Image Production

Image Production

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.

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).

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.


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.

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.

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.


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).

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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Feb 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Image Production

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