The present invention relates to MRI based imaging. In particular, the present invention provides MRI contrast agents targeted to a HaloTag protein with tunable relaxation properties thereby providing optimal relaxivity for low field strength imaging and the other optimal relaxivity for high field strength imaging. Moreover, the MRI contrast agents are used to detect gene expression (of a gene of interest) in real time in vivo, to detect changes in gene expression (of a gene of interest) over time in, for example, an individual organism, to detect gene expression changes (of a gene of interest) in response to therapeutics, in cell labeling for MR imaging, in clinical diagnostics, and in theranostics.