Final answer:
A decrease from 90 to 77 kVp results in a decrease in the scale of grays in an x-ray image, but does not necessarily affect the optical density, which is influenced more by the number of photons than their energy.
Step-by-step explanation:
A decrease from 90 to 77 kVp (kilovoltage peak) will result in a decrease in the scale of grays on the resulting x-ray image. The scale of grays, or contrast, is affected by the peak voltage of the x-ray tube because the kVp determines the energy and penetration ability of the photons produced. A lower kVp means that the x-rays have less energy and are more likely to be absorbed, which creates greater contrast between different tissues but a narrower range of grays. However, the change in kVp does not necessarily equate to a change in the optical density. Optical density is primarily controlled by the number of photons (mA and exposure time), not their energy. Therefore, a decrease to 77 kVp does not directly imply a change in optical density.