31P-nuclear magnetic resonance spectroscopy in vivo of four human melanoma xenograft lines: spin-lattice relaxation times

Phosphorus spin-lattice relaxation times (T(1)s) were measured in vivo by P-31-nuclear magnetic resonance spectroscopy in tumors from four amelanotic human melanoma xenograft lines grown subcutaneously in BALB/c-nu/nu mice. The T(1)s were analyzed in relation to tumor volume, fractional tumor water content, and fraction of necrotic tumor tissue. The following resonances were studied: phosphomonoesters (PME), inorganic phosphate (P-i), phosphodiesters (PDE), phosphocreatine (PCr), and nucleoside triphosphates gamma, alpha, and beta (NTP gamma, alpha, and beta). Two different techniques were used to measure the T(1)s: superfast inversion recovery (SUFIR) and conventional inversion recovery (IR). The SUFIR and IR methods gave similar results. Tumors in the volume range 100-3000 mm(3) were studied. The PME, P-i, PDE, and PCr resonances showed significantly longer T(1)s than the NTP gamma, alpha, and beta resonances at small tumor volumes. The T(1)s at small tumor volumes also differed significantly between the tumor lines. The T(1)s either decreased or remained unchanged with increasing tumor volume; the volume-dependence of the T(1)s differed significantly between the tumor lines but not between the resonances. Calculations based on the T(1)s measured here indicated that the errors in PCr/P-i and NTP beta/P-i resonance ratios due to partial saturation can vary with tumor volume but are usually <20% at a repetition time of 2.0 s and <15% at a repetition time of 3.0 s. There was no correlation between the T(1)s and fractional tumor water content. On the other hand, the T(1)s showed significant correlations to fraction of necrotic tumor tissue across the tumor lines; theon of the PDE resonance decreased with increasing fraction of necrotic tumor tissue.