Using collimation affects the primary signal by:

Using collimation significantly impacts the quality and clarity of the primary signal in imaging. When collimation is applied, it limits the X-ray beam to the area of interest, effectively reducing the amount of scattered radiation that can contribute to the signal.

This focused approach enhances contrast in the resulting images because collimation minimizes the exposure to peripheral tissues that do not contain relevant diagnostic information. As a result, the features of the structures of interest become more distinguishable from the background, leading to greater diagnostic clarity.

However, because collimation restricts the area being imaged, it can also lead to a decrease in signal intensity due to the reduction in the overall amount of radiation reaching the detector. While the targeted area may have improved contrast, the total signal measured decreases as the exposure is limited to a smaller region.

Therefore, the correct answer accurately reflects that collimation increases the contrast by sharpening the image quality while simultaneously decreasing the intensity of the signal due to the restricted field of view.