Linewidth narrowing for $^{31}$Phosphorus MRI of cell membranes

Most $^{31}$P Magnetic Resonance Spectroscopy studies of tissues try to avoid contamination by a relatively large, but broad, spectral feature attributed to cell membrane phospholipids\footnote{W.J. Thoma et al., J. MR \textbf{61}, 141 (1985); E.J. Murphy et al., MR Med \textbf{12}, 282 (1989); R. McNamara et al., NMR Biomed \textbf{7}, 237 (1994).}. MRI using this broad $^{31}$P membrane spectrum is not even attempted, since the spatial resolution and signal-to-noise would be poor, relative to conventional MRI using the narrow $^{1}$H water spectrum. This long-standing barrier has been overcome by a novel pulse sequence, recently discovered in fundamental quantum computation research\footnote{Y. Dong et al. Phys. Rev. Lett. \textbf{100}, 247601 (2008); D. Li et al. Phys. Rev. B \textbf{77}, 214306 (2008).}, which narrows the broad $^{31}$P spectrum by $\sim$1000$\times$. Applying time-dependent gradients in synch with a repeating pulse block enables a new route to high spatial resolution, 3D $^{31}$P MRI of the soft solid components of cells and tissues. So far, intact and sectioned samples of \textit{ex vivo} fixed mouse organs have been imaged, with (sub-mm)$^{3}$ voxels. Extending the reach of MRI to broad spectra in natural and artificial tissues opens a new window into cells, enabling progress in biomedical research.