Held within the scope of VetSummit 2025: New Horizons in Veterinary Clinical Sciences, organized under the sponsorship of Kito as part of the Healthy Pet Food Ecosystem, we have compiled the key takeaways from this session for you in this blog post. We would like to thank our esteemed speaker for his valuable presentation.
Sonographic Interpretation of Kidney Diseases: A Roadmap for Clinical Practice
In veterinary medicine, kidney diseases are among the most frequently encountered internal medicine problems, especially in cats and dogs. Ultrasonography is both a practical and a highly powerful tool in the diagnosis and follow-up of these conditions. However, the true value of ultrasound emerges not as a standalone diagnostic tool, but when it is interpreted together with the clinical picture and laboratory data.
Why Is Ultrasonography So Important in Kidney Diseases?
The prevalence of kidney diseases in cats and dogs is approximately 10–15%. The sensitivity of ultrasonography is particularly high in chronic kidney disease, with up to 86% of morphological changes detectable via ultrasound. In acute kidney injury, this rate ranges between 70–75%, and when Doppler imaging and the resistive index are included, the evaluation becomes much clearer and more accurate.
The advantages of ultrasonography are quite distinct:
- Non-invasive and painless
- Repeatable
- Easy to perform
- Provides guidance prior to biopsy
- A method that delivers rapid and reliable information throughout the clinical process
The Right Device – The Right Probe – The Right Settings
The success of ultrasonographic evaluation does not depend solely on the operator; device and probe selection are also of critical importance.
- Probe selection:
- Microconvex/convex probe in dogs (generally 5–8 MHz)
- Linear probe (the probe that provides the best renal detail; 7–12 MHz)
- Technical settings:
- The focal zone should be positioned over the renal cortex
- Gain (contrast) settings should be balanced according to liver echogenicity
- Depth should be adjusted so that the entire kidney fits within the imaging frame
Normal Sonographic Anatomy of the Kidney
- The first step in recognizing a pathological image is having a solid understanding of normal anatomy.
- In a normal renal ultrasound:
- The cortex has medium echogenicity
- (more hyperechoic than the liver, more hypoechoic than the spleen)
- The medulla appears more hypoechoic (darker than the cortex)
- The cortex-to-medulla ratio is approximately 1.5
- The renal pelvis is mostly anechoic
- Right and left kidney sizes should be symmetrical
- (the difference should not exceed 0.3 cm)
It is very important to remember the echogenicity hierarchy:
- Liver < Renal cortex < Spleen
Do Not Mistake Artifacts for Pathology
Many artifacts observed in urinary system ultrasonography can actually assist in diagnosis. However, some may be physiological.
Commonly encountered artifacts include:
- Acoustic shadowing: a very valuable finding for stones/mineralization
- Posterior acoustic enhancement: interpreted in favor of fluid or cysts
- Twinkle artifact (color twinkling): when Doppler is activated, color noise over a stone; pathognomonic for urolithiasis with approximately 90% diagnostic accuracy
Ultrasonographic Findings in Chronic Kidney Disease (CKD)
The most important sonographic changes associated with CKD include:
- Decreased kidney size
- in cats usually <3 cm
- in small dogs <5.5 cm
- Irregular contours (due to fibrosis)
- Hyperechoic cortex (increased fibrotic tissue)
- Loss of corticomedullary differentiation
- Small anechoic cysts (<5 mm) may be present
- Increased resistive index
- Marked cortical thinning
Ultrasonographic Findings in Acute Kidney Injury (AKI)
In acute processes, renal morphology behaves completely differently from chronic disease:
- Increased kidney size (approximately 0.35 cm enlargement)
- Hypoechoic cortex
- (darker appearance due to edema)
- Corticomedullary differentiation is preserved
- Perirenal fluid may sometimes accompany
- Hyperperfusion may be observed on Doppler imaging
- Resistive index is normal or low
- If the acute process becomes chronic (progressing to fibrosis), the imaging findings may reverse.
Acute vs Chronic Differentiation Table (Practical Summary)
- Findings
- Acute Kidney Injury
- Chronic Kidney Disease
- Size
- Increased
- Decreased
- Cortical echogenicity
- Hypoechoic
- Hyperechoic
- Cortex/medulla differentiation
- Preserved
- Lost
- Resistive index
- Normal or low
- High
- Clinical presentation
- Acute azotemia, vomiting
- PU/PD, weight loss, chronic symptoms
- Urine specific gravity
- Normal or mildly decreased
- Persistently low
Findings in Urolithiasis and Obstruction Cases
Key points to consider in stone and obstruction cases:
- Renal pelvis width >3 mm → mild dilation
- 5–10 mm or greater → evaluated as hydronephrosis
- Stones appear hyperechoic with acoustic shadowing and twinkle artifact
- Ureteral dilation >2.5 mm supports obstruction
- Prolonged obstruction leads to decreased cortical thickness
Resistive Index (RI): Why Is It Important?
RI is a measurement of vascular resistance against the main blood flow supplying the kidney.
The basic formula is:
- RI = (PSV – EDV) / PSV
Normal values:
- Cat: 0.55–0.70
- Dog: 0.56–0.75
Conditions associated with increased RI:
- Obstruction
- Fibrosis
- Hypertension
- Chronic kidney disease
Conditions associated with decreased RI:
- Inflammation
- Vasodilation
- Acute processes
Important note:
- An increase of 0.05 in RI correlates with approximately a 15% decrease in GFR.
Most Common Mistakes in Clinical Practice
Common errors that complicate diagnosis in renal ultrasonography include:
- Not using a reference organ
- (interpreting echogenicity without comparison to liver/spleen)
- Evaluating only one kidney
- (right and left must always be compared)
- Ignoring bladder filling
- (can mislead pelvis measurements)
- Mistaking artifacts for stones or vice versa
- Not using Doppler imaging
Final Note: Ultrasound Alone Is Not Enough
Ultrasound is a true compass in kidney diseases—but it is not the destination on its own.
- The most accurate approach includes:
- Clinical examination
- Laboratory data
- Urinalysis / UPC / SDMA
- Integrated interpretation of ultrasound findings
- With this approach, diagnostic accuracy can exceed 90%.
- We will continue to share key highlights from the sessions of VetSummit 2025: New Horizons in Veterinary Clinical Sciences, held under the sponsorship of Kito as part of the Healthy Pet Food Ecosystem.
We look forward to meeting you in future content that will contribute to your clinical practice.
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