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An imaging glossary

Yale Medicine Magazine, 1998 - Winter/Spring


Looking into the human body without opening it up has been a tool for medicine for more than a century. Exposing film to X-ray beams passed through the body provided readable images of solid structures like bones and masses of dense, soft tissue, such as muscle and brain. In many ways, the basic concepts in imaging technology have changed remarkably little.

Discovered by Wilhelm Conrad Roentgen in 1895, X-rays are beams of radiation that pass through different kinds of tissue in varying amounts. On photographic film, air and soft tissue will register in different shades of black while bone will show up as white. In dense organs like breast tissue, a radiologist will read the film for subtle gradations to spot anomalies that are potential indicators of disease.

Computed tomography (CT) takes an X-ray camera and spins it about the subject's body to take a cross-sectional image, or slice. The X-rays from the CT scanner are picked up by sensors linked to a computer that generates a highly detailed video image. A significant technical advance, a helical CT scanner moves over the patient, taking rapid, overlapping images of large portions of the body. When rendered on the computer, the images can be viewed from any angle and perspective. For instance, tours through a virtual, 3D image of an organ allow physicians to study areas of the body requiring reconstructive surgery prior to operating or to inspect precancerous polyps in the colon to determine appropriate treatment.

Using pharmaceuticals "tagged" with nuclear trace elements, single photon emission computed tomography (SPECT) and positron emission tomography (PET) scanners can detect and measure the site, speed and quantity of absorption of the radiopharmaceutical. The minute change can be recorded to show, for instance, the body repairing tiny fractures in bone structure or the rate of uptake of neurotransmitters in the brain. Yale researchers have used PET and SPECT to develop a new method for detecting Parkinson's disease prior to the onset of the shaking and other behavioral symptoms associated with it. Physicians can now begin treatment early enough, in many cases, to prevent the most devastating symptoms altogether.

The advent of Magnetic Resonance Imaging (MRI)–first used clinically in 1986, followed by a giant leap forward with the arrival of functional MRI (fMRI) in 1990–made the dynamism of life visible for the first time without significantly interfering with that dynamic. The tube-shaped magnet of the MRI machine creates a massive yet harmless magnetic field around the body that causes hydrogen atoms to line up uniformly. When the magnetic field is switched off, the atoms relax back to their normal patterns, giving off detectable signals as they do. A computer records the process, which occurs at varying rates depending on the tissue, and then generates highly detailed images. Radiologists can set the magnet for study of anatomy through sensitivity to tissue or they can observe function, or metabolic activity, through sensitivity to blood oxygenation. When a portion of the brain is activated, an fMRI scan can detect increased blood flow. By overlaying an anatomical with a functional study, researchers can map out which sites in the brain serve to control different activities, or they can compare function in a normal brain versus that of a person with a mental illness or such disorders as learning disabilities or autism.