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 andÂ reflected ultrasound beam. Thus, a stronger signal isÂ transmitted thusÂ and reflectedÂ back to the machine. What then are theÂ undulating hypoechoic “bowel looking”Â 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)Â hits the bowel, is reflected back to the transducer, and registers the bowel’s location and contour (solid blue curve)Â accurately. This bowel image is real.
AllÂ ultrasound beams, however, give off additional unwanted side beams (dotted red arrow). These extraneous beams are called side lobes. Being trueÂ sound waves (though smaller intensity than the “main” ultrasound beam), theyÂ can be reflected by true structures (solid blue curve) lying to the side of the main beam vector. Since the ultrasound machine assumes ALL reflectionsÂ of an ultrasound beamÂ arise only from theÂ axis of the beam,Â 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 aÂ full urinary bladder filled with “sediment” — which can beÂ simply side lobe artifacts from adjacentÂ hyperechoic bowel. TheyÂ can be given out as much as 45 degrees from the main beamÂ andÂ 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.