Renal Pathology: Interpreting Renal Ultrasound from Simple Cysts to Hydronephrosis

Dr. Segnini
Dec 22, 2025
3 min read

Dr. Segnini Renal ultrasound: simple cysts

Ultrasound, the First Look at the Kidney Renal ultrasound: simple cysts

Renal ultrasound is a fundamental, non-invasive, and accessible study that provides a detailed anatomical and structural evaluation of the kidneys. It is the first-line technique for investigating lower back pain, recurrent urinary tract infections, abnormalities in urinalysis or renal function, and for monitoring incidental findings. Its great value lies in its ability to differentiate common benign conditions from pathologies that require immediate attention.

Basic Ultrasound Evaluation: What is Looked At

The radiologist performs a systematic evaluation of each kidney:

Number, Size and Position: Check the presence of both kidneys, their size (normal: 9-12 cm) and their location.
Parenchyma (Cortex and Medulla): Evaluate thickness and echogenicity (degree of "whiteness" on the image). Normal renal parenchyma is less echogenic (darker) than the adjacent liver or spleen.
Collecting System (Renal Pelvis and Calyces): Review their morphology. They are normally "collapsed" or barely visible. Their dilation is a key sign.
Contour and Shape: Smooth surface and "bean" shape.

Classification of Findings: From Benign to Urgent

1. Renal Cysts: Simple vs. Complex

This is one of the most important distinctions. The Bosniak Classification System is used , which guides management according to the risk of malignancy.

Simple Renal Cyst ( Bosnakisk I):
- Ideal Ultrasound Characteristics: Perfectly anechoic (black in the image, like water), with imperceptible wall , well-defined edges and posterior acoustic enhancement (the area behind the cyst appears brighter).
- Meaning: An extremely common benign finding , especially with age. It does not require follow-up .
Complicated or Complex Cysts ( Bosnakis II, III, IV):
- Ultrasound Warning Signs: Any deviation from the characteristics of a simple cyst raises suspicion.
  - Thin or multiple septa: Lines inside the cyst.
  - Thick or irregular wall.
  - Echogenic content : The cyst is not black, it has " debris " or material inside.
  - Calcifications in the wall or in septa.
  - Solid vascular components (seen with Doppler ) attached to the wall.
- Significance: These findings require characterization with contrast-enhanced computed tomography (CT) for Bosniak classification . A Bosniak III or IV has a high probability of being renal cell carcinoma and usually requires surgery.

2. Hydronephrosis: Dilation of the Collecting System

This is an alarming finding that indicates an obstruction to urine flow. It is not a definitive diagnosis, but a sign that necessitates investigating the cause.

Ultrasound Appearance: The collecting system (pelvis and calyces), which is normally virtual, dilates and fills with urine, appearing as anechoic (black) structures that communicate with each other.
Severity Levels:
- Mild (Grade I): Dilation only of the renal pelvis.
- Moderate (Grade II): Dilation of pelvis and calyces, but the renal parenchyma is preserved.
- Severe (Grade III): Marked dilation with thinning of the renal parenchyma , indicating potential damage to kidney function.
Common Causes That Ultrasound Can Identify:
- Ureteral stone (renal colic): A bright , hyperechoic structure (the stone) that casts a posterior acoustic shadow (a dark area behind it), impacted at the junction of the kidney and ureter ( ureteropelvic junction ) or in the ureter. It is the most common cause of acute pain.
- Tumors: Masses that obstruct from within (renal pelvis) or from without (e.g., prostate cancer, cervical cancer).
- Ureteral Narrowing (Stenosis).

3. Other Relevant Findings

Renal Lithiasis (Parenchymal Stones): Small, bright, hyperechoic structures with acoustic shadowing within the renal tissue. They may be asymptomatic.
Polycystic Kidney Disease Autosomal Dominant (ERPAD): Characteristic finding with multiple cysts of varying sizes in both kidneys , which often distort the normal architecture.
Solid Renal Masses: Lesions that are not cystic. A solid mass, especially if it is heterogeneous and vascularized on Doppler , is highly suspicious for renal cancer and requires CT for staging .

Diagnostic and Management Flow

First Level Abdominal/Renal Ultrasound: Detects and characterizes the lesion (simple cyst, hydronephrosis, mass).
CT characterization: If ultrasound shows a complex lesion ( Bosnakisk cyst ≥IIF, solid mass) or hydronephrosis of unclear cause, contrast-enhanced CT is the method of choice for definitive evaluation.
Targeted Treatment: According to the diagnosis: follow-up for simple cysts, lithotripsy or ureteroscopy for stones, urinary diversion for obstructive hydronephrosis, or nephrectomy for cancer.

Conclusion: A Study That Makes a Difference

Renal ultrasound is a powerful diagnostic tool that serves as an effective triage or screening device . Its ability to reliably distinguish a simple benign cyst (which reassures the patient) from obstructive hydronephrosis (which requires urgent action) or a suspicious mass (which requires oncological evaluation) is invaluable. It provides a clear visual map that guides the urologist or nephrologist toward the correct diagnosis and the most appropriate treatment, optimizing resource use and improving clinical outcomes.