Analysis of 1H-NMR relaxation time distributions in L1 to L6 rat lumbar vertebrae

A better knowledge of the NMR relaxation behavior of bone tissue can improve the definition of imaging protocols to detect bone diseases like osteoporosis. The six rat lumbar vertebrae, from L1 to L6, were analyzed by means of both transverse (T2) and longitudinal (T1) relaxation of 1H nuclei at 20 MHz and 30°C. Distributions of relaxation times, computed using the multiexponential inversion software uniform penalty inversion, extend over decades for both T2 and T1 relaxation. In all samples, the free induction decay (FID) from an inversion-recovery (IR) T1 measurement shows an approximately Gaussian (solid-like) component, exp[−12(t/TGC)2], with TGC ≈12 μs (GC for Gaussian component) and a liquid-like component (LLC) with initially simple-exponential decay. Averaging and smoothing procedures are adopted to obtain the ratio α between GC and LLC signals and to get separate T1 distributions for GC and LLC. Distributions of T1 for LLC show peaks centered at 300–500 ms and shoulders going down to 10 ms, whereas distributions of T1 for GC are single broad peaks centered at roughly 100 ms. The T2 distributions by Carr-Purcell-Meiboom-Gill at 600 μs echo spacing are very broad and extend from 1 ms to hundreds of ms. This long echo spacing does not allow one to see a peak in the region of hundreds of μs, which is better seen by single spin-echo T2 measurements. Results of the relaxation analysis were then compared with densitometric data. From the study, a clear picture of the intratrabecular and intertrabecular 1H signals emerges. In particular, the GC is presumed to be due to 1H in collagen, LLC due to all the fluids in the bone including water and fat, and the very short T2 peak due to the intratrabecular water. Overall, indications of some trends in composition and in pore-space distributions going from L1 to L6 appeared. Published results on rat vertebrae obtained by fitting the curves by discrete two-component models for both T2 and T1 are consistent with our results and can be better interpreted in light of the shown distributions of relaxation times.