Transcallosal and Pericallosal Courses of the Anterior Cerebral Artery.

(1) Background: The anterior cerebral artery (ACA) has a precommunicating A1 segment, followed by a postcommunicating A2 segment. Anatomically, after it sends off from the callosomarginal artery (CMA), it continues as the pericallosal artery (PCalA). A detailed pattern of the anatomical variations of the PCalA are needed for practical reasons. (2) Methods: There were 45 retrospectively documented Computed Tomography Angiograms of 32 males and 13 females. (3) Results: In 90 sides, eleven different types of PCalA were documented: type 1: normal origin, above the genu of the corpus callosum (CC) (51.11%); type 2: low origin, below the rostrum of the CC (8.88%); type 3: late origin, above the body of the CC (3.33%); type 4, initial transcallosal course (3.33%); type 5, duplicated PCalA (1.11%); type 6, azygos PCalA (2.22%); type 7, absent PCalA (CMA type of ACA) (7.78%); type 8: CMA continued as PCalA (5.56%); type 9: PCalA continued as the cingular branch (1.11%); type 10: PCalA type of ACA, absent CMA (14.44%); type 11: triple PCalA, with an added median artery of the CC (1.11%). Different types of CMA were also documented: type 0, absent CMA (17.78%); type 1, CMA with frontoparietal distribution (45.56%); type 2, CMA with parietal distribution (22.22%); type 3, low origin of CMA, either from A1, or from A2 (8.88%); type 4, CMA continued as PCalA (5.56%). Ipsilateral combinations of PCalA and CMA types were classified as types A-P. In 33/45 cases (73.3%), the bilateral asymmetry of the combined anatomical patterns of PCalA and CMA was documented. Additional rare variations were found: (a) huge fenestration of A2; (b) bihemispheric ACAs (6/45 cases); (c) twisted arteries within the interhemispheric fissure. (4) Conclusions: The PCalA and CMA are anatomically diverse and unpredictable. Therefore, they should be documented on a case-by-case basis before surgical or endovascular approaches.