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: á 15% kVp = doubling of exposure to the film
â 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 = D22I2 D12 / I1 = old intensity
I2 = new intensity / D12 = old distance squared
D22 = new distance squared
b. Density maintenance formula
mAs1 = D12mAs2 D22 / or: mAs2 = mAs1 x D22
D12 / mAs1 = original mAs
mAs2 = new mAs / D12 = old distance squared
D22 = new distance squared
· Filtration: all types of filtration--inherent, added and total--alters density - density â when filtration á
· 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 á, radiographic density â.
· Grids: absorb scatter that would otherwise add density to the film. Grids reduce density.
a. Compensation
mAs1 = GCF1mAs2 GCF2 / mAs1 = original mAs
mAs2 = new mAs / GCF1 = old relative speed
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 á the amt of exposure required to maintain the same density decreases.
mAs1 = RS2mAs2 RS1 / or: mAs2 = RS1 x mAs1
RS2 / mAs1 = old mAs
mAs2 = new mAs / RS1 = old relative speed
RS2 = new relative speed
b. mAs conversion:
Screen speed100
200
400
600 / 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
\
B. CONTRAST: the difference between adjacent densities.
1. Scale of contrast:
· High Contrast (wrist)a. short scale or increased contrast
b. product of lower kVp
c. widely different 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. / · Lower Contrast (abd).
a. longer scale or decreased contrast
b. the product of higher kVp
c. many similar tissue densities.
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. á‘d developer temp, immersion time, and replenishment rates cause á fog and â 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 á, a wider range of photon energies is produced.
b. Any factor resulting in á fog will â 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 # = áabsorption)
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 / Change for visible Change / Change to equal % change30-50 kVp
50-90 kVp
90-130 kVp / 4-5%
8-9%
10-12% / 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 ás, contrast âs.
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.
4 IMAGE PROD