Computer-assisted measurement of sagittal pelvic alignment parameters from radiographic images

Abstract

Background: Sagittal pelvic alignment is an important aspect of the sagittal balance that can be quantitatively assessed by measuring pelvic geometrical parameters, i.e. the sacral slope (SS), pelvic tilt (PT) and pelvic incidence (PI). In this paper we present the results of a completely automated computer-assisted measurement of the parameters of sagittal pelvic alignment from radiographic images, and test the hypothesis stating that there are no statistically significant differences between the obtained and reference manual measurements. Methods: Automated computer-assisted measurements of the sagittal pelvic alignment parameters are based on the latest technologies in the field of medical image processing and analysis, namely on the convolutional neural networks as a special group of deep learning techniques. In each sagittal radiographic image of the pelvis, regions of interest (sacral endplate and both femoral heads) are first automatically defined, and then distinctive points are detected within these regions, i.e. the anterior edge, the center and the posterior edge of the sacral endplate, to which a line is fitted at a later stage, and the centers of both femoral heads with the corresponding midpoint representing the hip axis. From the hip axis, and the line along the sacral endplate and its center point we can finally compute SS, PT and PI. Results: Measurements were retrospectively performed on 90 sagittal radiographic images of the pelvis from 47 subjects (19 males and 28 females; mean age 71.3 years). Statistical analysis of reference manual and automated computer-assisted measurements of the sagittal pelvic alignment parameters revealed a relatively good agreement and low variability. For SS, PT and PI, the mean absolute difference (standard deviation) was 4.9 ° (3.4 °), 2.7 ° (2.5 °) and 5.5 ° (4.2 °) respectively, the correlation coefficient was 0.71, 0.91 and 0.81 (p < 10 -6 ), and the paired t-test always confirmed the null hypothesis (p > 0.05). Conclusion: The results showed that there are no statistically significant differences between the reference manual and automated computer-assisted measurements of the sagittal pelvic alignment parameters. Moreover, the deviation from reference manual measurements is within the repeatability and reliability of manual parameter measurements, and therefore the parameters of sagittal pelvic alignment can be accurately determined by the automated computer--assisted measurement. Nevertheless, verification and confirmation of measured values cannot be completely omitted, as the deviation can be in specific cases quite large, especially due to the natural biological variability of the human anatomy and properties of radiographic imaging.