Characterization of Normal Bone in the Equine Distal Limb with Effective Atomic Number and Electron Density Determined with Single-Source Dual Energy and Detector-Based Spectral Computed Tomography

Abstract

Single-source dual energy (SSDECT) and detector-based spectral computed tomography (DBSCT) are emerging technologies allowing the interrogation of materials that have different attenuation properties at different energies. Both technologies enable the calculation of effective atomic number (EAN), an index to determine tissue composition, and electron density (ED), which is assumed to be associated with cellularity in tissues. In the present prospective observational study, EAN and ED values were determined for 16 zones in normal subchondral and trabecular bone of 37 equine cadaver limbs. Using both technologies, the following findings were obtained: 1. palmar/plantar EAN zone values in the fetlock increased significantly with increasing age of the horse; 2. all EAN and ED values were significantly lower in the trabecular bone than in the subchondral bone of all phalanges; 3. in the distal phalanx and navicular bone, most EAN and ED values were significantly lower compared to the proximal and middle phalanx; and 4. some EAN and ED values were significantly different between front and hind limbs. Several EAN and ED values significantly differed between SSDECT and DBSCT. The reported EAN and ED values in the subchondral and trabecular bone of the equine distal limb may serve as preliminary reference values and aid future evaluation and classification of diseases.

Keywords: computed tomography; dual energy; effective atomic number; electron density; equids; subchondral bone.

Figure 1

Regions of interest (ROIs; circles) and zones (mean EAN calculated from all ROIs included in an anatomical area; lines) in the dorsal aspect of a front fetlock joint on a mid-dorsal monoenergetic image (detector-based spectral computed tomography, Philips): ROIs in the medial and lateral metacarpal/metatarsal condyle (dorsal MC/MT zone; continuous line), the proximal subchondral and trabecular bone of the proximal phalanx (proximal P1 subchondral/trabecular zone; dashed/dotted line), the marrow cavity of P1, and the proximal epiphyseal P1 zone (dotted rectangular line) and dorsal FJ zone (continuous rectangular line).

Figure 2

Regions of interest (ROIs; circles) and zones (mean EAN calculated from all ROIs included in an anatomical area; lines) in the palmar/plantar aspect of a front fetlock joint on a transverse monoenergetic image (detector-based spectral computed tomography, Philips) at the mid-level of the proximal sesamoid bones (PSBs): ROIs in the subchondral bone of the medial and lateral metacarpal/metatarsal condyle (palmar/plantar MC/MT zone; continuous line), the dorsal subchondral bone of the medial and lateral PSB (PSB zone; dashed line), and the palmar/plantar FJ zone (continuous rectangular line).

Figure 3

Regions of interest (ROIs; circles) and zones (mean EAN calculated from all ROIs included in an anatomical area; lines) in a front second (P2) and third phalanx (P3) on a mid-dorsal monoenergetic image (detector-based spectral computed tomography, Philips): ROIs in the proximal subchondral and trabecular bone of P2/P3 (proximal P2/P3 subchondral/trabecular zone; continuous/dashed lines) and the proximal epiphyseal P2/3 zone (continuous rectangular line).

Figure 4

Medial, central, and lateral region of interest (ROI; circles) within the spongious bone of a front navicular bone on a mid-dorsal monoenergetic image (detector-based spectral computed tomography, Philips) and navicular zones (mean EAN calculated from all 3 ROIs; continuous line).