Abstract: The present invention relates to a composition comprising a shell structure forming a cavity, wherein said shell structure comprises a drug and wherein said composition is associated with at least one contrast agent; wherein said shell structure is capable of releasing its contents into the exterior upon the application of an external stimulus and wherein said contrast agent comprises magnetic particles which are capable of being detected by Magnetic Particle Imaging (MPI), wherein at least more than 5% (w/w) of the magnetic particles comprised in said contrast agent have a magnetic moment of at least ?18 m 2 A, 10 wherein said magnetic particles are preferably composed of Fe, Co, Ni, Zn or Mn or alloys thereof or oxides of any of these.