Quinnipiac University Diagnostic ImagingRS 240L Lab #5

Radiographic Contrast- Manipulating kVp and mAs to visualize the Effect of Contrast

Name: Karen Finn

Objective: To demonstrate the effects of various changes in kVp on radiographic contrast.

Equipment Used

The equipment that was used during this lab was the portable x-ray, an abdomen elbow, a 14 by 17 Konica CR plate (400 RSI image receptor), AL step wedge, a table Bucky, and the dark room, which included the processor with the feed tray.

Procedure: The first step in this lab was to center the x-ray tube and place the abdomen phantom on the table centering to the Bucky. Next the SID was set to 40’ and the central ray was directed between L2 and L3 on the lumbar spine. The sides were collimated and the step wedge was placed in the open x-ray field above the lumbar spine, showing all steps. Then the kVp was set to 70 and the mAs was set to 20. The film was exposed and processed and marked with the label number 1. Next for radiograph number 2, decreasing the kVp by 15% and then decreasing the kVp by 15% again changed the original technical factors. The mAs was adjusted to compensate for the change by doubling the mAs once and doubling it again. The film was exposed and processed and marked with the label number 2. For radiograph number 3, the original exposure factors changed by increasing the kVp by 15% once and then it was increased by 15% again. The mAs was then cut in half then cut in half again to compensate for the change in kVp. The film was exposed and processed and marked with the label number 3.

Results / Discussion / Questions

1. Radiograph 1 had low long scale contrast, radiograph 2 had high short scale contrast with more white aspects on the film, and radiograph 3 had longer scale contrast than number 1 and higher contrast than radiograph 3.

2. A High kVp means more scatter, low contrast, and more penetration. A low kVp means less scatter, high contrast, and less penetration. Low kVp increases patient dose. An increase in kVp would make an image darker because a more penetrating beam (shorter wavelengths), so more radiation would get through the object and to the film.

3. kVp determines the quality of the beam, the x-ray penetration, controls the contrast of the radiographic image. 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). If kVp is increased by

15%, the mAs is reduced 50%. If kVp is reduced by 15%. The mAs is doubled. As the kVp is reduced, the contrast increases.

4. Radiograph 1 has long scale contrast there is good contrast throughout the image. Radiograph 2, is very light, doesn’t have as much contrast, and has short scale contrast. The steps are also more attenuated. Radiograph 3, has a longer scale than radiograph 1 and 1 has a higher contrast than 3 so there is more gray.

5. Increased wavelength means decreased frequency. As a photon wavelength increases, the photon energy decreases. Therefore the maximum x-ray energy is associated with the minimum x-ray wavelength. Low kVp means low energy which means weak penetration. High kVp means high energy and strong penetration. Higher kVp produces shorter wavelengths with ability to penetrate tissue. When kVp is increased, the penetrability of the x-rays is increased and relatively fewer x-rays are absorbed in the patient.