Novel anthropomorphic hip phantom corrects systemic interscanner differences in proximal femoral vBMD

Sources of error in bone mineral density estimates from quantitative CT

Bone mineral density (BMD) estimates from quantitative computed tomography (QCT) have proven useful for opportunistic screening of osteoporosis, treatment monitoring, and bone strength measurement. These estimates are subject to bias and variance from a variety of sources related to the imaging equipment, methods applied in the estimation procedure, and the patients themselves. In this article, we review the literature to describe the sources and sizes of error in spine and hip BMD estimates from single-energy QCT that can result from factors related to the scanner, imaging techniques, imaging subject, calibration phantom, and calibration approach. We also describe the baseline variance that can be expected based on repeatability and reproducibility studies. Though reproducible BMD estimates may be achievable with QCT, a thorough understanding of the potential sources of error and their size relative to the diagnostic task is essential to their appropriate and meaningful interpretation.

Hip load capacity and yield load in men and women of all ages

Quantitative computed tomography (QCT) based finite element (FE) models can compute subject-specific proximal femoral strengths, or fracture loads, that are associated with hip fracture risk. These fracture loads are more strongly associated with measured fracture loads than are DXA and QCT measures and are predictive of hip fracture independently of DXA bone mineral density (BMD). However, interpreting FE-computed fracture loads of younger subjects for the purpose of evaluating hip fracture risk in old age is challenging due to limited reference data. The goal of this study was to address this issue by providing reference data for male and female adult subjects of all ages. QCT-based FE models of the left proximal femur of 216 women and 181 men, age 27 to 90 years, from a cohort of Rochester, MN residents were used to compute proximal femoral load capacities, i.e. the maximum loads that can be supported, in single-limb stance and posterolateral fall loading (Stance_LC and Fall_LC, respectively) [US Patent No. 9,245,069] and yield load under fall loading (Fall_yield). To relate these measures to information about hip fracture, the CT scanner and calibration phantom were cross-calibrated with those from our previous prospective study of hip fracture in older fracture and control subjects, the Age Gene/Environment Susceptibility (AGES) Reykjavik cohort. We then plotted Stance_LC_, Fall_LC and Fall_yield versus age for the two cohorts on the same graphs. Thus, proximal femoral strengths in individuals above 70 years of age can be assessed through direct comparison with the FE data from the AGES cohort which were analyzed using identical methods. To evaluate younger individuals, reductions in Stance_LC, Fall_LC and Fall_yield from the time of evaluation to age 70 years can be cautiously estimated from the average yearly cross-sectional decreases found in this study (108 N, 19.4 N and 14.4 N, respectively, in men and 120 N, 19.4 N and 21.6 N, respectively, in women), and the projected fracture loads can be compared with data from the AGES cohort. Although we did not set specific thresholds for identifying individuals at risk of hip fracture, these data provide some guidance and may be used to help establish diagnostic criteria in future. Additionally, given that these data were nearly entirely from Caucasian subjects, future research involving subjects of other races/ethnicities is necessary.

The Influence of Reconstruction Kernel on Bone Mineral and Strength Estimates Using Quantitative Computed Tomography and Finite Element Analysis

Quantitative computed tomography has been posed as an alternative imaging modality to investigate osteoporosis. We examined the influence of computed tomography convolution back-projection reconstruction kernels on the analysis of bone quantity and estimated mechanical properties in the proximal femur. Eighteen computed tomography scans of the proximal femur were reconstructed using both a standard smoothing reconstruction kernel and a bone-sharpening reconstruction kernel. Following phantom-based density calibration, we calculated typical bone quantity outcomes of integral volumetric bone mineral density, bone volume, and bone mineral content. Additionally, we performed finite element analysis in a standard sideways fall on the hip loading configuration. Significant differences for all outcome measures, except integral bone volume, were observed between the 2 reconstruction kernels. Volumetric bone mineral density measured using images reconstructed by the standard kernel was significantly lower (6.7%, p < 0.001) when compared with images reconstructed using the bone-sharpening kernel. Furthermore, the whole-bone stiffness and the failure load measured in images reconstructed by the standard kernel were significantly lower (16.5%, p < 0.001, and 18.2%, p < 0.001, respectively) when compared with the image reconstructed by the bone-sharpening kernel. These data suggest that for future quantitative computed tomography studies, a standardized reconstruction kernel will maximize reproducibility, independent of the use of a quantitative calibration phantom.

