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

Recent work has shown that solid-state 1H and 31 P MRI can provide detailed insight into bone matrix and mineral properties, thereby potentially enabling differentiation of osteoporosis from osteomalacia. However, ³¹P 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, ³¹P MRI signal-to-noise ratio (SNR) was predicted on the basis of T₁ and T₂* (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, T₁ was measured in deuterium-exchanged bone and partially demineralized bone. 31 P T₂* was found to decrease from 220.3 ± 4.3 μs to 98.0 ± 1.4 μs from 1.5 to 11.7 T, and T₁ 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-P dipolar interactions. Lastly, hypomineralization was found to decrease T₁, which may have implications for 31 P MRI based mineralization density quantification. Despite the steep decrease in the T₂*/T₁ ratio, SNR should increase with field strength as B₀^0.4 for sample-dominated noise and as B₀ 1.1 for coil-dominated noise. This was confirmed by imaging experiments.