So we are scanning the left thorax in a patient with shortness of breath, in an effort to assess for pleural effusion. The following video was obtained:
The operator correctly noted the presence of a pleural effusion, and a bit of lung tissue can be seen towards the left side of the screen floating in fluid. In addition, there are THREE shadows evident, each from a different source. Can you spot them?
So let’s take these one at a time, with labels:
Is the easiest one. It extends almost from the first pixel at the top of the screen down to the far field. We can’t even see the characteristic echotexture of skin or subcutaneous tissue in the near field. There’s no contact here between the transducer and skin, possibly due to:
the probe not touching at all
clothing or an EKG lead getting in the way
not enough gel (the novice’s answer to everything but sometimes still true)
The most interesting one of the bunch. Probably two major factors at work here. First, this section of diaphragm is a particularly bright reflector so it can create a shadow behind it due to the sheer amount of reflection occurring. Second, the density difference between the diaphragm and pleural effusion is creating a refraction artifact, often referred to as an edge artifact. Beams of sound which were roughly parallel as they struck this interface get bent at different angles based on whether they hit the dense diaphragm or the less dense fluid. The space in between the formerly tightly spaced beams is displayed as blackness, or the absence of returning echoes.
That’s a rib shadow. Did you know that ribs grow back if you remove them?
Ultrasound is quite sensitive in detecting even very small pleural effusions; it has been demonstrated to perform better than chest x-ray and nearly as well as CT scan. In order to assess for pleural fluid, the transducer should be directed through the liver (Right side) or spleen (Left side) and diaphragm. In a normal thorax, a mirror image artifact will generally be seen above the diaphragm. When effusion is present, fluid eradicates this artifact, creating an anechoic appearance in the costophrenic angle.
The image above demonstrates a common pitfall inÂ abdominalÂ and thoracic ultrasound. The liver is visible in the near field, and a dark anechoic structure is evident just deep to the liver. Some see this fluid and may note a positive FAST examination or free intraperitoneal fluid.Â Others mayÂ see this appearance and diagnoseÂ pleural effusion or hemothorax. While it is true the anechoic area represents fluid, there is a more correct response.
The inferior vena cava can generally be seen posterior to the liver, towards the patient midline. As it is filled with blood it will appear anechoic. below the diaphragm it will course parallel and to the [patient’s] right of the Aorta. Just above the diaphragm itÂ willÂ quickly merge into the Right Atrium.
As with most scanning, fanning through multiple planes will generally sort out the true anatomy. In the clip below we see the IVC as the operator sweeps medially, and the the pleural effusion is more evident in the lateral portions of the sweep.Â One (of many) giveaways is that the hepatic veins drain into the IVC, and even in this brief sweep through the IVC a hepatic vein is visible anteriorly, draining into the IVC.
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.
The â€œlung pulseâ€ is an ultrasound sign first described by Dr Daniel Lichtenstein in 2003. Essentially, it is the detection of the subtle cardiac pulsation at the periphery of the lung (parietal pleura to be exact) on the M mode.
In a normally ventilating lung, this subtle transmission is present but NOT seen, as it is masked or â€œerasedâ€ by the air artifact generated by lung sliding. In a non-ventilated lung, however, lung sliding is abolished. Here, the pleura is perfectly still and this allows the underlying cardiac pulsation to be detected.
Since the heart is on the left side, lung pulse is more prominent on the left side than right (both images taken from a healthy volunteer while holding his breath).
Thus, for the lung pulse to be seen on M mode, the following two conditionsÂ must be present:
1. Absent ventilation (i.e. no lung sliding)
2. Apposition of visceral pleural and parietal pleural (i.e. no pneumothorax)
The clinical utility of the presence of a lung pulse is:
Diagnosis of non-ventilated lung (sensitivity of 93% and specificity of 100% in patients without previous respiratory disorders)