Flow Voids and Carotid MR Angiography

I read with interest the recent article by Nederkoorn and coworkers (1) regarding the occurrence of flow voids in 3D time-of-flight MR angiography (TOF-MRA) of the carotid bifurcation, although I was disappointed that they had not discovered a smaller series analyzing 2D TOF-MRA published by our group (2). In fact, the lower limit of percent stenosis associated with a flow void is similar between the two studies, probably primarily due to the relatively long echo time (TE = 6.9 ms) used in Nederkoorn’s 3D-TOF series. It is important to note that flow voids would be even more likely to represent greater than 70% diameter stenosis with a shorter TE, as is commonly employed by many practitioners for this purpose. In 10 of the 14 arteries in which flow voids were observed at less than 70% stenosis as determined by digital substraction angiography (DSA), the authors note that Doppler sonography suggested a more severe stenosis. A trend toward poor DSA image quality with increasing stenosis is suggested as a cause. Since only two or three projections were obtained of each artery at DSA, another possible cause would be underestimation of true stenosis by DSA due to lack of the optimal projection.