Determining material properties of the breast for image-guided surgery

We have previously proposed a system for image-guided breast surgery that compensates for the deformation of the breast during patient set-up. Since breast surgery is performed with the patient positioned supine, but MR imaging is performed with the patient positioned prone, a large soft tissue deformation must be accounted for. A biomechanical model can help to constrain the associated registrations. However the necessary material properties for breast tissue under such strains are not available in the literature. This paper describes a method to determine these properties. We first show that the stress-free or 'reference' state of an object can be approximated by submerging it in liquid of a similar density. MR images of the breast submerged in water and in a pendulous prone position are acquired. An intensity-based non-rigid image registration algorithm is used to establish point-by-point correspondence between these images. A finite element model of the breast is then constructed from the submerged images and the deformation to free-pendulous is simulated. The material properties for which the model deformation best fits the observed deformation are determined. Assuming neo-Hookean material properties, the initial shear moduli of fibroglandular and adipose tissue are found to be 0.4 kPa and 0.3 kPa respectively.