons are partially absorbed and their
remaining energy scattered: deflected from their
original path
at high photon energies in megavoltage
radiotherapy, scatter is in the forward direction and
contributes to absorbed dose
at low photon energies in diagnostic imaging,
scatter occurs in all directions and may contribute to
loss of image quality by increasing the overall
density of the image receptor
scattered x-ray photons contribute to the patient’s
dose (by internal scatter) and exposure of staff
scatter
lead-glass screen
x-ray source
air
Back scatter
patient
Side scatter
image receptor
Forward scatter
Scattered radiation reaching image receptor
blackening due to
collimated x-ray photons in
primary beam
object
low density blackening
around collimated beam
due to scattered x-ray
photons from collimators
and object
attenuation is exponential…
Attenuation in matter is due to some x-ray photons
being totally absorbed…
…and some x-ray photons being partially absorbed
and their remaining energy scattered
X-ray photons that are not attenuated, are transmitted
X-ray photons are attenuated differently in different
materials depending on various factors: the individual
x-ray photon’s energy, the material’s physical density,
electron density, proton number and its thickness
For a specific photon energy, an equal percentage (or
fraction) of the energy in the beam is attenuated in
equal thicknesses of material
This is an exponential relationship: equal changes in
one quantity give equal fractional changes in another
Exponential attenuation
The linear attenuation coefficient (m) gives the
fractional reduction in intensity of a homogenous
beam of x-ray photons per cm thickness of matter
For example, the linear attenuation coefficient of soft
tissue for 100 keV x-ray photons is approximately 0.2
(20%) (Bushong: table 29.1 in 6th ed)
20% of photon energy is attenuated in each cm
thickness of soft tissue; 80% is transmitted
100%
80%
1 cm
64%
1 cm
51%
1 cm
41%
1 cm
33%
1 cm
Exponential attenuation
The exponential relationship between the intensity
of transmitted x-ray photons and the thickness of a
specific material can be given as:
It = Io e -mx
Where
It = transmitted intensity
Io = original intensity
e = exponential constant (2.718)
m = linear attenuation coefficient of material
x = thickness of material
Provided the x-ray beam is i) homogenous
ii) parallel
Exponential attenuation in practice…
Exponential attenuation of a beam of x-ray photons
in equal thicknesses of aluminium is used to
measure half value thickness (HVT) in an x-ray
beam as part of routine quality assurance
The linear attenuation coefficient is calculated for
each individual voxel of tissue during a CT
(computerised tomography) scan and mapped to a
grey scale. Different tissues have slightly different
coefficients and therefore map to a different grey to
give the image
Interaction Processes
There are various interaction processes
of x-ray (and gamma ray) photons, that
may occur alongside each other:
coherent (elastic) scatter (negligible)
Compton (inelastic) scatter
photoelectric absorption
pair production
Relative importance of each
interaction process in water
x-ray photon
energy
10 keV
photoelectric
absorption
95%
Compton
scatter
5%
Pair
production
0
25 keV
50%
50%
0
60 keV
7%
93%
0
150 keV
0
100%
0
4 MeV
0
94%
6%
10 MeV
0
77%
23%
24 MeV
0
50%
50%
Mass attenuation coefficients in air
Graham & Cloke (2003) p 299
Photoelectric absorption
Occurs when an x-ray photon interacts with a
bound electron, in the inner shells of an atom
Only occurs if the energy of the x-ray photon
exceeds the binding energy of the shell
The x-ray photon disappears by transferring all its
energy to the bound electron
This energy is used to overcome the binding
energy of the electron, which then escapes the
atom as a photoelectron carrying kinetic energy
The photoelectron loses its KE via ionisation of
surrounding atoms
Photoelectric absorption
3
z

E3
incident x-ray
photon
photoelectron
carrying KE
photons of
electromagnetic radiation
3
z
The probability of photoelectric absorption 
E3
and is more likely to occur:
in beams of low energy when the average x-ray photon
energy is < 25 keV
in dense matter with atoms of higher proton number eg
bone, metal (shielding, filters), positive contrast media
(barium sulphate, iodine)
With a bound electron in an atom where the x-ray photon
energy is just above the binding energy:
this results in absorption edges: a large increase in
photoelectric absorption of x-ray photons with an energy just
above the binding energy of a specific shell – and an
increase in x-ray photons being transmitted just below the
binding energy
Photoelectric absorption:
absorption edges
Photoelectric absorption
only occurs when the x-ray
photon energy is equal to,
or slightly greater than, the
electron binding energy
This results in bursts of
absorption at the binding
energy of each shell
Compton scatter
Occurs alongside photoelectric absorption
An x-ray photon interacts with a bound electron,
whose binding energy is negligible in comparison
with the x-ray photon energy
Some of the x-ray photon energy is transferred to
the electron, to overcome its binding energy. It
escapes the atom as a Compton electron carrying
kinetic energy
The Compton electron loses its KE via ionisation of
surrounding atoms
The x-ray photon with reduced energy is deviated,
or scattered, from its original path and will interact
again until all its energy is lost and it disappears.
incident x-ray
photon
Compton scatter
 electron density
E
scattered x-ray photon
with reduced energy
angle of scatter
Compton
electron
carrying KE
The probability of Compton scatter
and is more likely to occur
 electron density
E
In higher energy x-ray beams (average energy >25 keV)
when electron binding energies in the attenuating material are
negligible in comparison;
as photon energy increases, the proportion of forward scatter
increases.
In diagnostic imaging, Compton scatter results in a loss of
image quality. In megavoltage radiotherapy, it results in poor
quality portal images but gives the main contribution to
absorbed dose
In materials containing atoms with a high electron density eg
hydrogen.
In diagnostic imaging, this results in more scatter in soft
tissues…increasing the importance of collimation and shielding.
In megavoltage radiotherapy, inhomogeneity correction is
important in treatment planning
Megavoltage imaging: example of DRR
and portal image
Pair production
A high energy x-ray photon (>1.02 MeV) interacts
with a nucleus
Its energy is converted into an electron and a
positron, carrying any excess energy as kinetic
energy
The positron annihilates with an electron: the two
particles are converted back into energy – two
gamma ray photons of 511 keV
Pair production only becomes important in very high
energy beams of photons above 10 MeV
incident x-ray
photon
Pair production
 E.z
g-ray photon
0.511 MeV
positron
+ KE
electron
+ KE
g-ray photon
0.511 MeV
Summary: Interaction of x-ray
photons with matter
When a beam of x-rays passes through
matter, its intensity is reduced: attenuated
attenuation is due to the interaction
processes of photoelectric absorption,
Compton scatter or pair production
Attenuation increases with thickness and
density of matter
interaction process depends on x-ray photon
energy, proton number and electron density
of the matter
at x-ray target
Energy transfer to matter
Electrical energy  KE of electrons
 x-ray production: characteristic/bremss
 x-ray photon energy and heat energy
in matter
 Interaction of x-ray photons
Photoelectric absorption
Compton scatter
Pair production
 Secondary electrons carrying KE
 x-ray photon energy deposited
Excitation and ionisation
 Chemical/biological effects