Cross-relaxation parameters in cortical bone assessed with different MR sequences.

Area of Science:

• Biophysics
• Magnetic Resonance Imaging
• Bone Biology

Background:

• Cortical bone's complex structure involves collagen and bound water.
• Understanding water-macromolecule interactions is crucial for bone characterization.
• Magnetic Resonance (MR) techniques offer non-invasive insights into bone tissue composition.

Purpose of the Study:

• To demonstrate magnetic resonance (MR) cross-relaxation effects in cortical bone.
• To compare different MR sequences for quantifying cross-relaxation parameters.
• To assess the potential of MR for in vivo collagen quantification.

Main Methods:

• Spectroscopic methods (inversion-recovery, off-resonance saturation) and variable flip angle (VFA) UTE imaging were used on bovine bone samples.
• A two-pool model with matrix algebra was employed for parameter assessment.
• Measurements were conducted on a 4.7 T laboratory scanner.

Main Results:

• Evidence of cross-relaxation was observed, attributed to collagen-bound water (25% water fraction).
• Off-resonance saturation provided the highest precision for model parameters, estimating 22-28% collagen methylene protons with a 11 μs transverse relaxation time.
• Magnetization transfer (MT) effects were detected across all sequences.

Conclusions:

• MR cross-relaxation is detectable in cortical bone.
• Off-resonance saturation is superior for precise quantification of collagen-related MR parameters.
• In vivo MT quantification may enable assessment of collagen content in bone.