Volumetric density is mathematically defined as the mass per unit volume, i.e grams/cm3. Areal density, is derived from area based measurements and thus has units of mass per unit area, i.e grams/cm2. Imaging modalities such as micro-computed tomography or microCT generate images three-dimensionally. When properly calibrated against an X-Ray attenuation phantom, skeletal parameters such as mineral density can be accurately determined. Since the origin of this density of based on voxel information, users are able to look at vBMD, or volumetric Bone Mineral Density.

But, in a two-dimensional imaging modality such as Dual Energy X-Ray Absorptiometry, or DEXA, images are solely pixel based. The resulting BMD is based on area thus resulting in areal BMD or aBMD. Readers may wonder what the informational differences are between vBMD and aBMD.

Head to Head comparison of vBMD vs aBMD

aBMD generally lacks the consideration of bone thickness and body orientation. Because of the lack of the z-dimension, changes in bone quality across specimens may not be true. The oversight is addressed in vBMD, however. In addition, higher resolution can generally be achieved in vBMD measurements because object orientation can be adjusted in X,Y,Z to achieve optimum physical magnification. Since it seems that vBMD is more robust and accurate in density measurements, readers may wonder why aBMD is even given attention.

Utility of BMD (aBMD) from DEXA 

DEXA is a powerful tool that exists in the pre-clinical and clinical landscape due to its cost-effectiveness and data acquisition speed. Scans can be as quick as 1 minute long which make for ease of patient or sample handling as well as less exposure to harmful ionizing (X-Ray) radiation. Image processing is automated since images are purely two-dimensional.

The added benefit of quick scans and minimal radiation allow researchers to track changes of BMD longitudinally. Furthermore, studies have found positive correlation between vBMD and aBMD. Liu et al. in 2018 compared clinical image data between CT and DEXA and found a very promising correlation (Figure 1).

Ott et al. directly compared physical density measurements of macaque vertebrae using calipers and water weighing and their DEXA scan counterparts. They found very strong statistical correlations as well, proving that DEXA results can be stand-alone and indubitable.

References:

Ott et al. “Evaluation of Vertebral Volumetric vs. Areal Bone Mineral Density During Growth.” Bone, vol. 20, no. 6, June 1997, pp 553-556

Liu et al. “Quantification of Volumetric Bone Mineral Density of Proximal Femurs Using a Two-Compartment Model and Computed Tomography Images.” BioMed Research International, vol 2018, Article ID 6284269, 8 pages