hdr 112 - xraykamarul...more damaging to tissue than low let radiations (electrons, gamma and...
TRANSCRIPT
HDR 112
CHAPTER 2
RADIATION BIOLOGY AND RADIATION PROTECTION
RADIATION INTERACTIONS
WITH MATTER
PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCE
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CHAPTER 2: Radiation Interactions with Matter
Interactions & Processes
which lead to Radiation Injury
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CHAPTER 2: Radiation Interactions with Matter
LEARNING OUTCOMES
At the end of the lesson, the student should be able to:-
Explain the interaction and processes which lead to radiation injury
Describe Photoelectric Effect
Describe Compton Scattering
Describe Linear Energy Transfer (LET)
Describe Relative Biological Effects (RBE)
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CHAPTER 2: Radiation Interactions with Matter
Particle Interactions
Energetic charged particles (e.g. electron, proton)
interact with matter by electrical forces and lose kinetic
energy via:-
Excitation
Ionization
Radiative losses
~ 70% of charged particle energy deposition leads to non-
ionizing excitation.
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CHAPTER 2: Radiation Interactions with Matter
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CHAPTER 2: Radiation Interactions with Matter
Specific Ionization
Number of primary and secondary ion pairs produced per
unit length of charged particle’s path is called specific
ionization.
Expressed in ion pairs (I.P.)/mm
Increases with electrical charge of particle.
Decreases with incident particle velocity.
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CHAPTER 2: Radiation Interactions with Matter
Specific Ionization for 7.69 MeV alpha particle from polonium214
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CHAPTER 2: Radiation Interactions with Matter
Charged Particle Tracks
Electrons follow tortuous paths in matter as the result of
multiple scattering events.
Ionization track is sparse and non-uniform.
Larger mass of heavy charged particle results in dense and
usually linear ionization track.
Path length is actual distance particle travels; range is
actual depth of penetration in matter.
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CHAPTER 2: Radiation Interactions with Matter
Path Lengths vs. Ranges
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CHAPTER 2: Radiation Interactions with Matter
Linear Energy Transfer
Amount of energy deposited per unit path length is
called the linear energy transfer (LET).
Expressed in units of eV/cm
LET of a charged particle is proportional to the square of
the charge and inversely proportional to its kinetic energy.
High LET radiations (alpha particles, protons, etc.) are
more damaging to tissue than low LET radiations
(electrons, gamma and x-rays).
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CHAPTER 2: Radiation Interactions with Matter
Bremsstrahlung
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CHAPTER 2: Radiation Interactions with Matter
Bremsstrahlung
Probability of bremsstrahlung production per atom is
proportional to the square of Z of the absorber.
Energy emission via bremsstrahlung varies inversely with
the square of the mass of the incident particle.
Protons and alpha particles produce less than one-
millionth the amount of bremsstrahlung radiation as
electrons of the same energy.
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CHAPTER 2: Radiation Interactions with Matter
Bremsstrahlung
Ratio of electron energy loss by bremsstrahlung production
to that lost by excitation and ionization = EZ/820
E = kinetic energy of incident electron in MeV
Z = atomic number of the absorber
Bremsstrahlung x-ray production accounts for ~1% of
energy loss when 100 keV electrons collide with a tungsten
(Z = 74) target in an x-ray tube.
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CHAPTER 2: Radiation Interactions with Matter
Neutron Interactions
Neutrons are uncharged particles.
They do not interact with electrons.
Do not directly cause excitation or ionization.
They do interact with atomic nuclei, sometimes liberating
charged particles or nuclear fragments that can directly
cause excitation or ionization.
Neutrons may also be captured by atomic nuclei.
Retention of the neutron converts the atom to a
different nuclide (stable or radioactive).
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CHAPTER 2: Radiation Interactions with Matter
Neutron Interaction
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CHAPTER 2: Radiation Interactions with Matter
X- and Gamma-Ray Interactions
Rayleigh scattering
Compton scattering
Photoelectric absorption
Pair production
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CHAPTER 2: Radiation Interactions with Matter
Rayleigh Scattering
Incident photon interacts with and excites the total atom
as opposed to individual electrons.
