Kilovolt Peak (kVp)

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Kilovolt Peak (kVp)

• kVp determine the quality of the beam.
• X-ray penetration is governed by the kVp setting.
• kVp controls the contrast of the radiographic
  image.
• Contrast is the difference in density of adjacent
  structures on the image.

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Kilovolt Peak (kVp)

• kVp and the density of the body part impacts the
  amount of scatter radiation.
• High kVp more scatter low contrast more
  penetration
• Low kVp less scatter high contrast less
  penetration

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Kilovolt Peak (kVp)

• Optimum kVp consistent scatter adequate
  contrast sufficient penetration
• Optimum kVp is based upon the area of the body
  and film size to assure adequate penetration.

4
Technique Selection

• Adjusting kVp and mAs is a balancing act.
• When kVp is increased, mAs must be decreased.
• We are balancing both density and contrast of the
  image.

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kVp Adjustment

• If kVp is increased without reducing the mAs,
  penetration increases and contrast is reduced.
• Over penetrated images lack contrast and have
  high density (black).
• Under penetrated images have very high contrast
  with dense structures not being penetrated.(white)

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kVp and Radiation Exposure

• The higher the kVp used, the less enter -action
  the photons will have with the tissue being
  exposed.
• When we increase kVp by 10, we reduce the
  exposure by about 25.
• Use as high as possible kVp that will provide
  adequate contrast.

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kVp and Tissue Density

• The tissue types will determine how much kVp is
  needed. There are four basic tissue types.
• Air filled least dense
• Fat more dense
• Muscle more dense
• Bone most dense

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kVp and Tissue Density

• As we age, we loose bone and muscle mass, the kVp
  is reduced to compensate for this.
• Very muscular patients require more kVp to assure
  proper penetration.
• Disease processes that impact bone and tissue
  density will require adjustment of the kVp.

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Over Penetrated Too High kVp

• Over penetrated view have too much kVp.
• They appear gray and lack contrast.
• The upper lumbar spine is hard to visualize.

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The 15 Rule or the Rule of 10

• If kVp is increased by 15, the mAs is reduced
  50.
• If kVp is reduced by 15. The mAs is doubled.
• Between 60 and 90 kVp and adjustment of 10 kVp
  equal 15.

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The 15 Rule or the Rule of 10

• Increase kVp 8 reducing mAs 25
• Decrease kVp 8 increasing mAs 25
• Remember that it take a 20 to 30 change in mAs
  to see a visible difference in the density of the
  image.
• It can be said that an 8 change in kVp is needed
  to make a visible change on the film.

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Adjusting Contrast Using the 15 Rule

• The kVp can be adjusted to enhance contrast .
• When the kVp is too low, the image is under
  penetrated.
• When the kVp is high, the image lacks contrast.

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Baseline kVp

• The kVp is adequate to penetrate the bone and
  visualize the soft tissues.
• Patient exposure is low.
• Contrast is adequate

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kVp increased 15

• There are more shades of gray making this a
  broad scale of contrast.
• There is lower contrast compared to the base line
  image.

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kVp Decreased 15

• There are less shades of gray making this a
  shorter scale of contrast.
• This image is higher contrast than the base line
  image.
• Also higher exposure to the patient.

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kVp Decreased 30

• This is a very high contrast image.
• It has the shortest scale of contrast.
• The carpal bones are under penetrated.
• Much higher exposure to the patient.

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Observations

• 1. Does Image 2 have more or less contrast?
• Less contrast, more shades of gray or broader
  contrast higher kVp.
• 2. Compare the appearance of image 3 and 4 to
  image 1.

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Observations

• As the kVp is reduced, the contrast increases.
  Low kVp High Contrast
• 3. Which Image is Best?
• Image 3 is best but the exposure is higher than
  image 1.

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What would you do to this image?

• This image provides a real challenge.
• It is over penetrated or iver exposed for the
  upper lumbars.
• It is properly exposed for L5 and sacrum.
• Next week we will see how to improve the image.

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The End

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