A pseudoaneurysm (false aneurysm) forms when a breach in the arterial wall allows blood to leak into surrounding tissue, forming a contained pulsatile hematoma communicating with the artery via a neck. On color Doppler, the classic finding is a yin-yang swirling color pattern within the sac. Spectral Doppler of the neck demonstrates to-and-fro (bidirectional) flow — forward during systole and reversed during diastole. Pseudoaneurysms most commonly occur as complications of arterial catheterization, most frequently at the femoral artery access site.