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8/14/2019 Unit 12 X-Ray and Its Medical Applications
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Unit XII
Ionizing radiation
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Ionizing radiation causes ionization in matter.
Radiation above10 eV is ionizing radiation.
high energy electromagnetic radiation (energeticphotons), including x-rays and gamma rays,
energetic particles of matter (alpha rays,
neutrons, beta rays, electrons, etc), coming from
radioactivity of some radioactive nuclei.
What is ionizing radiations?
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For electromagnetic radiation:
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X-rays in medicine
X-rays imagingX-rays therapy
X-rays imaging: bone images, mammography
X-ray therapy: x-rays are used to kill canceroustumors
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Observation of bone Mammography
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Rotating beam X-rays therapy
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The interaction of x-rays - matters
The production of x-rays The use of X-rays in imaging The use of X-rays in therapy
Contents
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The interaction of x-rays with matter
When x-rays pass through a medium, its intensity isreduced by interacting with that matter .
This is called attenuation.
There are four mechanisms of attenuation:
Simple scatterPhotoelectric effect
Compton scatterPair production
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The interaction of x-rays: simple scatter
low energy photon scattering process (
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The interaction of x-rays: the photoelectric effect
A high energy electron falls into the low orbit to take the
place of the electron that has been ejected,
during this process, a lower energy photon is emitted.
An x-ray photon collideswith an atom and transfer
all its energy to an inner
orbital electron.
The photon disappears. The electron is ejected. The lost of electrons
causes the ionization of
the atom.
(photon energy 1-100 keV)
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The interaction of x-rays: Compton scatter
Part of the photon energy is transferred to the ejected electron the photon leaves in a different direction with a less energy
a photon collides with an
atom interaction of x-ray photon
with the outer electrons an electron is ejected from the
atom causes the ionization of the
atom.
(photon energy 100-5000 keV)
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The photoelectric effectversus Compton scatter
interaction of x-ray
photon with the inner
orbital electrons
incident photondisappears
low energy photon
emitted due to electron
jump causing ionization
interaction of x-ray
photon with the outer
electrons
incident photon isscattered
low energy photon due to
scattering of the incident
photon causing ionization
The photoelectric effect Compton scatter
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The attenuation of x-rays
When x-rays pass through a medium, the transmittedintensity is reduced due to interaction, this is called
attenuation.
X-ray
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The linear attenuation coefficients
: the linear attenuation coefficient, unit m-
1. depends on
1. incident photon energy
2. nature of the medium.
I
Io
x
x
oeII
=
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The linear attenuation coefficients
The attenuation coefficient is related to the proton number
(Z-number).
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The photoelectric effect
x
oeII
=1
1
xoeII 22
=
Io
Z1x
Io
Z2 x
212121 IIZZ >
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Compton Scatter
x
oeII
=1
1
x
oeII 22
=
Io
Z1x
Io
Z2 x
212121 but IIZZ ==>
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The linear attenuation coefficients
x
oeII =
The half-value thicknessx1/2 : thickness forI=Io/2
2/1
2
1 xooeII
=
2/12 xLn = 693.02
2/1 ==Ln
x
ability of a medium to stop the x-raysSmallerx1/2, stronger attenuation
The half-value thicknessx1/2
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The Production of x-ray: x-rays tube
I= Vray tube-xofon)(comsumptiPower
ray tube-xofpower
beamray-xofpowerray tube-xofefficiency =
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Thefmax , corresponding to a collision where the
electron losses all its initial kinetic energyKo, can be
obtained from :
h
eVf =max
e: charge of an electron
V: x-ray tube voltage
h = 6.6310-34 J.s (Planks constant)
min
maxenergyphotonray-xmaximum
hchfeV ===
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The X-rays tube is evacuated to high vacuum level,
Electrons are produced at the heated cathode and
accelerated toward anode under a high electricpotential,
The movement of electrons from cathode to the target
forms a tune current, When the electrons hit the anode target, less than 1 %
of incident energy is converted into X-rays photon
energy,
The reminder of the energy is converted into thermalenergy and the anode target get heated,
To prevent the target from overheating, water or oil
circulation is made around the target to remove the
heat through convection
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The x-rays spectrum
The variation of x-rays intensity as a
function of x-rays photons energy
Continuous spectrum
Characteristic
x-ray peaks
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Two features arise from the x-rays spectrum : one is
the continuous spectrum, another is two sharp peaks,
they are due to different mechanisms, For the continuous spectrum , an electron is
accelerated under a potential difference V. It will gain
a kinetic energy equals eV. When the electron hits the target, it undergoes a range
of decelerations. the kinetic energy of the electron is transferred to a
nearby atom it passes by, x-ray photons of a range of energies are emitted from
the tube.
