Skip to 0 minutes and 0 secondsSPEAKER: For making an ultrasound image, a transducer has to be able to listen in a focused manner inside the tissue. But it should also be able to focus sound into the tissue. But how does it work. How can it focus. That will be the topic of this video.

Skip to 0 minutes and 22 secondsAn ultrasound transducer makes a thin slab of ultrasound. This slightly focused slab is made by a cylindrical lens. It is a fixed lens making a fixed focus.

Skip to 0 minutes and 35 secondsBut in the other direction, the beam is more stretched out. But how is the transducer able to distinguish the sound coming from point A from the sound coming from point B, for instance. Well this is the secret of the so-called dynamic focussing.

Skip to 0 minutes and 57 secondsTo understand this, let's zoom in on the piezo array. It is built up from tiny crystals, which have a size of a few millimetres in one direction, and the size of a human hair in the other direction. And the beautiful thing is that we can control each element separately. So with each element, we can emit ultrasound, and with each element we can detect ultrasound, independent from the other elements.

Skip to 1 minute and 27 secondsSo now suppose that we want to listen to the sound emitted from point A in the issue. We know where point A is, and we know the speed of sound. So we know the time a wave takes to travel from point A to each element in the array. And now for each element and for each signal, we create a delay that we can calculate. All the signals in the end arrive at the same time in the transducer cable, and we can add them up. And in this way, we've created a kind of focus. So in this manner, we've created a digital lens. And this also works the other way around. We can give an electrical pulse to each element.

Skip to 2 minutes and 14 secondsAnd in this case, the outer elements are fired, as we can say, earlier than the inner elements. And this creates a focused ultrasound beam, because now, in the end, all the ultrasound waves overlap in point A. This is a slightly more realistic simulation. Now we see some ripples and side lobes, but we have a focus.

Skip to 2 minutes and 42 secondsAnd here the elements are fired in such a way that we get a beam under an angle. And this angled beam is very important for the so-called compound imaging. This compound imaging will be the topic of a later video. focussing can be very important for giving images a better quality. Left, we have an image with a shallow focus. And on the right-hand side, we see an image with a deep focus. And although the difference is not tremendous, we see a difference in sharpness of these regions and these regions, with shallow focussing and with deep focussing. This way of focussing ultrasound is called dynamic focussing. And the transducer with which we can do this is called a phased array.

Skip to 3 minutes and 37 secondsAnd this manner gives you a flexibility for making various beam shapes, and for sending sound out in the direction that you want.

Skip to 3 minutes and 48 secondsDynamic focussing is a major technical feat, and it allows you to scan the tissue without moving the probe.

Focusing

We now know that ultrasound can be used to get lots of information from inside the human body. But we don’t yet know how to distinguish all the information that is coming back to us.

The secret to this is dynamic focusing. Focusing is the process of timing your piezo-electric elements such that the resulting wavefront adds up in intensity at some points. A specific depth can thus be in focus.

Using ultrasound in a focused manner, we can distinguish all the different points from which information is coming. Wiendelt Steenbergen explains with clear animations how this works, and how it is used to our advantage.

Eventually, the end-result of this feature can be observed in the following two images:

Shallow focus Shallow focus

Deep focus Deep focus

Notice the sharpness in the areas marked by the white rectangles. The focus triangle on the left in the red circle indicates the depth to which focus is set.

Share this video:

This video is from the free online course:

Ultrasound Imaging: What Is Inside?

University of Twente

Get a taste of this course

Find out what this course is like by previewing some of the course steps before you join: