Exploring bone density analysis on routine CT scans as a tool for opportunistic osteoporosis screening

Abstract

The primary aim of this study was to evaluate computed tomography (CT)-based bone density analysis at the level of thoracic vertebra 12 (Th12) as a screening method for decreased bone density in patients admitted to the intensive care unit (ICU). Interobserver variability was analyzed. Secondary aims were to assess the prevalence of CT-based low bone density upon ICU admission in a cohort of COVID-19 patients and to assess the potential effect of long-term ICU stay on bone density in these patients. Retrospective single-center cohort study. ICU of the Leiden University Medical Center (LUMC), the Netherlands. Patients admitted to the ICU of the LUMC between March 1st, 2020 and February 1st, 2022 with a diagnosis of COVID-19, and a length of ICU stay of ≥ 21 days. In the included patients both baseline chest CT scans (obtained upon ICU admission) and follow-up chest CT scans (obtained ≥ 21 days after ICU admission) were available for analysis. A total of 118 CT scans in 38 patients were analyzed. There was a good interobserver variability, with an overall mean absolute difference (between measurements of three observers) of 9.7 Hounsfield Units (HU) and an intraclass correlation coefficient (ICC) of 0.93 (95% CI 0.88-0.96). The effect of intravenous contrast administration on bone density measurements was small (+ 7.5 HU (95% CI 3.4-11.5 HU)) higher in contrast enhanced CT images compared to non contrast enhanced CT images). Thirty-seven percent of patients had a bone density < 140 HU, suggestive of osteoporosis. No significant difference was found between bone density upon ICU admission and bone density at follow-up (≥ 21 days after ICU admission). Vertebral CT-based bone density analysis using routine CT scans is an easily applicable method to identify ICU patients with decreased bone density, which could enable enrollment in osteoporosis prevention programs. A high prevalence of low bone density was found in our cohort of ICU patients. There were no changes observed in bone density between baseline and follow-up measurements.

Keywords: Bone density; COVID-19; Computed tomography; Intensive care; Osteoporosis.

Figure 1

Measurement of CT-based bone density by manually placed ROI. CT-based bone density was measured by first locating the Th12 vertebral body in the sagittal plane (A). Subsequently, in the axial plane, a circular region of interest (ROI) was placed stretching from the anterior aspect of the vertebral body to the anterior side of the basivertebral vein (B). The mean CT-based bone density within the ROI was determined using the IDS7 software.

Figure 2

Interobserver agreement plot. Modified Bland–Altman plot showing the interobserver agreement between three observers for vertebral CT attenuation measurements. Differences between all measurements and the mean of the three observers (y-axis) are plotted against the mean HU values of the three observers for all CT scans (x-axis). The horizontal dashed lines represent the 95% CI limits of agreement with the mean of the three observers and range from -15 to 15 HU. HU Hounsfield Units

Figure 3

Distribution of bone density upon ICU admission. CT-based bone density was measured at the vertebral body of Th12. Cut-off values were based on previous findings by Pickhardt et al. for distinguishing osteoporosis from non-osteoporosis. For vertebral level Th12, the study of Pickhardt et al. showed that a CT-attenuation threshold of 115 HU or less was 90% specific and 51% sensitive for osteoporosis. A threshold of 165 or less was 89% sensitive and 49% specific for osteoporosis. A threshold of 140 or less showed a balanced sensitivity and specificity of 75 and 74%, respectively. ICU intensive care unit