Quantitation of T 2 ′ anisotropic effects on magnetic resonance bone mineral density measurement

In this paper, the authors quantitate the anisotropy of susceptibility effects in an uniaxial trabecular bone model and show its relevance to clinical MR bone mineral density measurements. A physical model is described that quantitates the anisotropic MR behavior of uniaxial trabecular bone. To test the model, a phantom of parallel polyethylene filaments was scanned every 15° between 0° and 90° with respect to the system's main magnetic field ( B 0 ). The distal radial metaphysis of a healthy female volunteer was scanned in orthogonal projections. The signal from each phantom image and each radial image was separated in a pixel‐wise fashion into R 2 and R 2 ′ maps. As predicted, R 2 ′ relaxation showed anisotropic behavior and changed according to sin 2 (ϕ), confirming that columnar structures parallel with B 0 will cause no MR susceptibility effects. Scans of the distal radius showed that R 2 ′ relaxation was twice as great with the forearm perpendicular to B 0 as when it was parallel to it, demonstrating different contributions from struts and columns. For both phantom and radial bone scans, R 2 relaxation was isotropic and did not change with object orientation.