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

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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