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

Recent work has shown that solid‐state 1 H and 31 P MRI can provide detailed insight into bone matrix and mineral properties, thereby potentially enabling differentiation of osteoporosis from osteomalacia. However, 31 P 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, 31 P 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. 31 P T 2 * was found to decrease from 220.3 ± 4.3 µs to 98.0 ± 1.4 µs 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 31 P longitudinal relaxation rate is due to 1 H– 31 P dipolar interactions. Lastly, hypomineralization was found to decrease T 1 , which may have implications for 31 P 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 © 2013 John Wiley & Sons, Ltd.