Increasing the sensitivity of hyperpolarized [ N ]urea detection by serial transfer of polarization to spin-coupled protons.

Purpose: Hyperpolarized N-labeled molecules have been proposed as imaging agents for investigating tissue perfusion and pH. However, the sensitivity of direct N detection is limited by the isotope's low gyromagnetic ratio. Sensitivity can be increased by transferring N hyperpolarization to spin-coupled protons provided that there is not significant polarization loss during transfer. However, complete polarization transfer would limit the temporal window for imaging to the order of the proton T (2-3 s). To exploit the long T offered by storing polarization in N and the higher sensitivity of H detection, we have developed a pulse sequence for partial polarization transfer.

Methods: A polarization transfer pulse sequence was modified to allow partial polarization transfer, as is required for dynamic measurements, and that can be implemented with inhomogeneous B fields, as is often the case in vivo. The sequence was demonstrated with dynamic spectroscopy and imaging measurements with [ N ]urea.

Results: When compared to direct N detection, the sequence increased the signal-to-noise ratio (SNR) by a factor of 1.72 ± 0.25, where both experiments depleted ~20% of the hyperpolarization (>10-fold when 100% of the hyperpolarization is used). Simulations with measured cross relaxation rates showed that this sequence gave up to a 50-fold increase in urea proton polarization when compared to spontaneous polarization transfer via cross relaxation.

Conclusion: The sequence gave an SNR increase that was close to the theoretical limit and can give a significant SNR benefit when compared to direct C detection of hyperpolarized [ C]urea.