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, Radiopaedia.org, rID: 50201

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.

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.

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’.

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.

Nephrocalcinosis: by default, we apply this term to ‘medullary’ calcification (= medullary nephrocalcinosis), though it can occur in renal cortex (e.g. in renal cortical necrosis of pregnancy or chronic active glomerulonephritis). Nephrocalcinosis is associated with conditions that cause hypercalcemia, hyperphosphatemia, and increased excretion of calcium, phosphate, and/or oxalate in the urine. Hypocitraturia also may contribute, especially in those with distal (type 1) renal tubular acidosis. Citrate normally inhibits crystal formation by forming a complex with calcium.

Important conditions that cause Medullary nephrocalcinosis: With hypercalcemia + hypercalciuria: primary hyperparathyroidism, sarcoidosis, too much vitamin D. Conditions with hypercalciuria but no hypercalcemia: Distal renal tubular acidosis, medullary sponge kidney, too much furosemide.

On a renal sonogram, the medullary pyramids appear bright or white (hyperechoic) instead of anechoic or hypoechoic. Kidney appears like a stretched chrysanthemum flower. Acoustic shadowing may or may not be seen. Sometimes, the whole pyramid becomes hyperechoic while sometimes it’s only the rim (especially in early stages).