CTICasesInterrupted Bolus Phenomenon
Imaging findings
Contrast-enhanced chest CT demonstrates a transient dilution of contrast material within the lower lobe pulmonary veins and left atrium. Unopacified blood from the inferior vena cava (IVC) mixes with contrast-opacified blood from the superior vena cava (SVC), creating a dark 'smoke' or filling defect appearance in the pulmonary veins and left atrium, without evidence of true thrombosis.
Key takeaways
The interrupted bolus phenomenon (or transient contrast dilution) is a common imaging artifact on chest CT. It occurs when a patient performs a deep inspiration immediately before or during scanning, which increases venous return from the IVC. This draws unopacified blood into the right heart and pulmonary circulation, causing a transient decrease in contrast concentration that can mimic a pulmonary embolism or left atrial thrombus.
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