FIG 2. Case of a 63-year-old man.

A and B, Coronal 2D PC MR venogram with regular breathing (A) and breathholding (B). Right transverse and sigmoid sinuses are seen on both regular-breathing and breath-holding images. The left sigmoid sinus is not visualized on regular-breathing image, but is seen on the breath-holding image (arrow).

C, Velocity image shows signal intensities of sigmoid sinuses. The signal intensity of the right sigmoid sinus decreases slightly during breath holding. The signal intensity of the left sigmoid sinus with regular breathing is below zero, which indicates retrograde flow. With breath holding, the retrograde flow is normalized, but the signal intensities are small, indicating slow flow. indicates the right sigmoid sinus (regular breathing); •, the right sigmoid sinus (breath holding); , the left sigmoid sinus (regular breathing); , the left sigmoid sinus (breath holding).

D, Three-dimensional time-of-flight MR angiogram. The left internal jugular and sigmoid sinuses are seen on the maximum intensity projection image (arrow). Visualization of these veins can be explained by the in-flow effect of retrograde flow.