Tracers that can be used for PET or SPECT imaging of the distribution of Pgp are disclosed. The tracers are metalloprobes that can comprise a radioactive metal ion such as 67Ga or 68Ga. Methods of synthesizing the tracers, and methods of imaging heart and other tissues are also disclosed. The tracers can be used to obtain high signal-to-background ratios for imaging tissues in vivo such as heart or tumor tissue. In various embodiments, disclosed tracers can exhibit, a) enhanced first pass extraction into heart tissue compared to presently available probes, b) linearity with true blood flow, c) enhanced detection of myocardial viability compared to presently available probes, d) reduced liver retention compared to presently available probes, and e) more efficient clearance from non-cardiac and adjoining tissues compared to presently available probes.