Non-invasive Predictors of Human Cortical Bone Mechanical Properties: T2-Discriminated 1H NMR Compared with High Resolution X-ray

Recent advancements in magnetic resonance imaging (MRI) have enabled clinical imaging of human cortical bone, providing a potentially powerful new means for assessing bone health with molecular-scale sensitivities unavailable to conventional X-ray-based diagnostics. To this end, 1H nuclear magnetic resonance (NMR) and high-resolution X-ray signals from human cortical bone samples were correlated with mechanical properties of bone. Results showed that 1H NMR signals were better predictors of yield stress, peak stress, and pre-yield toughness than were the X-ray derived signals. These 1H NMR signals can, in principle, be extracted from clinical MRI, thus offering the potential for improved clinical assessment of fracture risk

Populations of planets in multiple star systems

Astronomers have discovered that both planets and binaries are abundant throughout the Galaxy. In combination, we know of over 100 planets in binary and higher-order multi-star systems, in both circumbinary and circumstellar configurations. In this chapter we review these findings and some of their implications for the formation of both stars and planets. Most of the planets found have been circumstellar, where there is seemingly a ruinous influence of the second star if sufficiently close (<50 AU). Hosts of hot Jupiters have been a particularly popular target for binary star studies, showing an enhanced rate of stellar multiplicity for moderately wide binaries (>100 AU). This was thought to be a sign of Kozai-Lidov migration, however recent studies have shown this mechanism to be too inefficient to account for the majority of hot Jupiters. A couple of dozen circumbinary planets have been proposed around both main sequence and evolved binaries. Around main sequence binaries there are preliminary indications that the frequency of gas giants is as high as those around single stars. There is however a conspicuous absence of circumbinary planets around the tightest main sequence binaries with periods of just a few days, suggesting a unique, more disruptive formation history of such close stellar pairs.Comment: Invited review chapter, accepted for publication in "Handbook of Exoplanets", ed. H. Deeg & J. A. Belmont

Axial μCT images are shown for cortical bone specimens from a 48 y.o. male donor (left) and an 82 y.o. male donor (right).

<p>For the 48 and 82 y.o. donors, respectively, avBMD was 1222 and 1135 mg-HA/cm³, and porosity was 4% and 11.3%.</p

Summary of T₂ spectra measured from 40 human cortical bone samples.

<p>All spectra exhibited a short-T₂ component (T₂≈60 µs), derived primarily from collagen protons, an intermediate T₂ components (T₂≈400 µs), derived primarily from collagen-bound water protons, and a broad distribution of long-T₂ components (1 ms2<1 s), derived from a combination of pore water and lipid protons.</p

From each cadaveric bone studied, one strip of cortical bone was extracted, three separate pieces of which were used for NMR, μCT, and mechanical testing.

<p>From each cadaveric bone studied, one strip of cortical bone was extracted, three separate pieces of which were used for NMR, μCT, and mechanical testing.</p

A summary of Pearson's r² for pairwise correlations between imaging measures (¹H NMR and X-ray) and mechanical properties.

<p>All correlations were significant (p<0.05) <i>except</i> those indicated with <b>^*</b>. The imaging measure that was most predictive (highest r²) of each mechanical measure is indicated with boldface type.</p

Correlations of measured peak stress and T₂ spectral component amplitudes (NMR, left) and avBMD measured by μCT (right).

<p>Blue, red, and green data show integrated amplitudes (<i>S_A</i>, <i>S_B</i>, and <i>S_C</i>) of the T₂-discriminated signals from pools A, B, and C, respectively. The black data show the total ¹H NMR signal (<i>S_A</i>+<i>S_B</i>+<i>S_C</i>), and the purple data are derived from μCT-based measures of avBMD. Each of the NMR signals amplitudes shows a significant linear correlation with peak stress and both <i>S_B</i> and <i>S_C</i> correlate more strongly with peak stress than does avBMD. Note that the total ¹H NMR signal does not correlate well with peak stress.</p

A representative stress vs. strain curve for cortical bone is shown (blue) along with graphical depictions of mechanical parameters.

<p>Flexural modulus is the slope of the initial linear mechanical response, yield stress is defined at 0.2% offset from the flexural modulus line, and peak stress is the maximum observed stress. Pre-yield toughness (see text for definition) is proportional to the area under the curve, up to the yield stress.</p