RADIOGRAPHIC CONTRAST

OBJECTIVES• Define radiographic contrast

• Factors that affect radiographic contrast

• Factors that affect film contrast

• Factors that affect subject contrast

• Effect of fog on image contrast

• Technical factors changes necessary to achieve optimal contrast

• Assess radiographic contrast on various radiographic images

• Recommend appropriate adjustments to improve contrast under various conditions

ASSESSING CONTRAST

• It is one of the two photographic properties that comprise visibility of detail

• It is defined as the difference between adjacent densities

DESCRIBING CONTRAST• High Contrast

- Few shades of gray

- Increased contrast

- Low kVp

- Short scale contrast

• Low contrast

- Many shades of gray

- Decreased contrast

- High kVp

- Long scale contrast

RADIOGRAPHIC CONTRAST

• It is the total amount of contrast acquired from both the anatomical part (subject) and the image receptor (film)

IMAGE RECEPTOR CONTRAST

• It is the range of densities that the image receptor (film) is capable of recording. Mathematically this is expressed as the slope of the D Log E curve

• It depends on four factors: intensifying screens, film density, the D log E curve, and processing

Intensifying screens• It create an inherently higher contrast image

• It is related to the way in which the film responds to the x-ray photons

• The D log E curve changes dramatically when a film designed for exposure by light is exposed directly to x-rays

• Changing screen speed has a negligible effect on contrast

Film Density• Film contrast changes with

changes in film density

• There is an optimal range of densities that permits contrast to be visualized at a maximum

• If the film exposed correctly, the film densities will fall within visible range of the D log E curve

• If the exposure places the developed densities on the toe or shoulder, the slope is not as steep, resulting in a decrease in contrast

D log E Curve• The primary determinant of

the shape of the D log E curve is the physical composition of the film emulsion

• As the slope of the curve becomes steeper, contrast is increased

• Film A is a high contrast, narrow latitude film while film B is a low contrast, wide latitude film

Processing• Developer temperature

and immersion time, replenishment rate, and developer contamination are the primary processing factors that cause objectionable fog levels

• As these factors increases, the amount of fog on the film increases and the contrast decreases

SUBJECT CONTRAST• It is the range of differences in the

intensity of the x-ray beam after it has been attenuated by the subject

• It depends on kVp and the thickness and type of irradiated material

Kilovoltage• It is the primary controller of

subject contrast

• High kVp, more penetration, more scatter radiation, wider range of densities, long scale contrast (low contrast)

• Low kVp , less penetration, less scatter radiation, short range of densities, produce high subject contrast (short scale contrast)

Amount of Irradiated Material• It depends on the thickness of the body part

and the field size

• Increased body part thickness or/and field size will results in a decrease in subject contrast

• Decreased body part thickness or/and filed size will results in increased subject contrast

Type of Irradiated Material

• It is influenced by the atomic number of the material and its density. Both these factors influence subject contrast

• Higher atomic number and density of the material, the higher the subject contrast (e.g bone and air in the lungs)

EVALUATING CONTRAST• The major consideration in

evaluating visible contrast is verification that a proper range of densities are visible throughout the anatomical area of interest on the radiograph

• Densities must be within the range of human visibility (OD 0.25-2.50)

• Proper contrast is not the same for all tissues. Knowledge of anatomy and physiology, pathology and technical factors, is critical for proper contrast evaluation

• 30% density difference has been used as the minimum change to cause a visible difference. 15% in other people

SELECTING THE APPROPRIATE kVp

• Fixed kVp technique system: low contrast

• Variable kVp technique system: high contrast

• Contrast varies as the kVp varies

• Contrast varies for the same kVp with different body part thickness

• A visible change in contrast will not be perceived until kVp is changed by 15%, depending on the kVp range

• The rule for contrast changes is to make adjustments in increments of 15 or 8%

FACTORS AFFECTING CONTRAST

SUMMARY