Bone imaging is typically used to assess bone mineral density (BMD), morphology, or microstructure. Imaging modalities that are used for bone health assessment include dual energy X-ray absorptiometry (DXA), dual-energy photon absorptiometry (DPA), and standard and high-resolution peripheral quantitative computed tomography (pQCT, HR-pQCT).
Dual-Energy Absorptiometry (DXA, DPA)
BMD assessment by dual energy X-ray absorptiometry (DXA) imaging is considered by the World Health Organisation as the “gold standard” to diagnose osteoporosis and is the most widely used assessment technique for determining treatment effectiveness. DXA is a non-invasive, relatively safe modality for measuring areal BMD (aBMD), which is defined as bone mineral content per unit area in g/cm2. DPA is an older technology for measuring aBMD that is sometimes reported in studies conducted prior to the 1990’s.
Increases in areal BMD (aBMD) are presumed to be a suitable surrogate outcome for fracture risk reduction when assessing the effectiveness of SLOP therapy. “Optimal therapeutic outcome” would be defined as an increase in knee region BMD above the fracture threshold in the absence of fragility fracture.
There are several established methods for measuring BMD at the knee (Garland et al. 1993; Moreno et al. 2001; Eser et al. 2004; Morse et al. 2009). Regardless of the methodology chosen, assessment of knee region BMD is crucial as it best predicts knee region fracture risk after SCI (Eser et al. 2005; Garland et al. 2005; Lala et al. 2013).
Peripheral Quantitative Computed Tomography (pQCT, HR-pQCT)
Peripheral QCT is another non-invasive, relatively safe imaging modality that can be used to diagnose osteoporosis. Whereas DXA measures areal BMD, pQCT measures volumetric BMD (vBMD), which is defined as bone mineral content per unit volume in g/cm3. vBMD stands alongside aBMD as a surrogate outcome for fracture risk reduction. In addition to assessing volumetric bone density, pQCT can also differentiate cortical bone from trabecular bone and quantify architecture. However, pQCT is available as a clinical diagnostic tool in only a few countries.
High-resolution pQCT (HR-pQCT) improves upon the resolution of standard pQCT imaging, and is now available with as fine as 80 µm resolution. This imaging modality gives detailed information on the microarchitecture of peripheral bone, but is not widely available outside of research applications.