Lab & analytical techniques · Reference
What is ultrasound? (the measurement physics)
Ultrasound imaging sends high-frequency sound waves into a medium and times their echoes from boundaries to build an image; the physics is the emission, reflection, and timing of sound.
The physics of the measurement
Ultrasound uses sound waves at frequencies typically in the megahertz range, well above human hearing. A transducer, containing a piezoelectric element, converts an electrical pulse into a mechanical sound wave and sends it into the medium. As the wave travels, it encounters boundaries between regions of different acoustic impedance — a property combining density and sound speed. At each boundary, part of the wave is reflected back as an echo. The same transducer then detects these returning echoes, turning them back into electrical signals.
From echoes to an image
Because the speed of sound in soft media is roughly constant, the time delay between sending a pulse and receiving its echo gives the depth of the reflecting boundary. The strength of the echo indicates how large the impedance difference is.
By sweeping the beam across the region and combining many such echo lines, the system builds a two-dimensional image in real time. This pulse-echo timing is the same principle as sonar and echolocation.
The measurement in research
As a measurement technique, ultrasound is attractive because it is non-invasive, real-time, and uses no ionising radiation. The Doppler effect — a frequency shift in echoes from moving reflectors — lets it measure motion and flow. Beyond imaging, ultrasonic methods are used in engineering for non-destructive testing of materials and in physics to measure distances and material properties. This entry covers only how the measurement is made and does not provide any diagnostic interpretation or clinical advice.
Key facts
At a glance
- Uses: high-frequency sound waves (above human hearing)
- Device: a piezoelectric transducer (emitter and receiver)
- Key property: differences in acoustic impedance cause echoes
- Depth from: the time delay of each echo
- Doppler mode: measures motion and flow
- Uses no ionising radiation
Common questions
FAQ
How does ultrasound imaging work?+
A transducer emits pulses of high-frequency sound into a medium. Where the sound meets a boundary between materials of different acoustic impedance, part reflects as an echo, and the timing and strength of the returning echoes are used to build an image.
What is the Doppler effect in ultrasound?+
The Doppler effect is a shift in the frequency of echoes returning from moving reflectors. Measuring this shift lets ultrasound estimate the speed and direction of motion, such as flow within a medium.
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