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