Effect of melanin on phosphorus T1s in human melanoma xenografts studied by 31P MRS

31P MRS resonance ratios of tumors depend on the T1s of the phosphorus compounds. The objective of the 31P MRS study reported here was to investigate whether the phosphorus T1s of melanomas are influenced by the presence of melanin. One amelanotic (COX-t) and one melanotic (ROX-t) human melanoma xenograft line were studied at two different tumor volumes: 200 and 1000 mm3. 31P MRS was performed in nonanaesthetized mice at 4.7 T. The T1s were measured by using the superfast inversion recovery technique. Fraction of necrotic tissue in the tumors was determined by histological examination. The ROX-t tumors showed shorter T1s than the COX-t tumors at a volume of 200 mm3, where the fraction of necrotic tissue in the tumors was insignificant. The difference was similar in magnitude for all resonances. The T1s were not significantly different for COX-t and ROX-t at a volume of 1000 mm3, where the tumors of both lines had developed significant necrosis. The phosphorus T1s of melanomas without necrosis can be shortened significantly by the presence of melanin. The magnitude of the T1 shortening is similar for all major compounds. 31P MRS resonance ratios of melanomas are not altered significantly by correcting for effects of partial saturation.