08 Ultrasound

Ultrasound attenuation

Soft-tissue rule of thumb: 0.5 dB/cm/MHz one way.

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Simulation

Ultrasound attenuation — Change the numbers; the scene follows.

Where it works

Ultrasound

Tissue path

Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus.

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Formula

A_{\mathrm{dB}} = \alpha f z,\quad I = I_0 \cdot 10^{-A/10}

Variables

α · Attenuation coefficientdB/cm/MHzf · FrequencyMHzz · Depth (one way)cmround · Round trip? 1 = yes

Results

A
Attenuation
40dB
I/I₀
Relative intensity
0.0001

Curve

Explanation

A_{\mathrm{dB}} = \alpha f z,\quad I = I_0 \cdot 10^{-A/10}

What it means

Soft-tissue ultrasound attenuation is about 0.5 dB cm⁻¹ MHz⁻¹ one way (1.0 dB round trip). A 5 MHz beam to 8 cm and back loses ~40 dB — TGC exists because of this exponential. This is a working relation in Ultrasound.

Where it is used

Clinically it sits on the Ultrasound — Tissue path. Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus. Ultrasound equations sit on the probe face and along the beam: impedance, Snell, Doppler, MI and TI. They explain why gel matters, why aliasing appears, and why a mechanical index is on the screen.

Ultrasound · Open this machine

How to use it

Set round-trip = 1 for pulse-echo. Fat ~0.6, muscle ~1.0, bone much higher. Frequency is a trade: penetration vs resolution. Change one input and watch the curve and the simulation follow.

Symbols

αAttenuation coefficient0.5 dB/cm/MHz
fFrequency5 MHz
zDepth (one way)8 cm
roundRound trip? 1 = yes1

Worked example

A typical case from the default values: α = 0.5 dB/cm/MHz (Attenuation coefficient); f = 5 MHz (Frequency); z = 8 cm (Depth (one way)); round = 1 (Round trip? 1 = yes). Substituting into the relation gives A = 40 dB; I/I₀ = 0.0001. These are teaching numbers — align them with your machine.

Typical values give

A = 40dB
I/I₀ = 0.0001

Where it comes from

The displayed formula is the working relation. Soft-tissue rule of thumb: 0.5 dB/cm/MHz one way. Usual reference: Bushberg. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Bushberg

Assumptions & limits

Constant α, no focusing gain, no harmonic generation. Real TGC also compensates beam spreading.

Pitfalls

Soft-tissue 1540 m/s is an assumption — not a measurement in that patient. Doppler angle 90° gives no shift. MI and TI are on-screen estimates, not absorbed dose. Constant α, no focusing gain, no harmonic generation. Real TGC also compensates beam spreading.

Keep this

Use 2z for round trip. α ≈ 0.5 dB cm⁻¹ MHz⁻¹.

In this specialty

Ultrasound