Occurs mainly with very low energy diagnostic x-rays, as
used in mammography (15 to 30 keV).
Less than 5% of interactions in soft tissue above 70 keV; at
most only 12% at ~30 keV.
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CHAPTER 2: Radiation Interactions with Matter
Rayleigh Scattering
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CHAPTER 2: Radiation Interactions with Matter
Compton Scattering
Predominant interaction in the diagnostic energy range
with soft tissue.
Most likely to occur between photons and outer
(“valence”) shell electrons.
Electron ejected from the atom; photon scattered with
reduction in energy.
Binding energy comparatively small and can be ignored.
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CHAPTER 2: Radiation Interactions with Matter
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CHAPTER 2: Radiation Interactions with Matter
Compton Scatter Probabilities
As incident photon energy increases, scattered photons and
electrons are scattered more toward the forward direction.
These photons are much more likely to be detected by the
image receptor, reducing image contrast.
Probability of interaction increases as incident photon energy
increases; probability also depends on electron density.
Number of electrons/gram fairly constant in tissue;
probability of Compton scatter/unit mass independent of Z
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CHAPTER 2: Radiation Interactions with Matter
Relative Compton Scatter Probabilities
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CHAPTER 2: Radiation Interactions with Matter
Compton Scattering
Laws of conservation of energy and momentum place
limits on both scattering angle and energy transfer.
Maximal energy transfer to the Compton electron occurs
with a 180-degree photon backscatter.
Scattering angle for ejected electron cannot exceed 90
degrees.
Energy of the scattered electron is usually absorbed near
the scattering site.
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CHAPTER 2: Radiation Interactions with Matter
Compton Scattering
Incident photon energy must be substantially greater than the
electron’s binding energy before a Compton interaction is likely to
take place.
Probability of a Compton interaction increases with increasing
incident photon energy.
Probability also depends on electron density (number of electrons/g
density)
With exception of hydrogen, total number of electrons/g fairly constant in
tissue
Probability of Compton scatter per unit mass nearly independent of Z
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CHAPTER 2: Radiation Interactions with Matter
Photoelectric Absorption
All of the incident photon energy is transferred to an
electron, which is ejected from the atom.
Kinetic energy of ejected photoelectron (Ec) is equal to
incident photon energy (E0) minus the binding energy of
the orbital electron (Eb)
Ec = Eo - Eb
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CHAPTER 2: Radiation Interactions with Matter
Photoelectric Absorption (Iodine-131)
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CHAPTER 2: Radiation Interactions with Matter
Photoelectric Absorption
Incident photon energy must be greater than or equal to the
binding energy of the ejected photon.
Atom is ionized, with an inner shell vacancy.
Electron cascade from outer to inner shells
Characteristic x-rays or Auger electrons
Probability of characteristic x-ray emission decreases as Z
decreases
Does not occur frequently for diagnostic energy photon
interactions in soft tissue
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CHAPTER 2: Radiation Interactions with Matter
Photoelectric Absorption
Although probability of photoelectric effect decreases
with increasing photon energy, there is an exception.
Graph of probability of photoelectric effect, as a function
of photon energy, exhibits sharp discontinuities called
absorption edges.
Photon energy corresponding to an absorption edge is the
binding energy of electrons in a particular shell or
subshell.
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CHAPTER 2: Radiation Interactions with Matter
Photoelectric Mass Attenuation Coefficients
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CHAPTER 2: Radiation Interactions with Matter
Photoelectric Absorption
At photon energies below 50 keV, photoelectric effect
plays an important role in imaging soft tissue.
Process can be used to amplify differences in attenuation
between tissues with slightly different atomic numbers,
improving image contrast.
Photoelectric process predominates when lower energy
photons interact with high Z materials (screen phosphors,
radiographic contrast agents, bone).
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CHAPTER 2: Radiation Interactions with Matter
Percentage of Compton and Photoelectric Contributions
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CHAPTER 2: Radiation Interactions with Matter
Pair Production
Can only occur when the energy of the photon exceeds
1.02 MeV.
Photon interacts with electric field of the nucleus; energy
transformed into an electron-positron pair.
