Linewidth Narrowing for $^{31}$Phosphorus MRI of Bones

Bone is a particularly challenging tissue to study with conventional MRI given the relatively low water density and wider linewidths of its solid components.\footnote{F. W. Wehrli, J. MRI \textbf{25}, 390 (2007); S. Anumula et al., Bone \textbf{42}, 405 (2008); D. Idiyatullin et al., J. Mag Res \textbf{193}, 267 (2008); E.E. Sigmund et al., NMR Biomed \textbf{22}, 436 (2009); Y. Wu et al., J. MRI \textbf{31}, 954 (2010)} Recent fundamental research in quantum computing gave rise to a new NMR pulse sequence that can be used to narrow the broad NMR spectrum of solids.\footnote{Y. Dong et al. Phys. Rev. Lett. \textbf{100}: 247601 (2008); D. Li et al. Phys. Rev. B \textbf{77}: 214306 (2008)} Here we narrow the spectrum of the $^{31}$P in natural bone mineral (by a factor of up to 1600x). This technique offers a new route to do high spatial resolution, 3D $^{31}$P MRI of bone which complements conventional MRI and x-ray based techniques to study bone physiology and structure. Thus far we have used our pulse sequence to do high spatial resolution (sub-250 $\mu$m)$^3$ 3D $^{31}$P MRI of \textit{ex vivo} dry bovine cortical bones, wet procine rib bones, and wet rabbit femoral bones at 4T. We have also explored the use of compressive sampling\footnote{M. Lustig et al., Mag Res Med \textbf{58}, 1182 (2007)} to push imaging time down to less than two hours without distracting artifacts.