Fuchss Radiographic Principles

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Fuchss Radiographic Principles

• THE 15 PERCENT RULE

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Definition

• A 15 percent change in kVp will result in a
  change in image density by a factor of 2.
  (Bushong, 2003)

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In other words

• If the kVp is increased by 15 percent, the
• radiographic density will be approximately
• double the original density if the mAs
• remains unchanged.
• If the kVp is decreased by 15 percent, the
• radiographic density will be approximately
• one-half the original density if the mAs
• remains unchanged.

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The relationship between kVp and image density is
an exponential relationship. (Bushong, 2003)

• Therefore, any change in kVp will affect
• 1. Penetration
• 2. Scatter radiation
• 3. Patient dose
• 4. Image contrast
• 5. Object density

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• The 15 percent rule can be considered fairly
  accurate in the 60kVp to 100kVp.

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As you work in the higher kVp ranges the
exposure latitude becomes greater.

• For example 100kVp x 1.15 115 kVp. A 15
  increase or decrease in kVp is only 15 kVp

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As you change kVp in the lower ranges the
exposure latitude becomes less.

• For example 60kVp x 1.15 59 kVp. A 15
  increase or decrease in kVp is only 9.0 kVp

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Applying the Rule in Steps When Increasing kVp

• Step 1 increase kVp by 15
• Take original kVp and multiply it by 1.15 (115)
• Step 2 decrease mAs by 50
• Take original mAs and multiply it by .50 (50)

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Example 1

• If you radiographed an ankle using 12mAs at
  60kVp. What would be your technique if you had to
  increase the kVp by 15?
• 1. 60kVp x 1.15 69kVp
• 2. 12mAs x .50 6mAs
• Final answer The new technique would be 6 mAs _at_
  69kVp

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Applying the Rule in Steps When Decreasing kVp

• 1. Step 1 decrease kVp by 15
• Take original kVp and multiply it by .85 (85)
• Step 2 increase mAs by 50
• Take original mAs and multiply it by 1.50 (150)

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Example 2

• If you radiographed an elbow using 5mAs at
  55kVp. What would be your technique if you had to
  decrease the kVp by 15?
• 1. 55kVp x 0.85 47kVp
• 2. 5mAs x 1.50 7.5mAs
• Final answer The new technique would be 7.5mAs
  _at_ 47kVp

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QUIZ (click on the HORN by the correct
answer to see if you are right.)1. The 15
Percent Rule is utilized to maintain the
density on the radiographic images.
1. True 2. False
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If you x-rayed an elbow using 14 mAs at 65 kVp.
What would be the new mAs you would need to use
in order to maintain density if the kVp was
decreased to 55kVp? 7 mAs 21 mAs
14 mAs 28 mAs
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If you used 40 mAs at 80 kVp to x-ray a supine
abdomen, what would be the new kVp you would need
to use if you decreased your technique to 20
mAs? a. 68 kvp c. 92 kVp b. 80
kVp d. 106 kVp
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References

• Bushong, S. (2001). Radiologic Science for
  Technologists Physics, Biology, and Protection.
  Mosby, Inc. 7th Ed. pg. 280, 282.
• Carroll, Q. (2003). Fuchss Radiographic
  Exposure, Processing, and Quality Control.
  Charles C. Thomas, Publisher, LTD. 7th Ed. Pg.
  103-109.