Reduced caliber of the internal carotid artery: a normal finding with ipsilateral absence or hypoplasia of the A1 segment.

PURPOSE To determine whether a relationship exists between normal variations in anatomy of the circle of Willis and the size of the internal carotid arteries (ICA). METHODS MR angiograms and axial MR images of the brains of 104 patients were reviewed. Included were 10 patients with unilateral absence of the A1 segment of the anterior cerebral artery, 10 with hypoplasia of one A1 segment, 28 with asymmetric A1 segments, nine with isolated unilateral fetal origin of the posterior cerebral artery, and 47 with balanced circulation. RESULTS The mean ICA diameter measurement for the total population was 4.62 +/- 0.68 mm. In patients with absent A1, the mean ipsilateral and contralateral ICA diameters were 3.63 +/- 0.41 mm and 5.25 +/- 0.52 mm, respectively. The mean percentages of the difference between the diameters of the right and left ICA (31% in the group with absent A1 and 21% in the group with hypoplastic A1) varied significantly from the differences in the ICA diameters among the rest of the population. The diameter differences produced by other common variations (unilateral small A1 segment or fetal origin of the posterior cerebral artery) did not differ significantly from those of the 47 patients with balanced intracranial circulation. CONCLUSIONS There is an association of unilaterally absent or hypoplastic A1 segments of the anterior cerebral artery with ipsilateral decrease in ICA caliber, and this can be seen on MR angiograms.