99mTc-MAA vs. 68Ga-MAA as Perfusion Agents

The use of Tc-99m Macroaggregated Albumin (MAA) as a perfusion agent has been evaluated since 1965. With the advent of Positron Emission Tomography (PET), an initial attempt to produce pharmaceutical grade Ga-68-MAA was successfully performed in 1989. However, a comparison of both perfusion agents, beyond the advantages of PET over Single Photon Emission Tomography (SPET) has not been performed to date. Both Tc-99m-MAA and Ga-68-MAA were used to perform lung perfusion studies in male Sprague Dawley rats. Images were taken at several time points. Animals were euthanized at 2 and 4 hours, organs collected and biodistribution determined. Biodistribution of both agents was very similar within the first hour; however, Tc-99m is released from the MAA after the first hour and it is excreted into the urine. Ga-68-MAA remains stable until Ga-68 decays and more than 95% of the injected activity is concentrated in lungs. The previously reported 6 hour "in vivo" half-life of MAA is challenged. Both imaging agents are suitable for lung perfusion studies. However, the observed half-life of MAA, greater than 6 hours, has significant implication for other applications. 68Ga-MAA is introduced as a possible candidate for Selective Internal Radiation Treatment Planning agent.