The human body supports the propagation of many kinds of waves, each of which can provide an image with a specific type of information. For example, ultrasonic waves reveal a tissue’s density and how it responds to compression forces, and mechanical shear waves indicate how tissues respond to shear forces. Low-frequency electromagnetic waves are sensitive to electrical conductivity; optical waves tell about optical absorption. In all those circumstances, physicists have striven to obtain the best overall contrast and resolution. Now, after decades of work, we are pushing against the physical limits inherent in each imaging modality. As described in the box on page 30, that limit is, in many cases, not determined by wavelength.
Physicians quickly realized that for medical imaging and diagnosis, one way to overcome the inherent limits of singlemode imaging is to combine different imaging modalities. The basic idea of multimodality imaging—for example, in the combination of...
