51 – AUTOMATED QUANTITATIVE ANALYSIS OF METASTATIC DISEASE IN RAT SPINE: THE EFFECTS OF STEREOLOGICAL MODEL AND SPATIAL RESOLUTION

Abstract

Purpose: To examine the effect of image resolution and structural model on quantifying architectural differences between healthy and metastatically involved vertebrae.

Method: Lumbar vertebrae of healthy(n=6) and meta-statically involved(n=6) rnu/rnu rats were utilized. Osteolytic vertebral metastases were developed via intracardiac injection of human MT1 breast cancer cells. μCT images of the vertebrae were acquired ex vivo at 14μ isotropic spatial resolution. The whole vertebrae were segmented using an automated atlas based demons deformable registration followed by level set curvature evolutions. A subsequent iteration of level set was used to yield a segmentation of the trabecular centrum. The individual trabecular network was further segmented using intensity based thresholding. Architectural parameters were computed from the segmented μCT images: Cortical Bone Volume(CBV), Trabecular Bone Volume(TBV), Trabecular Bone Surface Area and the degree of anisotropy based on Mean Intercept Length(MIL). From this, trabecular Thickness(TbTh), Trabecular Number(TbN) and Trabecular Separation(TbS) were calculated using the Parfitt Model (Parfitt, Bone & Mineral. 1987). TbTh was also calculated separately using the Hilderbrand model (Hilderbrand, J of Microscopy 1997). The degree of anisotropy was determined via Mean Intercept Length (MIL) measured utilizing a binary shift/subtraction approach. The measures of TbTh and MIL were compared for each image at 8.725(high), 17.45(medium) and 34.9(low) μm3 isotropic spatial resolutions.

Results: Parfitt’s plate model showed a significant decrease in TBV, TbN and CBV and a significant increase in TbS in the metastatic vertebrae in comparison to the healthy group at the highest resolution. In both Hilderbrand’s and Parfitt’s models at the highest resolution there was no significant difference in TbTh between the healthy and metastatic groups. In both models, TbTh and TbS values rose while TBV and TbN decreased as the resolution was lowered. Significant reductions were observed only in TbTh between the healthy and metastatic vertebrae at the medium and low resolutions. In all cases, the Hildebrand model yielded lower values of TbTh than the Parfitt model. However, achieving robust automated results using the Hildebrand method was limited in the final stage of the segmentation due to sensitivity to small islands of bone. Structural anisotropy remained consistent in all groups at all resolutions, with ~3x greater MIL in the superior/inferior direction. The degree of anisotropy was, however, consistent in both groups suggesting that the metastatic destruction does not have any directional preference.

Conclusion: The automated use of Parfitt’s plate model along with the MIL method can be used to yield quantitative analyses demonstrating differences in vertebral microstructure due to metastatic involvement. However the sensitivity of these architectural parameters to resolution motivates the need for high resolution scanning in future preclinical applications.