Feline Abdominal Ultrasound: Common Artifacts vs. Pathology

Feline ultrasonography is a high-stakes practice. With their smaller organs, rapid heart rates, and propensity for subtle, diffuse disease (like triads or small cell lymphoma), cats present a unique challenge. Distinguishing between a benign mirroring artifact and the early hyperechoic signs of pathology requires an advanced understanding of acoustic physics as well as anatomy.

The Crucial Role of Ultrasound in Feline Medicine

Unlike dogs, cats rarely present with massive, obvious solitary tumors in early disease stages. Feline disease processes—such as inflammatory bowel disease (IBD), cholangiohepatitis, and early-stage chronic kidney disease (CKD)—often cause diffuse, subtle changes in organ echogenicity or architecture. Because normal feline organs are so small, recognizing these changes requires excellent equipment, rigorous technique, and a healthy skepticism of artifacts.

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Common Artifacts and How They Mimic Disease

Artifacts are an inescapable phenomenon in ultrasonography. They are created when the sound waves interact with tissue in ways the machine's software does not anticipate.

1. Acoustic Shadowing: The Imitator of Mass Effects

Acoustic shadowing occurs when sound waves encounter a highly reflective (like gas) or absorptive (like bone or calculus) structure.

• Dirty Shadowing: Cast by gas within the gastrointestinal tract. Beginners frequently misinterpret this reverberating shadow as a distinct mass, severe wall thickening, or even a foreign body.
• Clean Shadowing: Cast by highly absorptive materials like uroliths or mineralization.
• The Fix: Apply gentle, graded pressure with the transducer to displace gas. If the "lesion" moves or disappears, it was gas. Evaluate the area from a different acoustic window.

2. Edge Shadowing in Kidneys

Refraction at the curved edges of the feline kidney produces edge shadowing. This normal artifact can artificially obscure the renal cortex or mimic a subcapsular fluid accumulation (like blood or urine from a ruptured kidney).

• The Fix: Adjusting your focal zone to the depth of the kidney's edge and gaining multiple acoustic windows (intercostal and subcostal) resolves this. Do not mistake an edge shadow for a structural defect in the renal capsule.

3. Slice-Thickness Artifact (Partial Volume Artifact)

The ultrasound beam is not a 2D slice; it has a thickness. If a fluid-filled structure (like the feline gallbladder or urinary bladder) and adjacent solid tissue (like liver or bowel) both lie within the beam's thickness, the machine averages the echoes. This creates the appearance of "sludge" or debris within an otherwise clear fluid.

• The Fix: Scan precisely through the center of the cystic structure. If the "sludge" disappears when you are dead-center, it was a slice-thickness artifact.

Evaluating the Feline Liver and Biliary System

The feline liver is normally hypoechoic to the falciform fat and isoechoic or slightly hyperechoic to the renal cortex. Feline cholangitis, hepatic lipidosis, and lymphoma all cause diffuse changes.

• Hepatic Lipidosis: Diffusely hyperechoic liver. The liver is often so bright that the sound beam attenuates rapidly, making the deeper structures (like the diaphragm) difficult to see.
• The Biliary Tree: The feline common bile duct is normally up to 3-4mm in diameter. Dilatation can occur with pancreatitis, cholelithiasis, or neoplastic obstruction (often at the major duodenal papilla). Note: older cats can have mildly dilated, tortuous bile ducts as an incidental finding.

Conclusion

Understanding acoustic physics is just as important as knowing your anatomy in feline ultrasonography. The moment you recognize why an artifact occurs, you eliminate the risk of misdiagnosis. Rely on rigorous measurement, multiplanar evaluation, and specialist consultation for ambiguous focal lesions before committing to invasive diagnostics or surgery.