MRI Measures of Cortical Bone Water Concentration: Dependence on Age and Pore Volume Fraction

Introduction: Age-related increase in cortical bone porosity is a major cause of the impaired strength of osteoporotic cortical bone. The majority of these pores, however, are below the resolution limit achievable in vivo. Water H NMR signal in bone at 3T results from two compartments: long T2 (T2 ≥ 1 ms), corresponding to free water in pores (‘pore water’); and short T2 (T2 ~ 300-400 μs), corresponding to motionally restricted water bound to bone matrix collagen (‘bound water’) [1], as shown in Fig. 1. Ultrashort echo-time (UTE) MRI is able to detect the short-T2 components in cortical bone [2] and enables measurement of bulk bone water content (BWC) [3], the sum of bound and pore water. BWC is expected to increase as pore volume expands, but because it is the sum of two components which change in opposite directions as bone tissue is lost, it is a less than optimal measure of bone health. The suppression ratio (SR), defined as the ratio of signal amplitudes without and with long-T2 suppression (the red line in Fig. 1), may provide a better measurement of bone porosity. We quantify BWC in 72 human subjects and SR in 40 human subjects and 13 bone specimens, and compare these measurements to x-ray-based modalities.