31P NMR relaxation of cortical bone mineral at multiple magnetic field strengths and levels of demineralization

Recent work has shown that solid-state H-1 and P-31 MRI can provide detailed insight into bone matrix and mineral properties, thereby potentially enabling differentiation of osteoporosis from osteomalacia. However, P-31 MRI of bone mineral is hampered by unfavorable relaxation properties. Hence, accurate knowledge of these properties is critical to optimizing MRI of bone phosphorus. In this work, P-31 MRI signal-to-noise ratio (SNR) was predicted on the basis of T-1 and T-2* (effective transverse relaxation time) measured in lamb bone at six field strengths (1.5-11.7 T) and subsequently verified by 3D ultra-short echo-time and zero echo-time imaging. Further, T-1 was measured in deuterium-exchanged bone and partially demineralized bone. P-31 T-2* was found to decrease from 220.3 +/- 4.3 mu s to 98.0 +/- 1.4 mu s from 1.5 to 11.7 T, and T-1 to increase from 12.8 +/- 0.5 s to 97.3 +/- 6.4 s. Deuteron substitution of exchangeable water showed that 76% of the P-31 longitudinal relaxation rate is due to H-1-P-31 dipolar interactions. Lastly, hypomineralization was found to decrease T-1, which may have implications for P-31 MRI based mineralization density quantification. Despite the steep decrease in the T-2*/T-1 ratio, SNR should increase with field strength as B-0(0.4) for sample-dominated noise and as B-0(1.1) for coil-dominated noise. This was confirmed by imaging experiments. Copyright (c) 2013 John Wiley & Sons, Ltd.