CTXA hip: the effect of partial volume correction on volumetric bone mineral density data for cortical and trabecular bone

This study compares the results of computed tomography X-ray absorptiometry (CTXA) hip volumetric BMD (vBMD) analyses of cortical and trabecular bone with and without partial volume correction. For cortical bone in some circumstances, corrected cortical volumes were negative and corrected vBMD was very high. For trabecular bone, the correction effects are smaller. CTXA volumetric data should be interpreted with caution.

PURPOSE

Previous studies have reported concerns about the reliability of CTXA hip cortical vBMD measurements generated using partial volume (PV) correction (the "default" analysis, with cortical PV correction). To date, no studies have examined the results of the alternative ("new") analysis (with trabecular PV correction). This study presents in vivo and phantom data comparing the corrected and uncorrected data for cortical and trabecular bone respectively.

METHODS

We used the commercial QCTPro CTXA software to analyze CT scans of 129 elderly Chinese men and women and an anthropomorphic European Proximal Femur phantom (EPFP) and accessed data for two alternative scan analyses using the database dump utility. The CTXA software gives the user two methods of performing the PV correction: (1) a default analysis in which only cortical bone results are corrected; (2) a new analysis in which only trabecular bone results are corrected. Both methods are based on a numerical recalculation of vBMD values without any change in volume of interest (VOI) placement.

RESULT

In vivo, the results of the two analyses for integral bone were the same while cortical and trabecular results were different. PV correction of cortical bone led to a decrease of cortical volume for all four VOIs: total hip (TH), femoral neck (FN), trochanter (TR), and intertrochanter (IT) volumes were reduced on average by 7.8 cm³, 0.9 cm³, 2.5 cm³, and 4.3 cm³ respectively. For TR, where cortex was thinnest, average corrected cortical volume was negative (- 0.4± 1.3 cm³). Corrected cortical vBMD values were much larger than uncorrected ones for TH, FN, and IT. Scatter plots of corrected cortical vBMD against cortical bone thickness showed that elevated results correlated with thinner cortices. When trabecular bone was corrected for the PV effect, trabecular volumes of TH, FN, TR, and IT were reduced on average by 7.9 cm³, 0.8 cm³, 2.6 cm³, and 4.4 cm³ respectively, while vBMD measurements were increased correspondingly. The trabecular volume and vBMD measurements of the two datasets both had highly positive correlations. For the EPFP, the PV-corrected FN data deviated from the nominal phantom value, but was closer for the TR and IT VOIs. Both corrected and uncorrected data overestimated trabecular vBMD, with the corrected results showing greater deviation from nominal values.

CONCLUSION

The default and new CTXA analyses for volumetric data generate different results, both for cortical and trabecular bone. For cortical bone, the uncorrected results are subject to partial volume effects but the correction method of the default analysis overcorrects the effect leading to in part unreasonable results for cortical bone volume and BMD. For trabecular bone, the correction effects are smaller. CTXA volumetric data should be interpreted with caution.

QCT of the femur: Comparison between QCTPro CTXA and MIAF Femur

QCT is commonly employed in research studies and clinical trials to measure BMD at the proximal femur. In this study we compared two analysis software options, QCTPro CTXA and MIAF-Femur, using CT scans of the semi-anthropometric European Proximal Femur Phantom (EPFP) and in vivo data from 130 Chinese elderly men and women aged 60-80 years. Integral (Int), cortical (Cort) and trabecular (Trab) vBMD, volume, and BMC of the neck (FN), trochanter (TR), inter-trochanter (IT), and total hip (TH) VOIs were compared. Accuracy was determined in the 5 mm wide central portion of the femoral neck of the EPFP. Nominal values were: cross-sectional area (CSA) 4.9 cm², cortical thickness (C.Th) 2 mm, CortBMD 723 mg/cm³ and TrabBMD 100 mg/cm³. In MIAF the so-called peeled trabecular VOI was analyzed, which excludes subcortical bone to avoid partial volume artefacts at the endocortical border that artificially increase TrabBMD. For CTXA uncorrected, so called raw cortical values were used for the analysis. QCTPro and MIAF phantom results were: CSA 5.9 cm² versus 5.1 cm²; C.Th 1.68 mm versus 1.92 mm; CortBMD 578 mg/cm³ versus 569 mg/cm³; and TrabBMD 154 mg/cm³ versus 104 mg/cm³. In vivo correlations (R²) of integral and trabecular bone parameters ranged from 0.63 to 0.96. Bland-Altman analysis for TH and FN TrabBMD showed that lower mean values were associated with higher differences, which means that TrabBMD differences between MIAF and CTXA are larger for osteoporotic than for normal patients, which can be largely explained by the inclusion of subcortical BMD in the trabecular VOI analyzed by CTXA in combination with fixed thresholds used to separate cortical from trabecular bone compartments. Correlations between CTXA corrected CortBMD and MIAF were negative, whereas raw data correlated positively with MIAF measurements for all VOIs questioning the validity of the CTXA corrections. The EPFP results demonstrated higher MIAF accuracy of cortical thickness and TrabBMD. Integral and trabecular bone parameters were highly correlated between CTXA and MIAF. Partial volume artefacts at the endocortical border artificially increased trabecular BMD by CTXA, especially for osteoporosis patients. With respect to volumetric cortical measurements with CTXA, the use raw data is recommended, because corrected data cause a negative correlation with MIAF CortBMD.