Quinnipiac University Diagnostic ImagingRS 240L Lab #3

Radiographic Density- Manipulating mA and Exposure Time

Name: Karen Finn

Objective: To demonstrate the effects of various exposure factors on radiographic density.

Equipment Used

The equipment that was used during this lab was the portable x-ray, a phantom elbow, a 14 by 17 Konica CR plate (100 RSI image receptor), AL step wedge, and the dark room, which included the processor with the feed tray.

Procedure: The first step in this lab was to place the extremity cassette on the x-ray table. Then the elbow phantom was placed on the cassette. Next, the AL step wedge was placed perpendicular to the elbow phantom and the SID was set to 40 inches. The central ray was properly centered to the phantom and the elbow was collimated. 58 kVp and 6.3 mAs were set for the technical factors. The film was then exposed and the film was labeled as image 1. Next another radiograph was taken but the technical factors were set as 67 kVp and the mAs was kept the same at 6.3. The film was then exposed and the film was labeled as image 2. Radiograph number three was taken with the mAs adjusted to compensate for the increase in kVp, which provided a comparable density to the first image. For radiograph 3, the kVp was kept at 67 and the mAs was halved to 3.2. The film was then exposed and the film was labeled as image 3.

Results / Discussion / Questions

1. Radiograph 1 and 3 look similar because in order to maintain density if the kVp is increased by 15% mAs is halved, which was done for radiograph 3 so they look similar. Radiograph 2 is denser because the kVp was increased by 15% but the mAs remained the same so it was denser than radiograph 1 and 3.

2. Radiograph 1 and 3 looked similar in density. Radiograph 2 was denser because the kVp increased by 15% and the mAs remained the same. Radiograph 2 was overexposed, the overall image appeared darker and so did the steps.

3. As mA is increased the density is increased. If the mA is doubled in order to maintain density, the exposure time should be deceased by 15%. As mA is doubled, the number of electrons able to cross the tube also doubles so the exposure time needs to be reduced. Increasing kVp will cause an increase in the speed and energy of the electrons applied across the x-ray tube. Therefore, the penetrability of the photon increases, as does the quantity (the intensity, # of photons, the density) and also gives a decreased contrast.

4. The mAs should be halved if the kVp was increased by 15%. 5. If the kVp was decreased by 15%, then the mAs should be doubled in order to

maintain radiographic density.

6. The step wedge was placed perpendicular to the elbow phantom due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side.