RADIOGRAPHIC FILM CONTRAST

FILM CONTRAST

The term radiographic contrast refers

to the difference in density between areas

in the radiograph. 

Radiographic contrast depends on three

factors:

1. subject contrast

2. film contrast

3. fog and scatter

Subject Contrast

Subject contrast, sometimes called radiation

contrast, is the difference in x-ray

intensity transmitted through one part of

the subject as compared to that transmitted

through another part

Subject contrast depends on

l. thickness difference

2. density difference

3. atomic number difference

4. radiation quality (kVp)

Thickness Difference. If an x-ray beam

is directed at two different thicknesses of

the same material, the number of x rays

transmitted through the thin part will be

greater than the number transmitted

through the thick part. This is a relatively

simple but important factor contributing to

subject contrast. If 15 is the intensity of the

x-ray beam (I) transmitted through the

thin (small) segment, and IL is the intensity

transmitted through the thick (large) segment,

subject contrast may be defined m

the following

:

Subject contrast =Is/IL

Density Difference.

The difference indensity between body tissues is one of the

most important factors in causing subject

contrast. The greater the density (i.e., mass

per unit volume) of a tissue, the greater is

its ability to attenuate x rays. Consider ice

and water. Because ice floats in water, it is

less dense than water. Water is about 9%

more dense than ice. Equal thicknesses of

ice and water will demonstrate subject contrast

to an x-ray beam because the water

attenuates 9% more of the beam than does

the ice. 

Atomic Number Difference.

Subjectcontrast depends on the relative difference

in attenuation of the x-ray beam by different

tissues in the body. In diagnostic radiology,

attenuation of the x-ray beam by the

photoelectric effect makes the most important

contribution to subject contrast.

Photoelectric absorption is increased in

substances with high atomic numbers, especially

when low-kVp x rays are used. The effective atomic

numbers of bone, muscle

(water), and fat are

Bone 13.8

Muscle 7.4

Fat 5.9

Contrast Media. 

The use of contrast materials

with high atomic numbers (53 for

iodine and 56 for barium) gives high subject

contrast. Photoelectric absorption of x

rays in barium and iodine will be proportionally

much greater than that in bone and

tissue because of the large differences in

atomic number.

Radiation Quality. The ability of an xray

photon to penetrate tissue depends on

its energy; high-kVp x rays have greater

energy. Selecting the proper kVp is one of

the most important matters to consider in

choosing the proper exposure technique.

If the kVp is too low, almost all the x rays

are attenuated in the patient and never

reach the film.

The kilovoltage selected has a great effect

on subject contrast. Low kVp will produce

high subject contrast, provided the

kVp is high enough to penetrate the part

being examined adequately

Film Contrast

Film contrast has been discussed as a

photographic property of x-ray film

 X-ray film will significantly

amplify subject contrast provided the exposure

(mAs) is correct (keep away from

the toe and shoulder of the characteristic

curve). Under good viewing conditions, a

density difference of about 0.04 (difference

in light transmission of 10%) can be

seen.

Fog and Scatter

The effect of fog and scatter is to reduce

radiographic contrast.

Scattered radiation is produced mainly

as a result of Compton scattering. The

amount of scatter radiation increases with

increasing part thickness, field size, and energy

of the x-ray beam (higher kVp). Scatter

is minimized by collimation of the x-ray

beam (use as small a field size as possible)

and the use of grids or air gaps. Scatter

radiation that reaches the x-ray film or

film-screen combination produces unwanted

density.

Fog is strictly defined as those silver halide

grains in the film emulsion that are developed

even though they were not ex posed by light or x rays. The amount offog in an unexposed x-ray film can be demonstrated

easily. Cut the unexposed film in

half. Develop (fully process) one-half of the

film and clear (fix, wash, and dry only) the

other. The density difference between the

two film halves represents the amount of

fog present. Fog produces unwanted film

density, which lowers radiographic contrast