Comparison of polarization transfer sequences for enhancement of signals in clinical 31P MRS studies

Several P-31 MRS studies in tumors in vivo have shown that levels of phosphocholine (PC) and other phosphomonoesters (PME) and phosphodiesters (PDE) are useful prognostic or early-response indicators. To improve sensitivity for such measurements, four polarization transfer (PT) sequences (insensitive nuclei enhanced by PT (INEPT), distortionless enhancement by PT (DEPT), reverse-INEPT, and heteronuclear single-quantum coherence (HSQC)) were assessed theoretically and experimentally. The presence of homonuclear (H-1-H-1) and heteronuclear (P-31-H-1) couplings of similar magnitude makes theoretical analysis very sensitive to precise model parameters, especially for the H-1-detected sequences. The H-1-H-1 coupling causes the splitting of H-1 peaks, and hence reduces the proton spectral resolution. This effect and a 50% signal loss from gradient-enhanced water suppression negate the usual advantages of H-1-detection. Among the PT methods, INEPT gave the higher signal enhancement. However, T-2 losses during the long echo times (TEs) required by the weak coupling limited the resulting signals from PC. (C) 2003 Wiley-Liss, Inc.