Convergent flow phenomenon mimics the appearance of venous thrombosis in gradient-echo images with or without the presence of a contrast agent

We have observed signal voids at the junction of the renal vein and the inferior vena cava in the Spoiled Grass images. They mimicked the magnetic resonance appearance of an intraluminal thrombus with and without the presence of a contrast agent. The patency of the vessels was unveiled by fast Spoiled Grass sequence with reduced echo time as well as by Doppler ultrasound. Phantom studies revealed patterns of counterrotating vortices at the confluence. The cause of this image artifact was subsequently deduced as the intravoxel spin phase dispersion arsing from the impinging flows of the renal vein and inferior vena cava. It is concluded that in regions where complex flow patterns reside, fast imaging sequences that reduce spin phase variations should always be conducted in addition to other routine sequences to exclude uncertainties in image interpretation.