When pulses of laser light pass through a turbid medium such as fog or body tissue, most of the photons are either scattered or absorbed.
However, across short distances, a few photons pass through the scattering medium in straight lines.
[1] Slightly scattered "quasi-ballistic" photons are often measured as well, to increase the signal 'strength' (i.e., signal-to-noise ratio).
Ballistic photons have many applications, especially in high-resolution medical imaging systems.
Ballistic scanners (using ultrafast time gates) and optical coherence tomography (OCT) (using the interferometry principle) are just two popular imaging systems that rely on ballistic photon detection to create diffraction-limited images.