VISUALS for TEACHING
Guide to using these visuals

 X-RAY IMAGE FORMATION AND CONTRAST

Description

Click on Image
for Full Size

The five radiographic image quality characteristics.  Contrast sensitivity is a major characteristic that determines what tissues and body structures are visible in a radiograph.

The many factors that have an effect on radiographic contrast sensitivity.
The total attenuation by soft tissue is determined by both photoelectric and Compton interactions.  Attenuation generally decreases with increased photon energy because of the energy dependence of the photoelectric effect.
The photon energy spectrum of an x-ray beam determines its quality.

An x-ray image is formed by the different attenuations of the x-ray beam within a patient's body.  Objects with increased attenuation produce shadows.
The image contrast produced by an object depends on its attenuation of the x-ray beam.
Generally increased penetration through an object decreases contrast.  In creased penetration through the total body generally decreases the radiation dose to the patient.

Penetration through the body is generally increased by increasing the KV.

Calculation of the amount of contrast coming from a patient's body and also recorded on film.
Changing KV is the principle control of contrast in radiography.

The x-ray image of an object in a patient's body is in the form of a shadow.
Object penetration and contrast can be changed by changing KV.
The x-ray attenuation and total body penetration changes with photon energy.
An object must have some form of physical contrast in order to be visible in an x-ray image.
The forms of physical contrast that contribute to contrast in x-ray imaging.
The penetration through soft tissue can be changed by changing the KV which changes the spectrum.
X-ray spectra produced by different KV values.
The spectrum of an x-ray beam is determined by: the anode material, the KV value, and the filtration.
The chest has high physical contrast primarily because of the air within the lungs.

Good chest radiographs are produced by using a high KV value and a heavily filtered beam with a spectrum that gives good penetration.

An image showing good penetration through the bones.

An image with high area contrast is not desirable.
Generally, relatively low contrast between large areas within the body produce better local object contrast and visibility.
Bone image.
Good contrast of iodine requires a spectrum with a significant amount of radiation above the K-edge energy of iodine.  This is usually produced with KV values in the range of 65-75.
KV values for the different radiographic procedures.
The breast generally has low physical contrast.  It is in the form of relatively small differences in density among the soft tissues and the low contrast of small calcifications.
The x-ray spectrum used for mammography is determined by the anode material, selected KV value, and type of filter.
The spectrum and the resulting breast penetration can be changed by alternating between different filter materials.
A mammography machine console.
A mammography machine console.
Manual selection of optimum KV for mammography.
Automatic selection of KV in mammography.
Factors affecting the x-ray spectrum in mammography.
Display of compressed breast thickness in mammography.