When ultrasound scans of the abdomen or kidneys are mentioned, calcifications or stones (such as kidney stones and gallstones in the figure above) are often first associated, but stones of comparable size may have different degrees of sound and shadow. For example, the different composition of the stone, or the influence of the smoothness of the surface of the stone. For whether these physical properties fundamentally determine the size of the sound and shadow, for the time being, we will analyze the performance of the sound and shadow in the shape of the ultrasonic beam itself.
First of all, the sound and shadow is popularly speaking, the ultrasonic beam emitted is blocked in the position of the stone, resulting in no ultrasonic illumination behind the stone, and naturally the tissues in these positions cannot produce echoes, thus producing sound and shadow. We know that the beam of ultrasonic emission is the thinnest in the focal point of the emission, and the beam in the area outside the focus gradually widens and appears saddle-shaped. As is customary, we still use the analogy of ultrasound imaging with cameras. Just as the lens aperture value of a SLR camera is smaller (the actual aperture is larger), the better the resolution of the focus point position, and the more pronounced the foreground and background bokeh. When photographing the animals inside the iron cage with a camera, did you notice that the iron cage became a translucent mesh on the photo? The picture below is a pair of monkeys and mothers photographed by the author in a cage at the Bangkok Wildlife Park, and if you don’t look closely, you may overlook the faint grids. But when we focus on the iron cage, the black iron cage really blocks the back. Those who are interested can go home and try to experience this experiment in different focus positions, just like the author in the picture below shooting a girl’s beggar doll across a fork.
Let’s go back to ultrasound imaging, in order to quantitatively study this problem, we use ultrasonic body molds (KS107BG) that measure penetration and resolution to demonstrate the phenomenon of sound and shadow, the target of this body model is a thin line that is not transparent, which can well simulate the effect of sound shadow. To better demonstrate the effect of occlusion, we use a high-frequency probe with a center frequency of 8.5MHz, because the high-frequency probe can obtain a finer ultrasonic beam (so it is also easy to obtain high lateral resolution).
First of all, we adjust the emission focus to a depth of 1cm, we can see the target at the 1cm position is the clearest, and the slightly darkened area can be faintly seen behind the target of about 5mm, but the target below 1cm is dragged by a long black channel, which is the so-called sound and shadow. The area within 1cm is like the foreground in photography, with the focus depth at 1cm and the background area after 1cm. Obviously, the foreground target within 1cm is like the cage in the monkey photo just now, and when we focus to a depth of 1cm, the ultrasound seems to be able to bypass it and continue to transmit energy forward almost unaffected. However, the area below the focus cannot be blocked around the target, resulting in almost no ultrasonic energy patronage behind the target, so there is no echo. In order to better confirm our hypothesis, we simulated the ultrasonic beams focused at this time, and the wavefronts of the ultrasonic pulse waves at different moments are shown in the following figure.
Apparently, at a depth of 1 cm, the energy of the emission focal point is concentrated, resulting in a thin beam, and the width of the beam gradually widens as it moves away from the depth of focus. When the depth of the target is less than 1cm, the target obscures part of the energy, but the size of the target is relatively small, and the energy that is not blocked on the side will continue to soar towards the focal point, so the sound and shadow of these targets will be very weak, and the closer to the surface of the probe, the less obvious the sound and shadow will be. When the target position is just at the depth of focus, the ultrasonic beam itself is very thin, so the energy that the target can block is relatively large, resulting in very little energy being able to continue around the target, which also makes the area behind this depth produce a real dark area. It’s like you’re focusing on the cage, and the area behind the cage grid is completely blocked.
