Principles of Doppler ultrasound and emerging blood flow imaging.

Doppler ultrasound is one of the oldest modalities for measuring and visualizing blood flow. This review paper explores the principles of Doppler ultrasound and blood flow imaging. Following a brief history of Doppler ultrasound, all Doppler modes, including B-Flow imaging, are discussed, with emphasis on spectral broadening and Doppler spectrum amplitude in relation to clinical implications. The paper also outlines sources of uncertainty in Doppler flow measurement. Emerging technologies in blood flow imaging are introduced, including microvascular flow imaging without contrast agents and four-dimensional vascular imaging from a two-dimensional transducer, which have been commercialized within the past decade, as well as row-column array transducer systems, high-frame-rate imaging, and photoacoustic imaging, which remain in preclinical and research stages.