“Medical device” is a recent terminology used to identify a wide variety of medical equipments. Medical devices are generally classified into three principal categories: diagnostic, therapeutic, and assistive (or rehabilitative).

Diagnostic Devices

Diagnostic devices are very common because of their usefulness when a physician first examines a patient. Perhaps the two most familiar diagnostic devices are the thermometer and the stethoscope. A thermometer measures body temperature, which is an indicator of the process of regulated burning of body fuel called metabolism. The stethoscope collects the feeble sounds made by some internal body organs and presents these sounds to the physician's ear. The timing and nature of these sounds give important clues to the physician regarding the function of these organs. A stethoscope is also used to measure blood pressure when an occluding cuff is wrapped around the upper arm and quickly inflated to a pressure high enough to occlude the artery under it. When the pressure in the cuff is lowered slowly, blood spurts through the collapsing artery, and a stethoscope is used to monitor the downstream arterial sounds, the appearance and disappearance of which indicate when to read the cuff pressure to determine the maximum (systolic) and minimum (diastolic) blood pressure.

There are some organs in the body which function in relatively inaccessible locations. These organs must be transilluminated for visualization. The lowenergy x-ray beam is used for this purpose, but because of the insensitiveness of the human eye to electromagnetic energy, a photographic surface or fluorescent screen is used to convert the absorption image to a visible one.