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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.

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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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