Analysing heart motion provides crucial insights on the condition of the cardiac function. Tagged-MRI and 2D-strain ultrasound enable quantitative assessment of the myocardium strain. But estimating 3D myocardium strain from cine-MRI remains attractive: cine-MRI is widely available and it yields detailed 3D+t anatomical images. This paper presents an image-based method to estimate myocardium strain from clinical short-axis cine-MRI. To recover non-apparent cardiac motions, we improve the diffeomorphic demons, a non-linear registration algorithm, by adding two physical constraints. First, myocardium near-incompressibility is ensured by constraining the deformations to be divergence free. Second, myocardium elasticity is modelled using smooth vector filters. The proposed physically-constrained demons are compared with the diffeomorphic demons and evaluated in a healthy subject against tagged MRI. The method is also tested on a patient with congenital pulmonary valve regurgitations and compared with 2D-strain measurements. In both cases, obtained results correlate well with ground truth. This method may become a useful tool for cardiac function evaluation.
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