Image Production

Image Production

brightness gain

coherent scattering

Compton electron

differential absorption

electrostatic focusing lenses

image intensification

minification gain

output phosphor

photoelectric effect

remnant radiation

secondary electron

Introduction

To produce a radiographic image, x-ray photons must pass through tissue and interact with an image receptor (a device that receives the radiation leaving the patient) such as a film-screen or digital system. Both the quantity and quality of the primary x-ray beam affect its interaction within the various tissues that make up the anatomic part. In addition, the composition of the anatomic tissues affects the x-ray beam interaction. The absorption characteristics of the anatomic part are determined by its composition, such as thickness, atomic number, and tissue density or compactness of the cellular structures. Finally, the radiation that exits the patient is composed of varying energies and interacts with the image receptor to form the latent or invisible image.

Differential Absorption

The process of image formation is a result of differential absorption of the x-ray beam as it interacts with the anatomic tissue.

Make the Physics Connection 8-1

Differential absorption is the difference between the x-ray photons that are absorbed photoelectrically versus those that penetrate the body.

The term differential is used because varying anatomic parts do not absorb the primary beam to the same degree. Anatomic parts composed of bone absorb more x-ray photons than parts filled with air. Differential absorption of the primary x-ray beam creates an image that structurally represents the anatomic area of interest (Figure 8-1).

FIGURE 8-1 Differential Absorption.
As the primary beam interacts with the anatomic part, x-ray photons are transmitted, absorbed, and scattered based on the tissue’s composition. Differential absorption of the primary x-ray beam creates an image that structurally represents the anatomic area of interest.

Critical Concept 8-1

Differential Absorption and Image Formation

A radiographic image is created by passing an x-ray beam through the patient and interacting with an image receptor, such as a film-screen or digital system. The variations in absorption and transmission of the exiting x-ray beam structurally represent the anatomic area of interest.

Beam Attenuation

As the primary x-ray beam passes through anatomic tissue, it loses some of its energy.

Fewer x-ray photons remain in the beam after it interacts with anatomic tissue. This reduction in the energy or number of photons in the primary x-ray beam is known as attenuation. Beam attenuation occurs as a result of the photon interactions with the atomic structures that compose the tissues. Two distinct processes occur during beam attenuation in the diagnostic range: absorption and scattering.

Absorption

As the energy of the primary x-ray beam is deposited within the atoms composing the tissue, some x-ray photons are completely absorbed. Complete absorption of the incoming x-ray photon occurs when it has enough energy to remove (eject) an inner-shell electron. The ejected electron is called a photoelectron and quickly loses energy by interacting with nearby tissues.

The ability to remove (eject) electrons, known as ionization, is one of the characteristics of x-rays. In the diagnostic range, this x-ray interaction with matter is known as the photoelectric effect.

Make the Physics Connection 8-2

Photoelectric interactions occur throughout the diagnostic range (e.g., 20 kVp to 120 kVp) and involve inner-shell orbital electrons of tissue atoms. For photoelectric events to occur, the incident x-ray photon energy must be equal to or greater than the orbital shell binding energy. In these events the incident x-ray photon interacts with the inner-shell electron of a tissue atom and removes it from orbit. In the process the incident x-ray photon expends all of its energy and is totally absorbed.

With the photoelectric effect, the ionized atom has a vacancy, or electron hole, in its inner shell. An electron from an outer-shell drops down to fill the vacancy. Because of the difference in binding energies between the two electron shells, a secondary x-ray photon is emitted (Figure 8-2). This secondary x-ray photon typically has very low energy and is therefore unlikely to exit the patient.

FIGURE 8-2 Photoelectric Effect.
The photoelectric effect is responsible for total absorption of the incoming x-ray photon.

Critical Concept 8-2

X-ray Photon Absorption

During attenuation of the x-ray beam, the photoelectric effect is responsible for total absorption of the incoming x-ray photon.

The probability of total photon absorption during the photoelectric effect depends on the energy of the incoming x-ray photon and the atomic number of the anatomic tissue. The energy of the incoming x-ray photon must be at least equal to the binding energy of the inner-shell electron. After absorption of some of the x-ray photons, the overall energy or quantity of the primary beam decreases as it passes through the anatomic part.

Scattering

Some incoming photons are not absorbed, but instead lose energy during interactions with the atoms composing the tissue. This process is called scattering. It results from the diagnostic x-ray interaction with matter known as the Compton effect. The loss of some energy of the incoming photon occurs when it ejects an outer-shell electron from a tissue atom. The ejected electron is called a Compton electron or secondary electron. The remaining lower-energy x-ray photon changes direction and may leave the anatomic part to interact with the image receptor (Figure 8-3).

FIGURE 8-3 Compton Effect.
During the Compton effect, the incoming x-ray photon loses energy and changes its direction.

Critical Concept 8-3

X-ray Beam Scattering

During attenuation of the x-ray beam, the incoming x-ray photon may lose energy and change direction as a result of the Compton effect.

Compton interactions can occur within all diagnostic x-ray energies and are therefore an important interaction in radiography. The probability of a Compton interaction occurring depends on the energy of the incoming photon. It does not depend on the atomic number of the anatomic tissue. For example, a Compton interaction is just as likely to occur in soft tissue as in tissue composed of bone.

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Tags: Essentials of Radiographic Physics and Imaging

Feb 27, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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