13C steady state free precession (SSFP) magnetic resonance imaging and effective spin-spin relaxation time (T2) mapping were performed using hyperpolarized [13C] urea and [13C, 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 [13C,15N 2] urea giving T2 values of 6.3±1.3s in the cortex and medulla, and 11±2s in the renal pelvis. The measured T2 in the aorta was 1.3±0.3s. [13C] urea showed shortened T 2 values in the kidney of 0.23±0.03s compared to 0.28±0.03s 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,15N2] urea giving a greater than four-fold increase in signal-to-noise ratio over [ 13C] urea.
- Dynamic nuclear polarization
- Steady state free precession (SSFP)