Mass in a cat's colonic wall: the fluid level on CT
8-year-old male American Shorthair cat with 3 days of constipation and anorexia; normal haematology. Radiographs show a soft-tissue mass caudodorsal to the bladder narrowing the descending colon. CT, transverse image:
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What does the denser content settling in the dependent part of the mass, with a horizontal interface, indicate?
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A. Dystrophic mineralisation typical of colonic adenocarcinoma
Mineralisation is far denser, close to bone, usually irregular or stippled, and does not form a horizontal level.
B. Impacted faeces within the descending colon lumen
Faeces contain gas and look mottled and heterogeneous; they do not form a sharp fluid level inside a mass in the wall.
C. Settling blood: the haematocrit effect of haemorrhage
Correct. A fluid–fluid level with a denser dependent layer forms when blood cells settle (haematocrit effect) and indicates haemorrhage within a cavity or a haematoma. In this cat it was an intramural colonic haematoma. It is also seen in bleeding tumours or haemorrhagic cysts, so histology is still needed.
D. Gas trapped in the necrotic wall of a colonic tumour
Gas is hypoattenuating (black) and rises to the non-dependent part; this content is denser and collects at the bottom.
lightbulb Key takeaway
A fluid–fluid level with a denser dependent layer (haematocrit effect) suggests haemorrhage on CT, ultrasound or MRI. Gas rises and cellular sediment sinks: note which part of the lesion each one occupies.
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Updated: November 1, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.