The spatial resolution describes the smallest distance between two points in the object that can be distinguished as separate details in the image, generally indicated as a length or a number of black and white line pairs per mm (lp/mm). In ultrasound imaging, the spatial resolution means how closely two reflecting tissues - or scattering regions, can be to one another while they can be identified as different reflectors.

The wavelength is a unit of relative distance equal to the length of a wave. This could be a light wave, a radio wave, or even a sound wave. For sound waves the formula is:
l=c/f (wavelength = propagation speed/frequency)
In ultrasound imaging is the wavelength the distance between the onset of peak compression or cycle to the next. The wave propagates as bands of compression and rarefaction. One wavelength is the distance between two bands of compression, or rarefaction. Maximum compression corresponds to maximum pressure. The wavelength (see also Angstrom) is important in image resolution.

See also Spectral Reflector.