2. December 10, 2004by Debbie Miller1 Fuchs’s Radiographic Principles THE 15 PERCENT RULE

3. December 10, 2004 by Debbie Miller2 Definition : A 15 percent change in kVp will result in a change in image density by a factor of 2. (Bushong, 2003)

4. December 10, 2004 by Debbie Miller3 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.

5. December 10, 2004 by Debbie Miller4 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

6. December 10, 2004 by Debbie Miller5. The 15 percent rule can be considered fairly accurate in the 60kVp to 100kVp.

7. December 10, 2004 by Debbie Miller6 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

8. December 10, 2004 by Debbie Miller7 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

9. December 10, 2004 by Debbie Miller8 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%)

10. December 10, 2004 by Debbie Miller9 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 @ 69kVp

11. December 10, 2004 by Debbie Miller10 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%)

12. December 10, 2004 by Debbie Miller11 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 @ 47kVp

13. December 10, 2004 by Debbie Miller12 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

14. December 10, 2004 by Debbie Miller13 2.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

15. December 10, 2004 by Debbie Miller14 3.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 kvpc. 92 kVp b. 80 kVpd. 106 kVp

16. December 10, 2004 by Debbie Miller15 References Bushong, S. (2001). Radiologic Science for Technologists: Physics, Biology, and Protection. Mosby, Inc. 7 th Ed. pg. 280, 282. Carroll, Q. (2003). Fuchs’s Radiographic Exposure, Processing, and Quality Control. Charles C. Thomas, Publisher, LTD. 7 th Ed. Pg. 103-109.