High Resolution $^{13}$ C MRI With Hyperpolarized Urea: In Vivo $T_{2}$ Mapping and

¹³C steady state free precession (SSFP) magnetic resonance imaging and effective spin-spin relaxation time (T2) mapping were performed using hyperpolarized [¹³C] urea and [¹³ C,¹⁵N₂] urea injected intravenously in rats. ¹⁵N labeling gave large T2 increases both in solution and in vivo due to the elimination of a strong scalar relaxation pathway. The T2 increase was pronounced in the kidney, with [¹³ C,¹⁵ N2] urea giving T2 values of 6.3±1.3 s in the cortex and medulla, and 11±2 s in the renal pelvis. The measured T2 in the aorta was 1.3±0.3 s. [¹³C] urea showed shortened T2 values in the kidney of 0.23±0.03 s compared to 0.28±0.03 s measured in the aorta. The enhanced T2 of [¹³C,¹⁵N2] urea was utilized to generate large signal enhancement by SSFP acquisitions with flip angles approaching the fully refocused regime. Projection images at 0.94 mm in-plane resolution were acquired with both urea isotopes, with [¹³C,¹⁵ N2] urea giving a greater than four-fold increase in signal-to-noise ratio over [¹³C] urea.