X-rays: Although more damaging to biological tissue than visible light, due to the fact that the associated photons have more energy, X-rays can see finer detail when used in a ``microscope'' due to their short wavelength. In the days before the dangers of X-rays were appreciated, they were used as a novelty in some shoe stores to accurately determine the foot size. In a CAT scan (computer assisted tomography), X-rays are taken from a variety of angles, and the individual slices are then fed into a computer to reconstruct a 3-dimensional image of the object under study.
Tumor treatment: Radiation is used to destroy tumors present in a body. Damage to healthy cells is controlled by using radioactive samples with specific half-lives which subsequently are intended to decay in the vicinity of the tumor when injected into the body.
Tracer techniques: The body absorbs various elements but does not discriminate between different isotopes. In tracer techniques a radioactive isotope of such an element, such as iodine, is injected into the body. The signals coming from the ensuing radiation then give a picture of the size and location of the area where the isotope was absorbed. Usually isotopes of a relatively short half-life, of the order of minutes or days, are used to minimize long-term radiation damage.
PET scans: PET (positron emission tomography) scans involve the injection into the body of an isotope which decays by positron emission. When this positron encounters an electron they annihilate each other, emitting two photons. The energy and path of these photons leaving the body can then be used to give an accurate picture of the area where the isotope was absorbed.
The advantages of these and other uses of radiation must be balanced against the health risks. As well, less dangerous diagnostic devices are being actively developed. One obvious one is the use of ultrasound for fetal monitoring, which because it involves sound waves is far less dangerous than X-rays. Another technique is MRI (Magnetic Resonance Imaging), which formerly was known as NMR (Nuclear magnetic Resonance). This technique exploits the fact that the nucleus has certain magnetic properties due to the spin of the nucleons, much like an atom has magnetic properties due to the orbiting electrons. When subject to a relatively large magnetic field, and subsequently probed with particular waves, these nuclei respond by emitting characteristic waves. These waves, which are not dangerous, are then tracked, and their properties are used to construct an image of the area being probed.