The kidneys are included in almost every abdominal ultrasound. Being able to recognize and accurately characterize renal pathology — and communicate findings clearly — is a core sonographer skill. This guide covers the most common renal findings you'll encounter.

Quick Reference: Normal Kidney Measurements

Parameter	Normal Value
Length	9–12 cm (adult)
Width	4–6 cm
Cortical thickness	≥ 10 mm
Echogenicity	Isoechoic or slightly hypoechoic compared to liver
Renal sinus	Echogenic central complex (fat + vessels)

Key rule: Kidney length is the single most important measurement. Discrepancy > 2 cm between right and left kidney is significant. Kidneys < 9 cm suggest chronic renal disease; kidneys > 13 cm suggest acute edema, infiltrative disease, or obstruction.

Hydronephrosis: Grading and Causes

Hydronephrosis is dilation of the collecting system due to obstruction (or, less commonly, vesicoureteral reflux without obstruction).

Grading (SFU grading system for pediatrics; descriptive for adults):

Grade	Appearance
1 (Mild)	Renal pelvis dilated, no calyceal dilation
2 (Mild-Moderate)	Renal pelvis and calyces dilated, normal parenchyma
3 (Moderate)	More severe pelvic and calyceal dilation, parenchyma preserved
4 (Severe)	Massive dilation, thinned cortex (parenchymal compression)

Adult reporting: Use descriptive grading: mild, moderate, severe hydronephrosis + note on cortical thickness.

Measurements to document:

Anteroposterior (AP) renal pelvis diameter (measured in the transverse plane, within the kidney)
AP pelvis < 10 mm in adults: borderline/mild
AP pelvis 10–20 mm: moderate
AP pelvis > 20 mm: severe
Cortical thickness (if thinned, note)

Common causes of hydronephrosis:

Urolithiasis (stone obstructing ureter)
Ureteral stricture
External compression (malignancy, retroperitoneal fibrosis)
Pelvic mass (uterine fibroids, ovarian mass, pregnancy)
BPH (bilateral, sometimes causing hydronephrosis via chronic outlet obstruction)
Congenital UPJ obstruction (ureteropelvic junction)

Pyonephrosis: Infected, obstructed collecting system. Hydronephrosis + echogenic debris or layering material within the dilated pelvis. This is an emergency.

Urinary Tract Stones

Renal stones appear as echogenic foci with posterior acoustic shadowing within the collecting system.

Typical appearance:

Hyperechoic (bright)
Clean posterior acoustic shadow
May cause "twinkling artifact" on color Doppler (rapid color aliasing behind the stone) — helpful when shadowing is incomplete

Location: Most commonly in the lower pole calyces. Stones at the ureteropelvic junction (UPJ) or ureterovesical junction (UVJ) can cause acute obstruction.

Measuring stones: Measure in the largest dimension in cm or mm.

Small stones vs calcifications in vessel walls: Vascular calcifications are in the wall of blood vessels (look like small rings on Doppler) and don't shadow as cleanly. True calculi are in the collecting system.

Echogenic sinus fat: The renal sinus contains echogenic fat and can simulate stones. Sinus fat does NOT shadow. If an echogenic area in the sinus doesn't shadow, it's not a stone.

Ureteral stones: The ureter is usually not visible unless dilated. Look for the UVJ (ureterovesical junction) in the bladder view — stones here appear as echogenic foci with shadowing in the posterior bladder wall area. Ask the patient if they have pain and note its location.

Renal Cysts: Simple vs Complex

Simple cyst criteria (ALL must be met for a simple cyst):

Anechoic (no internal echoes)
Round or oval
Imperceptible thin wall
Posterior acoustic enhancement
No internal septa or nodules

Simple cysts are Bosniak I — benign, no follow-up needed.

