Sources of the increased longitudinal relaxation rates observed in melanotic melanoma an in vitro study of synthetic melanins

Abstract

Researchers have suggested that the increased longitudinal relaxation rates (1/T1) of solvent water protons often found in melanoma result either from the paramagnetism of stable free radicals occurring in melanin or from that of methemo- globin in nonacute hemorrhagic regions of the tumor. However, field-cycling relaxometry and model Solutions of synthetic melanin produced data which show that free radicals in melanin do not contribute significantly to 1/T1; instead, aggregation of melanin into macromolecular particles and binding of biologically-common paramagnetic metal ions (ie Fe3+, Mnw, and Cu2+) to melanin effectively do increase 1/ T1. These data have been combined with published histochem- ical data on melanin-containing tissues, while disregarding any additional effect related to hemorrhage. The result indicates that in melanoma the expected contribution of melanin-bound Fe-3+ to 1/T1, at typical imaging fields, predominates under estimated in vivo conditions; furthermore, the total contribution from all sources, speciflcally due to the presence of melanin, is sufficient to account for reported measurements of 1/T1 in melanoma. Comparing the latter results with published data on T1 relaxation in model Solutions of methemoglobin suggests that co-existing regions of nonacute microhemorrhage also may contribute significantly to 1/T1 under certain conditions. Finally, the implications for 1/T2 of melanin occurring in vivo within discrete melanosomes is discussed. © Lippincott-Raven Publishers.