Increased sensitivity of 31P MRSI using direct detection integrated with multi-echo polarization transfer (DIMEPT)

Here, we show that the sensitivity of (31)P MRSI of (31)P spins J-coupled to protons can be increased by almost a factor of three when compared with an optimal direct detection free induction decay. By direct detection integrated with multi-echo polarization transfer (DIMEPT), multiple signals from polarization transfer and direct detection can be acquired in one repetition time, with minimal mutual interference, provided that the number of refocusing pulses in the multi-echo polarization transfer part is even. The DIMEPT sequence was implemented on a 7-T body scanner and tested on a phantom and on the breasts of five healthy volunteers. The in vivo signal-to-noise ratio (SNR) enhancement for the J-coupled phosphomonoesters was 270% when compared with an Ernst angle pulse-acquire sequence. However, the phosphodiester signals, presumably mainly mobile phospholipids, had T2 values that were too short to be enhanced. Uncoupled (31)P spins, with sufficiently long T2 values, such as inorganic phosphate, were SNR enhanced by a factor of 1.9 relative to an Ernst-angle excitation pulse-acquire sequence by multi-echo direct detection.