Ultrasound Imaging

An alternating potential difference makes a piezoelectric crystal change dimensions and emit ultrasound. Returning sound deforms the crystal and generates a potential difference. The transducer alternates between transmitting short pulses and receiving echoes.

The time between transmission and an echo includes the outward and return journeys. Estimate boundary depth using half the total distance. An A-scan plots echo amplitude against time or depth along one line. A B-scan uses echo strength to control pixel brightness along many scan lines, building a two-dimensional section.

A change in acoustic impedance produces reflection. A large mismatch can return most of the incident intensity, leaving little to image beyond the boundary. Coupling gel removes the air gap between skin and transducer because an air–tissue boundary would reflect very strongly.

The standard intensity reflection formula below assumes normal incidence. Some transmitted sound is also attenuated by absorption and scattering as it travels. Deep echoes can therefore be weaker because of travel distance as well as the boundary’s reflectivity.

Higher frequencies have shorter wavelengths and can resolve smaller structures, but are attenuated more strongly in tissue, reducing useful penetration. Short pulses improve separation of echoes from neighbouring depths; a long ringing pulse blurs them together.

Ultrasound uses non-ionising radiation and provides convenient real-time images, for example of a developing fetus. It performs poorly through gas and bone and depends on access and operator technique. Compare it with CT or MRI through the task, resolution and penetration required, rather than claiming one method is best in every situation.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.