Many people looking for an ultrasound scanner come to us with a very precise description of the frequency they need. For example, "I need a probe which is 10Mhz for ocular scanning." When we reply that we can offer them an 8 - 12 MHz probe, for example, there seems to be some confusion as to what this means. Similarly, when adjusting the frequency on scanners anywhere from the small practice SonoScape A5v right up to the echocardiographer's SonoScape S9, it can feel frustrating that the scanner is reading 3 - 5 MHz. Why can't you set it at 3MHz precisely? In the following article, we address the question of central frequency - what is it, and why can't you specify a single frequency?

Ultrasound works on the pulse-echo principle. You may recall the following diagram from your secondary school Physics or Maths days: boat on the water, sea bed below, a line to represent the signal leaving the boat hitting the sea bed, before being reflected back up to the boat. The purpose of the exercise is to find the depth of the water, given that you know the speed of sound in water and can easily measure the time it takes for the echo to return to you.

An ultrasound probe works in a similar way. It emits a pulse, and then receives the corresponding echo back. The ultrasound machine knows the speed of sound in tissue, so by measuring the time taken between emitting a signal and receiving echoes back, it can build up a 2D picture of where the reflections are in the body in relation to one another, displaying a representation on the ultrasound monitor.

For this concept to work and for interfaces to be resolved separately, the ultrasound probe has to be able to produce distinct echoes in short pulses. If the signal were to be continuous (i.e. a pure sine wave), a graph of wave amplitude against frequency would show a specific value at that one frequency, and zero at all other frequencies. You would indeed have your single frequency probe. It would not, however, be suitable for most scanning applications.

A pulsed wave, on the other hand, is made up of a range of frequencies. This is necessary to produce a pulse, which is created due to the interaction between different frequencies. It is for this reason that your probe comes with a stated frequency range, centred around one central frequency, and not one fixed point frequency. Without it, your transducer could not emit a pulse and generate the high resolution 2D image required.