Re-evaluation of the Couinaud classification for segmental anatomy of the right liver, with particular attention to the relevance of cranio-caudal boundaries

Background Although the Couinaud classification of liver segments has been challenged by several studies, whether the cranio-caudal boundaries can be delineated in the right liver has not yet been assessed. This study scrutinized the third-order branching pattern of the portal vein in the right liver with attention to the validity of cranio-caudal segmentation. Methods Three-dimensional reconstruction of the portal vein and hepatic vein, using non-contrast-enhanced magnetic resonance imaging was performed in 50 healthy participants. Results In the right paramedian sector, the portal vein ramified into 2 thick P8s (P8vent and P8dor) in all the participants. Additional thick P8s that ran laterally and/or medially (P8lat and/or P8med) were observed in 18 (32%) participants. In contrast, multiple thin P5s, ranging in number from 2 to 6 (median, 4), branched from the right paramedian trunk, the right portal trunk, and/or even from P8s. In the right lateral sector, an arch-like type in which multiple P6s ramified from a single thick P7 was observed in 26 (52%) participants. A bifurcation type composed of a single P7 and a single P6 was observed in 23 (46%) participants, and a trifurcation type was observed in 1 participant. Conclusion No clear cranio-caudal intersegmental plane could be delineated in the right liver in most of the participants. The resection of a whole Couinaud segment in the right liver should not be regarded as a systematic, anatomic resection from an oncologic viewpoint. In contrast, detailed information on the third-order portal vein ramification pattern is likely to be helpful when performing smaller anatomic resections.