EL CAMINO COLLEGE RADIOLOGIC TECHNOLOGY
DENSITY AND CONTRAST
1
A. DENSITY: the degree of overall blackening from the black metallic silver deposited in the emulsion.
1. Optical Density: range of human visibility--.25 TO -2.5 (straight line portion on H&D curve)
2. Controlling factor: mAs
a. mAs determines the quantity of x-rays
3. Minimum change necessary to cause a visible shift in density is 30% of mAs, or any other influencing factors that
would equal this change.
4. Influencing factors:
 kVp
a. 15% rule:  15% kVp = doubling of exposure to the film
 15% kVp = halving of exposure to the film
b. 15% rule will always change the contrast of the image because kV is the primary method of changing image
contrast.
 Focal spot size: has very little influence on image density.
 Anode Heel effect: alters the intensity of radiation and therefore the density between the anode and cathode ends of the
x-ray tube.
 Distance:
a. SID alters the intensity of the beam according to the inverse square law
I1 = D22
I1 = old intensity
D12 = old distance squared
I2 D12
I2 = new intensity
D22 = new distance squared
b. Density maintenance formula
mAs1 = D12 or: mAs2 = mAs1 x D22 mAs1 = original mAs
D12 = old distance squared
2
2
mAs2 D2
D1
mAs2 = new mAs
D22 = new distance squared
 Filtration: all types of filtration--inherent, added and total--alters density - density  when filtration 
 Beam restriction: collimation reduces scatter and reduces the overall density of the image
 Anatomical part: There is an inverse relationship between tissue thickness/type and radiographic density.
a. as tissue thickness, average atomic number of tissue and/or tissue density , radiographic density .
 Grids: absorb scatter that would otherwise add density to the film. Grids reduce density.
a. Compensation
mAs1 = GCF1
mAs1 = original mAs
GCF1 = old relative speed
mAs2 GCF2
mAs2 = new mAs
GCF2 = new relative speed
b. mAs conversion factor for grids
no grid =1; 5:1=2; 6:1=3; 8:1=4; 10/12:1=5; 16:1=6.
 Film/Screen combination
a. Compensation: as RS  the amt of exposure required to maintain the same density decreases.
mAs1 = RS2
or: mAs2 = RS1 x mAs1
mAs1 = old mAs
RS1 = old relative speed
mAs2 RS1
RS2
mAs2 = new mAs
RS2 = new relative speed
b. mAs conversion:
Screen speed
100
200
400
600
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mAs
2
1
½ (0.50)
¼ (0.25)
To determine New mAs with change in screen speed
speed screen 1 = mAs 2
speed screen 2
mAs 1
EL CAMINO COLLEGE RADIOLOGIC TECHNOLOGY
DENSITY AND CONTRAST
2
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B. CONTRAST: the difference between adjacent densities.
1. Scale of contrast:
 High Contrast (wrist)
 Lower Contrast (abd).
a. short scale or increased contrast
a. longer scale or decreased contrast
b. product of lower kVp
b. the product of higher kVp
c. widely different tissue densities
c. many similar tissue densities.
contrast influenced by:
 grids: increased grid ratio =‘s increased
contrast (absorbs more scatter).
 air gap: increases contrast.
 beam restriction: increases contrast (scatter
reduced).
 increase kVp =’s decreased contrast
 increase mAs =‘s decreased contrast
 increase development time =‘s decreased
contrast.
2. Physical contrast: the total range of density values recorded by the image receptor. It is the maximum contrast
possible and is the most accurate representation of the varying intensities present in the x-ray beam after it has
passed through the subject.
3. Visible contrast is the total range of density values that can be perceived by the human eye in a single image. It is a
portion of the physical contrast and comprises the information from which the diagnosis is made.
4. Film (image receptor) contrast.
 Intensifying screens: create an inherently higher contrast image.
 Film density: Optimal ranges of densities permits contrast to be visualized at a maximum. Excessive or inadequate
density decreases contrast.
 D log E curve (H&D Curve) : determined by the physical composition of the film emulsion. As the slope becomes
steeper, contrast is increased.(shorter scale) lower slope – lower contrast (more latitude)
 Processing: Increased film developer time, temperature, or replenishment rate will increase chemical fog on the film.
When any processing factor causes a change in base fog, contrast is affected.
a. ‘d developer temp, immersion time, and replenishment rates cause  fog and  contrast.
5. Subject contrast: the range of differences in the intensity of the x-ray beam after it has been attenuated by the subject.
a. Dependent on kVp and amount and type of irradiated material.
 kVp: Primary controller of subject contrast.
a. as kVp , a wider range of photon energies is produced.
b. Any factor resulting in  fog will  contrast (developer temp, immersion time, and replenishment)
 Amt of irradiated material: thickness of body part and field size.
 Types of irradiated material: the atomic number of the material and its density (high atomic # = absorption)
a. tissue density describes how tightly the atoms of a given substance are packed together.
6. Evaluating contrast: Adequate contrast must demonstrate enough distinctly different densities within the range to
satisfy diagnostic quality requirements for the particular exam.
7. Selecting the appropriate kVp
 a visible change in contrast will not be perceived until kVp is changed by 4-12%, depending on the kVp range.
a. Changes Necessary to Produce Visible Contrast Differences
Level
30-50 kVp
50-90 kVp
90-130 kVp
Change for visible Change
4-5%
8-9%
10-12%
Change to equal % change
1-3 kVp
4-8 kVp
9-16 kVp
8. The rule for contrast changes is to make adjustments in increments of or 8%
9. Controlling Factor Affecting Contrast: kVp
as kVp s, contrast s.
E. RECORDED DETAIL: the degree of geometirc sharpness or accuacy of the structural lines acutrally recorded in the
radiographic image.
F. DISTORTION: the misrepresentation of the size or shape fo the sturctures being examined.
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EL CAMINO COLLEGE RADIOLOGIC TECHNOLOGY
DENSITY AND CONTRAST
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