How Do Medical Image Machines Work?
Radiographs, utilizing X-rays, are often used to view the heart and lungs.
The ability to perceive the state of structures within the human body without opening it surgically has transformed medicine's ability to diagnose disease. While the practice of medicine has existed in some form for thousands of years, medical imaging has been around only since 1895. Starting with radiography, medical imaging has branched into several modalities, each best suited for various conditions. Kinds of imaging machines share similarities and differences in the way they work.
- Generally, most medical imaging functions by permeating the body with energy that is either absorbed, transmitted through or reflected back. The subtle changes in this energy is detected by sensitive chemicals, electronic sensors or other detectors and sometimes analyzed by computers. The type of energy used varies according to the type of pathology suspected, area to be examined or other factors.
- The use of X-rays in medical examination dates to 1895. Dr. Wilhelm Conrad Rontgen (a physicist, not a physician) was experimenting with vacuum tubes that emitted radiation when he discovered that some of the radiation was not only invisible but could penetrate cloth, wood, paper and even flesh. As he continued to experiment with the equipment, he found that the rays could be directed at an object (or person) to be examined and would produce a shadow that could be captured on film. Where the rays encounter little resistance, they leave a dark area. Thin tissue, air, lungs and other such structures appear dark. Denser tissue, such as heart muscle, visceral organs and bones, absorb the energy and appear on developed film as light areas.
- Computerized tomography, also called CT or CAT scan, works by employing the same kind of energy. X-ray frequency radiation is essentially the same kind of radiation as visible light but is at a much more energetic frequency, allowing it to penetrate objects opaque to light. In computerized tomography, X-ray frequency radiation directed through the body is detected by sensors. Both the emitters and detectors rotate around the body of the subject, and this motion produces data about the structures from a variety of perspectives. No film is produced as in the simpler radiograph (X-ray film). Instead, computers use sophisticated algorithms to produce a mathematical model of the patient, which is displayed on computer screens or printed for easier viewing or transport.
- The waves of energy capable of penetrating the body don't have to be electromagnetic in nature. Sonography, also known as ultrasound, uses sound waves, which are easily transmitted by fluid and, because the human body is composed mostly of water, most parts of the anatomy. Soft tissue is most easily imaged by sonography, which also has the advantage of being far more portable than computerized tomography or X-ray. Though early work into sonography was done in the 1940s, a practical scanner for producing images was not fielded until the 1960s.
- Magnetic resonance imaging (MRI) is sometimes regarded as the most esoteric of the modalities in common use. The specifics of MRI are quite complicated; in short, though, the subject is placed in a very strong magnetic field and then exposed to radio-frequency radiation. This process causes the hydrogen atoms in the body to emit radiation. The sensors in the machine detect the location, quantity and frequency of these emissions, and a powerful computer uses this raw data to construct a virtual model of the subject's body. MRI is effective for viewing both soft and bony tissue. Though the word "radiation" is used, it's important to note that ordinary light and radio waves are radiation, and they are harmless. Like them, the radiation used by MRI has not been shown to cause side effects or illness, and it is a major advantage of the technique.
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