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In the 1970s, Holasek, Gans, Purnell, and Sokollu expressed the notion that the frequency content of medical-ultrasound echo signals might be exploited to characterize soft tissue; they developed a method of color-encoding of B-scan images to depict spectral content of RF echo signals and termed the method ‘‘spectra color.’’By characterize, they essentially meant to distinguish one type of soft tissue from another, for example, to distinguish a cancerous lesion from a benign one, to track changes in tissue over time, or to monitor the progression or regression of disease. Unfortunately, the available analog technology of that era made testing, validating, and implementing this concept practically impossible. However, the notion of characterizing soft tissue based on features that could be extracted from echo signals caught the imagination of many other investigators, and several innovative efforts were launched in …