Flow Voids and Carotid MR Angiography
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* Joseph E. Heiserman

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.

## References

1.  Nedekoorn PJ, van der Graaf Y, Eikelboom BC, van der Lugt A, Bartels LW, Mali WPTM. **Time-of-flight MR angiography of carotid artery stenosis: Does a flow void represent severe stenosis?** AJNR 2002;23:1779–1784
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiYWpuciI7czo1OiJyZXNpZCI7czoxMDoiMjMvMTAvMTc3OSI7czo0OiJhdG9tIjtzOjIyOiIvYWpuci8yNC84LzE3MjcuMS5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=) 

2.  Heiserman JE, Zabramski JM, Drayer BP, Keller PJ. **Clinical significance of the flow gap in carotid magnetic resonance angiography.** J Neurosurg 1996;85:384–387
    
    [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=8751620&link_type=MED&atom=%2Fajnr%2F24%2F8%2F1727.1.atom) 
    
    [Web of Science](http://www.ajnr.org/lookup/external-ref?access_num=A1996VD34700003&link_type=ISI) 

#  {#article-title-2}

*Reply* 

We agree with the clinical important findings described by Heiserman et al (1). Their study, however, focused on 2D-TOF MRA, where we describe the results of 3D-TOF MRA. We have only used 2D-TOF MRA to confirm flow void artifacts found on the three-dimensional images. In our opinion maximal-intensity-projections of 3D-TOF MRA are actually used in clinical practice to determine the degree of stenosis and therefore flow voids often are recognized with this technique. However, their conclusions certainly are in line with our findings and we apologize for not having cited their article. We agree with their suggestion that DSA in two or three projections might underestimate stenosis. Moreover, we recently studied the same hypothesis (2) and found it confirmed.

## References

1.  Heiserman JB, Zabrarnski JM, Drayer BP, Keller PJ. **Clinical significance of the flow gap in carotid magnetic resonance angiography.** J Neurosurg 1996;85:384–387
    
    [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=8751620&link_type=MED&atom=%2Fajnr%2F24%2F8%2F1727.1.atom) 
    
    [Web of Science](http://www.ajnr.org/lookup/external-ref?access_num=A1996VD34700003&link_type=ISI) 

2.  Nederkoorn PJ, Elgersma OE, Mali WP, Eikelboom BC, Kappelle LJ, van der Graaf Y. **Overestimation of carotid artery stenosis with magnetic resonance angiography compared with digital subtraction angiography.** J Vasc Surg 2002;36:806–13
    
    [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=12368742&link_type=MED&atom=%2Fajnr%2F24%2F8%2F1727.1.atom) 
    
    [Web of Science](http://www.ajnr.org/lookup/external-ref?access_num=000178617900025&link_type=ISI) 

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