Monthly Archives: July 2019

Renal milk of calcium cysts

Renal milk of calcium refers to the viscous colloidal suspension of calcium salts found either within a calyceal diverticulum or within a simple renal cyst. Unlike stones, milk of calcium is typically asymptomatic and does not require intervention. In fact, shock wave lithotripsy is ineffective for the treatment.

The etiology of milk of calcium is unclear; however, it may be related to urine stagnation (obstruction) and infection. As a colloid suspension, the calcium salts gravitate to the most dependent portion of the cavity. This layering (gravitational) effect leads to the radiological finding of crescent-shaped dense lesion with a fluid level at the upper border of the stone and may change positions with patients’ position. This finding is reported in KUB, ultrasound and more evident on CT scan. Acoustic shadowing is usually absent but may be seen with larger depositions. Reverberation artefact is seen sometimes.

This entity can be seen in the gall bladder also, and may be associated with chronic cholecystitis. However, ultrasound is non-specific for differentiating gall bladder sludge from milk of calcium, and CT or plain radiographs usually suggest the diagnosis.

Hypertrophic column of Bertin

The renal column of Bertin is a medullary extension of the renal cortex in between the renal pyramids. Hypertrophic column of Bertin (HCB) is a normal variant that appears as a mass that extends towards renal sinus.

How to differentiate it from a real renal mass?

HCBRenal tumor
1. Usually isoechoic with rest of the cortex Can be isoechoic or heterogeneous but usually demarcates itself from the renal parenchyma. May contain calcifications or necrotic areas.
2. Protrudes inwards into the sinus fat and the renal outline is preserved Grows outwards and usually distorts the renal outline
3. On Doppler, the vascular pattern is similar to that of the surrounding renal parenchyma. Use power Doppler for better identification of flow. The vasculature is prominent in the periphery of the mass
4. May wrap around the medullary pyramid without distorting its architecture Distinct from pyramid, may compress

Some HCBs may demonstrate atypical characteristics on greyscale ultrasound and may need contrast-enhanced CT/MRI/ultrasound to make sure it is similar to adjacent cortical tissue.

Post-biopsy bleeding

Compared with biopsy of other sites, native renal biopsy has the greatest risk of post-procedure hemorrhage (1.2%). However, the incidence of major bleeding and death due to bleeding is very low (0.3 and 0.02% respectively).

Post-biopsy bleeding can occur into the perinephric space or the collecting system or in the form of a subcapsular hematoma.

On a sonogram, fresh hematomas appear anechoic and organizing ones are of mixed echogenicity due to clotting. There will be little to no Doppler flow because the blood is not flowing.

Note that urinoma can appear similar to fresh hematoma (urine is anechoic as well) and should be included in the differentials depending on the context (e.g. recent kidney transplant).

Patent track sign: a color Doppler signal along the course of a biopsy, indicating that the needle track has not collapsed and that there is still active bleeding from the procedure. The track usually extends out of the biopsied organ into a surrounding hematoma. Case courtesy of Dr Matt A. Morgan,, rID: 50201

The jellyfish sign: atelectasis

In moderate to large pleural effusions, it’s not uncommon to see lung atelectasis on the ultrasound. This collapsed lung within the effusion moves with respiration and appears like a jellyfish. Also known as the ‘whale tail’ sign.

Important things to note:

Jellyfish sign favors atelectasis over pneumonia as the collapsed lung can change its shape with respiration whereas pneumonia is more firm.

It also favors transudative etiology of the effusion as the viscosity of exudative effusions tend to hamper mobility of the lung tissue.

Here are more examples

Ultrasound signs of urolithiasis

Although sonography is less sensitive than CT for detecting kidney stones, it is the preferred initial imaging modality as there is no risk of radiation, is reproducible, inexpensive, and the outcome is not significantly different for patients with suspected urolithiasis undergoing initial ultrasound exam compared to those undergoing CT scan.

On gray-scale images, stones appear as hyperechoic or bright structures with a posterior “acoustic shadow”. Acoustic shadowing is the black area or signal void seen beyond structures that do not transmit ultrasound waves.

In the Doppler mode, stones exhibit “twinkling sign” or artifact, which refers to a rapidly alternating focus of color Doppler signals mimicking turbulent flow and is more pronounced with rougher stones. It is of note that this sign is more sensitive than shadowing for detection of small stones, which I found to be very helpful in my practice.

Pneumonia versus atelectasis: the differentiation can be difficult

As mentioned before, hepatization of the lung i.e., lung looking like liver tissue can occur in both pneumonia and atelectasis. Dynamic air bronchograms, when present, point toward pneumonia but static air bronchograms can be seen in both conditions. The differentiation can be very difficult at times and the management should be guided by the clinical picture.

Dynamic air bronchograms are not always as obvious as previously demonstrated. The probe angle needs to be adjusted as necessary to differentiate out of plane lung motion from moving air bronchograms. Other things to note:

  • More pleural effusion and less consolidated tissue suggest collapsed lung i.e., relaxation or passive atelectasis.
  • More consolidated tissue with less effusion, especially when the patient has fever or signs suggestive of infection points toward pneumonia even in the absence of dynamic air bronchograms.
  • Fibrin strands or loculated pleural effusion suggest infectious etiology.
  • Increased color flow in the consolidated tissue favors pneumonia while little to no flow suggests atelectasis.
Dynamic air bronchograms better seen in some planes. Note the Doppler flow in this case of pneumonia. Image courtesy: Dr. Lars Mølgaard Saxhaug

Here is another example of pneumonia with dynamic air bronchograms that require careful observation. Color flow is overall increased though there is some interference from tissue motion while the patient is breathing rapidly.

Pleural effusion and consolidation with fibrin stranding suggestive of exudative etiology (e.g. infection, hemorrhage). Image courtesy: Dr. Rohit Patel

Also note that small subpleural consolidations do not show typical dynamic air bronchogram pattern. For example, this lung ultrasound image obtained with the straight linear array probe (5–13 MHz) over right anterior chest demonstrates lung sliding and a small subpleural hypoechoic area with ragged margin separating it from the surrounding normal lung. This is described as the “shred sign” because of its distinctive irregular boundary with the normal lung.

Do kidneys sweat?

An extracapsular, hypoechoic or anechoic rim of simple-appearing fluid surrounding the kidneys, first described on ultrasonography in some patients with renal failure (serum creatinine >2 mg/dL), was termed kidney sweat. The rim is thought to represent perirenal edema, and the differential includes more significant perirenal fluid collections, such as hematoma or abscess. It can also be identified on CT and MR scans, with typical imaging features of simple fluid. The amount of fluid varies and when a large volume of fluid surrounds the kidney, the term ‘floating kidney’ has been applied. The pathogenesis and prognostic significance of this finding remain unclear. It can be seen both in chronic kidney disease and acute kidney injury.