  Bone health related skeletal disorders are commonly diagnosed by X-ray
imaging, but the radiation limits its use. Light excitation and optical imaging
through the near-infrared-II window (NIR-II, 1000-1700 nm) can penetrate deep
tissues without radiation risk, but the targeting of contrast agent is
non-specific. Here, we report that lanthanide-doped nanocrystals can be
passively transported by endothelial cells and macrophages from the blood
vessels into bone marrow microenvironment. We found that this passive targeting
scheme can be effective for longer than two months. We therefore developed an
intravital 3D and high-resolution planar imaging instrumentation for bone
disease diagnosis. We demonstrated the regular monitoring of 1 mm bone defects
for over 10 days, with resolution similar to X-ray imaging result, but more
flexible use in prognosis. Moreover, the passive targeting can be used to
reveal the early onset inflammation at the joints as the synovitis in the early
stage of rheumatoid arthritis. Furthermore, the proposed method is comparable
to {\mu}CT in recognizing symptoms of osteoarthritis, including the mild
hyperostosis in femur which is ~100 {\mu}m thicker than normal, and the growth
of millimeter-scale osteophyte in the knee joint, which further proves the
power and universality of our approach in diagnosis of bone diseases
