A new scanning electron microscopy approach to the quantification of bone mineral distribution: backscattered electron image grey-levels correlated to calcium K alpha-line intensities.

The introduction of backscattered electron (BSE) imaging in scanning electron microscopy (SEM) has led to new possibilities for the evaluation of mineral distributions in bone on a microscopic level. The different grey-levels seen in the BSE-images can be used as a measure for the local mineral content of bone. In order to calibrate these BSE-grey-levels (BSE-GL) and correlate them to mineral contents, various attempts, using reference samples with known weighted mean atomic number and/or using simulated bone tissues with known hydroxyapatite concentrations, have been made. In contrast, a new approach is presented here based on measurements of the X-ray intensities of the calcium K alpha-line on selected areas of real bone samples; the measured intensities are then related to the corresponding BSE-GL. A linear positive correlation between weight percent (wt%) calcium and BSE-GL was found. When the BSE-mode is standardized using carbon and aluminum as references, the different mineral contents in bone samples can be recorded as BSE-GL, calibrated to wt% of calcium or hydroxyapatite (HA), respectively. The resulting mineral concentration histograms have a dynamic range from 0 to 89 wt% HA and have a binwidth resolution of 0.45 wt% HA. The presented modifications of the BSE method strongly enhance its feasibility in the field of bone research and its application as a special diagnostic tool for bone diseases.