Interactions of x-rays

What is attenuation?

Absorption of x-rays as it passes through material

Sharp decrease in intensity as material thickens

What is a half value layer?

Shielding powers of an object when 50% of energy is absorbed

What are the two types of attenuation?

Scattering

Absorption

What are the two types of scattering?

Coherent

Compton

What are the features of coherent scattering?

Incident photon is absorbed, exciting the atom

Energy is released in the form of a scattered photon

Does not cause ionisation

What are the features of Compton scattering?

Incident photon ionises an atom in patient tissue

Causes scattered photon to be released

Causes electron to be ejected

What is the relationship between angle of scatter and energy loss in Compton scattering?

Angle of scatter is directly proportional to energy loss of photon

What is the relationship between scatter probability and electron density?

The probability of Compton scatter is directly proportional to electron density multiplied by 1/energy of the photon

What are the two types of absorption?

Photoelectric effect

Pair production

What are the features of the photoelectric effect?

X-ray photon ionises atom, releasing electron from inner shell

Inner shell vacancy is filled by an outer shell electron

Energy difference is released as a characteristic x-ray

What are the features of pair production?

Photon interact with nucleus

Ejects electron-positron pair that travel in exactly opposite directions

Only ejected if photon has greater energy

What is the type of scatter and type of absorption that affect image quality?

Compton scattering

Photoelectric effect

Why does different anatomy have different attenuation?

Determined by differences in effective atomic number and therefore differences in probability of the photoelectric effect

How does increasing kVp affect the contrast?

Decreases contrast because there is less attenuation and more transmission of x-rays

Lower probability of the photoelectric effect

What is the function of contrast agents?

Increase attenuation and provide additional contrast to the x-ray image

What is the intensity of an x-ray?

The amount of radiation passing through an area over time

What is the trend of the photon energy graph?

As photon energy increases, probability of attenuation decreases

What is the k-edge of the photon energy graph?

K shell electrons have the highest binding energy because they are the innermost electrons so cause a sudden increase in attenuation

What does the mass attenuation coefficient indicate?

Amount of contrast between different materials

What is the relationship between energy and contrast?

As energy increases, the attenuation difference decreases so contrast decreases