X-ray imaging (radiography)

Short pulses of radiation (X-rays) are shot through a body
with a photographic or digital film behind. As the X-rays hit the film, it turns from white to black, leaving a pattern of bright (where few X-rays hit) and dark (where many X-rays hit) on the film. Different tissues within the body absorb the X-rays to varying degrees, resulting in the contrast seen in an X-ray image.

CT (computed tomography) or CAT (computed axial tomography)

The fundamental limitation of X-ray radiography is that it
generates 2-D images of 3-D objects. The CT scanner eliminates this problem by taking a series of 2-dimensional X-ray “slices” through the body from many angles and a computer uses these 2-D to generate a 3-D view.

MRI (magnetic resonance imaging)

MRI forms images of inside the body by exposing the patient to changing magnetic fields, which cause water molecules in the body to absorb and give off energy. The scanner first sends in radio waves and then measures those given off by the tissues. Different tissues give off radio waves in different ways, generating the contrast seen in an image. MRI can reveal exquisite soft tissue details, and does not expose patients to potentially harmful radiation.

Nuclear imaging techniques: PET (positron emission tomography) and SPECT (single photon emission computed tomography)

PET and SPECT scans utilize radioactively labeled compounds (tracers) to obtain information about the biology of body tissues. In the most common use of PET, a radioactive sugar is taken up and metabolized more readily by tumor cells (since cancer cells burn more energy than healthy cells), distinguishing a cancerous tissue from its normal surroundings.

Ultrasound (US)

Ultrasound techniques direct high-frequency sound waves into the body, which, upon hitting tissues, are reflected back to a detector. A computer then creates an image of the tissue. Ultrasound can’t penetrate deeply into the body, but has become essential for visualizing tumors in tissues that lie just under the skin (e.g., breast and thyroid).