X-ray Interaction with Matter

Knowledge of X-ray interactions with matter must exist on what two levels?

Large scale and atomic scale

What two main processes occur in the large scale?

Transmission and attenuation

Define transmission:

Photons pass through atoms with no interaction at all

Why do photons pass through atoms with no interaction?

Atoms are mostly empty space

The majority of diagnostic energy X-rays are ________

Transmitted

Define attenuation:

A loss of X-ray photons upon interaction with matter per unit area

What are 3 processes that are responsible for attenuation?

Absorption, scattering, and divergence

Define absorption:

Photons are absorbed by orbital electrons of the nucleus itself

Define scattering:

Change in direction of a photon with or without a loss of energy

Define divergence:

X-rays are continually diverging from their source so as distance through matter increases, the quantity of photons per unit area will decrease

What 5 main processes occur in the atomic scale?

1. Coherent or unmodified scatter

2. Photoelectric effect

3. Compton effect

4. Pair production

5. Photodisintegration

What is coherent or unmodified scatter also known as?

Classical scatter, Rayleigh scatter, or Thomson scatter

When does coherent or unmodified scatter occur?

When a low energy X-ray photon (usually under 30 keV) strikes an orbital electron and is momentarily absorbed

What happens with the absorbed energy in coherent or unmodified scatter?

It is then re-radiated in the form of another photon with no change in its energy, but usually in a different direction from the original incident photon.

True or false

Coherent or unmodified scatter results in no useful information in diagnostic radiology

True

What is photoelectric effect also known as?

True absorption

What is the most dominant interaction and why?

Photoelectric effect because it will occur more often than any other interaction

What happens during the photoelectric effect?

The incident X-ray strikes an inner shell electron and is completely absorbed

The incident X-ray in the photoelectric effect must possess energy of how much?

At least equal to or greater than the binding energy of the inner shell electron

When is the photoelectric effect most likely to occur?

When the photon energy and the electron binding energy are nearer to each another

What 3 probability factors of a photoelectric interaction depend on?

1. Energy of the incident X-ray

2. Atomic number or target atom

3. Mass density of subject

As the energy of the incident X-ray (kVp) increases, the probability of photoelectric interaction _______

Decreases

When is the photon most likely to undergo photoelectric interaction?

When the energy of the photon is equal to or slightly greater then the binding energy of the electron (K-edge effect)

As the atomic number of the absorber material increase, the probability of the photoelectric interaction will ______

Increase

What is differential absorption?

The difference in x-ray interactions with matter (what is absorbed and what is transmitted)

As the mass density of the object being struck by X-rays increases, the amount of photoelectric interaction will ______

Increase

Ec =

Ek - Eb

Ephotoelectron =

Ei - Eb

Ei=

Incident photon energy

Eb=

Electron binding energy

What is the Compton effect also known as?

Compton modified scatter or incoherent scatter

When does the compton effect occur?

Within the diagnostic energy range and will begin to dominate with energy's above 180 keV

When does the compton effect take place?

When the incident X-ray interacts with an outer shell electron that is more loosely bound that a K-shell electron

When can the compton effect occur in an inner shell electron?

If they have a low binding energy

What happens with the energy in the compton effect with the incident X-ray?

Some of the energy of the incident X-ray is absorbed (partial absorption) while the rest of it will continue in a different direction

The photon after is it partially absorbed in the compton effect is now known as ______ radiation

Scatter radiation

For the compton effect , what is the free electron called?

Recoil electron

Equation of the energy transfer in the compton effect:

Ei = Es + Eb + Eke

Es =

Energy of Compton scattered photon

Eke =

Kinetic energy of the recoil electron

In the diagnostic range, the greatest percentage of Compton scatter photons tend to scatter in the ______ direction

Forward direction

The greater the scattering angle, the ____ the energy of scattered radiation

Lower

As the energy of the incident photon is increased, the _____ the energy of the scattered radiation

Greater

What 2 probability factors are Compton interaction dependent on?

Photon energy and mass density

As the photon energy increase, the total amount of Compton interaction occurring will ______, but the percentage of Compton vs photoelectric interaction will ______

Decrease , increase

As the mass density of the subject material increases, the amount of Compton interaction will _______

Increase

The degree of Compton scattering is independent of the _______

Atomic number

When does pair production occur?

Only with very high energy incident x-rays and gamma rays (above 1.02 MeV)

When does pair production begin to predominate?

Above 24 MeV

In pair production, what do incident x-rays interact with?

Nuclear force field and is completely absorbed

In pair production, what does the energy transform into?

Two particles of matter; an electron and a positron

What is annihilation reaction?

When a negatron and positron combine, they will mutually destroy each other

When does photodisintegration occur?

Only with high energy X-ray and gamma ray photon with an energy above 10 MeV

In photodisintegration, what happens with the high energy photons?

Can penetrate the nuclear force field and enter the nucleus itself

In photodisintegration, what happens with the incident X-rays?

They are completely absorbed and transferred into the nucleus

In photodisintegration, what does the energy result in?

A nuclear fragment being ejected from the nucleus

What is secondary radiation?

Radiation existing after the interaction of the primary beam with matter. This can contribute to the overall total dosage

What secondary radiation occurs during the photoelectric effect?

Photoelectrons and characteristic radiation

What secondary radiation occurs during the Compton effect?

Recoil electrons and scattered photons

What secondary radiation occurs during pair production?

Positron, negatron, and annihilation reaction photons

What secondary radiation occurs during photodisintegration?

Nuclear fragment

Positive contrast agents:

Barium and iodine

Structures containing positive contrast agents would be more visible because of an _______ in photoelectric effect

Increase

Negative contrast agents:

Air and various gases

Structures containing negative contrast agents would be more visible because of a _______ in photoelectric effect

Decrease