5.1 & 5.2 - Photon Interactions

What happens when the primary X-ray beam enters the patient?

Some photons pass through and reach the film (transmission), while others are attenuated (absorbed or scattered). This produces varying shades of gray due to differential absorption.

What is attenuation in radiography?

any interaction that removes a photon from the primary beam, including absorption and scatter.

What is transmission in the context of X-rays?

Transmission is the opposite of attenuation; it refers to photons that pass through the patient without interaction and reach the image receptor, contributing to the image.

What are the five types of attenuation

- Coherent Scatter

- Compton Scatter

- Photoelectric Effect

- Pair Production

- Photodisintegration

What is Coherent Scatter?

A process where low energy photons excite an atom, which then releases a new photon of the same energy in a different direction.

What energy level of photons is associated with Coherent Scatter?

Low energy photons (<10 keV).

Does Coherent Scatter significantly affect imaging?

No

What is Compton Scatter?

A photon strikes an outer shell electron, ejects it, and is deflected with reduced energy.

What percentage of interactions does Compton Scatter account for?

~90%

What effect does Compton Scatter have on atoms?

It ionizes the atom.

What is a consequence of Compton Scatter in imaging?

It leads to image fog due to scattered photons.

Backscatter in Compton Scatter

A deflected photon at an angle >90°, where more energy is lost and the photon travels backward.

What is the photoelectric effect?

About 10% of interactions involve a photon hitting an inner shell electron, ejecting it, and being absorbed.

What happens to the electron vacancy created by the photoelectric effect?

The electron vacancy is filled by an outer electron, emitting secondary radiation.

How does the photoelectric effect contribute to image contrast?

It produces areas of low optical density, resulting in lighter areas in an image.

Effect of Increasing kVp on Compton and Photoelectric Interactions

- Compton scatter percentage increases

- Photoelectric effect percentage decreases

- Transmission increases.

What happens to image darkness with more transmission?

The image becomes darker (higher optical density).

What happens to image darkness with less transmission?

The image becomes lighter (lower optical density).

What is the relationship between transmission and absorption?

More transmission means less absorption.

Effect of Scatter on Radiographic Image Quality

Scattered photons add unwanted exposure to the film (fog), reducing image contrast without contributing meaningful information.

What does differential absorption refer to?

The varying degrees of absorption by different tissues.

What factors influence differential absorption?

Density, atomic number, and thickness of tissues.

What effect does differential absorption have on radiographs?

It produces contrast on the radiograph.

What happens to photons when they interact with materials?

Some photons are fully absorbed.

What is it called when photons are partially absorbed by a material?

Attenuation.

What determines whether photons are absorbed or pass through a material?

The energy of the photons and the properties of the material.

Photon Interaction Determinants

The photon's energy and the object's composition (atomic number, mass density, thickness).

What happens to low energy photons?

They are more likely to be absorbed.

What happens to high energy photons?

They are more likely to scatter or be transmitted.

What factor influences whether a photon is absorbed or scattered?

Kilovoltage peak (kVp) - it controls the energy of the X-rays and affects how they interact with tissues.

What are the four main factors that affect the shade of gray in radiography?

- kVp of the beam (controllable)

- Mass density of the object

- Atomic number of the object

- Thickness of the object.

What effect does high kVp have on wavelength?

High kVp results in shorter wavelengths.

What is the effect of high kVp on penetration?

High kVp leads to greater penetration.

How does high kVp affect contrast?

High kVp results in lower contrast (long gray scale).

What effect does low kVp have on wavelength?

Low kVp results in longer wavelengths.

What is the effect of low kVp on penetration?

Low kVp leads to less penetration.

How does low kVp affect contrast?

Low kVp results in higher contrast (short gray scale).

What is the recommended kVp for hand/finger imaging?

60 kV

What is the recommended kVp for cervical spine imaging?

70 kV

What is the recommended kVp for AP lumbar imaging?

80 kV

What is the recommended kVp for lateral lumbar imaging?

90 kV

How does mass density affect absorption and image appearance?

Higher mass density = more atoms = more interactions = more absorption = whiter appearance on film (lower optical density).

How does atomic number affect X-ray absorption?

Higher atomic number means more electrons per atom, leading to more photon interactions and greater absorption (whiter image).

How does tissue thickness affect absorption and image contrast?

Thicker tissue contains more atoms, causing more absorption and appearing whiter on the film.

If a pathology doesn't affect mass density, thickness, or atomic number, will it be visible on a radiograph?

No, because it won't cause differential absorption to create contrast between normal and abnormal tissue.

How can uniformity in tissue density be useful in radiography?

It may help establish a baseline or identify subtle pathologies when compared with known normal appearances.

When would mass density and atomic number vary between patients?

Conditions like osteoporosis, metallic implants, tumors, or fluid accumulation can alter these properties.

How do we compensate for patient size/thickness differences to ensure consistent image density?

Adjust the kVp or mAs (exposure settings) to increase photon penetration for thicker patients.

What factors do you control in radiography?

kVp

What are the subject variables that influence radiation absorption?

Atomic number, mass density, thickness

What do subject factors in radiography vary by?

Anatomy and pathology