X-Ray Interaction with Matter

Beam quality

Penetrating power, beam energy, controlled by kVp

Beam quantity

Tube current, intensity, beam photon number, controlled by mAs

Absorption

Transfer of EM energy from photon to biologic tissue

Attenuation

Reduction of photon number in beam as partial energy is transferred to biologic tissue

Transmission

No EM transfer from photon to body

Absorbed dose (D)

Amount of EM energy absorbed per unit of body mass

Primary radiation

The x-ray beam emerging from the tube

Attenuation

Primary beam reduced by absorption or scatter

Direct transmission

Photon reaches the IR unchanged

Absorption

Total loss of energy

Scatter

Partial loss of energy and change of photon direction

Five types of interactions with matter

Coherent (classical) scatter, compton scatter, photoelectric absorption, pair production, and photodisintegration

Which two interactions with matter are important in the diagnostic range?

Compton scattering and photoelectric absorption

Coherent Scatter (Classical Scatter)

No loss of KE as photon scatters. Usually occurs at less than 10 keV. Slight change of direction (20° or less) in forward direction. Little importance in the diagnostic range.

Coherent scatter

Compton Scatter

Outer shell photon-electron interaction. Produces Compton electron & scattered photon. Does not contribute diagnostically (direction and energy change). Both can cause additional biologic interactions or reach IR.

Compton scatter

Formulas for compton scatter

• Energy of Compton photon = the difference between incoming x-ray and ejected electron

• Energy of the ejected electron = its binding energy plus the kinetic energy with which it leaves the atom

• Ei = Es + (Eb + EKE)

What does compton scatter contribute to?

Image fog and occupational exposure

Photoelectric Absorption - Body

An inner shell photon-electron interaction created as an incoming x-ray photon knocks loose an inner-shell electron, creating a vacancy in the inner shell and leading to an electron cascade. This vacancy is filled by outer-shell electrons, causing radiation in the form of characteristic photons or an ejected electron.

What is photoelectric absorption responsible for?

Patient dose and image contrast

Photoelectric absorption

The ______the difference in the amount of photoelectric absorption, the ______ the contrast in the radiographic image will be between adjacent structures of differing atomic numbers.

greater; greater

As absorption ______, so does the potential for ______.

increases; biologic damage

How to ensure both radiographic image quality and patient safety

Choose the highest-energy x-ray beam that permits adequate radiographic contrast for computed radiography, digital radiography, or conventional radiography

Photoelectric absorption depends on _____ AND _____

incident photon energy; atomic number of tissue

Photoelectric absorption ______ as incident photon energy _______

increases; decreases

Why does photoelectric absorption have an inverse relationship with incident photon energy?

Because lower energy x-ray photons will be absorbed more easily

What is the Z# for compact bone?

Z = 13.8

What is the Z# for soft tissue?

Z = 7.4

What is the Z# of air?

Z = 7.6

Mass density

How compact the atoms in a given tissue are

What patient characteristics can cause absorption to differ on a macro scale?

Different body structures and pathology (ex. differences in chest densities or tissue density loss due to decrease in bone calcium)

How does contrast media act to increase photoelectric absorption?

Contrast media has a higher atomic number than surrounding tissue, causing more x-rays to be absorbed by contrast media. This leads to sharper outlines and better visibility in structures with contrast media.

What is the Z# for barium?

Z = 56

What is the Z# for iodine?

Z = 53

Probability of compton effect

Inversely proportional to x-ray energy. Inversely as E1 ; as kVp/keV increases, Compton interactions decrease proportionally by a power of 1.

Probability of photoelectric effect

Inversely proportional to the third power of the x-ray energy. Inversely as E^3; as kVp/keV increases, photoelectric interactions decrease proportionally by a power of 3.

Compton scattering is ———- — tissue atomic number and depends solely on ——— ——-

independent of; x-ray energy

True/False: Photoelectric effect depends on both tissue atomic number and x-ray energy

True

As kVp increases, Compton scattering _____ relative to photoelectric absorption of tissues

increases

Which interaction is this?

Photoelectric effect

Which interaction is this?

Compton

Which interaction is this?

• Depends on x-ray energy & tissue atomic number

• Dominates in bone

• Dominates at low x-ray energies

Photoelectric absorption

Which interaction is this?

• Depends on x-ray energy only

• Dominates in soft tissue

• Dominates at high x-ray energies

What happens as kVp is increased?

All numbers decrease, but you are left with mainly Compton (retains 2/3 original energy)

Pair production

A higher energy incident x-ray (above 1.02 MeV) interacts with nucleus (attractive force). This causes the X-ray to disappear, and two electrons are created (one positive – positron – and the other negative - electron). The electron continues to ionize & eventually will fill an empty shell. The positron unites with a free electron & converts to energy via a process called annihilation radiation.

What is annihilation radiation?

A positron, a form of antimatter that is a positive electron, interacts with an electron. The opposite charges are cancelled out, converting their matter into photons (energy) of 0.511MeV each.

In what setting does pair production and annihilation radiation occur?

Nuclear medicine or PET (positron emission tomography)

Photodisintegration

An incident x-ray energy above 10 MeV is absorbed directly by nucleus of an atom. A nucleon or nuclear fragment is then emitted by excited nucleus

In what setting does photodisintegration occur?

Radiation therapy

Which two interactions with matter are important in conventional radiology?

Compton and photoelectric

True/False: Compton Scatter is diagnostic

False

What is involved in differential absorption?

Compton scattering, photoelectric effect, and direct transmission.

What does this image show?

Differential absorption

Compton scattering causes _____

Noise

How do photons that do not reach the IR add diagnostic info to the image?

They represent anatomy with high Z# (in case of the photoelectric interaction) and are visualized via light on the image.

Direct transmission

Photons that penetrate the body, but do not interact with matter. These produce dark areas of the image.

A radiographic image is produced from the difference between _____ and _____.

Photoelectric effect; direct transmission

About __of radiation incident on the patient reaches the IR

1%

Fewer than ____ of radiation incident interact to form an image

Half

_____ kVp leads to _____ differential absorption

Increasing; reduced

What is high differential absorption best for?

Exams requiring large differences in tissue contrast (ex. extremity work, mammography)

What is low differential absorption best for?

Exams requiring small differences in tissue contrast (ex. KUB, barium studies)

____ Mass density, ____ chance of interaction

Double; double

Roughly __% of photons are attenuated for every 5 cm of tissue

50%

Photoelectric, pair production, and photodisintegration are _____ processes

absorption

Compton is a ____ ____ process

partial absorption

Attenuation

Total reduction in x-ray intensity after passing through a given tissue thickness; product of absorption and scattering