ï»¿Abstract. Magnetic resonance guided high intensity focused ultrasound (MRgHIFU) is a new type of minimally invasive therapy, which can be employed to ablate liver tissues. Since the ribs on the beam path can com- promise an eective therapy, detecting them and tracking their motion on MR images is of great importance. However, due to poor magnetic signal emission of bones, one cannot observe ribs entirely and accurately in MR. We have proposed a method to take advantage of the accuracy of CT in capturing the ribs by combining a geometric ribcage model built from CT data with an appearance model of the neighbouring structures of ribs in MR to reconstruct realistic centerlines in MRIs. We have im- proved our previous method by employing a more sophisticated appear- ance model, a more exible ribcage model, and a more eective strategy to nd the optimal ribcage parameters for an MR image. This enabled us to decrease the mean error to 2.5 mm, making the method suitable for clinical application. Finally, we propose a rib registration method which conserves the shape and length of ribs, and imposes realistic constraints on their motions based on our geometric ribcage model, achieving 2.7mm error on MRI with 2.5mm slice thickness.