Post on 09-Feb-2016
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Exposure Factors or Prime Factors
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PRIME FACTORS
What is “technique” ?
How does it affect the “image”2
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?5
“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
Determines the number of photons, radiation quantity, OD & patient dose
Changing mA does not change the kinetic energy of e-
Available 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
mAs determines the number of photons in the primary beam
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Distance
Affects exposure of the IR because of the Inverse Square Law
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 TechniqueThe Patient Factor The most difficult task for technologists…
evaluating your patient
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
Technique charts are based on the “average patient”
The thicker the part the more x-radiation is required to penetrate. Calipers should be used
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
Extremity (soft tissue & bone) = low kVp
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
Pathology can severely affect the technologist technique selection
Always question your patients about health status
If prior images are available…check them!
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Pathology
Can appear with increased radiolucency or radiopacity
Some pathology is destructive causing tissue to be radiolucent
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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