Transmitting Ultrasound Pulses

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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.

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Shape of the Ultrasound Pulse as a Function of Time[edit]

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.

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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.

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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™ Transducers.

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Temporal Shape of the Pulse on Sonix Platforms[edit]

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 (See here). dt which is the shortest unit of time used in the pulse is also adjustable, and determined by the transmit frequency. Needless to say, the transmit frequency has to be a multiple of the clock cycle of the system (20, 40, or 80 MHz). The very first system we shipped had a clock cycle of 20 MHz. Almost all of the operational Sonix systems in the field at this time (2013) have a clock cycle of 40 MHz. SonixDAQ's clock cycle can be chosen through software to be either 40 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.

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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:

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Also see:

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Shape of the Ultrasound Pulse as a Function of Space[edit]

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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.
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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.

Speaking in terms of optics the transducer has a beam width: It can illuminate objects which are right in front of it but cannot illuminate objects which are to the sides. This is a desirable feature, however: Imagine if the beam width could be decreased to a narrow beam, like a laser, the transducer would be able to image a single line of the tissue! An array of such transducers would be able to scan multiple lines, forming a cross-sectional image of the tissue. As one would expect, this ideal situation never happens. However through techniques collectively known as beam forming the spatial shape of the pulse is adjusted to make it as close as possible to such a narrow beam.

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One way to narrow the beam is through the use of physical lenses. Just as an optical lens can be used to narrow a diverging beam of light, an ultrasound lens, attached to the transducer surface can narrow the beam, and focus it at a certain point. This is called mechanical beam forming.

A second way to narrow the beam is through the use of multiple transducers, which transmit at different times. This technique is called electronic beam forming. The next figure shows the concept. Five transducer elements are placed side-by-side. Similar pulses are applied to the transducer elements, however the electrical pulse is applied to the outer elements before the inner elements. As a result, the spherical wave fronts will intersect at a focal point. The figure also shows schematically how the beam would look like in this situation. The main advantage of electronic beam forming over mechanical beam forming is that by changing the time delays, the focal point can be changed.

Ultrasound transducers are commonly arranged in a 1D array, the transducer array. Each of the transducers in the array is called an element. The next figure shows a typical arrangement in a linear probe. Typical dimensions of the elements and pitch are also given.

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With such an arrangement, it is possible to carry out electronic beam forming in the lateral direction, which is the direction in which the elements are arranged in the array. However the beam would still be divergent in the elevational direction. To overcome this problem, mechanical beam forming is used in the elevational direction. In other words, a fixed physical lens is present in ultrasound transducers which focuses the beam in the elevational direction at a fixed elevational focus depth. Electronic beam forming is used to focus the beam in the lateral direction.

Beam Steering[edit]

Another technique commonly used in ultrasound imaging to determine the spatial shape of the ultrasound pulses is beam steering. Through this technique, the angle of the beam with respect to the transducer can be changed. The technique is implemented in transducer arrays through electronic beam forming. The time delays of the electric pulses sent to the individual transducer elements are adjusted to steer the beam in one direction or the other. This is particularly useful for looking at anatomical structures from a different angle by illuminating them from the side.

BeamSteering.jpg

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Spatial Shape of the Pulse on Sonix Platforms[edit]

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Sonix platforms provide a number of different parameters which can be used for electronic beam-forming using transducer arrays. This is achieved through the application of identically-shaped temporal pulses to different transducer elements in an array, with differing time delays.

The elements to which the pulse will be applied can be selected via a mask. A center element needs to be specified which determines the position of the center of the beam with respect to the transducer array.

The relative delays for different elements can be determined either automatically, or (manually) by the researcher. In the case of automatic delays, a focal depth needs to be specified for the Sonix software. The Sonix software then calculates the delays that would focus the ultrasound beam at the specified depth.

In the case of manual time delays, the researcher can specify the time delays (needless to say, as a multiple of the clock cycle of the system).

Another adjustable parameter is the aperture, which determines how many elements will be involved in the formation of the ultrasound pulse. These elements will always be chosen evenly on both sides of the center element.

When automatic time delay calculation is used, the angle of the beam can also be changed via a parameter passed to the software. This parameter can be used to steer the ultrasound beam.

Also see:

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