Antimatter annihilates whenever it encounters matter and causes an explosion that emits gamma rays.
Hospital PET scans, one of the most common applications of antimatter, use positrons, which are a subatomic, antimatter particles.
Positron Emission Tomography (PET) is the process used to inject positrons into a brain and watch for gamma rays that flash when the positrons encounter electrons of normal matter and then explode. The two destroy each other, giving off a light pattern that is different in an afflicted brain than in a normal one, thus revealing neurological aberrations.
Antimatter is used almost daily in hospitals around the world.