What causes the yin-yang sign on colour Doppler ultrasound?
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Answer:
The yin-yang sign is the swirling, bidirectional colour flow that fills the sac of a pseudoaneurysm on colour Doppler ultrasound, appearing as adjacent red and blue halves within a rounded perivascular cavity. Blood entering the sac in systole and leaving it in diastole circulates around the cavity, so at any instant flow is directed towards the transducer on one side and away from it on the other, and the colour map splits into two complementary halves. The sign is typically seen after arterial puncture at the common femoral or radial artery, after biopsy or trauma, and in mycotic pseudoaneurysms of visceral vessels. Its companion finding is the to-and-fro waveform sampled at the neck on spectral Doppler: forward flow in systole and reversed flow in diastole, which is more specific than the colour appearance. The same swirling pattern is visible on contrast-enhanced CT and MR.
Why is it called so?
The two complementary halves of red and blue flow within a round sac resemble the yin-yang symbol of Chinese philosophy.
Pathophysiology
A pseudoaneurysm is a contained arterial wall breach in which blood escapes into a surrounding cavity that has no true arterial wall and communicates with the artery through a narrow neck. During systole the arterial pressure exceeds sac pressure and blood jets in; during diastole the pressure gradient reverses and blood drains back out. The jet entering the sac sets up a rotating vortex, so blood on one side of the cavity moves towards the probe and on the other away from it, encoded as opposing colours. Sampling at the neck captures the alternating inflow and outflow directly as the to-and-fro waveform.
Alternative names: Yin-yang appearance
Other associated named signs: To-and-fro waveform at the pseudoaneurysm neck, Perthes sign, and the colour bruit of an arteriovenous fistula
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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