Micron-resolution Imaging of Cortical Bone under 14 T Ultrahigh Magnetic Field

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 ¹ H signals. This work demonstrates the first MR research on cortical bones under an ultrahigh magnetic field of 14 T. The ¹ H 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 3D anatomy of the Haversian canals. The T₂ * relaxation characteristics further allow spatial classifications of collagen, pore water and lipids in human specimens. The 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.