Ionising radiation may be destructive, but it definitely has its uses in the modern world.  Discussed here are the uses of ionizing radiation in nuclear medicine and sterilisation.  Note that ionising radiation is also used to for non-medical imaging, but the processes used closely parallel those described in the nuclear medicine section, and as such will not be repeated. 

Nuclear Medicine:
 

Nuclear medicine is an umbrella term that encompasses the various medical uses for ionising radiation.  Our focus will be on medical x-ray imaging, PET, CT and radiation therapy.

Medical X-ray Imaging

X-rays are used to image radiopac tissures in the body.  Wilhelm Röntgen discovered x-rays in 1895 and pioneered their use in imaging the human skeletal structure .  Modern x-ray imaging has moved far beyond the skeletal structure and is now used to identify lung diseases, blockages of the intestine, air and fluid pockets in the abdomen and even as a real-time imaging source (supplanting the traditional real-time imaging technology- ultrasound) during barium enema treatments (used to image intestines) as well as during angioplasty treatments.

Positron Emission Tomography

Positron emission tomography (PET) is an imaging technique in which a patient is injected with a quickly decaying radioactive isotope that creates positrons (via ¥^- decay )  that annihilate with electrons within the patient's tissue.  These annihilation events produce two characteristic gamma rays that travel in a path separated by exactly 180¥.  A specialised machine collects data describing the locations and relative frequencies of these events and uses this information to create precise three-dimensional maps of the tissue in question.  This technology is unique because the radioactive isotopes are chemically attached to metabolically active molecules such as sugars and as such the images collected can be used to visualise the metabolic processes within a patient's body.  To image the anatomic structure of a patient CT scans are often used.

Computed Tomography

Computed tomography (CT) is an imaging technique in which a series of x-ray images are taken about the same axis of rotation and combined using computational methods to create a three dimensional image.  This process was formally known as a Computed Axial Tomography scan (CAT scan).  The crux of this technology is to take a series of x-ray images at different angles relative to a fixed axis and use a computer to combine them, "filling in the blanks" between the different layers.  Because the area being imaged is hit with many successions of x-rays, this imaging system results in a very large radiation dose to a patient compared to conventional 2-D x-ray imaging.

Radiation Therapy

Radiation therapy refers to the use of ionising radiation for medical treatments.  The most common use of ionising radiation is for therapeutic or palliative cancer treatments.  In therapeutic cancer treatments the radiation is directed at a specific area of the body to kill cancer cells or to prevent the reoccurrence of cancer after a previous treatment.  In palliative cancer treatments radiation is used on terminal patients to treat areas more extensively to help relieve symptoms and make a patient's last days more comfortable. 

Radiation is less commonly used in cases unrelated to cancer due to fears of causing cancer in the patients.  For instance, before a patient receives a bone marrow transplant they receive Total Body Irradiation (TBI).  In TBI the patient's entire body is treated, killing all of the patient's bone marrow.  Immediately afterward they receive a bone marrow transplant from a genetically suitable individual, or alternately they may receive previously harvested bone marrow producing stem cells from their own body.  Radiation therapy is also used to treat pterygium, a condition characterised by the overgrowth of the conjunctiva in the eye, as well as treatment of heterotopic skin and bone growth (overgrowth of skin or bone tissue).

Sterilisation:

Ionising radiation is increasingly becoming a viable alternative for the sterilisation of products ranging from mail to food.

Food

The deadly effects of radiation exposure can be harnessed to kill microbes that cause food spoilage, in a process called irradiation.  Because x-ray and gamma radiation do not render the target radioactive, these are the primary means of irradiating foods.  Electron beams are used as well, although the energy of the electrons are limited to prevent inducing radioactivity.  The use of radiation for pasteurisation is mainly applied to meats.  Note that pasteurisation is not identical to sterilisation, with the goal being to greatly reduce the levels of bacteria and other spoiling agents.  Doses of radiation necessary to completely sterilise food are rarely applied due to exposure concerns.  Herbs and spices are among a small group of foods exposed to partially sterilising levels of ionising radiation. 

Irradiation is also used to treat fruits and vegetables susceptible to fungal growth.  This process has replaced the use of methyl bromide (which was dropped because of its ozone depleting effects) to fumigate fruits and vegetables just after harvest.  Ionising radiation is considered a viable alternative to fumigating processes similar to this, although public perception is a limiting factor to the full-scale application of many food irradiation techniques.

Mail

In September of 2001 a series of letters containing weaponised anthrax spoors were mailed to US senators and members of the media.  The result was five deaths and a huge change to the way the United States Postal Service did business.  Beginning in November 2001, the USPS has been irradiating all mail sent to government offices.  This irradiation is intended to kill any biological agents like anthrax.  The effect of the irradiation on goods within the mail was very pronounced.  Electronics, biological compounds and samples, plastics and papers were all damaged by the irradiation.  Some plastics melted and certain types of paper were damaged.  Although the USPS initially irradiated huge swaths of mail following the attacks and announced their intention to expand the program to cover all mail, the program was reduced and is only used on mail sent to government offices at this time.