Bone volume–to–total volume ratio measured in trabecular bone by single‐sided NMR devices
Dual‐energy X‐ray absorptiometry (DXA) is considered the gold standard for the diagnosis of osteoporosis. It uses ionizing radiations and provides a measure of the areal bone mineral density (BMD), but does not provide any information about the microstructure of the bone 3, while the strength of trabecular bone to resist fractures depends not only on BMD, but also on the architecture of the trabecular structure. Much epidemiologic evidence has shown that low values of BMD can explain approximately 50% of the incident fracture cases 5. This is because bones might have the same BMD, but very different structure parameters, like porosity.
Microcomputed X‐ray tomography (micro CT) is particularly suited to investigate the structure of calcified tissues at different scale lengths, but it requires a high X‐ray dose and is performed on small samples, and therefore not feasible in vivo.
Nuclear magnetic resonance appears to be a good choice, as it does not use ionizing radiations, and laboratory studies have shown the ability of NMR to assess structural properties of bone with different techniques, ranging from relaxometry 6 to diffusometry 10 and magic angle spinning spectroscopy 12. Magnetic resonance imaging allows one to assess bone structure in vivo 2, but most of the work has been done using high‐field whole body scanners. One proposed way to perform more manageable MRI analyses is the use of more compact and lower magnetic field scanners, for imaging trabecular bone structure on local body parts 22. Unfortunately, all of these devices could hardly be used for wide screening campaigns.
In this work, a method is proposed and validated to assess the bone volume–to–total volume (BV/TV) ratio (also called bone volume fraction) on trabecular bone specimens, through single‐sided NMR scanners. The BV/TV ratio is an important parameter to evaluate the microstructure of the trabecular bone 27.
The main feature that differentiates a single‐sided apparatus from others NMR scanners is the detection of the signal from a sample external to the magnet. These devices 28 are based on open NMR sensors and are equipped with a permanent magnet, able to detect NMR signal outside the magnet, regardless of the sample sizes, outside the laboratory or the hospital. The idea is to combine the advantages offered by a compact, portable, low‐cost, low magnetic field apparatus, with the possibility of spatial localization of the NMR signal, offered by the intrinsic magnetic field gradient of the device. The portability of these scanners would offer the further advantage of allowing the screening of population with easy and low‐cost protocols.
Single‐sided sensors have been applied to detect water contained in the skulls of mummies, and it has been suggested that these devices have the potential to replace destructive measurements by mercury intrusion porosimetry 30, highlighting their potential to assess microstructure‐related parameters.