Cortical Bone under an Ultrahigh Magnetic Field: Relaxation, Spectroscopy and Micron-resolution Imaging

Compact, mineralized cortical bone tissues are often concealed on magnetic resonance (MR) images. Recent development of MR instruments and pulse techniques has yielded significant advances in acquiring anatomical and physiological information from cortical bone despite its poor 1H signals. This work demonstrates the first MR research on cortical bones under an ultrahigh magnetic field of 14 T. The 1H signals of different mammalian species exhibit multi-exponential decays of three characteristic T2 or T2* values: 0.1–0.5 ms, 1–4 ms, and 4–8 ms. Systematic sample comparisons attribute these T2/T2* value ranges to collagen-bound water, pore water, and lipids, respectively. Ultrashort echo time (UTE) imaging under 14 T yielded spatial resolutions of 20–80 microns, which resolves the three-dimensional anatomy of the Haversian canals. The T2* relaxation characteristics further allow spatial classifications of collagen, pore water and lipids in human specimens. Our study achieves a record of the spatial resolution for MR imaging in bone and shows that ultrahigh-field MR has the unique ability to differentiate the soft and organic compartments in bone tissues.