Concerning resection guidance, with the exclusion of the mass-forming type of cholangio-carcinoma for which the role of IOUS guidance is similarly relevant as for CLM, the above-mentioned limitations are actually real and linked to the features of the target. Indeed, perihilar cholangiocarcinomas are generally isoechoic (see Chap.​  4), and thus less visible even by IOUS (Fig. 9.4a). However, more important, these tumors tend to expand within the bile duct wall frustrating any attempt to disclose the real tumor margin in a reliable way (Fig. 9.4b), and thus accounting for the still considerable rate of palliative resections [5]. This limitation together with the low visibility of the main lesion impacts also the capability of IOUS to disclose an eventual vascular encasement by the tumor (Figs. 9.4c–e, 9.5a, b), which accounts for the need of extemporary vascular reconstruction, in spite of the encouraging results of such a policy [5]. However, as IOUS can recognize the intrahepatic biliary tree anatomy, it can also play a role in avoiding its damage by providing fundamental information for planning the surgical strategy (see Chap.​ 7) or guiding the liver dissection (see Chap.​ 8) (Fig. 9.6a–g). Another aspect which is relevant both for planning the surgical strategy and guiding the resection is its ability of reveal even minimal bile duct dilation: this is particularly the case for tumor-vessel relationships for establishing the risk of tumor invasion of the glissonian sheath, and, as a consequence, the need for its resection and for the extension of the resection area according to the criteria mentioned earlier [6] (see Chap.​ 7). IOUS allows the precise definition of this occurrence in patients with tumors in contact with an intrahepatic glissonian sheath (Fig. 9.7) but also its exclusion (Figs. 9.8, 9.9).