To address the gap in identifying patients at risk of fracture in the normal and osteopenic groups, Medimaps has developed a new clinical software tool: TBS iNsight™, which estimates fracture risk based on a determination of bone texture (an index related to bone microarchitecture). TBS iNsight can be used in addition to risks determined by DXA bone density and clinical risk factors.1,2 The result is expressed as a Trabecular Bone Score (TBS).
How it Works
TBS is a textural index that evaluates pixel gray-level variations in the lumbar spine DXA image, providing an indirect index of trabecular microarchitecture. Simply stated, TBS principles could be compared to an aerial view of a forest. An aerial view of a forest cannot discern individual elements of that forest (i.e., trees); the DXA image cannot discern the individual elements of its components (trabeculae). Although both of these ‘low power’ views do not have sufficient resolution to identify individual trabeculae (by the spine DXA image) or trees (in the forest aerial view), the areas of missing bone in the trabecular compartment or clearings in the forest are clearly noticeable (Figure 1).
Figure 1: Areas of a compact forest (A) and one with open clearings (B) is analogous to the patterns observed in highly dense (C) and porous (D) bone.
Figure 2: The TBS value is derived by an algorithm that analyzes the spatial organization of pixel intensity, which in turn corresponds to the differences in the X-ray absorption power of an osteoporotic bone versus a normal trabecular pattern.3
Applying this principle to the specifics of TBS, a dense trabecular microstructure projected onto a plane generates an image containing a large number of pixel-to-pixel gray level variations of small amplitude. Conversely, a 2D projection of a porous trabecular structure produces an image with a low number of pixel-to-pixel gray-level variations, but of much higher amplitude (Figure 2). A variogram of those projected images, calculated as the sum of the squared gray-level differences between pixels at a specific distance, can estimate a 3D structure from the existing variations on the 2D projected images. TBS is derived from the mathematical so-called “experimental variogram” of 2D projection images. TBS is calculated as the slope of the log-log transform of the 2D variogram, where the slope characterizes the rate of gray-level amplitude variations. A steep variogram slope with a high TBS value is associated with better bone structure, while low TBS values indicate worse bone structure. TBS iNsight integrates seamlessly with most existing DXA scanners. The exam, performed at the same time as DXA, requires no additional scan time or additional radiation exposure. Once the standard DXA spine scan is completed, TBS results are displayed automatically within seconds.
- Hans D, Barthe N, Boutroy S, Pothuaud L, Winzenrieth R, Krieg MA. Correlations between trabecular bone score, measured using anteroposterior dual-energy X-ray absorptiometry acquisition, and 3-dimensional parameters of bone microarchitecture: an experimental study on human cadaver vertebrae. J Clin Densitom. 2011Jul-Sep;14(3):302-12.
- Winzenrieth R, Michelet F, Hans D. Three-dimensional (3D) microarchitecture correlations with 2D projection image gray-level variations assessed by trabecular bone score using high-resolution computed tomographic acquisitions: effects of resolution and noise. J Clin Densitom. 2013 Jul-Sep;16(3):287-96.
- Silva BC, et al, Trabecular Bone Score: A non-invasive, analytical method based on the DXA image. J Bone Miner Res 2014; 29(3): 518-530.