Radiation is energy, in the form of particles or waves, that moves from place to place. Most people are familiar with light, heat, and sound as types of radiation in everyday life.
When radiation travels through an object it is generally affected by the material inside that object. We can use this behavior to “see” inside patients. By measuring changes in the intensity of x-rays after they pass through a patient, we can obtain a 2D picture of that person’s insides (see “What is x-ray imaging?“). If we rotate around the patient to acquire x-ray pictures at many different angles, we can build 3D maps of internal anatomy (see “What is computed tomography (CT)?“). We can also send sound waves into a patient’s body and then analyze the reflections of those waves to obtain images of patient anatomy (see “How does an ultrasound scanner work?“).
Alternatively, we sometimes use radiation that is generated inside the body. Magnetic resonance imaging (MRI) stimulates the body’s tissues to emit radiofrequency waves, and we can use the locations and properties of these waves to create maps of the patient’s internal anatomy (see “How does an MRI scanner work?“). Radiation-emitting “beacons” can be attached to molecules that participate in biological processes, and by tracking the location of these beacons in the body we can study behaviors such as metabolism and blood circulation. Examples of such technologies include positron emission tomography (PET) and single photon emission computed tomography (SPECT).
Many types of radiation are thus available for use in diagnosing disease. Some methods, such as x-rays, provide anatomic information; others, such as PET, provide physiological information. The choice usually depends on both the clinical question and the part of the body being examined. Your health care team will determine the best modalities to use for your case.