Paramagnetic metal scavenging by melanin: MR imaging

PURPOSE: To quantitate the binding of metals to synthetic melanin in vitro, which is believed to be the reason why melanotic melanomas are hyperintense on T1-weighted magnetic resonance (MR) images, and to test whether such binding by natural melanin can be detected in cultured melanoma cells in vivo with MR imaging. MATERIALS AND METHODS: Seven synthetic metallomelanins were prepared and their metal contents and relaxivities determined. Melanotic PC1A and amelanotic B16 melanoma cells were incubated with increasing concentrations of iron. MR images of synthetic melanin and cell phantoms were obtained. RESULTS: The iron-binding capacities and relaxivities of the different synthetic metallomelanins varied considerably, which reflects the heterogeneous structure of melanin and the complexity of its binding of metals. Nevertheless, the MR signal intensities of the synthetic melanin and cell phantoms show marked increases that scale, respectively, with increasing iron content and iron concentration in the incubation medium. CONCLUSION: Melanotic melanomas are hyperintense on T1-weighted images because of paramagnetic metal scavenging. This observation has implications for the interpretation of MR images, the improved detection of melanomas, and the development of imaging marker genes.