Ultrasound imaging (A-level only) Flashcards – AQA A-level Physics (7408)

What is the acoustic impedance of a medium in terms of its density and speed of sound?

$Z=\rho c$, where $\rho$ is density and $c$ is the speed of sound.

What is the unit of acoustic impedance?

$\mathrm{kg\,m^{-2}\,s^{-1}}$, also called the rayl.

What happens to an incident sound wave at a boundary between tissues with different acoustic impedances?

Part is reflected and part is transmitted.

At normal incidence, what fraction of sound intensity is reflected at a boundary between two media?

$\frac{I_r}{I_i}=\left(\frac{Z_2-Z_1}{Z_2+Z_1}\right)^2$

$I_i$ and $I_r$ are incident and reflected intensities; $Z_1$ and $Z_2$ are the media’s impedances.

How does impedance mismatch affect the reflected intensity fraction at a boundary?

A greater mismatch gives a larger reflected fraction. Equal impedances give no reflection.

If boundary absorption is neglected, what is the transmitted intensity fraction in terms of the reflected and incident intensities?

$1-\frac{I_r}{I_i}$

How does a piezoelectric ultrasound transducer generate an ultrasound pulse?

A short electrical pulse applies a potential difference across the piezoelectric element, making it vibrate and produce an ultrasound pulse.

How does a piezoelectric ultrasound transducer detect a returning echo?

The echo compresses and stretches the element, producing an electrical signal that can be processed for echo information.