One major drawback to these approaches was that they carried the risks associated with exposure to ionizing radiation, which even in relatively small amounts can cause irreparable damage to cells and tissues. But about the same time that CAT scanning was becoming a practical tool, a less harmful—and indeed more revealing—imaging technology appeared. Magnetic resonance imaging (MRI) relies not on X rays or gamma rays but on the interaction of harmless radio waves with hydrogen atoms in cells subjected to a magnetic field. It took a great deal of work by radiologists, scientists, and engineers to iron out all the wrinkles in MRI, but by the 1980s it too was proving to be an indispensable diagnostic tool. Not only was MRI completely noninvasive and free of ill effects, it also could create images of nearly any soft tissue. In its most developed form it can even chart blood flow and chemical activity in areas of the brain and heart, revealing details of functioning that had never been seen before.

Completing the gamut of medical imaging techniques is ultrasound, in at least one way a unique member of the family. As its name implies, rather than using electromagnetic radiation, ultrasound imaging relies on sound waves at high frequencies. The history of seeing with sound traces back to the early years of the century, when several different European engineers discovered that high-frequency sound waves bounced off metallic, underwater objects. Timing how long an echo takes to return to a transmitter/detector made it possible to determine the object's distance; other refinements eventually gave more detailed views of size and shape. Although sound navigation and ranging (sonar), a later term, was employed to some extent in World War I to detect submarines and underwater mines, it didn't really become refined enough for practical benefit until World War II. Meanwhile, as with X rays, sonar was also being used to detect flaws in metals and welded joints.

Not until the 1940s and 1950s did researchers seriously apply the principles and technology of sonar to the medical realm. One of the many pioneers was a Scottish gynecologist named Ian Donald, known by some of his colleagues as Mad Donald for his seemingly eccentric interest in all sorts of machines and inventions. Experimenting with tissue samples and an ultrasonic metal flaw detector owned by one of his patients, Donald realized that the detector could be used to create images of dense masses and growths within less dense tissue. Some of his early clinical efforts proved disappointing, but one noted success, when he correctly diagnosed an easily removed ovarian cyst that had been misread as inoperable stomach cancer, changed everything. As Donald himself said, "There could be no turning back." In 1959 he went on to discover that particularly clear echoes were returned from the heads of fetuses, and within a decade the use of ultrasound to chart fetal development throughout pregnancy was becoming more commonplace. Today, it is considered one of the safest methods of imaging in medicine and is a routine procedure in many doctors' offices and hospitals in the developed world.