Soft tissue has a density of 1060 kg m⁻³ and a speed of sound of 1540 m s⁻¹. Calculate its acoustic impedance.

The acoustic impedance is about 1.63 × 10⁶ kg m⁻² s⁻¹. The working is Z = ρc = 1060 × 1540 = 1.63 × 10⁶ kg m⁻² s⁻¹.

What is the intensity reflection coefficient, and how is it calculated?

The intensity reflection coefficient R is the fraction of the incident ultrasound intensity reflected at a boundary. The equation is R = (Z₂ − Z₁)² ÷ (Z₂ + Z₁)².

Fat has Z = 1.38 × 10⁶ kg m⁻² s⁻¹ and muscle has Z = 1.70 × 10⁶ kg m⁻² s⁻¹. Calculate R at a fat–muscle boundary.

The reflection coefficient is about 0.011, so about 1.1% of the intensity is reflected. The working is R = (1.70 − 1.38)² ÷ (1.70 + 1.38)² = 0.32² ÷ 3.08² = 0.011.

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Medical Physics

Ultrasound A-scans, B-scans and acoustic impedance

3 things to know
A2 3

Overview

Ultrasound frequencies and probes, specific acoustic impedance and the intensity reflection coefficient with worked values, coupling gel, A-scans and B-scans, echo depth, frequency choice and wavelength.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is ultrasound, and what frequencies are used in medical imaging?

Key term

How is ultrasound produced and detected in a probe?

Key term

Define specific acoustic impedance.

Questions from this subtopic

Each one with the model answer underneath.

1
What is ultrasound, and what frequencies are used in medical imaging?
Model answer
Ultrasound is sound with a frequency above 20 kHz, beyond the range of human hearing. Medical imaging typically uses frequencies from 1 MHz to 18 MHz.
2
How is ultrasound produced and detected in a probe?
Model answer
A piezoelectric crystal vibrates when an alternating voltage is applied, producing ultrasound. Echoes reaching the probe make the same crystal vibrate and generate a voltage, so the crystal also detects them.
3
Define specific acoustic impedance.
Model answer
Specific acoustic impedance Z is the density of a material multiplied by the speed of sound in it. The unit of Z is kg m⁻² s⁻¹.
4
Soft tissue has a density of 1060 kg m⁻³ and a speed of sound of 1540 m s⁻¹. Calculate its acoustic impedance.
Model answer
The acoustic impedance is about 1.63 × 10⁶ kg m⁻² s⁻¹. The working is Z = ρc = 1060 × 1540 = 1.63 × 10⁶ kg m⁻² s⁻¹.
5
What is the intensity reflection coefficient, and how is it calculated?
Model answer
The intensity reflection coefficient R is the fraction of the incident ultrasound intensity reflected at a boundary. The equation is R = (Z₂ − Z₁)² ÷ (Z₂ + Z₁)².
6
Fat has Z = 1.38 × 10⁶ kg m⁻² s⁻¹ and muscle has Z = 1.70 × 10⁶ kg m⁻² s⁻¹. Calculate R at a fat–muscle boundary.
Model answer
The reflection coefficient is about 0.011, so about 1.1% of the intensity is reflected. The working is R = (1.70 − 1.38)² ÷ (1.70 + 1.38)² = 0.32² ÷ 3.08² = 0.011.

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