Abstract: The present invention relates to the field of in vivo determination of enzyme activity. It also allows visualization of organisms, organs, tissues and cells. In particular, the present invention provides a method of in vivo visualization and a composition suitable for in vivo determination and/or visualization of enzyme activity by methods such as Magnetic Resonance Imaging, also called Magnetic Resonance Tomography (MRI or MRT), or Magnetic Particle Imaging (MPI). In particular, the activity of the enzyme lipoprotein lipase affects the signals received and allows conclusions on the lipid metabolism of an organism, an organ system, an organ, a tissue and a cell of interest, This method can be employed, e.g., for diagnosis of cardiac disorders, of tumor prognosis and of disorders of the lipid metabolism. The composition used comprises superparamagnetic iron oxide nanocrystals (SPIO) incorporated in the core of lipid micelles designated nanosomes.