  Purpose: The balanced steady-state free precession sequence has been
previously explored to improve the efficient use of non-recoverable
hyperpolarized $^{13}$C magnetization, but suffers from poor spectral
selectivity and long acquisition time. The purpose of this study was to develop
a novel metabolite-specific 3D bSSFP ("MS-3DSSFP") sequence with
stack-of-spiral readouts for improved lactate imaging in hyperpolarized
[1-$^{13}$C]pyruvate studies on a clinical 3T scanner.
  Methods: Simulations were performed to evaluate the spectral response of the
MS-3DSSFP sequence. Thermal $^{13}$C phantom experiments were performed to
validate the MS-3DSSFP sequence. In vivo hyperpolarized [1-$^{13}$C]pyruvate
studies were performed to compare the MS-3DSSFP sequence with metabolite
specific gradient echo ("MS-GRE") sequences for lactate imaging.
  Results: Simulations, phantom and in vivo studies demonstrate that the
MS-3DSSFP sequence achieved spectrally selective excitation on lactate while
minimally perturbing other metabolites. Compared with MS-GRE sequences, the
MS-3DSSFP sequence showed approximately a 2.5-fold SNR improvement for lactate
imaging in rat kidneys, prostate tumors in a mouse model and human kidneys.
  Conclusions: Improved lactate imaging using the MS-3DSSFP sequence in
hyperpolarized [1-$^{13}$C]pyruvate studies was demonstrated in animals and
humans. The MS-3DSSFP sequence could be applied for other clinical applications
such as in the brain or adapted for imaging other metabolites such as pyruvate
and bicarbonate.
