The utility of triple-quantum (TQ)-filtered 23 Na NMR spectroscopy for discriminating between intra-and extracellular Na + (Na + i and Na + e , respectively) in a solid tumor in vivo was evaluated using TmDOTP 5-as a 23 Na shift reagent. Infusion of 80 mM TmDOTP 5-without added Ca 2+ produced baseline-resolved Na + i and Na + e peaks in both single-quantum (SQ) and TQ-filtered 23 Na spectra. The Na + i signal represented 22 ± 4% of the SQ spectrum, but 59 ± 10% of the TQ-filtered spectrum. Therefore, the Na + i contribution in TQ-filtered spectra is much higher than in SQ spectra.
Both SQ and TQ-filtered Na
- i signals increased by about 75% 1 h after sacrificing the animal. The TQ-filtered relaxation times did not change during this time, indicating that changes observed in TQ-filtered spectra collected with a preparation time of 3 ms represent changes in the concentration of sodium ions contributing to the TQ-filtered signal. Similar experiments were conducted without TmDOTP 5-to determine changes in the Na + e signal in the absence of the shift reagent. The changes in total SQ and TQ-filtered signals 1 h after sacrificing the animal showed that the SQ Na + e signal decreased by approximately 35%, while the TQ-filtered Na + e signal did not change significantly. This demonstrates that the TQ-filtered 23 Na signal is relatively insensitive to changes in Na + e content. To our knowledge, this work represents the first evaluation of multiplequantum-filtered 23 Na spectroscopy to discriminate between intraand extracellular Na + in a solid tumor in vivo.