RADIOGRAPHIC DENSITY
OBJECTIVES
• Define density
• Factors affecting density
• Evaluating radiographic density
• Appropriate adjustments to compensate for
variation in the controlling factors that affect
radiograhpic density
ASSESSING DENSITY
• It is the degree of overall blackening from the black
metallic silver deposited in the emulsion
• The major consideration in assessing density is
verification that proper densities are visible
throughout the anatomical area of interest on the
radiograph. They should be within the range of
human visibility
(OD .25- 2.5)
• Whenever a choice must be made between excess
and insufficient density, the wise decision is always
the choice that will produce the darker image
FACTORS AFFECTING DENSITY
mAs as a controlling factor
• Density is determined by exposure, exposure is
directly proportional to mAs, mAs is used as the
primary controller of radiographic density
• The minimum change necessary to cause a visible
shift in density is 30% of mAs, or any other
influencing factors that would equal this change
• The general rule of thumb for mAs changes is to
make adjustments in increments of doubles or
halves
kVp as an influencing factor
• It controls the quantity as well as the quality of x-ray
beam
• 15% rule apply
• 15% rule may vary from a 15% rule to a 25% rule
• 15% rule will always change the contrast of the
image
• The configuration of the generator is an important
consideration in how kVp affects density
OTHER INFLUENCING FACTORS
• Focal Spot Size
- Large and small focus
- Large focus utilize a greater incident
electron stream than small focal spot.
This is compensated by adjusting the
actual mA at the filament for dual-focus
tubes
ANODE HEEL EFFECT
DISTANCE
• SID (FFD), OID
• Inverse square law
• Density maintenance
formula
FILTRATION
• Inherent, added and total filtration- alter
density
• Density decreases when filtration is
increased
BEAM RESTRICTION
• Reduce the primary field size and then reduces the
total number of photons available. This reduce the
amount of scatter radiation and then the overall
density
• Scatter increase
anatomical part
with
high
kVp
and
thicker
• Technical factors compensation for changes in
density is required only under the following
circumstances:
-
Large anatomical part
High kVp
Low grid ratio effiicncy
Non-grid examinations
BEAM RESTRICTION
• The
compensation
necessary in mAs for
the effect of beam
restriction on image
density is less than
30% for nearly all
images
produced
with grids at 8x10“ or
larger beam sizes
ANATOMICAL PART
•
It depends on the thickness
and type of the tissue
•
Use of contrast media or the
pathology can alter the
thickness/type of tissue
•
There
is
an
inverse
relationship between tissue
thickness/type
and
radiographic density. This is
not a linear relationship
•
Tube angle
GRIDS
• Absorbs scatter radiation to improve contrast
• The more efficient the grid, the less the density
• Grids with high ratio, low frequency and dense
interspace material; moving grids and improperly
used grids all reduce density
• Compensation for varying grid ratios is generally
accomplished by increasing mAs
• mAs1/mAs2 = GCF1/GCF2
FILM/SCREEN COMBINATION
• Increasing F/S combination relative
speed will increase density
• This is accomplished by changing the
size of phosphor, phosphor layer
thickness, and phosphor concentration
• mAs1/mAs2=RS2/RS1
FILM PROCESSING
• Density will increase when developer
solution
temperature
increases,
immersion
time
increases,
or
replenishment rates increases
• Contamination will decrease solution
strength and density decrease
SUMMARY