HDR 112 - xraykamarul...more damaging to tissue than low LET radiations (electrons, gamma and x-rays). Slide 11 of 52 TOPIC CHAPTER 2: Radiation Interactions with Matter Bremsstrahlung

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

Slide 2 of 52

TOPIC

CHAPTER 2: Radiation Interactions with Matter

Interactions & Processes

which lead to Radiation Injury

Slide 3 of 52

TOPIC


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)

Slide 4 of 52

TOPIC


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.

Slide 5 of 52

TOPIC


Slide 6 of 52

TOPIC


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.

Slide 7 of 52

TOPIC


Specific Ionization for 7.69 MeV alpha particle from polonium214

Slide 8 of 52

TOPIC


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.

Slide 9 of 52

TOPIC


Path Lengths vs. Ranges

Slide 10 of 52

TOPIC


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).

Slide 11 of 52

TOPIC


Bremsstrahlung

Slide 12 of 52

TOPIC


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.

Slide 13 of 52

TOPIC


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.

Slide 14 of 52

TOPIC


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).

Slide 15 of 52

TOPIC


Neutron Interaction

Slide 16 of 52

TOPIC


X- and Gamma-Ray Interactions

Rayleigh scattering

Compton scattering

Photoelectric absorption

Pair production

Slide 17 of 52

TOPIC


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.

Slide 18 of 52

TOPIC


Rayleigh Scattering

Slide 19 of 52

TOPIC


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.

Slide 20 of 52

TOPIC


Slide 21 of 52

TOPIC


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

Slide 22 of 52

TOPIC


Relative Compton Scatter Probabilities

Slide 23 of 52

TOPIC


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.

Slide 24 of 52

TOPIC


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

Slide 25 of 52

TOPIC


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

Slide 26 of 52

TOPIC


Photoelectric Absorption (Iodine-131)

Slide 27 of 52

TOPIC



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

Slide 28 of 52

TOPIC



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.

Slide 29 of 52

TOPIC


Photoelectric Mass Attenuation Coefficients

Slide 30 of 52

TOPIC



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).

Slide 31 of 52

TOPIC


Percentage of Compton and Photoelectric Contributions

Slide 32 of 52

TOPIC


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.

Slide 33 of 52

TOPIC


Pair Production

Slide 34 of 52

TOPIC


TYPES OF RADIATION INJURY

Slide 35 of 52

TOPIC


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.

Slide 36 of 52

TOPIC


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.

Slide 37 of 52

TOPIC


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.

Slide 38 of 52

TOPIC


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+

Slide 39 of 52

TOPIC


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.

Slide 40 of 52

TOPIC


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)

Slide 41 of 52

TOPIC


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)

Slide 42 of 52

TOPIC


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.

Slide 43 of 52

TOPIC


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.

Slide 44 of 52

TOPIC


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.

Slide 45 of 52

TOPIC


End of Chapter 2

Documents

HDR 112 - xraykamarul...more damaging to tissue than low LET radiations (electrons, gamma and x-rays). Slide 11 of 52 TOPIC CHAPTER 2: Radiation Interactions with Matter Bremsstrahlung