X-ray Interaction with Tissue

What is scattering?

A change in direction of an X-ray photon after it interacts with matter.

What is absorption?

When X-ray energy is completely absorbed by matter

What is attenuation?

The reduction in the intensity of an X-ray beam as it passes through matter due to absorption and scattering

What is ionisation?

The process of removing an electron from an atom, producing an ion

What can happen when X-rays strike matter?

They can be scattered, absorbed, partly absorbed and scattered, or pass through unchanged

What is the Compton effect?

An interaction where an X-ray photon interacts with a free or loosely bound outer-shell electron, causing the photon to lose some energy and change direction

What is another name for the Compton effect?

Incoherent scatter

What happens during a Compton interaction?

An X-ray photon hits an electron. The electron is knocked out, and the photon loses energy and changes direction

What happens to the Compton-scattered photon?

It has less energy than the original photon and travels in a different direction.

Why is Compton scatter undesirable in radiography?

It increases radiation dose to the patient and operator and reduces image quality by reducing image contrast

How does Compton scatter affect image contrast?

It reduces image contrast because scattered photons reach the image receptor from unwanted directions

What is the main radiation protection concern with Compton scatter?

Scattered radiation can increase radiation exposure to both the patient and operator

What is the photoelectric effect?

An interaction where an X-ray photon is completely absorbed by an inner-shell electron, causing the electron to be ejected

What are the released electrons in the photoelectric effect called?

Photoelectrons

What happens to the X-ray photon during the photoelectric effect?

It is completely absorbed and its energy is transferred to the electron and the atom

What happens after an inner-shell electron is ejected?

An electron from an outer shell moves into the empty inner shell, releasing energy.

What can be released when electrons move between shells?

Energy can be released as characteristic X-rays or other forms of energy such as heat.

What is the difference between Compton scatter and the photoelectric effect?

Compton scatter involves partial energy transfer and a scattered photon, while the photoelectric effect involves complete absorption of the X-ray photon

What is radiation risk?

The risk of harm resulting from exposure to ionising radiation, based on factors such as the radiation type, energy absorbed, and tissues exposed

What is dosimetry?

The measurement of the amount of radiation energy absorbed by an object or person

What is absorbed dose (D)?

The amount of radiation energy absorbed per unit mass of tissue

What unit is absorbed dose measured in?

Gray (Gy)

What is equivalent dose (H)?

A measure that takes into account the different biological effects of different types of radiation

What unit is equivalent dose measured in?

Sievert (Sv)

Why is equivalent dose needed?

Different types of radiation can cause different amounts of biological damage for the same absorbed dose

What is effective dose (E)?

A measure that takes into account the different sensitivities of different tissues and organs to radiation

What unit is effective dose measured in?

Sievert (Sv)

What is collective effective dose?

The total radiation exposure and associated radiation detriment across a group or population

Which cells are most sensitive to radiation damage?

Actively dividing cells

Which tissues are particularly sensitive to radiation?

Skin, hair, blood-forming cells, and the lining of the intestine

Which tissues are relatively resistant to radiation?

Muscle, bone, brain, and connective tissue

What is direct radiation damage?

Damage caused when ionising radiation directly interacts with important cellular molecules such as DNA.

What is somatic damage?

What can somatic radiation damage potentially cause?

It can cause cell death, malfunction, or mutations that may contribute to cancer

What is genetic or hereditary damage?

Radiation damage to reproductive cells that can potentially be passed on to offspring.

What is indirect radiation damage?

Damage caused when radiation interacts with water molecules, producing reactive free radicals that can damage cells

What is radiolysis of water?

The breakdown of water molecules caused by ionising radiation

What are free radicals?

Highly reactive molecules or atoms that can damage cellular structures

How can free radicals damage cells?

They can react with oxygen and other molecules, causing damage to cellular components including DNA

What are deterministic effects?

Radiation effects that have a threshold dose. Below the threshold, the effect does not occur; above it, severity increases with dose

What is another name for deterministic effects?

Tissue reactions

What causes deterministic effects?

Large numbers of cells being damaged or killed, causing tissue or organ malfunction

Give examples of deterministic effects

Skin burns and hair loss (alopecia)

What are stochastic effects?

Radiation effects where the probability of occurrence increases with dose, but the severity does not depend on the dose

Do stochastic effects have a threshold?

No. They are treated as having no safe minimum dose, meaning any exposure carries some risk

What causes somatic stochastic effects?

Non-lethal DNA damage or random mutations in somatic cells

Give examples of somatic stochastic effects

Radiation-induced cancer and leukaemia

What is the latent period?

The time between radiation exposure and the appearance of a radiation-induced effect e.g. malignancy

Why can radiation-induced cancers take years to appear?

They can have a long latent period between the original radiation exposure and development of the disease

What are genetic stochastic effects?

Probabilistic effects caused by radiation damage to reproductive cells that can potentially be passed to future generations

What cells are involved in genetic effects?

Reproductive cells, such as sperm and egg cells

Give examples of possible genetic effects

Hereditary defects, chromosomal abnormalities, and inherited genetic disorders

What does somatic mean?

An effect that occurs in the exposed person and is not inherited by their children

What does genetic/hereditary mean?

An effect that can be passed from an exposed person's reproductive cells to their offspring