Complicated / complex cysts:

Bosniak Category	Description	Malignancy Risk	Management
I	Simple cyst	~0%	No follow-up
II	Minimally complex — thin hairline septa, minimal calcification, < 3 cm hyperdense cyst	~0%	No follow-up
IIF	More complex septa, thicker calcification, larger hyperdense	~5%	Follow-up imaging
III	Thick irregular septa, uniform thickening	~55%	Surgical or urologic evaluation
IV	Clearly malignant — solid components, enhancement	~85–100%	Surgical resection

Practical note: Bosniak classification was designed for contrast-enhanced CT, not ultrasound. On ultrasound, your role is to describe what you see. If a cyst has septa, nodularity, or internal echoes, recommend CT or MRI for further characterization.

Common cyst findings:

Thin single septum: likely Bosniak II, usually benign
Thick septum or mural nodule: Bosniak III or IV, needs CT/MRI
Internal echoes without other features: complicated cyst — hemorrhage or infection can cause this; needs follow-up

Renal Masses

Any solid mass in the kidney requires further imaging (CT or MRI). Ultrasound identifies a solid mass but cannot characterize it definitively.

Renal cell carcinoma (RCC): Most common malignant solid renal mass.

Variable echogenicity — can be hyperechoic, isoechoic, or hypoechoic relative to renal cortex
Heterogeneous with larger lesions (necrosis, hemorrhage)
May distort the renal contour

Angiomyolipoma (AML): Benign lesion — fat-containing tumor.

Classically hyperechoic (brighter than renal sinus fat)
Well-circumscribed
Hyperechoic lesions > 1 cm: likely AML, but must exclude fat-poor RCC with CT
Associated with tuberous sclerosis (multiple AMLs)

Oncocytoma: Benign, but indistinguishable from RCC on ultrasound or CT.

Renal lymphoma: Can be multifocal, hypoechoic masses; may be primary or secondary.

Transitional cell carcinoma (urothelial carcinoma): Arises from the collecting system lining. May appear as a hypoechoic mass in the renal pelvis/sinus area, or cause focal wall thickening of the renal pelvis.

Renal Cortical Echogenicity

Normal: Equal to or slightly hypoechoic compared to liver.

Increased echogenicity (hyperechoic cortex): Suggests chronic renal disease (CKD), medical renal disease (glomerulonephritis, amyloid). The classic teaching: cortex brighter than liver = medical renal disease.

Decreased cortical thickness: Chronic atrophy. Combined with increased echogenicity = chronic kidney disease.

Doppler for Renal Evaluation

Renal artery Doppler:

Resistive Index (RI): normal 0.50–0.70
Elevated RI > 0.70: increased resistance — hydronephrosis, ATN, renal vein thrombosis, rejection
Tardus-parvus waveform intrarenal: suggests proximal renal artery stenosis

Renal vein: Should flow toward the IVC. Loss of flow or reversal = renal vein thrombosis.

Transplant Kidney Evaluation

Transplant kidneys are placed in the iliac fossa (right or left). The anatomy is reversed — artery and vein are on the anterior surface.

Standard protocol: Length measurement, cortical echogenicity and thickness, hydronephrosis grading, Doppler RI at upper/mid/lower pole arteries (average), perinephric collections.

Normal RI in transplant: Same thresholds as native kidneys.

Perinephric collections:

Lymphocele: most common — anechoic, often with thin septa
Urinoma: at anastomosis site, anechoic
Hematoma: complex, echogenic material
Abscess: complex, septated, with internal debris

Documentation Template

For each kidney:

Length (cm)
Cortical thickness (mm)
Echogenicity (normal / mildly increased / markedly increased)
Collecting system (normal / hydronephrosis — grade + AP pelvis measurement)
Any focal lesion (location, size, characterization)
Stones (location, size, shadowing)

Reference kidney measurements and Doppler RI normal values in SonoBuddy's Measurements section.

Kidney Pathology by Ultrasound: Hydronephrosis, Stones, Masses, and Cysts