Prime Factors

1
Chapter 11

• Prime Factors

2
Prime Factors

• Prime factors affect x-ray emission under the
  control of the radiographer.
• Miliamperage-second (mAs)
• Kilovoltage (kVp)
• Distance (d)
• Related to tube design and construction.
• Tube housing
• Target material
• Filtration
• Voltage waveform

3
X-Ray Quantity

• X-ray quantity measure of the number of x-ray
  photons in the useful beam.
• AKA x-ray output, intensity, or exposure
• Unit Roentgen (R)
• Directly affected by
• Miliamperage-second (mAs)
• Kilovoltage (kVp)
• Distance (d)

4
X-ray quality

• X-ray quality measurement of the penetrating
  ability of the x-ray beam.
• Describes the distance an x-ray beam travels in
  matter.
• High energy x-ray photons travel farther in
  matter- more penetrating
• Numerically represented by the Half-value layer
  (HVL).

5
Half-Value Layer

• Half-value layer of an x-ray beam is that
  thickness of absorbing material needed to reduce
  the x-ray intensity to half its original value.
• Affected by
• Kilovoltage
• Filtration
• Not controlled from exposure to exposure

6
Milliamperage

• Milliamperage (mA) measurement of x-ray tube
  current.
• The number of electrons crossing the tube from
  cathode to anode per second
• Directly proportional to tube current
• Ampere equal to an electrical charge of 1
  coulomb flowing through a conductor per second.
• 1 Coulomb 6.3 x 1018 electron charges.

7
Exposure

• Exposure time is directly proportional to the
  number of electrons crossing the tube and is
  therefore directly proportional to the number of
  x-rays created.
• The number of x-rays that will be created at the
  target is a product of the number of electrons
  crossing the tube (tube current) and how long the
  electrons are allowed to cross (exposure time).
• Measured in mAs- primary controller of x-ray
  quantity.

8
Density Relationship to mAs

• Radiographic film density is the degree of
  blackening of an x-ray film
• Created by deposits of black metallic silver on
  an x-ray film that has been exposed to light or
  x-ray and then processed
• Densities are the result of an x-ray exposure to
  the film and intensifying screens.
• Film density is determined by the amount of
  silver deposition in the emulsion due to
• Film type
• Exposure conditions
• Exposure (mR)
• Processing

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Density Relationship to mAs

• If the exposure to a film is increased, the
  density to that film will increase until the
  point where the film reached its maximum density
  (Dmax).
• mAs is used as the primary controller of
  radiographic film density.
• By maintaining a specific exposure relative to
  the speed of the image receptor, consistent film
  density can be achieved.

10
Reciprocity Law

• The reaction of a photographic film to light is
  equal to the product of the intensity of the
  light and the duration of the exposure.
• The density on the x-ray film should remain
  unchanged as long as the intensity and duration
  of the x-ray exposure remains unchanged.
• Fails for exposures made at extremely short
  exposure times (less than 1/100 second) or
  extremely long exposure times (more than a
  second)
• Law failure in not very significant in diagnostic
  radiology because exposures are seldom at those
  extremes

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Kilovoltage

• Increasing the kilovoltage on an x-ray control
  panel will cause an increase in the speed and
  energy of the electrons applied across the x-ray
  tube.
• Increased energy of the electrons results in the
  production of x-ray photons with greater energy.
• Controls both the quantity and quality of the
  x-ray beam.
• Quantity more interactions will occur at the
  target as kVp increases.
• Quality each electron has more energy resulting
  in a beam with greater penetrability

12
Density Relationship to kVp

• Changes in kilovoltage create changes in beam
  penetrability.
• kVp is the primary controller of the differences
  in radiographic density.
• kVp should NOT be used to control radiographic
  film density (contrast)
• As kVp increases, causes increase in
  penetrability, which will result in less contrast.

13
15 percent rule

• An increase in kVp by 15 percent will cause a
  doubling in exposure, the same effect as doubling
  the mA or doubling exposure time.
• Hypothetically, if kVp is doubled, the x-ray
  quantity would increase by a factor of four, but
  this does not take into account the increased
  penetrability of the bean with increasing kVp.
• As a result, radiographic density is more
  significantly affected.
• To maintain exposure with changes in kVp, the 15
  percent rule can be applied,
• To maintain density, if the kVp is increased 15
  percent, the mAs must be reduced to one-half its
  original value.

14
Distance

• The intensity of x-rays varies greatly with
  changes in distance.
• Measurement of the x-ray intensity is obtained
  using a dosimeter.
• X-ray photons are most concentrated at the target
  and from there they spread out in all directions.
• X-ray intensity (quantity) begins to diminish.
• Photons that exit the tube port constitute the
  primary useful beam.

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Inverse Square Law

• Intensity of radiation at a given distance from
  the point source is inversely proportional to the
  square of the distance.
• I1 / I2 D22 / D12
• I1 original intensity
• I2 new intensity
• D22 original distance
• D12 new distance

16
Distance Relationship to Distance

• As the distance increases, intensity decreases
  which causes a decrease in exposure to the image
  receptor.
• Since mAs is the primary controller of x-ray
  intensity and radiographic film density, mAs can
  be adjusted to compensate for changes in distance.

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Exposure (film density) maintenance formula

• mAs should increase proportionally to the square
  of the change when distance increases. (direct
  square law)
• mAs1 / mAs2 D12 / D22
• mAs1 original mAs
• mAs2 New mAs
• D22 original distance
• D12 new distance

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Density Relationship to mAs, kVp d

• The radiographer should select the kVp based on
  the desired contrast, and adjust mAs to provide
  the appropriate total exposure to the receptor.