Receiving Ultrasound Data

As an ultrasound pulse propagates in the tissue, it interacts with organs and cellular structures. As a result of this interaction, part of the ultrasound energy is reflected back towards the transducer. This is the so-called ultrasound echo. The transducer converts this mechanical energy into an electrical signal.

Shape of the Ultrasound Data as a Function of Time
The transmitted ultrasound pulse typically consists of a short duration (one or a few cycles) sinusoid. As soon as this pulse enters tissue, echos start getting back to the transducer. As the transmitted pulse travels deeper and deeper into tissue, weaker and weaker echos keep getting back to the transducer. Therefore, unlike the transmitted pulse, the echo signal is not of a short duration. As a matter of fact, the echo looks like a continuous sinusoids whose amplitude and phase are changing over time. Because of the similarity of the temporal shape of the echo signal to the temporal shape of Radio Frequency (RF) signals used in telecommunications, the echo signal is called the RF Signal and the received ultrasound data by the transducers, the RF data in the jargon of ultrasound imaging.

In addition to the attenuation of the ultrasound by tissue, which manifests itself as a decreasing amplitude in the RF data, a second major effect is present in the RF data. This is the downshifting of the carrier frequency.

Receive Beam Forming
As with the transmit of ultrasound pulses, a single element transducer usually has a wide field of view. That means when the ultrasound pulse bounces back from different reflectors that are located at the same distance from the transducer, all of the echos arrive at the transducer at the same time. Therefore it would be impossible to distinguish between these reflectors by using the RF data collected by the transducer.

It is ideal that the ultrasound transducer would only see the reflectors that are located on a narrow beam right in front of it. The RF data from such a transducer can be used to generate an image of the tissue features on a narrow line. An array of such transducers would generate a plane image, for instance. As with the transmit of ultrasound pulses, techniques that are used to narrow the field of view of the transducer are called beam forming, or receive beam forming in this case. The two common ways to do so are through physical lenses and electronic beam forming.

One way to narrow the field of view to a smaller area is through the use of physical lenses. Just as an optical lens can be used to focus the optics on a focal zone at a certain distance, an ultrasound lens, attached to the transducer surface can narrow the field of view and focus it on a certain zone. This is called mechanical focusing.

A second way to narrow the field of view is through the use of multiple transducers. This technique is called electronic beam forming. The next figure shows the concept. Five transducer elements are placed side-by-side. For simplicity, let us assume that there is no interaction between the emitted ultrasound pulse and the medium, except from a reflection off the single reflector shown in the figure. As can be seen in this figure, the reflected waves from this reflector arrive at the transducers at different times, because of the difference in the distance from the reflector to the individual transducers. The RF data collected by each transducer shows a pulse at a different time. These are called the pre-beam forming RF data. To focus the received data on the reflector, the pre-beam forming RF data are shifted in time so that the pulses are matched. Then the signals are added. The resulting signal is the beam formed RF data.

As one might notice, there is a caveat in this simplified scenario. we do not know where the reflector is (otherwise we wouldn't be imaging it), so we cannot possibly know how much to shift each of the RF data to get the pulses to match. The key here is to note that we can not possibly be focusing the ultrasound beam on the reflector. The ultrasound beam is out of our hands, and we do not know the location of the reflector. What we can do is to focus the pre-beam forming RF data on a specific point in space.

The pre-beam forming RF data which are also called Channel Data are acquired and stored in some memory by the ultrasound machine. The receive beam forming consists mostly of computational methods to process this data. We first choose a certain point in space (the focal point). Then we calculate the distances to the

Temporal Shape of the RF data on Sonix Platforms
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