Hip load capacity cut-points for Astronaut Skeletal Health NASA Finite Element Strength Task Group Recommendations

Concerns raised at a 2010 Bone Summit held for National Aeronautics and Space Administration Johnson Space Center led experts in finite element (FE) modeling for hip fracture prediction to propose including hip load capacity in the standards for astronaut skeletal health. The current standards for bone are based upon areal bone mineral density (aBMD) measurements by dual X-ray absorptiometry (DXA) and an adaptation of aBMD cut-points for fragility fractures. Task Group members recommended (i) a minimum permissible outcome limit (POL) for post-mission hip bone load capacity, (ii) use of FE hip load capacity to further screen applicants to astronaut corps, (iii) a minimum pre-flight standard for a second long-duration mission, and (iv) a method for assessing which post-mission physical activities might increase an astronaut’s risk for fracture after return. QCT-FE models of eight astronaut were analyzed using nonlinear single-limb stance (NLS) and posterolateral fall (NLF) loading configurations. QCT data from the Age Gene/Environment Susceptibility (AGES) Reykjavik cohort and the Rochester Epidemiology Project were analyzed using identical modeling procedures. The 75^th percentile of NLS hip load capacity for fractured elderly males of the AGES cohort (9537N) was selected as a post-mission POL. The NLF model, in combination with a Probabilistic Risk Assessment tool, was used to assess the likelihood of exceeding the hip load capacity during post-flight activities. There was no recommendation to replace the current DXA-based standards. However, FE estimation of hip load capacity appeared more meaningful for younger, physically active astronauts and was recommended to supplement aBMD cut-points.

Practical considerations for obtaining high quality quantitative computed tomography data of the skeletal system

Quantitative CT (QCT) analysis involves the calculation of specific parameters such as bone volume and density from CT image data, and can be a powerful tool for understanding bone quality and quantity. However, without careful attention to detail during all steps of the acquisition and analysis process, data can be of poor- to unusable-quality. Good quality QCT for research requires meticulous attention to detail and standardization of all aspects of data collection and analysis to a degree that is uncommon in a clinical setting. Here, we review the literature to summarize practical and technical considerations for obtaining high quality QCT data, and provide examples of how each recommendation affects calculated variables. We also provide an overview of the QCT analysis technique to illustrate additional opportunities to improve data reproducibility and reliability. Key recommendations include: standardizing the scanner and data acquisition settings, minimizing image artifacts, selecting an appropriate reconstruction algorithm, and maximizing repeatability and objectivity during QCT analysis. The goal of the recommendations is to reduce potential sources of error throughout the analysis, from scan acquisition to the interpretation of results.

Quantitative Computed Tomography Protocols Affect Material Mapping and Quantitative Computed Tomography-Based Finite-Element Analysis Predicted Stiffness

Quantitative computed tomography-based finite-element analysis (QCT/FEA) has become increasingly popular in an attempt to understand and possibly reduce vertebral fracture risk. It is known that scanning acquisition settings affect Hounsfield units (HU) of the CT voxels. Material properties assignments in QCT/FEA, relating HU to Young's modulus, are performed by applying empirical equations. The purpose of this study was to evaluate the effect of QCT scanning protocols on predicted stiffness values from finite-element models. One fresh frozen cadaveric torso and a QCT calibration phantom were scanned six times varying voltage and current and reconstructed to obtain a total of 12 sets of images. Five vertebrae from the torso were experimentally tested to obtain stiffness values. QCT/FEA models of the five vertebrae were developed for the 12 image data resulting in a total of 60 models. Predicted stiffness was compared to the experimental values. The highest percent difference in stiffness was approximately 480% (80 kVp, 110 mAs, U70), while the lowest outcome was ∼1% (80 kVp, 110 mAs, U30). There was a clear distinction between reconstruction kernels in predicted outcomes, whereas voltage did not present a clear influence on results. The potential of QCT/FEA as an improvement to conventional fracture risk prediction tools is well established. However, it is important to establish research protocols that can lead to results that can be translated to the clinical setting.

