Advanced imaging surrogates of bone strength

Bone 41 (2007) S33 www.elsevier.com/locate/bone

Abstract Advanced imaging surrogates of bone strength Ralph Müller Institute for Biometrics, ETH Zürich, Zürich, Switzerland

tomography (μCT) at 1–100 μm and micromagnetic resonance imaging (μMR) at 20–200 μm. μCT

Noninvasive and/or nondestructive imaging techniques can provide structural information about bone, beyond standard bone mineral densitometry (BMD). While BMD is known to be an important determinant of bone strength, other factors also play a central role in explaining bone strength and fracture resistance. • Quantitative assessment of macrostructural characteristics such as geometry and section modulus and microstructural features such as trabecular volume density, trabecular spacing, number and connectivity may improve our understanding and ability to estimate bone strength and predict fractures. • The rationale for imaging bone macro/micro structure is to obtain information beyond BMD, improve fracture risk prediction, clarify the pathophysiology of skeletal disease, define the skeletal response to therapy and assess biomechanical relationships. • The methods for quantitatively assessing the macrostructure of bone (besides conventional radiography) include computed tomography (particularly high resolution computed tomography [hrCT] at 100–400 μm), volumetric quantitative computed tomography (vQCT) and high resolution magnetic resonance imaging (hrMR) at 100–200 μm. hrCT/vQCT/hrMR Strengths

Limitations

Widely available Noninvasive, nondestructive Provide macrostructure and bone density information Moderately accurate

Modest or even no exposure to Ionising radiation Lack of derived microstructural information Approximate microstructural parameters Considerable resolution dependence

Permit serial measurement of almost any body site

• The methods for assessing the microstructure of bone noninvasively and/or nondestructively include micro-computed

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Strengths

Limitations

Automated 2D and 3D evaluation Nondestructive imaging nature

High exposure to ionising radiation At highest resolution need for invasive biopsy with sampling errors Expense and limited availability of equipment

Permitting mechanical or other testing of the sample Highly accurate measurement

• The strengths and weaknesses of μMR are similar, except for the absence of ionising radiation, and the greater complexity and expense of this technology. • Despite the considerable progress made in bone imaging over the past decade, a number of challenges remain. • Technically, the challenges reflect the balances and trade-offs between spatial resolution, sampling size, signal-to-noise, radiation exposure and acquisition time or between the complexity and expense of the imaging technologies versus their availability and accessibility. • Clinically, the challenges for bone imaging include balancing the advantages of standard densitometric information versus the more complex architectural features of bone, or the deeper research requirements in the laboratory versus the broader needs in clinical practice. • The biological differences between the peripheral appendicular skeleton and the central axial skeleton and their impact on the relevant bone imaging methods must be further clarified. The relative merits of these sophisticated imaging techniques must be weighed with respect to their applications as diagnostic procedures, requiring high accuracy or reliability, versus their applications as monitoring procedures, requiring high precision or reproducibility. doi:10.1016/j.bone.2007.08.010