Pulsing of the transmitted sound is necessary for real-time imaging because which is true?

• A. The transducer overheats
• B. The crystal will break under continuous emissions
• C. Lateral resolution is improved by pulsed transmission
• D. The depth of the interface from which the echo originated can be determined

Answer: (D)

Pulsing provides time-of-flight information that lets you know where echoes come from. When a short ultrasound pulse is emitted, echoes return from interfaces at various depths. By measuring the time between emission and reception and using the known speed of sound in tissue, you can calculate how far away the reflecting interface is. Since the sound must travel to the interface and back, depth equals speed × round-trip time divided by two. For example, if the round-trip time is about 20 microseconds and the speed is roughly 1540 m/s, the depth is around 1.54 cm. This time-based localization is essential for real-time imaging because you can emit many pulses per second and construct an image line by line. If you transmitted continuously, there would be no discrete echoes tied to specific depths, so depth information couldn’t be determined. Lateral resolution, while influenced by beam width and focusing, isn’t the primary reason pulsing enables real-time depth localization.