Patient-Specific Phantomless Estimation of Bone Mineral Density and Its Effects on Finite Element Analysis Results: A Feasibility Study

Abstract

Objectives This study proposes a regression model for the phantomless Hounsfield units (HU) to bone mineral density (BMD) conversion including patient physical factors and analyzes the accuracy of the estimated BMD values. Methods The HU values, BMDs, circumferences of the body, and cross-sectional areas of bone were measured from 39 quantitative computed tomography images of L2 vertebrae and hips. Then, the phantomless HU-to-BMD conversion was derived using a multiple linear regression model. For the statistical analysis, the correlation between the estimated BMD values and the reference BMD values was evaluated using Pearson's correlation test. Voxelwise BMD and finite element analysis (FEA) results were analyzed in terms of root-mean-square error (RMSE) and strain energy density, respectively. Results The HU values and circumferences were statistically significant (p < 0.05) for the lumbar spine, whereas only the HU values were statistically significant (p < 0.05) for the proximal femur. The BMD values estimated using the proposed HU-to-BMD conversion were significantly correlated with those measured using the reference phantom: Pearson's correlation coefficients of 0.998 and 0.984 for the lumbar spine and proximal femur, respectively. The RMSEs of the estimated BMD values for the lumbar spine and hip were 4.26 ± 0.60 (mg/cc) and 8.35 ± 0.57 (mg/cc), respectively. The errors of total strain energy were 1.06% and 0.91%, respectively. Conclusions The proposed phantomless HU-to-BMD conversion demonstrates the potential of precisely estimating BMD values from CT images without the reference phantom and being utilized as a viable tool for FEA-based quantitative assessment using routine CT images.

Calibration with or without phantom for fracture risk prediction in cancer patients with femoral bone metastases using CT-based finite element models

“…Lee et al . tested a non-patient-specific phantomless calibration function on femoral FE models, and found it to be reliable as a replacement for phantom calibration [15]. However, they used one CT scanner and well protocolized CT scans, and therefore it has not yet been investigated whether non-patient-specific calibration methods are also useable for CT scans obtained on different CT scanners and with different settings.…”

“…Several methods have been developed for calibrating CT scans without a calibration phantom [8–15]. Some studies calibrate with the use of a calibration function obtained from a separate scan containing a calibration phantom [8], determine calibration factors based on CT scans that contain a calibration phantom and apply this calibration factor to CT scans without calibration phantoms [9], or calculate BMD using a regression model based on previous phantom calibration [15]. Another phantomless option is to use patient-specific internal calibration methods, which are based on HU of specific tissues, such as fat and muscle tissue [10–13] or external air and either aortic blood or visceral fat [14].…”

“…Another phantomless option is to use patient-specific internal calibration methods, which are based on HU of specific tissues, such as fat and muscle tissue [10–13] or external air and either aortic blood or visceral fat [14]. Studies comparing phantom calibration with phantomless calibrations showed that they yielded comparable results [8–15]. These studies used a single CT scanner or multiple CT scanners with a standardized protocol.…”

“…They found good correspondence between phantom and phantomless calibrations, with a mean difference between the calibrations of 30 N (0.8%) [14]. Another study used a general calibration function to calibrate CT scans for femoral FE models and found errors of total strain energy of 0.91% in comparison with a phantom calibration [15].…”

Is bone mineral density lower in the necrotic lesion in pre‐collapse osteonecrosis of the femoral head?

“…Hounsfield unit (HU) values on plain computed tomography (CT) represent a normalized index of X-ray attenuation based on a scale of −1000 defined for air and 0 for water. 7 HU values have been shown to correlate with BMD obtained using various measurement methods, such as DXA and quantitative CT. [8][9][10][11] Evaluation of HU values can be performed using any slice of clinically obtained plain CT scans, which enables a detailed BMD assessment of various bone structures even though these cannot be evaluated by DXA. 8,12,13 To the best of our knowledge, no previous reports have investigated the HU values of the necrotic lesion in pre-collapse ONFH.…”