Physics A2 · Medical physics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A piezo-electric crystal changes shape when a p.d. is applied across it. An alternating p.d. makes it vibrate and send out ultrasound. The effect also works in reverse: a returning wave squeezes the crystal and it generates an e.m.f. So one transducer both sends and detects.
Short pulses are sent into the body. At each boundary between two tissues part of the pulse is reflected. The time for an echo to return gives the depth of the boundary, and its strength tells us about the tissues.
How much is reflected depends on the specific acoustic impedance of the two media. A big difference means a strong reflection. The beam also gets weaker as tissue absorbs it.
Students forget to halve the echo time when finding depth, or forget to square the bracket in the reflection coefficient. In I = I0e−μx, x must be in metres if μ is in m−1.
Answer: About 0.0044, which is 0.44%. Almost all of the beam is transmitted.
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