Quantitation of the susceptibility difference between trabecular bone and bone marrow: experimental studies

Area of Science:

• Biophysics
• Medical Imaging
• Materials Science

Background:

• Trabecular bone's susceptibility differences create magnetic field inhomogeneities.
• These inhomogeneities affect magnetic resonance (MR) signal intensity in gradient-echo images.
• Proton diffusion and transverse magnetization dephasing are key relaxation mechanisms.

Purpose of the Study:

• Quantify relaxation mechanisms affecting signal intensity in bone marrow with trabecular bone.
• Investigate the relationship between trabecular bone density and MR relaxation times (T1, T2, T2*).
• Differentiate susceptibility-induced relaxation from chemical-shift effects in bone marrow.

Main Methods:

• In vitro experiments using human vertebrae (varying bone densities) in saline or oil-water emulsion.
• Measurement of MR relaxation times T1, T2, and T2*.
• In vivo studies on normal volunteers to assess bone marrow relaxation rates in epiphysis and diaphysis.

Main Results:

• In vitro T1 and T2 showed no significant variation with bone density.
• In vitro relaxation rate (1/T2*) increased with bone density (0.2 s-1/mg/cc) due to susceptibility effects.
• In vivo, fat and water T2* were shorter in epiphysis than diaphysis, correlating with bone density variations.

Conclusions:

• Susceptibility-induced dephasing is the primary mechanism shortening T2* in trabecular bone.
• Bone density significantly influences T2* in bone marrow, impacting gradient-echo image signal intensity.
• In vivo findings confirm the influence of bone density on bone marrow T2* in different skeletal regions.