Liquid Calibration Phantoms in Ultra-Low-Dose QCT for the Assessment of Bone Mineral Density

Abstract

Introduction: Cortical bone is affected by metabolic diseases. Some studies have shown that lower cortical bone mineral density (BMD) is related to increases in fracture risk which could be diagnosed by quantitative computed tomography (QCT). Nowadays, hybrid iterative reconstruction-based (HIR) computed tomography (CT) could be helpful to quantify the peripheral bone tissue. A key focus of this paper is to evaluate liquid calibration phantoms for BMD quantification in the tibia and under hybrid iterative reconstruction-based-CT with the different hydrogen dipotassium phosphate (K₂HPO₄) concentrations phantoms.

Methodology: Four ranges of concentrations of K₂HPO₄ were made and tested with 2 exposure settings. Accuracy of the phantoms with ash gravimetry and intermediate K₂HPO₄ concentration as hypothetical patients were evaluated. The correlations and mean differences between measured equivalent QCT BMD and ash density as a gold standard were calculated. Relative percentage error (RPE) in CT numbers of each concentration over a 6-mo period was reported.

Results: The correlation values (R² was close to 1.0), suggested that the precision of QCT-BMD measurements using standard and ultra-low dose settings were similar for all phantoms. The mean differences between QCT-BMD and the ash density for low concentrations (about 93 mg/cm³) were lower than high concentration phantoms with 135 and 234 mg/cm³ biases. In regard to accuracy test for hypothetical patient, RPE was up to 16.1% for the low concentration (LC) phantom for the case of high mineral content. However, the lowest RPE (0.4 to 1.8%) was obtained for the high concentration (HC) phantom, particularly for the high mineral content case. In addition, over 6 months, the K₂HPO₄ concentrations increased 25% for 50 mg/cm³ solution and 0.7 % for 1300 mg/cm³ solution in phantoms.

Conclusion: The excellent linear correlations between the QCT equivalent density and the ash density gold standard indicate that QCT can be used with submilisivert radiation dose. We conclude that using liquid calibration phantoms with a range of mineral content similar to that being measured will minimize bias. Finally, we suggest performing BMD measurements with ultra-low dose scan concurrent with iterative-based reconstruction to reduce radiation exposure.

Keywords: cortical bone; hybrid iterative reconstruction; quantitative computed tomography; tibia; ultra-low dose.