  Morphological evolution of expanding shells of fast-mode magnetohydrodynamic
(MHD) waves through an inhomogeneous ISM is investigated in order to
qualitatively understand the complicated morphology of shell-type supernova
remnants (SNR). Interstellar clouds with high Alfv\'en velocity act as concave
lenses to diverge the MHD waves, while those with slow Alfv\'en velocity act as
convex lenses to converge the waves to the focal points. By combination of
various types of clouds and fluctuations with different Alfv\'en velocities,
sizes, or wavelengths, the MHD-wave shells attain various morphological
structures, exhibiting filaments, arcs, loops, holes, and focal strings,
mimicking old and deformed SNRs.
