Medical imaging has been transformed over the past 30 years by the advent of computerized tomography (CT), magnetic resonance imaging (MRI), and various advances in x-ray and ultrasonic techniques. An enabling force behind this progress has been the (so far) exponentially increasing power of computers, which has made it practical to explore fundamentally new approaches. In particular, what our group terms “model-based” modalities— which rely on iterative, convergent numerical modeling to produce an image from data that is related nonlinearly to a target volume—have become increasingly feasible