Up to now MR images report about the body anatomy; the use of currently available contrast agents allows to add relevant physiological information related to perfusion, flow, vascular permeability, etc. The next generation of MR agents will allow to acquire images that represent maps of the physico-chemical/biological parameters of interest. As representative example of the above depicted approach, we looked at pH-mapping as this parameter is known to change in the presence of several important diseases like tumor, stroke, infections. One of the crucial points to be considered designing a ¿smart¿ MR- contrast agent, is the need of knowing its local concentration in order to unambiglusly assign the observed relaxation rate variations to changes in the parameter of interest. In this work we overccame this drawback through a dual MRI/SPECT probe where the SPECT active moiety acts as a reporter of the concentration of the MRI responsive one. By properly designing the structure of the L ligand, the proof of concept acquired with this dual MRI/SPECT pH-responsive agent can be extended to different applications, for instance as responsive to enzymatic activity or metabolites concentration. ; JRC.DG.I.5-Nanobiosciences