# DENSITY Density

```Principles of Imaging Science II (RAD 120)
EXAM 1 Content
Spring 2011
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DENSITY
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One of the photographic properties that
determines visibility of detail
Overall blackness or darkness of the entire
radiographic image or a specific area
When evaluating an image for proper
radiographic density, the density of the entire
image is considered
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Density



Only the anatomical parts should be
evaluated for proper density
The collimated border surrounding the
structure represents x-ray beam attenuation
in air
The radiologist evaluates the image for
anatomic or pathologic changes that cause a
change in normal density
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1
DENSITY

CONTROLLING FACTOR: mAs (Quantity)

mAs = mA X time (sec)
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25 mAs = 100 mA X 0.25 sec (250 ms)
25 mAs = 200 mA X 0.125 sec (125 ms)
25 mAs = 400 mA X 0.0625 sec (62.5 ms)
mA, time inversely proportional to maintain the
same mAs or density
Why change mA, time when a desired mAs is
known?
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Density Controlling Factors

mA = mAs/time

Calculate the mA, given 50 mAs, 100 ms

Calculate the mA, given 100 mAs, 100 ms
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500 mA
1000 mA
time = mAs/mA

Calculate the time, given 20 mAs, 200 mA

Calculate the time, given 20 mAs, 400 mA


0.1 sec, 100 ms
0.5 sec, 50 ms
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Density Evaluation
Your initial reaction should determine
if the radiograph demonstrates proper
density
 If it is not correct, then it is too dark
(overexposed), or too light
(underexposed)
 You must determine the extent to
which it is to dark or too light.

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2
Density Evaluation

Over and underexposure is generally
expressed as either 50% or 100%
incorrect



If a radiograph is very light or dark, it is 100%
over or underexposed
Underexposure - correct by multiplying the
mAs by 2 (40 mAs vs 80 mAs)
Overexposure – correct by multiplying the
mAs by 0.5 (40 mAs vs 20 mAs)
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Density Evaluation

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If a radiograph is minimally light or dark and
warrants a repeat, it is 50% over or
underexposed
Underexposure - correct by multiplying the mAs
by 1.5 (40 mAs vs 60 mAs)
Overexposure – correct by multiplying the mAs
by 0.75 (40 mAs vs 30 mAs)
Remember that at least a 30% change in mAs is
needed to demonstrate a visible difference in
density. Known as the 30% rule.
mAs is the factor of choice for density changes
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mAs
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3
Density: Influencing Factors

Kilovoltage



Controls the energy, penetrability or quality of
the x-ray beam.
As kVp is increased or decreased, the quantity
of x-ray photons produced increases or
decreases
15% rule application


15% change in kVp results in 100% change in
density
7.5% change in kVp results in 50% change in
density
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Density Evaluation


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When evaluating density determine if the
structure is properly penetrated. This applies
to underexposed images. An image cannot
be overexposed if it is not adequately
penetrated
Look for structures behind another
If it is penetrated, change the mAs; if not
change the kVp according to the 15% rule
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Density:
INFLUENCING FACTORS

kilovoltage
I1
==
I2
I1:
Beginning Intensity
I2 :
New Intensity
kVp12
kVp22
kVp1: Beginning kilovoltage
kVp2 : New kilovoltage
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4
Density: kilovoltage calculations
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Density: Influencing Factors

Source-Image Distance (SID)

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Distance between the x-ray source(FS) and the
image receptor
Affects the intensity of the x-ray beam according
to the inverse square law
If the SID is changed, and density should be
maintained, apply the
Density Maintenance Formula
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Density:
INFLUENCING FACTORS

Source - Image Distance
mAs1
==
mAs2
D12
D22
mAs1:
Beginning mAs
mAs2 :
New mAs
D1:
Beginning distance
D2 :
New distance
mAs 2 = mAs1 (D22)
(D12)
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5
Density: Distance calculations
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.
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Density: INFLUENCING FACTORS
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There is a direct relationship between RSS
and density
Common RSS range from 50 - 800


As RSS increases, density increases
As RSS decreases, density decreases


Based upon no compensatory changes in mAs
A radiograph will be dark if you intended to use an
extremity cassette and you incorrectly selected a
high speed cassette.
