Prediction of Rib Motion During Free-Breathing from Liver Observations Using 4D MRI

Magnetic resonance guided high intensity focused ultrasound (MRgHIFU) is a new therapy for treating malignant liver tissues. However, the motion of the ribs in the beam path may compromise an effective and safe treatment. Due to poor visibility of bones in MR and US liver images, tracking them in real time is currently not feasible. We propose a method for modeling and registration of the respiratory motion of the ribs. Moreover, we show that it is possible to predict the ribs’ motion based on a few tracked points in the liver. Our registration had a mean error of 1.06 mm for deep inhalations with an average motion of 2.71 mm. We developed subject-specific and population-based modeling methods, which recover 60% and 40% of the respiratory motion extracted through registration, respectively. To the best of our knowledge, this is the first time the ribs’ motion due to respiration has been directly studied during free breathing over a relatively long time (100 breathing cycles).