Kidney pathology

Renal Cysts

Bosniak Classification of Renal Cystic Disease

Category I. : Category I lesions are simple benign cysts showing homogeneity, water content, and a sharp interface with adjacent renal parenchyma, with no wall thickening, calcification, or enhancement.

Category II. : This category consists of cystic lesions with one or two thin (≤ 1 mm thick) septations or thin, fine calcification in their walls or septa (wall thickening > 1 mm advances the lesion into surgical category III) and hyperdense benign cysts with all the features of category I cysts except for homogeneously high attenuation. A benign category II lesion must be 3 cm or less in diameter, have one quarter of its wall extending outside the kidney so the wall can be assessed, and be nonenhancing after contrast material is administered.

Category IIF : This category consists of minimally complicated cysts that need follow-up. This is a group not well defined by Bosniak but consists of lesions that do not neatly fall into category II. These lesions have some suspicious features that deserve follow-up up to detect any change in character.

Category III:  Category III consists of true indeterminate cystic masses that need surgical evaluation, although many prove to be benign. They may show uniform wall thickening, nodularity, thick or irregular peripheral calcification, or a multilocular nature with multiple enhancing septa. Hyperdense lesions that do not fulfill category II criteria are including in this group.

Category IV : These are lesions with a nonuniform or enhancing thick wall, enhancing or
large nodules in the wall, or clearly solid components in the cystic lesion. Enhancement was considered present when lesion components increased by at least 10 H.

Ultrasound Image- Small simple cortical cyst:

Well circumscribed, anechoic with posterior enhancement.

Ultrasound Image- Complex, cortical renal cyst:
Hypoechoic; Well circumscribed; Posterior enhancement.
No discernable internal vascularity when interrogated with low PRF colour doppler.
Any non-simple lesion should be further investigated with a contrast CT scan or at least followed up with ultrasound.

Ultrasound Image- A large right renal cyst…..pre aspiration

Ultrasound Image- ….Same cyst, post therapeutic aspiration.

Ultrasound Image- Parapelvic cysts can mimic hydronephrosis


  • The most common benign solid tumour of the kidney.
  • As the name suggests they contain, blood vessels, smooth muscle and fat.
  • On ultrasound: Generally <15mm, rounded echogenic lesion in the cortex.

Ultrasound image- An angiomyolipoma at the lower pole of the kidney.


Ultrasound image- Diffuse fatty deposition within the renal pelvis is common and known as ‘renal sinus lipomatosis’. This should not be confused with a circumscribed fatty mass(lipoma).

Ultrasound image- Experiment with different B-made color maps (chroma) to assist in defining subtle lesions.

Ultrasound image- A large lipoma within the renal pelvis.

Renal Cell Carcinoma

Is often an incidental finding as less than 10% produce symptoms.
Usually rounded, well circumscribed and generally subtly more echogenic than the cortex but have varying appearances.
Check the renal vein for tumour invasion.
Beware misdiagnosing an early RCC as an angiomyolipoma. A renal cell carcinoma should always be the primary differential diagnosis.

Ultrasound image- Power doppler of the RCC.

Ultrasound image- Importantly be cautious of the solid lesion that may be behind or adjacent to a cyst. The cyst is shown in blue, the RCC in green.

Ultrasound image- A renal cell carcinoma on the lower pole.

Ultrasound image- A large renal cell carcinoma on the left side of a horseshoe kidney.

Ultrasound image- A large renal cell carcinoma arising from the upper pole.

Ultrasound image- The renal cell carcinoma has invaded the collecting system and extends into the pelvo-ureteric junction.

Ultrasound image- A renal cell carcinoma with extension and thrombosus into the renal vein.

Ultrasound image- Using altered ‘chroma’ or colored B-mode maps can help to better demonstrate subtle tumors.

Ultrasound image- Colour doppler demonstrating flow through the material in the renal vein confirms tumour.

Ultrasound image- The tumor and thrombus extending into the IVC.


Hydronephrosis is dilatation of the collecting system of the kidney due to back pressure resulting from abnormal drainage of urine or from ureteric reflux.

Hydronephrosis may be associated with hydroureter (dilated ureter) depending on the level of obstruction.

