  Magnetic Resonance Imaging has become nowadays an indispensable tool with
applications ranging from medicine to material science. However, so far the
physical limits of the maximum achievable experimental contrast were unknown.
We introduce an approach based on principles of optimal control theory to
explore these physical limits, providing a benchmark for numerically optimized
robust pulse sequences which can take into account experimental imperfections.
This approach is demonstrated experimentally using a model system of two
spatially separated liquids corresponding to blood in its oxygenated and
deoxygenated forms.