No consequence in diagnostic x-ray imaging because of
high energies required.
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CHAPTER 2: Radiation Interactions with Matter
Pair Production
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CHAPTER 2: Radiation Interactions with Matter
TYPES OF RADIATION INJURY
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CHAPTER 2: Radiation Interactions with Matter
LEARNING OUTCOMES
At the end of the lesson, the student should be able to:-
Explain types of radiation injury.
Describe Direct Action of radiation.
Describe Indirect Action of radiation.
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CHAPTER 2: Radiation Interactions with Matter
The Effects of Radiation on the Cell at the Molecular Level
When radiation interacts with target atoms, energy is
deposited, resulting in ionization or excitation.
The absorption of energy from ionizing radiation produces
damage to molecules by direct and indirect actions.
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CHAPTER 2: Radiation Interactions with Matter
For direct action, damage occurs as a result of ionization
of atoms on key molecules in the biologic system. This
causes inactivation or functional alteration of the
molecule.
Indirect action involves the production of reactive free
radicals whose toxic damage on the key molecule results
in a biologic effect.
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CHAPTER 2: Radiation Interactions with Matter
Direct Action
Direct ionization of atoms in molecules is a result of
absorption of energy by photoelectric and Compton
interactions. Ionization occurs at all radiation qualities but
is the predominant cause of damage in reactions involving
high LET radiations. Absorption of energy sufficient to
remove an electron can result in bond breaks. Ionizing
radiation+RH R- + H+
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CHAPTER 2: Radiation Interactions with Matter
Indirect Action
These are effects mediated by free radicals.
A free radical is an electrically neutral atom with an
unshared electron in the orbital position. The radical is
electrophilic and highly reactive. Since the predominant
molecule in biological systems is water, it is usually the
intermediary of the radical formation and propagation.
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CHAPTER 2: Radiation Interactions with Matter
Indirect Action- Radiolysis of Water
Free radicals readily recombine to electronic and orbital neutrality.
However, when many exist, as in high radiation fluence, orbital
neutrality can be achieved by:
1. Hydrogen radical dimerization (H2)
2. The formation of toxic hydrogen peroxide (H2O2).
3. The radical can also be transferred to an organic molecule in the
cell.
H-O-H H+ + OH- (ionization)
H-O-H H0+OH0 (free radicals)
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CHAPTER 2: Radiation Interactions with Matter
Indirect Action
H0 + OH0 HOH (recombination)
H0 + H0 H2 (dimer)
OH0 + OH0 H2O2 (peroxide dimer)
OH0 + RH R0 + HOH (Radical transfer)
The presence of dissolved oxygen can modify the
reaction by enabling the creation of other free
radical species with greater stability and lifetimes
H0+O2 HO20 (hydroperoxy free radical)
R0+O2 RO20 (organic peroxy free radical)
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CHAPTER 2: Radiation Interactions with Matter
Indirect Action - The Lifetimes of Free Radicals
The lifetimes of simple free radicals (H0 or OH0) are
very short, on the order of 10-10 sec.
While generally highly reactive they do not exist long
enough to migrate from the site of formation to the
cell nucleus.
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CHAPTER 2: Radiation Interactions with Matter
Indirect Action - The Lifetimes of Free Radicals
However, the oxygen derived species such as hydroperoxy
free radical does not readily recombine into neutral
forms.
These more stable forms have a lifetime long enough to
migrate to the nucleus where serious damage can occur.
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CHAPTER 2: Radiation Interactions with Matter
Indirect Action- Free Radicals
The transfer of the free radical to a biologic molecule can
be sufficiently damaging to cause bond breakage or
inactivation of key functions
The organic peroxy free radical can transfer the radical
form molecule to molecule causing damage at each
encounter. Thus a cumulative effect can occur, greater
than a single ionization or broken bond.
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CHAPTER 2: Radiation Interactions with Matter
End of Chapter 2