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?
When evaluating for possible elevation in intracranial pressure, it has been shown that optic nerve sheath diameter (ONSD)Â measurements correlate with elevated intracranial pressures.(1,2) Â The optic nerve attaches to the globe posteriorly and is wrapped in a sheath that contains cerebral spinal fluid.Â The optic nerve sheath is contiguous with the dura mater and has a trabeculated arachnoid space through which cerebrospinal fluid slowly percolates.
ONSD Normal Ranges
< 5 mm
Children >1 yr
< 4.5 mm
Infants < 1 yr
The ONSD is measured 3 mm posterior to the globe for both eyes.Â A position of 3 mm behind the globe is recommended because the ultrasound contrast is greatest. Â It is best to average two measurements of each eye.Â An average ONSD greater than 5 mm is considered abnormal and elevated intracranial pressure should be suspected.
In severe cases of elevated ICP, one can see anÂ echolucentÂ circle within the optic nerve sheath separating the sheath from the nerve due to increased subarachnoid fluid surrounding the optic nerve. Â Ophthalmologists refer to this as theÂ crescent sign.
Â The Case
40 yo female patient presents with several months of frontal headache associated with photophobia and blurry vision. Â Symptoms have gotten much worse over the last few days and she has had difficulty reading and watching TV because of her visual symptoms. Â She denies fevers, chills, nausea, vomiting, or focal weakness. Â Pt is hypertensive 170/100. Â Her vital signs are otherwise normal.
This patient had enlarged ONSD (measurements were 6 mm bilaterally) as well as papilledema(arrow).
Lumbar puncture was performed. Â Opening pressure was 44. Â 30 cc’s of CSF was drained and the closing pressure was 11. Â The patient’s headache and visual symptoms improved . Â She was started on acetazolamide and admitted to the neurology service. Â MRI brain prior to lumbar puncture showed posterior scleral flattening bilaterally with protrusion of the optic nerve in the the globes bilaterally consistent with increased ICP.
This patient’s papilledema and increased ONSD correlated with a markedly increased opening pressure during lumbar puncture and suggests that ocular ultrasound may play a role in the ED management of patients with suspected pseudotumor cerebri.
Elevated intracranial pressure in the abscence of intracranial mass lesion.Â Most common in young, over weight women. If the diagnosis is missed, persistently elevated intracranial pressure can lead to optic atrophy and blindness.
Lumbar puncture to drain CSF to a normal opening pressure.
Medical:Â Diomox (acetazolamide), high dose steroids
The ability to diagnose papilledema using bedside sonography is useful to emergency physicians, as manyÂ non-ophthalmologistÂ clinicians do not feel confident in their ability to perform an accurate nondilated fundoscopic examination. (3) Â Ultrasound provides a useful alternative means of determining the presence or absence of papilledema in a patient in whom fundoscopy cannot be adequately performed.
 Geeraerts T, Launey Y, Martin L, et al. Ultrasonography of the optic nerve sheath may be useful for detecting raised intracranial pressure after severe brain injury. Intensive Care Med 2007;33(10):1704-11 [electronic publication 2007 Aug 1].Â PMID:Â 17668184
 Kimberly HH, Shah S, Marill K, Noble V. Correlation of optic nerve sheath diameter with direct measurement of intracranial pressure. Acad Emerg Med 2008;15(2):201-4.Â PMID:Â 18275454
 Wu EH, Fagan MJ, Reinert SE, Diaz JA. Self-confidence in and perceived utility of the physical examination: a comparison of medical students, residents, and faculty internists. J Gen Intern Med 2007;22 (12):1725-30 [electronic publication 2007 Oct 6]. Â PMID:Â 17922165
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.
This is a longitudinal view of trachea, the air-mucosa interface just beneath the tracheal wall. What are the structures “A” and “B”?
A â€“ Reverberation artifact from air-mucosal interface
B â€“ Mirror image of cricoid cartilage.
Reflection at the tissue interfacesÂ 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 (1st 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 1st image is away from the transducer (at around 1.15cm) and registers it. The skin-transducer interface itself also results in the 1st reflected beam being partially reflected back into the air-mucosa interface, which again gets reflected back to the transducer as a 2nd reflection. This 2nd 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 2nd image, the reverberation artifact, at twice the depth (around 2.3cm in this case). Lichtenstein called these artifacts â€œAâ€ 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 2nd reflection occurs at the cartilage-soft tissue interface.
Whatâ€™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:
They occur when there is a bright reflective surface
They are always deeper than the real image
They are always less distinct than the real image
Next time, look out forÂ the mirror in the tracheal wall.
Artifacts are ultrasound imagesÂ 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:Â e.g. “an aorta” isÂ “the aorta”Â 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:Â e.g. inÂ Â 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 allowsÂ Â 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 certainÂ assumptions that that ultrasound machine makes. When they are all met, you get a real image; whenÂ any assumptionÂ is not, well, you get an artifact. And thankfully, there are only four such assumptions. Here’s a quick review of themÂ as we begin this series of what’sÂ real and what’s not.
A pulse of ultrasound beam emitted by the transducer travels in a straight line, is reflected atÂ an interface, and travels back to the transducer (exactly along the path it was emitted, only in the reverse direction)
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)
The speed of ultrasound beam (emitted and/or reflected) is always and exactly 1540m/s
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 notÂ really there to find out what’s really going on.