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Density:
INFLUENCING FACTORS

Film/Screen Combination
mAs1
mAs2
==
mAs 2 = mAs1 (RSS1)
mAs1:
Beginning mAs
mAs2 :
New mAs
Beginning Film/Screen Speed
New Film/Screen Speed
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Density: INFLUENCING FACTORS
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Grids are used to absorb scattered photon
emitted from the patient before they interact
with the image receptor (IR)
Placed between the patient and IR
When a grid is used, you must adjust mAs to
compensate for the loss of density that would
be produced by scatter
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Density: INFLUENCING FACTORS
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Inverse relationship
between grid use and
density
An image would be
more dense if no grid
were used compared to
using a grid
Using a grid requires
higher mAs according
to the following:

Grid Ratio
GCF
5:1
6:1
8:1
10:1
12:1
16:1
Non-grid
2X
3X
4X
4X
5X
6X
1X
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Density:
INFLUENCING FACTORS

Grids
mAs1
==
mAs2
mAs1:
GCF1
GCF2
mAs 2 = mAs1 (GCF2)
(GCF1)
Beginning mAs
mAs2 :
New mAs
GCF1:
Beginning grid conversion factor
GCT2 :
New grid conversion factor
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Density: Grid calculations
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Density: String Formula

mAs2 = mAs1 (D2)2 (GCF2) (RSS1)
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Density: String Formula
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Density: String Formula
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Density: String Formula

mAs2 = mAs1 (D2)2 (GCF2) (RSS1)
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Density: Influencing Factors

Generator Type
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An AP projection of the hip is taken using a three
phase, 12 pulse high frequency generator.
Technical factors are 70 kVp, 24 mAs, 400 RSS,
40”SID, 10:1 grid. Using the correct conversion
factor(2.0), calculate the mAs needed when
using a single phase, 2 pulse unit.
 To maintain image density use the formula
mAs2 = mAs1 X conversion factor

mAs2 = 48
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Generator Type Conversions &amp;
Voltage Ripple
Voltage waveforms resulting from
various power supplies. The ripple
of the kilovoltage is indicated as a
percentage for each waveform.
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Density Influencing Factors
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Beam Restriction (Collimation)

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Less scatter radiation is produced and density
is decreased
14” X 17” to 8” X 10”, increase mAs 50-60%
14” X 17” to 10” X 12’’, increase mAs 30-40%
When collimation is increased, the amount of
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10
Density: Influencing Factors
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Filtration
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Indirect relationship with density

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Increased filtration = Decreased density
Film Processing
Direct relationship with density for developer time,
temperature, and replenishment rates
Focal Spot Size, Anode Heel Effect

Minimal effect
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Anode Heel Effect
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Concept Point Check
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Density: Influencing Factors
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Anatomical Part Thickness or Tissue

Indirect relationship with density
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Pathology
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Larger patients attenuate more x-ray photons
Higher atomic # tissues attenuate more x-ray photons
Additive Disease - Density decreases
Destructive Disease Density increases
Contrast Media
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Positive Contrast – Density decreases
Negative Contrast – Density increases
Refer to density chart in Carlton text
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A. Increased density
B. Decreased density
C. No change in density
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85 kVp to 60 kVp
100 milliseconds to 50 milliseconds
800 mA, 0.04 seconds to 600 mA, 53 ms
40” SID to 100 cm SID
Table-top radiography to a 10:1 bucky grid
2.5 mm focal spot size to 1.2 mm focal spot size
90 sec film processing to 3 min film processing
200 mA to 1000 mA
Normal tissue to additive pathologic condition
15 cm abdomen patient to 28 cm abdomen patient
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Greatest Density Calculation
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Which of the following sets of technical
factors will demonstrate the greatest density?
mA
time
A. 200 100 ms
40” 100 5:1
B. 400 200 ms
40” 100 8:1
C. 500 40 ms
48” 200 8:1
D. 500 20 ms
48” 200 10:1
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Greatest Density Calculation Practice
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Which of the following sets of technical
factors will demonstrate the greatest density?
mAs
A. 12.5 40” 400 10:1
B. 15
72” 400 12:1
C. 50 72” 400 12:1
D. 62.5 40” 400 10:1
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Digital Image Receptor Systems
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Density and contrast

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Controlled through post-processing
Still, the image receptor must receive proper
exposure

Extremely high or low exposures do not produce
acceptable image quality
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Digital Image Receptor Systems
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kVp based on desired contrast
Distance selected based on desired beam
geometry
mAs is adjusted to meet exposure needs
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Concept Point Check
Part II: Matching Identify the effect on radiographic density when a single change
A. Radiographic density will increase
B. Radiographic density will decrease
C. No change in density will result
____
40” SID to 60” SID
____
400 mA 20 milliseconds to 800 mA 30 milliseconds
____
1.0 mm Focal Spot Size to 1.2 mm Focal Spot Size
____
____
50 kVp to 65 kVp
____
5:1 ratio grid to 16:1 ratio grid
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