Spin-Lattice Relaxation Time of Inorganic Phosphate in Human Tumor Xenografts Measured in Vivo by ³¹P-Magnetic Resonance Spectroscopy Influence of oxygen tension

Abstract

Previous ³¹P-magnetic resonance spectroscopy (³¹P-MRS) studies have suggested that the spin-lattice relaxation time (T₁) of the inorganic phosphate (P_i) resonance is shorter in well-oxygenated than in poorly oxygenated tumors. Amelanotic human melanoma xenografts were therefore subjected to ³¹P-MRS to investigate whether the T₁ of the P_i resonance might be a useful parameter for assessment of tumor oxygenation status. It was searched for possible correlations between the T₁ of the P_i resonance and oxygen tension or parameters closely related to oxygen tension, including ³¹P-MRS tumor energy status and blood supply per viable tumor cell. Oxygen tension, tumor energy status, and blood supply per viable tumor cell decreased with increasing tumor volume. In contrast to previous suggestions, the T₁ of the P_i resonance decreased with increasing tumor volume and decreasing oxygen tension, tumor energy status, and blood supply per viable tumor cell, possibly because the tumors developed necrotic regions concomitantly with the decrease in oxygenation status, resulting in increased concentrations of freely dissolved para-magnetic ions in the tissue. Consequently, the T₁ of the P_i resonance can probably not be utilized to estimate the oxygenation status of tumors, at least not in tumors with necrotic regions.