Kidney Stones (Renal Lithiasis): Ultrasound as a Detection and Monitoring Tool

Dr. Segnini
Dec 26, 2025
4 min read

A Luminous Diagnosis in the Kidney

Kidney stones, commonly known as renal calculi, are a prevalent condition that causes intense pain (renal colic) and can lead to complications such as infection or obstruction. Renal ultrasound is the first-line study for its evaluation, especially in cases of acute pain, follow-up of patients with a history of kidney stones, and in populations sensitive to radiation (pregnant women, children). Its main advantage is detecting kidney stones by their characteristic sonic "glow . "

The Physical Principle: Why Calculations Look So Bright

Kidney stones, composed primarily of calcium, uric acid, or struvite , have a very different acoustic density than kidney tissue and urine. This extreme difference causes them to almost completely reflect the ultrasound beam, creating two pathognomonic signs:

Hyperechogenicity : The stone appears as an intensely bright (white) spot or structure on the image.
Posterior Acoustic Shadow: Behind the bright stone, a conical area of no echoes (black) is projected . This occurs because the stone absorbs and reflects most of the ultrasound energy, "stealing" the signal from the tissues behind it. The presence of this shadow confirms that the bright structure is a stone and not something else.

Key Ultrasound Findings: Location and Consequences

1. Detection of the Calculation Proper:

In the Renal Parenchyma (Nephrolithiasis): They appear as hyperechoic spots or small masses with acoustic shadowing, embedded in the renal tissue. They can be single or multiple.
In the Collecting System (Renal Pelvis or Calyces): These are more clinically relevant here, as they can obstruct urine flow. They are identified as shiny structures within the renal pelvis (which is normally black/ anezioic when full of urine) or at the junction of the calyces.
In the Ureter ( Ureterolithiasis ): This is the most important finding in acute renal colic. The radiologist follows the course of the ureter from the renal pelvis to the bladder, looking for the hyperechoic structure with shadowing that is causing the obstruction. It is usually found at the natural narrow points: the ureteropelvic junction , where the iliac vessels cross, and the ureterovesical junction .

2. Signs of Complication: Hydronephrosis

The main complication of an obstructive stone is hydronephrosis , the dilation of the renal collecting system due to the accumulation of retained urine.

Ultrasound Appearance: The renal pelvis and calyces, which are normally collapsed or barely visible, dilate and appear as anechoic (black) structures that communicate with each other.
Grades: Ultrasound allows for grading the severity:
- Mild: Only pelvic dilation.
- Moderate: Pelvic and calyceal dilation.
- Severe: Marked dilation with thinning of the renal parenchyma , indicating a risk of permanent damage.

3. Additional Assessment During the Study:

Bladder: It is evaluated to see if the stone has descended and is now in the bladder ( cystolithiasis ).
Contralateral Kidney: The other kidney is always examined to rule out bilateral pathology.
Renal Blood Flow ( Doppler ): In cases of acute obstruction, it may show an increased resistance index in the affected kidney.

Strengths and Limitations of Ultrasound Compared to Other Techniques

VS. Plain Abdominal X-ray ( X-ray ):
- Advantages of Ultrasound: It detects non-radiopaque stones (such as uric acid stones), which are invisible on X-ray . It also evaluates the renal parenchyma and the presence of hydronephrosis.
- Limitation: It may miss ureteral stones in its mid-segment, where intestinal gas interferes.
VS. Computed Tomography (CT) without Contrast:
- Advantage of CT: It is the "gold standard" for kidney stones in adults. It is more sensitive (detecting >95% of stones, regardless of their composition), more accurately determines the size and exact location, and diagnoses other causes of abdominal pain.
- Advantages of Ultrasound: It does not use ionizing radiation. For this reason, it is the first-line technique for pregnant women, children, and young adults who require repeated follow-up. It is excellent for monitoring known kidney stones.

Ultrasound-Guided Clinical Management

The ultrasound report answers key questions that guide the urologist:

Is there a calculation? ( Hyperechogenicity + acoustic shadowing).
Where is it? (Parenchyma, pelvis, ureter).
What size is it? (Precise measurement in millimeters).
Is it causing obstruction? (Presence and degree of hydronephrosis).
Are there signs of associated infection? ( Perirenal fluid , changes in kidney echogenicity ).

With this information, you can choose between:

Conservative management: For small (<5-6mm) non-obstructive stones, with analgesia and hydration.
Intervention: Extracorporeal lithotripsy (shock waves), ureteroscopy or percutaneous nephrolithotomy for large or obstructive stones.

Conclusion: A Fundamental and Safe First Look

Renal ultrasound is the ideal entry point for diagnosing kidney stones. It combines a high detection rate (especially for obstructive stones) with the absolute safety of not using radiation. Its ability to simultaneously diagnose the stone and its consequences (hydronephrosis) makes it a rapid and effective decision-making tool in emergency settings. For monitoring chronic patients or vulnerable populations, its role is irreplaceable, offering safe and repeatable monitoring over time.