Transmitting Ultrasound Pulses

Ultrasound pulses are mechanical waves which can propagate in tissue. In order to create these pulses, ultrasound transducers are used. An ultrasound transducer can typically convert mechanical energy into electrical energy and vice versa.

To image tissue with ultrasound waves, two main types of ultrasound waves are used:
 * pulse waves
 * continuous waves

An ultrasound pulse wave is generated by a applying a short duration electrical signal (a pulse) to an ultrasound transducer. The pulse can be a a signal like this:

u(t) = sin(2pi5MHzt)

No transducer can convert the electrical pulse into a perfectly matching pulse of the same shape. What determines the shape of the emitted pulse wave (as a function of time) from the transducer is not just the shape of the applied electrical pulse, but also the frequency response of the transducer. Applying an electrical pulse to a transducer is sometimes compared to banging a bell with a stick. The bell will keep ringing even after the stick is no longer there. The figure shows schematically the input electrical pulse and output mechanical pulse from a transducer.

Although to quantify the frequency response of a transducer, a graph of energy vs. frequency is needed, what is typically assumed in most ultrasound applications is that the ultrasound transducer is a bandpass filter. That means it will pass frequencies in a certain range, but block very low and very high frequencies. Therefore one only needs to know the lower and higher cut-off frequencies of the transducer.

Ultrasonix has a very straight forward convention for communicating the lower and higher cut-off frequencies of its transducers to its customers. It is coded right in the naming convention of our transducers. These are the two numbers separated by a dash. For example L14-5/38 means that the lower cutoff frequency of the transducer is at 5 MHz and the higher cutoff frequency is at 14 MHz.

Back to A Reference on Ultrasound Imaging