A hundred years ago, a doctor had no way to look within the body of a patient other than to slice it open. That changed radically at the turn of the century, with the discovery of X-rays. X-ray and other forms of diagnostic imaging technology developed slowly but steadily from then until the 1970s, at which point a revolution occurred. Made possible largely by the availability of powerful but inexpensive computers, the rapid and widespread adoption of computed tomography (CT) and, a decade later, of magnetic resonance imaging (MRI) greatly expanded the power of clinical imaging, and even...