What happens when the target is behind the focal point (background area)? Some people will say that the sound beam is also very wide, and the target can only cover part of it, will it be the same as the foreground area, can the energy bypass the target to reduce the sound and shadow? The answer is obviously no, just like the targets in the left oblique row in the above figure are all after 1cm depth, and the sound and shadow generated are no less than the targets in the 1cm position. At this time, we carefully observe the shape of the ultrasonic beam, and the wavefront of the beam before and after the focus is not flat, but resembles an arc shape centered on the focus. The beam close to the surface of the probe is converged toward the focal point, while the wave array deeper than the focal point is spread outwards with the focal point. That is to say, when the target is in the foreground area when the sound wave that is not obscured by the target will continue to propagate in the direction of the focus, and the sound wave that is not obscured by the target in the background area will continue to propagate in the direction of deviating from the scanning line, we only receive the echo signal on the scanning line, so the energy that deviates from the scanning line cannot be received, so the sound and shadow are formed.
When we adjusted the launch focus to a depth of 1.5cm, the sound and shadow behind the target at a depth of 1cm were also significantly reduced, but the target after 1.5cm was still dragging a long black tail. Below is a beam plot of ultrasonic emissions, Let’s Try to analyze the phenomenon of sound and shadow in combination with the morphology of the beam.
When the depth of focus is further increased to 2cm, the sound and shadow behind the target within 2cm are significantly weakened. The figure below is the corresponding ultrasonic emission beam plot.
The image of the previous example is only the focus depth adjusted, and the conditions on the other interfaces remain unchanged, but when adjusting the focus depth, the background also implies a condition, that is, as the depth of the emission focus becomes deeper, the aperture of the emission will also increase (the front number in the title of the beam diagram is the focus depth, and the number behind is the number of array elements corresponding to the emission aperture), and by observing the beam width of the probe surface, we can also find the actual emission aperture change. In general, the aperture of the emission focus is proportional to the depth of focus, just like a zoom lens with a constant aperture.
So what is the effect on the sound and shadow when the same focus depth and aperture size are different? Taking the same 1.5cm depth focus as an example, by adjusting the internal parameters of the machine, the size of the emission aperture is doubled
We should have learned to analyze the phenomenon of target sound and shadow through beam mapping through the above example, so we can look directly at the beamogram for this example. As the aperture becomes smaller, the beam of focus depth is broadened, but the saddle bend becomes less. The warping of the same foreground and background beams becomes inconspicuous, and observing how well the beam’s wavefront curves, it can be seen that the ultrasonic energy is somewhat like a plane parallel to the surface of the probe propagating forward. Therefore, the evil consequence is that although the ultrasonic energy in the original foreground area is partially blocked by the target, it can still continue to propagate around the target towards the focus position, but when the small aperture is small, the width of the foreground beam is narrowed first, the proportion of energy that is blocked is increased, and the sound waves on the side do not converge towards the launch focus position, so although the ultrasonic energy that is not obscured continues to propagate forward, it has almost no contribution to the echo of the scan line position, which also leads to the reduction of the aperture. Even the sound and shadow of the target in the foreground area will become more and more obvious. Just like when we take a picture of a caged bird with a mobile phone across the cage, no matter how large the aperture of the mobile phone claims, it will leave a noticeable dark grid of the cage on the photo, because the actual aperture of the mobile phone camera is too small.
Earlier, we only did some experimental analysis on the position of the emission focus and the size of the emission aperture on the sound and shadow, combined with the actual ultrasonic scanning, for the scanning of small stones, in order to obtain better sound and shadow effects, it is generally impossible to change the size of the aperture, but it may be possible to consider the focus position as close as possible to the front of the stone. Or when the sound and shadow are not obvious, it is not necessarily because the stones are too small, or it may be because the focus is not in the right position. In addition, as mentioned at the beginning, there may be many influencing factors of sound and shadow strength, such as the most direct nature is the size of the stone, in addition, the fundamental sound and shadow is often much weaker than the harmonic sound and shadow ,and so on, so it cannot be generalized.
So choose ultrasound products, its imaging quality is the most important, good harmonic imaging will make your medical career to a higher level, welcome to consult with you about the ultrasound products you are interested in and other medical equipment.
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Post time: Sep-08-2022