In vivo observation of lactate methyl proton magnetization transfer in rat C6 glioma

Magnetic resonance spectroscopy (MRS) measurements of the lactate methyl proton in rat brain C6 glioma tissue acquired in the presence of an off-resonance irradiation field, analyzed using coupled Bloch equation formalism assuming two spin pools, demonstrated the occurrence of magnetization transfer. Quantitative analysis revealed that a very small fraction of lactate (f = 0.0012) is rotationally immobilized despite a large magnetization transfer effect. Off-resonance rotating frame spin-lattice relaxation studies demonstrated that deuterated lactate binds to bovine serum albumin and the proteins present in human plasma, thereby providing a possible physical basis for the observed magnetization transfer effect. These results demonstrate that partial or complete saturation of the motionally restricted lactate pool (as well as other metabolites) by the application of an off-resonance irradiation field, such as that used for water presaturation, can lead to a substantial decrease in resonance intensity by way of magnetization transfer effects, resulting in quantitation errors.