Metalloporphyrins

The metalloporphyrin most commonly used as a MRI contrast agent is Mn(III)TPPS4 (manganese(III) tetra-[4- sulfanatophenyl] porphyrin) because of its low toxicity (compared to Fe(III)TPPS4 for example). A safety ratio of about 6:1 is estimated in mice. This material appears to work best with tumors that are isointense to surrounding structures on T1-weighted sequences. Incidentally, the fluorescent and tumor localizing characteristics of porphyrin derivatives have been exploited in phototherapy of tumors.