It is subjectively classified as:

  1. Mild – Fullness of the renal pevis and minor calyces.
  2. Moderate – Distension into the major calyces.
  3. Marked – Clubbing of the major calyces with compression of the medullary fat.

Ultrasound image- If the hydronephrosis is severe and longstanding, there is a loss or compression of the cortex & the kidney will become dysfunctional.

Ultrasound image- A markedly hydronephrotic kidney.
The medullary fat is compressed and there is clubbing of the major calyces (smooth convexity)

Ultrasound image- Mild hydronephrosis.

Ultrasound image- Pelvic lymphadenopathy causing extrinsic compression of the mid-distal ureter.

Ultrasound image- A ‘pigtail stent’ placed from the urinary bladder up to the renal pelvis to decompress the hydronephrosis.

Ultrasound image- The stent is visible as parallel echogenic lines.

Transitional Cell Carcinoma (Urothelial Carcinoma)

Is a malignancy of the epithelial cells (urothelium) lining the renal tract from the collecting system into the bladder.
Often results from exposure of the urothelium to cancinogenic contaminants in the urine (Eg smoking and petro-chemical exposure).
Frank, painless haematuria is the most common presenting sign.
If in the upper tract, a nephrectomy is generally performed an prognosis is then based on staging (see link below).

Ultrasound image- A large mass within the ureter causing obstruction, hydroureter and hydronephrosis.
These transverse views show the proximal dilated ureter (left) and ureteric mass (right).

Ultrasound image- The same ureteric TCC in longitudinal.
Most TCC’s are more subtle and are a cause for painless haematuria (macro/microscopic).

Renal Calculi

Renal calculi are the result of crystalisation and aggregation of dietary minerals in the urine. Usually an excess of solutes than can dissolve in the urine

The main types are:

  1. 80% – Calcium containing stones (primarily calcium oxalate)
  2. 10-15% – Struvite stones (infection stones): Some bacteria will increase the pH of the urine which promotes struvite aggregation.
  3. 5-10% – Uric acid stones

Ultrasound image- A large renal calculus in the renal pelvis.
A calculus will appear as an echogenic focus with shadowing.
TIP: Beware of using compound imaging as this reduces posterior artefact Medullary sponge kidney casts such shadowing or enhancement.

Ultrasound image- Ultrasound image- A large renal calculus in the renal pelvis.
A calculus will appear as an echogenic focus with shadowing.
TIP: Beware of using compound imaging as this reduces posterior artefact Medullary sponge kidney casts such shadowing or enhancement.

Ultrasound image- Non obstructing renal calculus in the superior pole of the right kidney.

Medullary Sponge Kidney

Is the congenital formation of microscopic cysts in the collecting tubules (thus ‘sponge’).

  • Generally asymptomatic other than high risk of nephrolithiasis.
  • May be normal on all other imaging.
  • On ultrasound: there is increase intermedullary echogenicity which may surround the pyramids. This is suggestive of, but not specific to medullary sponge kidneys.

Associated with nephrocalcinosis which is the deposition of fine calcifications in the renal cortex. Nephrocalcinosis may also be called Albrights Disease which is the depostion of calcium salts resulting from hyperparathyroidism..

Ultrasound image- Medullary Sponge Kidney: The medullary tissue extends out into the cortex surrounding the pyramids.

Ultrasound image- Medullary Sponge Kidney

Polycystic Kidney Disease (PCKD)

  • PCKD is a disorder with a dominant gene giving a 50% chance of inheritance. It has no apprecial preponderance Male-Female.
  • Increase risk of also cysts in the liver (50%), pancreas and testes.

There are 2 types:

  1. Autosomal Dominant PCKD (ADPCKD)
  • ADPCKD usually clinically manifests in the 3rd-4th decade of life. The cysts, however, may be visible far earlier than that on imaging tests.
  • The typical ultrasound appearance is of multiple prominent cysts engulfing both kidneys with subsequent renal enlargement.

Gradual deterioration in renal function and renal failure requiring dialysis or transplant is the eventual outcome.

2. Autosomal Recessive PCKD (Infantile PCKD)

  • Involves fibrotic ectasia of the tubules giving a microcystic appearance.
  • Almost always has associated changes in the bile ducts and portal vein (Congenital Hepatic Fibrosis)
  • Severity is variable and the degree of kidney V’s liver involvement is inversely proportional.

