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Ultrasound Basics

General Introduction. Ultrasound imaging is a medical analog of sound navigation and ranging (SONAR). Specifically, ultrasound pulses are generated and detected (after reflecting off tissue) by a “transducer,” which records the “echo time” (the time elapsed between a pulse’s creation and detection). The transducer also records the echo amplitude. The echo time is used to calculate tissue depth, and the echo amplitude is used to create contrast, with strong echoes appearing white, weaker echoes as variable shades of grey, and “absent” echoes as black (see Fig. 1). 

There are several medical ultrasound modes that are used in different medical applications. In all cases, tissue depth is quantified using the simple kinematic relationship:

2D = v x t   (Eq. 1)

where D = tissue depth,  v = speed of sound in soft tissue = 1540 m/s, and t is the elapsed time between pulse creation and detection (i.e., the echo time). The factor of two arises because the pulse traverses the depth twice before returning to the transducer; see Fig. 2. For the moment, strong echoes can be thought of as arising from reflections at interfaces that are highly mismatched (e.g., bone/soft tissue interfaces) and weak echoes as arising from reflections at interfaces that are similar (e.g., soft tissue interfaces).

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