Clinically compatible MRI strategies for discriminating bound and pore water in cortical bone

Abstract

Advances in modern MRI pulse sequences have enabled clinically-practical cortical bone imaging. Human cortical bone is known to contain a distribution of T1 and T2 components attributed to bound and pore water, although clinical imaging approaches have yet to discriminate bound from pore water on the basis of their relaxation properties. Herein, two clinically-compatible MRI strategies are proposed for selectively imaging either bound or pore water by utilizing differences in their T1s and T2s. The strategies are validated in a population of ex vivo human cortical bones, and estimates obtained for bound and pore water are compared to bone mechanical properties. Results show that the two MRI strategies provide good estimates of bound and pore water that correlate to bone mechanical properties. As such, the strategies for bound and pore water-discrimination shown herein should provide diagnostically useful tools for assessing bone fracture risk, once applied to clinical MRI.

In Vivo Quantitative MR Imaging of Bound and Pore Water in Cortical Bone

“…However, it is necessary to distinguish between the bound and pore water signals, because their sum has little or no relationship to mechanical properties (20). Bound and pore water signals can be discriminated on the basis of relaxation times by using wide-bandwidth T2-selective adiabatic radiofrequency (RF) pulses (21). Use of these pulses in conjunction with an ultrashort echo time (UTE) acquisition allows imaging of bound and pore water signal (22).…”

“…The pulse sequence diagram for these sequences is given in Figure 1. Signal equations and a more detailed description of these methods can be found in prior works (21,22).…”

Assessment of femoral bone osteoporosis in rats treated with simvastatin or fenofibrate

“…There are few reports on the association of peaks from T 2 distributions measured on rats' femurs. Horch et al [17,18] showed that, for human cortical bone, the T 2 values under millisecond correspond to bound water (primary to collagen) while the T 2 values larger than 1 ms (up to 1 s) correspond to pore water and lipids (bone marrow). Moreover, the majority of studies on human cortical bone are focused on the association of ultrashort T 2 values to collagen methylene protons (150 ms ≤ T 2 ≤ 1 ms), collagen-bound water protons (50 ms ≤ T 2 ≤ 1 ms) water protons in pores in lipid protons (1 ms ≤ T 2 ≤ 1 s) [16].…”

“…The 1 H CPMG (CarrPurcell-Meiboom-Gill) pulse sequence is a NMR method which correlated with Laplace inversion analysis lead to a relaxation T 2 spectrum that can be used to determine the porosity and to assess the pore size distribution in bone [10]. There are just a few studies by NMR spectroscopy [11] or relaxometry [12] on rats' bones or rat optical nerve and frog sciatic nerve [13], but there are many studies of human cortical bone by 1 H NMR relaxometry in particular 1D T 2 distribution [14][15][16][17][18][19] or 2D T 2 -T 2 exchange maps [20,21].…”

Three-dimensional adiabatic inversion recovery prepared ultrashort echo time cones (3D IR-UTE-Cones) imaging of cortical bone in the hip

“…This is done by inverting the longitudinal magnetization of the long T 2 signal components (i.e., muscle and bone marrow fat) while saturating the signal from cortical bone [27][28][29][30][31]. The Cones acquisition starts after an inversion time (TI) delay, which is used to null the long T 2 ⁎ components while permitting detection of recovered cortical bone signal (Fig.…”