Tag Archives: Kidney

Maternal Physiologic hydronephrosis in Pregnancy

Pregnancy-related hydronephrosis, more precisely physiologic maternal renal pelvis dilatation is common and the incidence is estimated to be as high as 80%. The dilatation of the pelvis and ureter typically develops toward the end of second trimester and disappears within a few weeks after delivery. It is more common in primigravid women and usually more prominent on the right side. Etiology includes extrinsic ureteral compression by the gravid uterus on top of reduced ureteral smooth muscle tone and peristalsis due to progesterone effect.

Right sided predominance may be due to dextrorotation of the uterus by the sigmoid colon, kinking of the ureter as it crosses the right iliac artery, and/or proximity to the right ovarian vein. Following figure gives an idea of the anatomy.

In these cases, it is often difficult to trace the ureter to the point of obstruction. However, in pathologic hydronephrosis, one may visualize the source of obstruction (e.g. stone). In addition, if the pelvic diameter exceeds 10 mm, pathologic dilatation should be suspected.

Here is a case of mild bilateral hydronephrosis in a primigravid pregnant woman during third trimester. Note weak left ureteral jet compared to normal. Color Doppler could not pick up the right jet despite maintaining the probe position for approximately 2 minutes consistent with obstruction.

Renal abscess

A middle-aged man with diabetes mellitus type 2 presented with fever, chills and malaise. Urinalysis revealed pyuria and was positive for nitrite. Renal sonogram demonstrated a heterogeneous mass-like structure in the mid right kidney extending over the lower pole.

CT scan and MRI with contrast were suggestive of renal abscess. A drainage catheter was placed and patient started on intravenous antibiotics. Fluid culture grew Klebsiella pneumoniae and the therapy was tailored accordingly. 1-week later, a repeat sonogram demonstrated near-complete resolution of the collection.

On a sonogram, renal abscess appears as a well-defined hypoechoic area often with thick walls and internal echoes. Can be associated with surrounding diffusely hypoechoic, enlarged kidney due to pyelonephritis or hydronephrosis interspersed with echogenicities if it ruptures into the collecting system = pyonephrosis. It may appear similar to an organizing hematoma or renal cell carcinoma as in the above case where history becomes crucial and further imaging should be obtained. Small abscesses can be totally missed on ultrasound. Following are two more examples.

For renal abscesses <5 cm in diameter, antibiotic therapy alone without drainage is appropriate initial management. These lesions usually respond well to prolonged antibiotic treatment, and moreover, radiographic localization for drainage can be difficult. If clinical symptoms and radiographic findings persist after several days of antimicrobial therapy, drainage should be considered.

Renal abscesses >5 cm should be managed with percutaneous drainage in addition to antimicrobial therapy.

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 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.