Answer
When one uses shorter wavelength waves to form an image, more detail can be resolved. Ultrasound has short wavelengths, allowing for better images. This is why blue light is sometimes used in optical microscopes, and why using x-rays or electrons produce far better microscopic images.
If you are curious, the reason that short wavelengths produce better images is that microscopes are "diffraction-limited". Try looking up the term "Rayleigh criterion" for a quantitative treatment of the problem.
