Monthly Archives: June 2019

Performance of lung ultrasound: Let’s talk numbers

The diagnostic accuracy of lung ultrasound for common pathologies such as pleural effusion, pulmonary edema, pneumothorax, and pneumonia is superior to chest radiograph and is comparable to chest CT scan. In addition, POCUS is helpful to evaluate the progress of lung pathology and response to treatment over time.

In ESRD patients on dialysis, lung ultrasound greatly outperforms the physical exam findings that we commonly rely on to ensure adequate ultrafiltration. It’s time to upgrade our physical exam!

Dromedary hump

Dromedary hump is a prominent focal bulge on the lateral border of the left kidney caused by splenic impression, which can mimic renal neoplasm. It is similar in appearance to the hump of a dromedary camel and thus the name. It is a benign anatomic variant and exhibits the same imaging characteristics as adjacent renal cortex with normal blood flow pattern on Doppler sonography. On the other hand, malignant lesions are usually heterogenous in echogenicity (though can be perfectly isoechoic to cortex sometimes) and the blood flow tends to be prominent in the periphery of the lesion.

Following Doppler loop shows that a medullary pyramid is extending into the hump with normal blood supply around it. If it was a tumor, the pyramid wouldn’t extend into the mass and the blood flow, if you see would be ‘around the mass’ and not ‘around the pyramid’.

Urinary bladder wall thickness: what is the number to remember?

It is not uncommon to see a thickened urinary bladder wall in cases of chronic bladder outlet obstruction. Have you ever wondered how thick is thick? In a study, a bladder wall thickness of 5 mm appeared to be the best cutoff point to diagnose bladder outlet obstruction. Note that we should not comment on the wall thickness or other characteristics when the bladder is empty. In this study for example, the bladder was filled to 150 ml before performing the scan. Prior to that, uroflowmetry and pressure-flow studies were performed.

Thickened gall bladder wall: not always acute cholecystitis

In clinical practice, a cut off of 3mm is commonly used to define the upper limit of normal gall bladder wall thickness and acute cholecystitis is one thing that comes to our mind when we see increased wall thickness. Along with a compatible clinical picture, ultrasonographic features that suggest acute cholecystitis are a distended gallbladder, thickened walls, biliary sludge and lithiasis, pericholecystic fluid and the sonographic Murphy’s sign, which is defined as maximal abdominal tenderness from pressure of the ultrasound probe over the visualized gallbladder.

However, this is not the only condition that causes gall bladder wall thickening. As POCUS is not necessarily performed in a fasting patient, we must be aware that the gall bladder wall is thick in non-fasting state. In addition, any systemic disease associated with generalized hypervolemia such as congestive heart failure can lead to gall bladder wall thickening. In these patients, think of alternate etiology if the clinical picture does not fit. The presence of gall stones also does not mean much if there are no symptoms. Following image shows thickened gall bladder wall and ascites in a patient with congestive heart failure exacerbation. Note that the liver floating in ascites appears like a fish with gall bladder as its mouth. Though the wall diameter is measured in transverse plane in this example, it is generally recommended to be measured in long axis, and of the anterior wall (the one on the top).

Renal cyst: simple or complex?

The diagnosis of a simple benign renal cyst on ultrasound requires the presence of all the following findings: a well-defined, roundish, anechoic structure, imperceptible near wall and thin echogenic far wall, and increased through transmission manifested by acoustic enhancement. Acoustic enhancement refers to the hyperechoic or bright area relative to surrounding tissues, distal to structures that are excellent transmitters of sound waves. This artefact is not restricted to cysts and can be seen with any fluid containing space such as a blood vessel or urinary bladder. Any lesion that does not meet criteria for a simple cyst is considered a complex cyst and may be characterized by findings such as irregular thickened walls, septations, internal echoes, and calcifications.

What is the utility of sonography in Bosniak classification of cysts?

The Bosniak classification system helps in the diagnosis and management of renal cysts. As the detection of neovascularization in malignant lesions, indicated by contrast enhancement (on CT) of solid components, septa or walls, is an essential part of the classification, ultrasound cannot be used instead of CT. However, it is known that ultrasound may demonstrate internal septa better than CT and MRI. Therefore, it has been suggested that simple and minimally complex (Bosniak I and II) cysts may be followed with sonography alone.

Bosniak classification illustration: Case courtesy of Dr Matt Skalski, Radiopaedia.org, rID: 20989

Mirror image artefact

This is one of the commonest artefacts encountered in NephroPOCUS. It is generated by the false assumption that an echo returns to the transducer after a single reflection. In this scenario, the primary beam encounters a highly reflective interface, the reflected echoes then encounter the “back side” of the structure and are reflected back toward the reflective interface before being reflected to the transducer for detection. The display shows a duplicated structure equidistant from but deep to the strongly reflective interface. Remember, time = distance in the ultrasound world, which means if the echoes take longer time to reach back to the probe, that structure is displayed farther from the top of the screen and vice versa. In other words, structures closer to the probe are displayed at the top of the image.

In this example, the diaphragm sitting next to the air-filled lung that is reflective acts as a mirror, and the mirror image artefact appears as hepatic parenchyma (or spleen on the left) in the expected location of lung. However, when there is pleural effusion, this artefact disappears (fluid is an excellent transmitter of ultrasound waves unlike air) and you’ll see an anechoic area above the diaphragm representing the effusion. In these cases, identification of thoracic spine sign confirms the diagnosis of pleural effusion.

You can watch my video 10 ~7:58 for more examples and better understanding of the mechanism of this artefact.