1. A method for registering 3D image data, the method comprising; a) receiving a first 3D image and a second 3D image; b) determining a rendering transform for applying to each of the first 3D image and the second 3D image; c) applying the rendering transform to i) the first 3D image to provide a first 2D projection and ii) the second 3D image to provide a second 2D projection; and, d) determining an elastic registration for mapping locations of the first 2D projection to homologous locations in the second 2D projection.