The typical ultrasound appearance is of enlarged echogenic kidneys. Depending on severity, this may become evident in the 3rd trimester or have a slower onset into infancy or early childhood. The earlier the clinical manifestation, the worse the prognosis.

Ultrasound image- Autosomal Dominant Polycystic Kidney Disease (Adult)-ADPCKD. The loss of cortico-medullary differentiaton and kidney completely engulfed by cysts.

Ultrasound image- A panoramic view of a typical, markedly enlarged kidney, essentialy replaced by multiple macroscopic cysts (note it is 33cm in length).

Renal Scarring

Fibrosis of the cortex as a result of chronic renal disease (CRD). This can occur in infants as a result of Vessico-ureteric reflux (VUR), or any age as a result of nephritis (pyelonephritis, glomerulonephritis)or vascular compromise (renal artery stenosis). Surgical scarring will also be visible, so taking a good history is important in considering a cause for scarring.

A pitfall is mistaking ‘fetal’ lobulations for scarring. A good general rule is that lobulations will span the pyramids. Of a cortical dip occurs into a pyramid it is a scar.

The arrows indicate the extensive upper-mid pole scarring with complete cortical loss.

An atrophic kidney.

Note the diffusely thinned cortex from chronic disease.

Atrophy from congenital/neonatal disease usually results in a very small almost invisble kidney.

Atrophic kidney with cortical cyst

Vascular Abnormalities

Arteriovenous malformations, aneurysms and accesory renal arteries will all be encountered.

A transverse ultrasound view of what looks like a dilated renal pelvis. B-flow doppler quickly proves it to be a vascular lesion. See the image to the right.

Ultrasound image- The aneurysmal junction with the IVC is readily apparant.
(Image courtesy Helen Charlewood AMS)

Adrenal Adenocarcinoma

Ultrasound image- Adrenal adenocarcinoma:
A poorly defined adrenal tumor.

Ultrasound image- A post operative adrenal cyst.
The surgical clips are visible as brightly echogenic focii with ring down artefacts (red arrows)

Adrenal Adenoma

Adrenal cortical adenoma is a common benign tumor arising from the cortex of the adrenal gland most commonly in adults.
Adrenal cortical adenomas are not considered to have the potential for malignant transformation. (ref 2)
They are homogeneous and hypoechoic. A CT is recommended to ensure it is benign.

Ultrasound image- Sagittal View of Rt Adrenal Adenoma

Ultrasound image- Transverse View of the same adrenal tumour showing its position between the IVC and superior pole of the right kidney.


  • Pyelonephritis
  • Glomerulonephritis
  • Nephitic syndrome

Ultrasound image- There is increased echogenicity of the renal cortex relative to both the liver and the medullary fat.

Ultrasound image- There are fetal lobulations present (normal variant).These should span the pyramids.
In this case however, the cortical defect is directly to a pyramid consistant with renal scarring.

Adrenal Pheochromocytoma

The 2004 WHO classification of endocrine tumors defines pheochromocytoma as a tumor arising from chromaffin cells in the adrenal medulla.

Patients commonly present with hypertension. Headaches, excessive truncal sweating and palpitations are the most common symptoms. Other symptoms or signs include pallor, dyspnea, nausea, constipation and episodes of anxiety or panic attacks. Pheochromocytomas are rare.

It is difficult with ultrasound to determine between benign and malignant pheochromocytoma on the basis of acoustic features alone. Abdominal sonography, however,does provide evidence of malignancy in patients with malignant tumors by finding regional or distant metastases. Pheochromocytoma is associated with a broad spectrum of possible sonographic presentations. (Ref 1)

Ultrasound image- Rt Adrenal Pheochromcytoma

The Right side is more evident as the liver acts as a great window.

Ultrasound image- Left Possible Adrenal Pheochromocytoma

These are not seen well with ultrasound due to stomach gas.
It may be difficult to tell a pheochromocytoma from an adenoma.


1. J Ultrasound Med. 1994 Jul;13(7):517-21.
Adrenal pheochromocytomas: a broad spectrum of sonographic presentation.
Schwerk WB, Görg C, Görg K, Restrepo IK.
Department of Internal Medicine, Philipps University, Marburg, Germany.