P REFACE BACKGROUND In 400 b.c. the Greek physician Hippocrates placed his hand on a patient's forehead and used the sense of touch to estimate body temperature. The five human senses were effectively the only sensors available to the "father of medicine" and his descendents until the seventeenth century when the first objective biomedical sensor in the form of a crude thermometer was devised. However, it was only toward the end of the nineteenth century that developments in science made possible significant advances in bio- medical sensor technology. The twentieth century and the modern era of biomedical technology may have been ushered in when, in November 1895, Wilhelm Röntgen astonished the medical profession and the world with an X-ray image of his wife's hand that he obtained using photographic film as a sen- sor. Twenty-four centuries after Hippocrates, sensors now extend the range of human senses to make possible diagnostic and therapeutic techniques that the ancient Greeks could never have envisioned. Almost all modern biomedical measurement and imaging systems depend upon sensors of one kind or another, although in many cases they are not immediately evident, often being hidden deep within the medical instrument. Today it is quite common for there to be a number of sensors embedded within the same medical device; for example, the ubiquitous hospital blood gas analyzer will routinely incor- porate more than half a dozen sensors, one for each gas and substance to be analyzed. WHAT IS A SENSOR? The broadest and simplest definition of a sensor is "anything that responds to an input of interest." However, a general-purpose dictionary definition is "a device that detects or measures some condi- tion or property and records, indicates, or otherwise responds to the information received." Although this is an excellent everyday definition, for biomedical engineering and medical physics purposes it is too imprecise. A device that merely "detects some condition or property," presumably by registering the presence or absence of a physical quantity with a simple yes-or-no response, would be called a "detector," not a sensor. Although detectors have important uses in medicine, especially as the basis of alarms, they are not generally regarded as sensors. For practical biomedical applications a sensor is bet- ter defined as "a device that responds to a physical input of interest with a recordable functionally related output that is usually electrical or optical." In a biomedical context, of course, the term physical input is taken to include chemical and biochemical quantities and concentrations. A device with an electrical output satisfies the "records or indicates" criterion in the general dictionary definition because electri- cal signals can be amplified and processed readily to give a display on a monitor, an output on a chart ix