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.

Shape of the Ultrasound Pulse as a Function of Time
To image tissue with ultrasound waves, two main types of ultrasound waves are used:
 * pulse waves Pulse.jpg
 * continuous waves CW.jpg

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. See Naming Convention of Ultrasonix&.

Temporal Shape of the Pulse on Sonix Platforms
Sonix Platforms provide a number of adjustable parameters for adjusting and designing the temporal shape of the ultrasound pulse. The Sonix platforms do not include digital to analog convertors (D2A's) on board and it is not possible to apply arbitrary shaped electrical signals to the transducers. The shape of the electrical pulse that is applied to the transducers is always a square wave shape. However, the shape of the square-wave pulse can be adjusted flexibly through a number of different parameters. The limited bandwidth of the transducer causes the emitted mechanical pulse to be of approximately a sinusoidal shape.

In the following figure, the a+ and a- are the positive and negative amplitudes of the signal, which are adjustable. Also dt which is the shortest unit of time used in the pulse is also adjustable. Needless to say, this has to be a multiple of the clock cycle of the system (40, 60 or 80 MHz). The shape of the pulse itself can be programmed as a pattern consisting of the three characters +, 0, and - as shown in the bottom of the figure.



Sonix systems provide yet another feature for designing longer pulses: A certain pulse pattern, for instance "++-0-" can be repeated an arbitrary number of times to design a longer pulse. For example if the above pulse is repeated, the following pulse is obtained:



Also see: Transmit Control, Texo Parameter Effects, Texo Parameters

Shape of the Ultrasound Pulse as a Function of Space
When an ideal point source of waves emits ultrasound waves in a medium, the energy will propagate on the surface of an expanding sphere. Moreover, the amplitude of the pulse will be the same on all points on this sphere: Ultrasound transducers are not ideal point sources. They come in different shapes and sizes such as rectangular and cylindrical. When a single ultrasound transducer emits mechanical energy in the form of the ultrasound wave, the energy still propagates on the surface of an expanding surface, which is very close in shape to a sphere, specially if the transducer is small.

However, the amplitude of the pulse is not the same in different directions. In other words, there is a directionality pattern associated with the transducer:

Spatial Shape of the Pulse on Sonix Platforms
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