The medical scanner was a medical device utilized by Starfleet medical personnel in the 2260s. Similar in size and function to the hand scanner of the medical tricorder, the medical scanner was a small, portable unit designed to function separately from the tricorder and diagnose by means of auditory emissions.
With this device, a trained physician could diagnose an illness by scanning the patient and interpreting the sounds it produced to diagnose, for example, arthritis. (TOS: "The Deadly Years") It was phased out as integration between the scanner and the medical tricorder became more advanced and commonplace.
Radioactive elements or other chemicals could set off the scanners. In 2266, Ruth Bonaventure set off Leonard McCoy's medical scanner just by walking past it, prompting him to ask whether she was wearing an unusual perfume or something radioactive. Later, he doubted that the unusually beautiful women were alien illusions, saying that any alien smart enough to do so would also be able to prevent his scanner from "going beep." (TOS: "Mudd's Women")
To accurately take readings of non-Human individuals, the scanner would have to be recalibrated. McCoy mentioned this to Spock in 2269 when the Vulcan returned from a trip to his past. Spock informed him that he was lucky, because if times were different, he'd have to recalibrate for an Andorian (first officer Thelin in an alternate timeline). McCoy didn't understand what he meant by that, having lost all memory of what had occurred. (TAS: "Yesteryear")