Ultrasound is longitudinal sound waves of frequencies > 20 kHz (beyond human hearing). The scans are harmless and non-invasive.They are used to create 2D images using the refraction (between substances), reflection (at the boundaries of substances), and diffraction (between small apertures of around 2mm) of the ultrasound waves.

Piezoelectric effect

Some crystals such as Quartz produce an e.m.f when compressed, stretched, twisted or distorted. This effect is reversible, so if a p.d is applied across the opposite ends of a crystal the electric field can stretch or compress the crystal (the strain won’t be greater than 0.1%).

Ultrasound Transducer

This device emits a pulses of ultrasound (~5000/s) of frequency 5MHz using the piezoelectric effect of a crystal. The frequency of the oscillations are set at the natural frequency to maximize their intensity.
The transducer also receives the ultrasound waves converting them using the piezoelectric effect in to e.m.f’s and then into images.
Amplitude (A) scans use a single transducer to record a straight line through a patient, it can be used to determine the thickness of bone or the distance between the lens and the retina in an eye. The scan works by waves being partly transmitted and reflected to produce ‘echo’s’. Voltage can be plotted against time using an oscilloscope, if the velocity is known length can be calculated.
This scan is the most common type of ultrasound scan. It produces a 2D image as the transducer is moved over the skin, it is connected to a high speed computer. For each of the positions of transducer a row of dots is produced corresponding to a boundary between tissues, the brightness of the dot is proportional to the intensity of the reflected ultrasound. The dots from the transducer positions makes a 2D image.