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 var iations in anatomy of the circle of Willis and the size of the interna l carotid arteries (ICA). METHODS: MR angiograms and axial MR images o f the brains of 104 patients were reviewed. Included were 10 patients with unilateral absence of the A1 segment of the anterior cerebral art ery, 10 with hypoplasia of one A1 segment, 28 with asymmetric A1 segme nts, nine with isolated unilateral fetal origin of the posterior cereb ral artery, and 47 with balanced circulation. RESULTS: The mean ICA di ameter measurement for the total population was 4.62 +/- 0.68 mm. In p atients with absent A1, the mean ipsilateral and contralateral ICA dia meters were 3.63 +/- 0.41 mm and 5.25 +/- 0.52 mm, respectively. The m ean percentages of the difference between the diameters of the right a nd 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 p roduced by other common variations (unilateral small A1 segment or fet al origin of the posterior cerebral artery) did not differ significant ly from those of the 47 patients with balanced intracranial circulatio n. CONCLUSIONS: There is an association of unilaterally absent or hypo plastic A1 segments of the anterior cerebral artery with ipsilateral d ecrease in ICA caliber, and this can be seen on MR angiograms.