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Thefmax , corresponding to a collision where the
electron losses all its initial kinetic energyKo, can be
obtained from :
h
eVf =max
e: charge of an electron
V: x-ray tube voltage
h = 6.6310-34 J.s (Planks constant)
min
maxenergyphotonray-xmaximum
hchfeV ===
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For the two sharp peaks labeled as characteristic peaks, they are related to
following processes: Some electrons penetrate into the atoms of the target materials,
Some inner orbit electrons are ejected due to energetic incident electrons Outer electrons jump to the vacancies formed due to the evacuation of the
inner electrons. X-ray photons are emitted.
These x-ray photons have
a specific energy which is
determined by thedifference between energy
levels of the electron
before and after jumping.Various sequences of
emissions, grouped as K-
lines and L-lines ,
correspond to different
types of electron transition.
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The principle of X-ray imaging relies upon individual parts ofan absorbing medium attenuating the incident X-ray beam
differently. The typical x-ray machine uses a voltage of 80-
100keV. This produces 30-50keV photons. The dominant
attenuating mechanism at this range is photoelectric effect.
The amount of attenuation that occurs in a medium dependsprincipally :
the thickness of the medium through which the X-
rays pass, the atomic number of the atoms in the medium, the energy of the X-ray photons incident on the
medium.
The x-rays imaging
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the attenuation coefficient is Z3 dependent, somedium of different Z number will results in
very different absorption behaviour of x-ray.
Soft tissue will attenuated very differently than
bone due to their large difference of Z-numbers. a good contrast can be created between bone
and soft tissue. ForCompton scatter, the attenuation coefficient
is independent of Z number, so this mechanism
does not contribute to the contrast between
different components of the tissue.
Thephotoelectric effectis the principal mechanism
involved in x-ray imaging.
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While essentially an X- ray photograph is a 'shadow
photograph', there are shades of grey between the black
and white, formed where the X- ray beam has been
partially attenuated in the medium.
h h
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The x-rays therapy
affect the function of molecules such as DNAcause cell deathcells in the process of dividing are easier to be
damaged by x-rays cancerous cells divide faster than healthy cells, a dose
of x-ray will kill more cancerous cells than healthycells.
When X- rays interact with matter, ionization is resulted.
Radiotherapy is the use of ionizing radiation to treat disease.
The principle of the x-ray therapy is based on the fact that
cancer cells are most susceptible to damage by x-rays.
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When x-rays are used for radiotherapy, the photon
energies are in the range 0.5-5 MeV.This is the energy range for Compton scattering
Attenuation is independent of the Z-number of theabsorbing medium in this energy range.
The Compton scatteris the principal mechanism
involved in x-ray therapy.
If lower photon energy is used, photoelectric effect isdominant, bone will absorb much more x-ray energy
than surrounding tissues, more damage will be done to
the bone than to the tumour to be treated.
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The tumour is accurately located and then X-rays are
aimed at the tumour from different directions: multiple
beam therapy
The X-rays tube is rotated about the patient with thetumor at the centre of the rotation: rotating beam
therapy
In both case the purpose is that the tumor receive larger dose.
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Biological effect of ionizing radiation
Ionizing radiation can directly damage DNA, RNA and enzymes.
However ionizing radiation can also ionize water to form H+ and