Image-guided procedures within the thoracic cavity require accurate registration of a pre-operative virtual model to the patient. Currently, surface landmarks are used for thoracic cavity registration; however, this approach is unreliable due to skin movement relative to the ribs. An alternative method for providing surgeons with image feedback in the operating room is to integrate images acquired during surgery with images acquired pre-operatively. This integration process is required to be automatic, fast, accurate and robust; however inter-modal image registration is difficult due to the lack of a direct relationship between the intensities of the two image sets. To address this problem, Computed Tomography (CT) was used to acquire pre-operative images and Ultrasound (US) was used to acquire peri-operative images. Since bone has a high electron density and is highly echogenic, the rib cage is visualized as a bright white boundary in both datasets. The proposed approach utilizes the ribs as the basis for an intensity-based registration method -- mutual information. We validated this approach using a thorax phantom. Validation results demonstrate that this approach is accurate and shows little variation between operators. The fiducial registration error, the registration error between the US and CT images, was < 1.5mm. We propose this registration method as a basis for precise tracking of minimally invasive thoracic procedures. This method will permit the planning and guidance of image-guided minimally invasive procedures for the lungs, as well as for both catheter-based and direct trans-mural interventions within the beating heart.