Increasing the sensitivity of hyperpolarized [¹⁵ N₂ ]urea detection by serial transfer of polarization to...

Hyperpolarized ¹⁵ N-labeled molecules have been proposed as imaging agents for investigating tissue perfusion and pH. However, the sensitivity of direct ¹⁵ N detection is limited by the isotope's low gyromagnetic ratio. Sensitivity can be increased by transferring ¹⁵ 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 T₁ (2-3 s). To exploit the long T₁ offered by storing polarization in ¹⁵ N and the higher sensitivity of ¹ H detection, we have developed a pulse sequence for partial polarization transfer.

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 B₁ fields, as is often the case in vivo. The sequence was demonstrated with dynamic spectroscopy and imaging measurements with [¹⁵ N₂ ]urea.

When compared to direct ¹⁵ 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.

The sequence gave an SNR increase that was close to the theoretical limit and can give a significant SNR benefit when compared to direct ¹³ C detection of hyperpolarized [¹³ C]urea.