X-ray Interaction with Matter

Electromagnetic Radiation interacts with structures with similar size to the

wavelength of the radiation.Interactions have wavelike and

particle like properties.X-rays have a very small wavelength,

no larger than 10-8 to 10-9 .

X-ray Interaction with Matter

The higher the energy of the x-ray, the shorter the wavelength.

Low energy x-rays interact with whole atoms.

Moderate energy x-rays interact with electrons.

High energy x-rays interact with the nuclei.

Five forms of x-ray Interactions

Classical or Coherent ScatteringCompton EffectPhotoelectric EffectPair productionPhotodisintegration

Two Forms of X-ray Interactions Important to Diagnostic X-ray

Compton EffectPhotoelectric Effect

Classical or Coherent ScatteringLow energy x-rays

of about 10 keV interact in this

manner.Incident photon

interacts with the atom.

There is a change in direction.

Classical or Thompson Scattering

There is no loss of energy and no

ionization.Photon scattered

forward.Because these are

low energy x-rays, they are of little

importance.

Classical ScatteringAt 70 kVp only a

few percent of the x-rays undergo this

form of scattering.Classic Scatter may

contribute to the graying of the

image called film fog.

Compton EffectModerate energy x-

ray photon through out the diagnostic

x-ray range can interact with outer

shell electron.This interaction not

only changes the direction but

Compton Effectreduced its energy

and ionizes the atom as well. The

outer shell electron is ejected. This is called Compton

Effect or Compton

Scattering.

Compton Scattering

The x-ray continues in an altered direction with decreased energy.

The energy of the Compton-scattered x-ray is equal to the difference

between the energy of the incident x-ray and the energy imparted to the

electron.

Compton Scattering

The energy imparted to the electron is equal to its binding energy plus the kinetic with which it leaves the atom.

During Compton-scattering most of the energy is divided between the

scattered photon and the secondary electron .

The Secondary Electron is called a Compton Electron.

Compton ScatteringThe scattered

photon and secondary electron will retain most of

its energy so it can interact many times before it losing all of it’s

energy .

Compton Effect

The scattered photon will ultimately be absorbed photoelectrically.

The secondary electron will drop into a hole in the outer shell of an atom

created by an ionizing event.Compton-scattered photons can be

deflected in any direction.

Compton Effect

A zero angle deflection will result in no energy loss.

As the angle approaches 180 degrees, more energy is transferred

to the secondary electron.Even at 180 degrees, 66% of the

energy is retained.

Compton Effect

Photons scattered back towards the incident x-ray beam are called

Backscatter Radiation.While important in radiation therapy,

backscatter in diagnostic x-ray is sometimes responsible for the hinges on the back of the the cassette to be

seen on the x-ray film

Compton EffectThe probability of

Compton Effect is about the same for soft tissue or bone.

This decreases with increasing photon

energies.Compton scatter

decreases with increased kVp.

Features of Compton ScatteringMost likely to occur

As x-ray energy increases

With outer-shell electrons

With loosely bound electrons.

Increased penetration through tissue w/o

interaction.Increased Compton

relative to photoelectric scatter .

Reduced total Compton scattering.

Features of Compton ScatterAs atomic number

of the absorber increases

As mass density of absorber increases

No effect on Compton Scatter

Proportional increase in

Compton Scatter.

Photoelectric EffectX-rays in the

diagnostic range can undergo

ionizing interactions with

inner shell electron of the target atom.

It is not scattered but totally absorbed.

Photoelectric Effect

The Photoelectric Effect is a photon absorption interaction.

Photoelectric Effect

The electron removed from the target atoms is called a photoelectron.

The photoelectron escapes with kinetic energy equal to the

difference between the energy of the incident x-ray and the

binding energy of the electron.

Photoelectric Effect

Low anatomic number target atoms such as soft tissue have low binding

energies.Therefore the photoelectric electron is

released with kinetic energy nearly equal to the incident x-ray.

Higher atomic number target atoms will have higher binding energies.

Photoelectric Effect

Therefore the kinetic energy of the photoelectron will be proportionally

lower.Characteristic x-rays are produced

following a photoelectric interaction to those produced in the x-ray tube.

These characteristic x-rays are also secondary radiation and acts like

scatter.

Photoelectric Effect

The probability of a photoelectric interaction is a function of the photon energy and the atomic

number of the target atom.A photoelectric interaction can

not occur unless the incident x-ray has energy equal to or

greater than the electron binding energy .

Photoelectric Effect

The probability of photoelectric interaction is inversely proportional to

the third power of the photon energy.

The probability of photoelectric interaction is directly proportional to

the third power of the atomic number of the absorbing material

Effective Atomic Numbers

Human TissueMuscleFatBoneLung

Other MaterialAirConcreteLead

Effective Atomic #7.46.313.87.4

7.61782

Photoelectric Effect

A probability of interaction to the third power changes rapidly.

For the photoelectric effect this means that a small variation in atomic

number or x-ray energy results in a large changes in chance of an

interaction.This is unlike Compton interactions.

Features of the Photoelectric Effect

Most likely to occur With inner-shell electrons

With tightly bound electrons.

When the x-ray energy is greater

than the electron-binding energy .

Features of the Photoelectric Effect

As the x-ray energy increases

Increased penetration

through tissue without interaction.

Less photoelectric effect relative to

Compton effect.Reduced absolute

absorption.

Features of the Photoelectric Effect

As the atomic number of the

absorber increases As mass density of

the absorber increases

Increases proportionally the

cube of the Z.Proportional

increase in photoelectric

effect.

Pair ProductionIf the incident x-ray

has sufficient energy, it may

escape the electron cloud and come

close enough to the nucleus to come

under the influence of the strong

electrostatic field of the nucleus.

Pair ProductionThe interaction

with the nucleus strong electrostatic

field causes the photon to

disappear and in its place appear

two electrons.

Pair ProductionOne is positively

charged and called a positron while the other remains

negatively charged. This is

called Pair Production.

Pair ProductionIt take a photon

with 1.02 MeV to undergo Pair

Production.Therefore it is not

important to diagnostic x-ray.

Photodisintegration

High energy x-ray photons with energies above 10 MeV can escape interaction with

both the electrons and nucleus electrostatic fields.

PhotodisintegrationIt is absorbed

into the nucleus that

excites the nucleus

resulting in the release of

a nucleon or other nuclear material. This is referred to

as:

Photodisintegration

Photodisintegration. Like pair production, the

high energy needed to cause

this makes it unimportant to

diagnostic radiography.