How Ultrasound Builds an Image
An ultrasound scan does not take a photograph inside the body. It sends a short sound pulse, waits for echoes, and uses those echoes to infer where boundaries are.
Send a pulse, then listen
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Ultrasound is sound with a frequency above human hearing. The probe contains a piezoelectric crystal that acts as both a transmitter and a detector.
- An alternating potential difference repeatedly changes the crystal's shape. The vibration produces ultrasound.
- The pulse travels through the body and part of it reflects at a boundary between tissues.
- A returning wave deforms the crystal. The crystal produces a voltage (an e.m.f.), so the echo is detected (9702/42/F/M/23 Q9(a)).
Key idea
Echo time and echo size tell different stories
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- Echo delay gives the boundary depth. A later echo has travelled farther.
- Echo intensity gives information about the change in material at the boundary (9702/42/M/J/24 Q10(a)).
- The recorded time includes the outward and return journeys. The pulse travels twice the depth.
Move the boundary: what changes?
The pulse speed stays fixed. A deeper boundary only makes the echo return later.
- Round-trip distance
- 10.0 cm
- Echo delay
- 65 μs
Worked example
Find a boundary depth
An echo returns after s. The speed of ultrasound in tissue is 1540 m s−1.
- The pulse travels out and back, so use half the total distance.
Ultrasound travels at 1500 m s−1. An echo returns after 80 μs. Calculate the depth of the boundary.
Show worked answer
The boundary is 0.060 m, or 6.0 cm, below the probe.
Common mistake