There was a time not so long ago, well within the memory of many of us, when medical imaging was an analog process in which X-rays, or reflected ultrasound signals, exiting from a patient were intercepted by a detector, and their intensity depicted as bright spots on a fluorescent screen or dark areas in a photographic film. The linkage between the exiting radiation and the resulting image was direct, and the process of forming the image was easily understandable and controllable. Teaching this process was straightforward, and learning how the process worked was relatively easy.
In the 1960s, digital computers began to migrate slowly into medical imaging, but the transforming event was the introduction of X-ray computed tomography (CT) into medical imaging in the early 1970s.With CT, the process of detecting radiation exiting from the patient was separated from the process of forming and displaying an image by a multitude of computations that only a computer could manage. The computations were guided by mathematical algorithms that reconstructed X-ray images from a large number of X-ray measurements across multiple imaging planes (projections) obtained at many different angles. X-ray CT not only provided entirely new ways to visualize human anatomy; it also presaged the introduction of digital imaging methods to every imaging technique employed in medicine, and ushered the way for new imaging technologies such as magnetic resonance and optical imaging.