Tag Archives: Kidney

Renal angiomyolipoma

A classic angiomyolipoma (AML) has abundant fat and therefore appears hyperechoic on a sonogram, similar to the sinus fat. Acoustic shadowing may be seen in up to a 3rd of the lesions. Unlike stones, these lesions do not typically exhibit twinkle artefact on color Doppler. Definitive diagnosis of AML is possible only by CT or MRI because they ‘diagnose’ fat and ultrasound can only ‘suggest’ fat. On non-contrast CT scan, the presence of regions of interest containing attenuations less than -10 HU allows confident identification of fat. Following are some examples of classic AMLs.

In patients with tuberous sclerosis, multiple renal AMLs are seen and often difficult to characterize on ultrasound. Corticomedullary differentiation is typically lost. Management should be guided by CT or MRI in such cases. Following is an example.

Small, well circumscribed bright lesions can be followed by ultrasound alone while any suspicion for malignancy warrants CT or MRI. The following lesion looks hyperechoic and most likely is AML but I would get a CT if possible because it’s more hypoechoic compared to sinus fat.

Renal cell carcinomas are hypoechoic to heterogeneous. They can be isoechoic also, which can be easily missed on quick scans. Therefore, it is important to image kidneys from multiple scan planes. Hardly takes an additional minute of your time! In addition, if you see anechoic rim surrounding the lesion or intralesional cysts or calcifications, think renal cell carcinoma. Following are some examples of RCC.

“Fat-poor AMLs” can be of heterogeneous echotexture or just isoechoic to renal parenchyma. It is challenging to differentiate them from malignancy and therefore, almost always require further imaging.

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.

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 the same (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, Radiopaedia.org, rID: 50201

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 refer 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.

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.