Physical and technical principles.

Ultrasound is an important technique for studying neurovascular pathology. As with any measurement or imaging technique, it has strengths and weaknesses, and there are a number of potential pitfalls for those interpreting its results. This chapter describes the basic physics and instrumentation behind both imaging and Doppler ultrasound techniques, with a special emphasis on their application to the cerebral circulation. The nature of ultrasound propagation in tissue is described, and the speed of ultrasound, its attenuation, and its behaviour at boundaries of various types are discussed. A description of pulse-echo B-mode techniques includes a section on transducers and artefacts. Doppler ultrasound is particularly important in the study of blood flow, and embolus detection, and its basic principles and various instrument types are described. The uses of transcranial Doppler for the measurement of velocity, flow changes, cerebrovascular resistance, and embolus detection are described. Finally the safety of ultrasound techniques in the context of cerebral vessels and in particularly transcranial Doppler is discussed.