It is an object of the invention to improve tissue classification in MRI images. In particular it is an object of the invention to improve the classification of bone and air in MRI images. This object is achieved by a method for tissue classification in a region of interest in a magnetic resonance (MR) image comprising a first region and a second region, wherein the first region represents air and the second region represents bone. The method comprises the steps of: —acquiring a first magnetic resonance image comprising the first and the second region and; —acquiring a second magnetic resonance image comprising the first and the second region, wherein the first region has a different shape in the second magnetic resonance image compared to the first magnetic resonance image and; —identifying the first region and second region in the first and second image and; —comparing the first image and the second image with respect to the first region and the second region and; —classifying the first region as region representing air based on a presence of a shape difference between the first and the second image and classifying the second region as bone based on an absence of the shape difference between the first and second image.