Quantifying the material and structural determinants of bone strength

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ranelate is incorporated into the mineral substance of bone [86], it is not known whether these
observations are true changes in bone morphology or artefacts due to altered edge-detection and/or
partial volume effects.
Altogether, HR-pQCT is promising for assessment of trabecular and cortical architecture in vivo with
high precision. The disadvantages are that it requires specialised scanners, and measurements are
limited to peripheral skeletal sites. Furthermore, motion artefacts are common, requiring re-scanning
of many patients.
Magnetic resonance imaging (MRI)
Magnetic resonance (MR) imaging is a non-ionising method that uses a strong magnetic field in
combination with specialised sequences of radio-frequency pulses to generate 3D images of bone
structure [87]. Because free hydrogen in water provides the ‘signal’ in this type of MR imaging and
since the water content of bone is minimal, there is generally little signal provided by bone in
standard MR imaging. As a result, the bone structure is assessed indirectly via measurements of
the surrounding marrow and other soft tissues (Fig. 4). Advances in the past decade have focussed
on image acquisition and analysis techniques to overcome inherent obstacles in MR imaging of
bone [88].
High-resolution MR imaging is performed at peripheral skeletal sites (e.g., distal radius, distal tibia
and calcaneus) using clinical MR scanners combined with specially designed coils. In vivo resolutions of
150–300 mm in plane and a slice thickness of 300–500 mm have been achieved [89,90]. With this
resolution, it is not possible to produce accurate values for most features of trabecular architecture.
Nonetheless, the ‘apparent’ trabecular properties that are derived from these images correlate with
trabecular architecture obtained with higher-resolution techniques [91,92]. Until recently, evaluation
of trabecular bone morphology was limited to appendicular sites. However, innovative surface coils
and pulse sequences show potential for MR-based assessments of trabecular structure in the proximal
femur [93–95] (Fig. 5).
MR-derived trabecular microarchitecture measurements reflect age- and disease-specific differences
[96,97], and differentiate patients with hip and vertebral fractures from controls with the best
performance achieved by combinations of structural parameters and BMD [98–103]. There are no
studies demonstrating prospective fracture risk prediction, and there are limited data on treatmentrelated
changes [104,105].
Fig. 4. Magnetic resonance image of the distal tibia (voxel size: 156 156 500 mm). Trabecular bone appears dark on a light
background of marrow. Image courtesy of Dr. Sharmila Majumdar, UCSF.