CT Angiography Reveals Anatomic Features that Account for the Distribution of Emboli in the Anterior Cerebral Circulation
Background: Correlative analysis of vascular anatomy on CT angiography (CTA) with transcranial Doppler ultrasonography (TCD) patterns of embolism may elucidate dynamic embolic particle behavior in humans. Methods: CTA was performed in 81 consecutive patients (median age 62 years, range 8–90; 54% female), employing Vitrea™ software to obtain 2D multiplanar-reformatted and 3D volume-rendered images. Two independent observers measured angulation and luminal diameters of the supraclinoid carotid (ICA), and proximal anterior (ACA) and middle (MCA) cerebral arteries. Based on previously described TCD methods, ipsilateral ACA and MCA were monitored for emboli in 10 patients with extracranial ICA stenosis (mostly solid emboli) and 8 with prosthetic cardiac valves (mostly gaseous emboli). Results: Pathologic CTA findings at the terminal ICA bifurcation were noted in 12% (20/162), with hypoplastic A1 segments observed in 11% (16/142) of normal bifurcations. Diameter of the A1 segment was significantly smaller than the M1, mean 2.55 mm (95%CI ±.05) vs 3.19 mm (±.07), p<.0001. A1 diameter contralateral to a diseased A1 was significantly larger, 2.70 mm (p=.05). ACA-ICA angulation was more acute than MCA-ICA, 81.13° (±1.50) vs 134.87° (±1.10), p<.0001. Angle relative to gravity was greater for ACA than MCA in natural head posture (63.73° (±3.50) vs 30.59° (±14.00), p<.0001) and supine TCD position (26.27° (±3.50) vs -59.41° (±14.00), p<.0001). ICA emboli exhibited a greater tendency to enter the MCA than those from cardiac valves: ACA/MCA emboli ratio of .32 (44/138) vs .54 (241/449), p<.0001. Conclusions: Vascular anatomy and embolic particle composition interact to determine embolus destination at the terminal ICA bifurcation. Larger, heavier than plasma, solid emboli are proportionately more likely to enter the MCA due to its wider diameter, downward gravitational position, and lesser path angularity; smaller, lighter than plasma, gaseous emboli are more likely to enter the ACA due to its upward gravitational position, and their lesser susceptibility to momentum and vessel diameter influences.