Download Prime Factors - El Camino College

Exposure Factors or Prime
Factors
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PRIME
FACTORS
What is “technique” ?
How does it affect the “image”
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Exposure Factors – 3 or 4

The four prime exposure factors are:
Voltage = kVp*
Current = mA*
Exposure time = seconds or fractions of a sec*

Source-to-image distance = SID
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PRIME FACTORS
• KVP
• MAS
• DISTANCE
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kVp

Kilovolts controls how fast the electrons
are sent across the tube

Controls, quality, penetrability & contrast

Increasing kVp also increases scattered
photons reducing image quality
Does kVp influence OD?
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“SHORT” VS “LONG” SCALE
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kVp
• Low kVp (50 – 60)
• Short scale
• High contrast
• “Bone work”
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kVp
• High kVp (90 – 120)
• Long scale
• Low contrast
• “Chest images”
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mA
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Determines the number of photons, radiation
quantity, OD & patient dose
Changing mA does not change the kinetic
energy of eAvailable mA stations are usually 50, 100,
200, 300, 400 & 600
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Exposure Time

Should be kept as short as possible, for most
examinations. To minimize the risk of patient
motion
mA X s = mAs
 mAs controls OD
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mAs determines the number of photons in the
primary beam
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Distance
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Affects exposure of the IR because of the
Inverse Square Law
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SID largely determines the intensity of
photons at the IR

Distance has no effect on radiation quality
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INTENSITY IS SPREAD OUT…
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Inverse Square Law Formula
Intensity #1
Intensity #2
Distance #2 Squared
Distance #1 Squared
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SID Changes
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Direct Square Law
• New mAs = New distance 2
Old mAs
Old distance 2
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Focal-Spot Changes
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Producing optimal radiographs …
key for diagnosing disease
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How much of the radiation
received by the patient
Actually reaches the IR ?
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About 1%
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Creating the IMAGE
• When x-rays pass through a patient's body,
three things can happen:
• (1) the x-ray photon is transmitted, passing
through the body, interacting with the film, and
producing a dark area on the film;
• (2) the x-ray photon is absorbed in an area of
greater tissue density, producing lighter areas
on the film; and
• (3) the x-ray photon is scattered and reaches the
film causing an overall gray fog.
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IMAGES
• DENSITY = THE AMOUNT OF
BLACKENING “DARKNESS” ON THE
RADIOGRAPH
• CONTRAST – THE DIFFERENCES
BETWEEN THE BLACKS TO THE
WHITES
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mAs Changes
• at least 20 - 30 % mas change
needed to see a visible change in
density
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mAs DOUBLED =
DENSITY DOUBLED
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+ 25%
+ 50 % mas
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mAs
Changes
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Tube voltage (kVp)
• Determines the maximum energy
in the beam
• spectrum and affects the quality
of the output spectrum
• Efficiency of x-ray production is
directly related to tube voltage
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Influencing factors: kVp
15% rule:
 15% kVp = doubling of exposure to the film
 15% kVp = halving of exposure to the film
15% rule will always change the contrast of the
image because kV is the primary method of
changing image contrast.
Remember :
15% change ( ) KVP has the same effect as
doubling or ½ the MAS on density
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kVp Changes
• The kVp setting must be changed by at
least 4% to produce visual changes an
image
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kVp
Changes
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+ 15% kvp
- 15% kvp
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kVp Changes
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4% kVp Changes
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Determining Radiographic Technique
The Patient Factor

The most difficult task for technologists…
evaluating your patient
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The patient size, shape, and physical
condition greatly influences the technique
selection
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4 general states of body habitus
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Radiographic Technique
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Technique charts are based on the “average
patient”
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The thicker the part the more x-radiation is
required to penetrate. Calipers should be used
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Keep in mind not only the measurement but the
type of tissue you need to penetrate (fat vs
muscle)
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Technique

In general, Soft tissue = low kVp and high
mAs
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Extremity (soft tissue & bone) = low kVp
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Chest (high subject contrast) = high kVp
Abdomen (low subject contrast) = middle kVp
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Low Subject
Contrast
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OD = amount
of black on image
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Film Screen
• Overexposed
• Referring to a radiograph that is too dark
because too much x-radiation reached the
image receptor
• Underexposed
• Referring to a radiograph that is too light
because too little x-radiation reached the
image receptor
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Technique - Pathology
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Pathology can severely affect the
technologist technique selection
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Always question your patients about health
status
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If prior images are available…check them!
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Pathology
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Can appear with increased radiolucency or
radiopacity
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Some pathology is destructive causing tissue
to be radiolucent
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Others can be additive causing tissue to be
radiopaque
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OD vs tissue
attenuation
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Technique selection – Fixed kVp

For each anatomic part there is an optimum
kVp

mAs is varied based on part thickness or
pathological condition
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