Significance of X-ray Absorption in Biologic Tissue

What are X-rays?

X-rays are manmade electromagnetic energy.

What can happen when X-rays touch tissue?

1. Photons can be absorbed by tissue (absorption).

2. Photons can interact with tissue and scatter (scatter).

3. Photons can pass through the body without interacting and reach the image receptor (transmission).

What is absorption?

When an X-ray photon interacts with an atom and transfers its energy to tissues; this is called absorption.

What is absorbed dose?

The amount of energy per unit mass that is absorbed by tissue.

Why is absorption important?

Absorption is necessary to create an image. Different tissues absorb photons differently because of their varying atomic numbers (differential absorption).

What is differential absorption?

Different tissues absorb X-ray photons differently based on their atomic number and density; this allows image contrast.

How is primary radiation produced?

A diagnostic X-ray beam is produced when a stream of very energetic electrons bombards a positively charged tungsten or tungsten-rhenium target (the anode) in a highly evacuated glass tube.

Why are tungsten and tungsten-rhenium used as target materials?

They have high melting points and high atomic numbers, which make them efficient for X-ray production.

What is primary radiation?

The X-ray photon beam that emerges from the X-ray tube and is directed toward the image receptor.

Do all photons in the X-ray beam have the same energy?

No, the photons have a range of energies. The most energetic photons have no more energy than the electrons bombarding the target.

What determines the energy of the electrons in the X-ray tube?

The electrical voltage applied across the tube, expressed in kilovolts (kV), typically characterized by the kilovolt peak (kVp).

What is attenuation?

A reduction in the number of photons in the primary X-ray beam as it passes through matter.

What are the types of transmission?

Direct transmission: photons that pass through without interacting and reach the IR.

Indirect transmission: photons that scatter in the patient but still reach the IR, lowering image contrast.

What are the types of scatter?

Compton scatter and coherent scatter.

What is Compton scatter?

A type of scatter that contributes no useful information to the image and increases patient dose.

What is coherent scatter?

A type of scatter that has little or no effect on image quality.

What happens in direct transmission?

Photons pass through the patient without interacting and reach the image receptor.

What happens in indirect transmission?

Photons scatter within the patient and still reach the IR, decreasing image contrast.

What happens when photons are absorbed?

Photons interact with atoms and deposit energy in tissue, not reaching the IR.

What happens when photons are attenuated?

They are absorbed or scattered and fail to reach the image receptor.

Is photon interaction predictable?

No, individual photon interactions are random, but the average behavior of many photons can be predicted to determine image characteristics.

What unit is used for absorbed dose?

The gray (Gy) or rad (radiation absorbed dose) are used for absorbed dose measurement.

What is the gray (Gy)?

A unit of absorbed dose representing joules of energy absorbed per kilogram of tissue.

What is the rad?

An older unit of absorbed dose (1 rad = 0.01 gray).

What should radiographers do to minimize biologic damage?

Keep the amount of electromagnetic energy transferred to the patient as small as possible (ALARA principle).