Physics of ultrasound

Wavelength, frequency, resolution — what drives image quality.

6 min read

Learning objectives

You know the basic quantities of a sound wave (frequency, wavelength, amplitude).
You understand the trade-off between penetration and resolution.
You can name typical transducer frequencies for adult TTE.

The physics of ultrasound is the foundation of every echocardiographic exam: to optimise image quality on purpose, you have to understand how frequency, wavelength and penetration depth are related.

The basic quantities

A sound wave is a periodic pressure fluctuation, described by:

Frequency f — oscillations per second, in Hz.
Wavelength λ — distance between two maxima, in mm.
Amplitude A — peak pressure deviation; determines intensity.
Speed of sound c — roughly constant in soft tissue at ≈ 1540 m/s.

The relation is c = f · λ, hence λ = c / f.

Frequency vs. penetration

Frequency	Penetration	Axial resolution
2 MHz	~ 30 cm	coarse
3.5 MHz	~ 20 cm	medium
5 MHz	~ 12 cm	good
7.5 MHz	~ 6 cm	high

Trade-off: higher frequency means better resolution, but stronger attenuation. Adult TTE typically uses 2–3.5 MHz, TEE 5–7 MHz, paediatric probes 5–8 MHz.

Resolution

Axial resolution ≈ ½ pulse length → improves with higher frequency.
Lateral resolution depends on beam width → varies with focus.
Temporal resolution (frame rate) trades off against sector size and line density.

PLAX rendered as a B-mode image by the sister project EchoSchema. The edit link toggles between the grayscale image (“echo”) and the vector schematic (“schema”) — the same plane once as the machine draws it, once as you memorise it.Open in echo generator

Mini-quiz

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How are frequency and wavelength related in soft tissue?

Take-aways

Higher frequencies give better axial resolution but less penetration.
Speed of sound in soft tissue: c ≈ 1540 m/s.
Adult TTE probes typically operate at 2–5 MHz; paediatric/TEE probes are higher.

Mark the lesson once you have worked through it – synced across devices with an account.