Final answer:
When it comes to soft-tissue contrast in medical imaging, contrast resolution is the most crucial factor as it determines the imaging system's ability to distinguish between similar densities in soft tissues.
Step-by-step explanation:
The most important factor for soft-tissue contrast is contrast resolution. Contrast resolution refers to the ability of an imaging system to distinguish between differences in intensity in an image. This is crucial for differentiating between soft tissues, which may have very similar densities and require high-quality contrast resolution to be adequately visualized.
Spatial resolution, while important, is more about the detail within the image and how finely it can resolve small structures. Exposure time can impact the overall quality of the image, but it is primarily related to how long the detector is exposed to radiation and doesn't inherently affect the contrast resolution. Noise can obscure details and affect the image quality, but it is not as critical as contrast resolution for soft-tissue contrast.