The Estimation of Second-Generation HR-pQCT From First-Generation HR-pQCT Using In Vivo Cross-Calibration

Second-generation high-resolution peripheral quantitative computed tomography (HR-pQCT) provides the highest resolution in vivo to assess bone density and microarchitecture in 3D. Although strong agreement of most outcomes measured with first- (XCTI) and second- (XCTII) generation HR-pQCT has been demonstrated, the ability to use the two systems interchangeably is unknown. From in vivo measurements, we determined the limits of estimating XCTII data from XCTI scans conducted in vivo and whether that estimation can be improved by linear cross-calibration equations. These data are crucial as the research field transitions to the new technology. Our study design established cross-calibration equations by scanning 62 individuals on both systems on the same day and then tested those cross-calibrations on the same cohort 6 months later so that estimated (denoted as XCTII*) and "true" XCTII parameters could be compared. We calculated the generalized least-significant change (GLSC) for those predictions. There was strong agreement between both systems for density (R²  > 0.94), macroarchitecture (R²  > 0.95), and most microarchitecture outcomes with the exception of trabecular thickness (Tb.Th, R²  = 0.51 to 0.67). Linear regression equations largely eliminated the systematic error between XCTII and XCTII* and produced a good estimation of most outcomes, with individual error estimates between 0.2% and 3.4%, with the exception of Tt.BMD. Between-system GLSC was similar to within-XCTI LSC (eg, 8.3 to 41.9 mg HA/cm³ for density outcomes). We found that differences between outcomes assessed with XCTI and XCTII can be largely eliminated by cross-calibration. Tb.Th is poorly estimated because it is measured more accurately by XCTII than XCTI. It may be possible to use cross-calibration for most outcomes when both scanner generations are used for multicenter and longitudinal studies. © 2017 American Society for Bone and Mineral Research.

Intestinal Calcium Absorption Decreases Dramatically After Gastric Bypass Surgery Despite Optimization of Vitamin D Status

Roux-en-Y gastric bypass (RYGB) surgery has negative effects on bone, mediated in part by effects on nutrient absorption. Not only can RYGB result in vitamin D malabsorption, but the bypassed duodenum and proximal jejunum are also the predominant sites of active, transcellular, 1,25(OH)2 D-mediated calcium (Ca) uptake. However, Ca absorption occurs throughout the intestine, and those who undergo RYGB might maintain sufficient Ca absorption, particularly if vitamin D status and Ca intake are robust. We determined the effects of RYGB on intestinal fractional Ca absorption (FCA) while maintaining ample 25OHD levels (goal ≥30 ng/mL) and Ca intake (1200 mg daily) in a prospective cohort of 33 obese adults (BMI 44.7 ± 7.4 kg/m(2)). FCA was measured preoperatively and 6 months postoperatively with a dual stable isotope method. Other measures included calciotropic hormones, bone turnover markers, and BMD by DXA and QCT. Mean 6-month weight loss was 32.5 ± 8.4 kg (25.8% ± 5.2% of preoperative weight). FCA decreased from 32.7% ± 14.0% preoperatively to 6.9% ± 3.8% postoperatively (p < 0.0001), despite median (interquartile range) 25OHD levels of 41.0 (33.1 to 48.5) and 36.5 (28.8 to 40.4) ng/mL, respectively. Consistent with the FCA decline, 24-hour urinary Ca decreased, PTH increased, and 1,25(OH)2 D increased (p ≤ 0.02). Bone turnover markers increased markedly, areal BMD decreased at the proximal femur, and volumetric BMD decreased at the spine (p < 0.001). Those with lower postoperative FCA had greater increases in serum CTx (ρ = -0.43, p = 0.01). Declines in FCA and BMD were not correlated over the 6 months. In conclusion, FCA decreased dramatically after RYGB, even with most 25OHD levels ≥30 ng/mL and with recommended Ca intake. RYGB patients may need high Ca intake to prevent perturbations in Ca homeostasis, although the approach to Ca supplementation needs further study. Decline in FCA could contribute to the decline in BMD after RYGB, and strategies to avoid long-term skeletal consequences should be investigated.