Mount Sinai Emergency Medicine Ultrasound

This right paracolic gutter image is taken from a patient with significant ascites. Notice how bright the bowel walls are (solid purple arrows). This is because the air in the bowel acts as a strong reflector, and because ascites (being fluid) only minimally attenuates the incident and reflected ultrasound beam. Thus, a stronger signal is transmitted thus and reflected back to the machine. What then are the undulating hypoechoic “bowel looking” structures just adjacent to it (dotted green arrows)?

The “structures” are simply side lobe artifacts. They are phantom, not real.

Here’s how it happens. The ultrasound beam (solid blue arrow) hits the bowel, is reflected back to the transducer, and registers the bowel’s location and contour (solid blue curve) accurately. This bowel image is real.

All ultrasound beams, however, give off additional unwanted side beams (dotted red arrow). These extraneous beams are called side lobes. Being true sound waves (though smaller intensity than the “main” ultrasound beam), they can be reflected by true structures (solid blue curve) lying to the side of the main beam vector. Since the ultrasound machine assumes ALL reflections of an ultrasound beam arise only from the axis of the beam, reflections from side lobes are depicted as if they arise from the main beam, thus generating the phantom images (dotted red curve).

Side lobes artifacts are ubiquitous. The typical example is a full urinary bladder filled with “sediment” — which can be simply side lobe artifacts from adjacent hyperechoic bowel. They can be given out as much as 45 degrees from the main beam and are found to a larger or smaller extent in all transducers. As you have seen, these artifacts degrade lateral resolution of the image. Although it is often difficult to eliminate side lobe artifacts, examining the same area from different angles will often improve the overall image acquisition.

Remember, side lobe artifacts occurs if the adjacent structures are hyperechoic. If there is doubt about whether it is side lobe artifact OR debris (e.g. sludge in gall bladder, sediments in bladder), turn the patient to another side. “Real” sludge or sediment will flow to the dependent position; slide lobe artifacts don’t.

This is a longitudinal view of trachea, the air-mucosa interface just beneath the tracheal wall. What are the structures “A” and “B”?

Answers:

• A – Reverberation artifact from air-mucosal interface
• B – Mirror image of cricoid cartilage.

Reflection at the tissue interfaces occur when there is a difference in acoustic impedance between two tissues. The greater the difference, the stronger the reflection, the brighter the image.

A . A transmitted ultrasound beam hits the air-mucosa interface and is reflected back to the transducer (1^st reflection). Based on the time taken for the reflected beam to return (assuming a constant speed of 1540 m/s), the machine calculated the distance this 1^st image is away from the transducer (at around 1.15cm) and registers it. The skin-transducer interface itself also results in the 1^st reflected beam being partially reflected back into the air-mucosa interface, which again gets reflected back to the transducer as a 2^nd reflection. This 2^nd reflection takes twice the time compared to the first; therefore the machine (assuming all beams travel only once to and from an object) registers a 2^nd image, the reverberation artifact, at twice the depth (around 2.3cm in this case). Lichtenstein called these artifacts “A” lines when they arise from the pleura.

B. A similar explanation accounts for the mirror image of the cricoid cartilage below the air-mucosa interface, only that the 2^nd reflection occurs at the cartilage-soft tissue interface.

What’s the difference between the two? The reproduction of tissue interfaces is called reverberation artifact; whereas the reproduction of objects is termed mirror image. Both artifacts follow the same principles:

1. They occur when there is a bright reflective surface
2. They are always deeper than the real image
3. They are always less distinct than the real image

Next time, look out for the mirror in the tracheal wall.

Artifacts are ultrasound images on the screen that do not correspond exactly what is in the body. Artifacts can be useful in determining true anatomy:

1. The presence of some artifacts can help us to identify anatomy:  e.g. “an aorta” is  “the aorta” because it’s resting on the spine, which is “the spine” because it casts a shadow (what if the spine does not cast a shadow….?)

2. The absence of artifacts can also reveal pathology:  e.g. in  FAST with right hemothorax, loss of the mirror image of the liver above the diaphragm not only reveals the blood and superior aspect of the diaphragm, it also allows  the vertebral column (above the diaphragm) to show up! The spine above the diaphragm is never seen because the normal aerated lung scatters all of the ultrasound energy above the diaphragm.

3. Both the real image and artifact arise because of certain assumptions that that ultrasound machine makes. When they are all met, you get a real image; when any assumption is not, well, you get an artifact. And thankfully, there are only four such assumptions. Here’s a quick review of them as we begin this series of what’s real and what’s not.

Assumption ONE:

A pulse of ultrasound beam emitted by the transducer travels in a straight line, is reflected at an interface, and travels back to the transducer (exactly along the path it was emitted, only in the reverse direction)

Assumption TWO:

All the returning echoes of the beam are presumed to have arisen only from the center (i.e. axis) of the beam and hence are displayed as such (i.e. along a vertical line on the screen that represents the axis)

Assumption THREE:

The speed of ultrasound beam (emitted and/or reflected) is always and exactly 1540m/s

Assumption FOUR:

The intensity of the displayed echo is dependent on the acoustic properties and size of the interface where it is being reflected

And with that, we’ll make good use of what’s not really there to find out what’s really going on.