European Forearm Phantom Research Articles

A new method to determine cortical bone thickness in CT images using a hybrid approach of parametric profile representation and local adaptive thresholds: Accuracy results.

MotivationCortical bone is an important contributor to bone strength and is pivotal to understand the etiology of osteoporotic fractures and the specific mechanisms of antiosteoporotic treatment regimen. 3D computed tomography (CT) can be used to measure cortical thickness, density, and mass in the proximal femur, lumbar vertebrae, and distal forearm. However, the spatial resolution of clinical whole body CT scanners is limited by radiation exposure; partial volume artefacts severely impair the accurate assessment of cortical parameters, in particular in locations where the cortex is thin such as in the lumbar vertebral bodies or in the femoral neck.MethodModel-based deconvolution approaches recover the cortical thickness by numerically deconvolving the image along 1D profiles using an estimated scanner point spread function (PSF) and a hypothesized uniform cortical bone mineral density (reference density). In this work we provide a new essentially analytical unique solution to the model-based cortex recovery problem using few characteristics of the measured profile and thus eliminate the non-linear optimization step for deconvolution. Also, the proposed approach allows to get rid of the PSF in the model and reduces sensitivity to errors in the reference density. Additionally, run-time and memory effective computation of cortical thickness was achieved with the help of a lookup table.ResultsThe method accuracy and robustness was validated and compared to that of a deconvolution approach recently proposed for cortical bone and of the 50% relative threshold technique: in a simulated environment with noise and various error levels in the reference density and using CT acquisitions of the European Forearm Phantom (EFP II), a modification of a widely used anthropomorphic standard of cortical and trabecular bone compartments that was scanned with various scan protocols.ConclusionResults of simulations and of phantom data analysis verified the following properties of the new method: 1) Robustness against errors in the reference density. 2) Excellent accuracy on ground truth data with various noise levels. 3) Very fast computation using a lookup table.

A new algorithm to improve assessment of cortical bone geometry in pQCT

High-resolution peripheral quantitative computed tomography (HR-pQCT) is now considered the leading imaging modality in bone research. However, access to HR-pQCT is limited and image acquisition is mainly constrained only for the distal third of appendicular bones. Hence, the conventional pQCT is still commonly used despite inaccurate threshold-based segmentation of cortical bone that can compromise the assessment of whole bone strength. Therefore, this study addressed whether the use of an advanced image processing algorithm, called OBS, can enhance the cortical bone analysis in pQCT images and provide similar information to HR-pQCT when the same volumes of interest are analyzed. Using pQCT images of European Forearm Phantom (EFP), and pQCT and HR-pQCT images of the distal tibia from 15 cadavers, we compared the results from the OBS algorithm with those obtained from common pQCT analyses, HR-pQCT manual analysis (considered as a gold standard) and common HR-pQCT analysis dual threshold technique. We found that the use of OBS segmentation method for pQCT image analysis of EFP data did not result in any improvement but reached similar performance in cortical bone delineation as did HR-pQCT image analyses. The assessments of cortical cross-sectional bone area and thickness by OBS algorithm were overestimated by less than 4% while area moments of inertia were overestimated by ~5–10%, depending on reference HR-pQCT analysis method. In conclusion, this study showed that the OBS algorithm performed reasonably well and it offers a promising practical tool to enhance the assessment of cortical bone geometry in pQCT.

Recommendations for Thresholds for Cortical Bone Geometry and Density Measurement by Peripheral Quantitative Computed Tomography

Peripheral quantitative computed tomography (pQCT) is widely used for clinical and research purposes. For accurate determination of bone geometry (bone cross-sectional area, cortical thickness, and cortical area), volumetric bone mineral density (vBMD) and cortical bone mineral content (BMC), it is important to select the appropriate thresholds. A Stratec XCT-2000 scanner was used to compare current standard practice with new optimized thresholds. Currently, a single threshold of 710 mg/mL for the measurement of cortical vBMD and geometry is used. We hypothesised that this threshold may not be optimal and used the European Forearm Phantom (EFP) and patient data to test more appropriate thresholds. A single slice (1.2 mm width, 0.4 mm pixel size) was made at section 4 of the EFP (representing the diaphyseal portion of a long bone). The EFP has a known cortical thickness of 2.5 mm and, therefore, the correct threshold for geometry would be that which measures cortical thickness as 2.5 mm. Thresholds were altered at approximately the 50% value between soft tissue (60 mg/mL) and peak density (879 mg/mL), and cortical thickness versus threshold was plotted; the correct threshold for geometry was 460 mg/mL. By expressing this threshold as a percentage of the range of density values in the EFP ([460-60]/[879-60] = 49%) and then applying this percentage to in vivo data, the optimum threshold for geometry can be determined: ([1240-79] x 0.49) + 79 = 648 mg/mL. For cortical vBMD of in vivo bone measurements at the midshaft site of the radius, thresholds were varied around the peak value (1240 mg/mL), and the threshold was set to that which gave a cortical density of 1240 mg/mL; the threshold for cortical density was, therefore, 1200 mg/mL. A subset of radius scans from a population of young healthy females was analyzed using the new thresholds (648 mg/mL for bone geometry, 1200 mg/mL for cortical vBMD) versus the current threshold (710 mg/mL). For bone geometry, the mean difference between the analysis based on the new threshold and that based on the manufacturer-recommended threshold ranged between 2.1% and 14% (total area = 2.1%, cortical thickness = 14%, cortical area = 3.7%). Although there was a 10% difference between the analysis based on the new threshold and that based on the manufacturer-recommended threshold, this difference was not systematic. Thresholds will significantly affect results obtained from pQCT. The current threshold of 710 mg/mL is inadequate for accurate determination of bone geometry and cortical vBMD. New thresholds of 648 mg/mL for geometry and 1,200 mg/mL for cortical vBMD should be used.

Bone mineral density measures in longitudinal studies: The choice of phantom is crucial for quality assessment. The Tromsø study, a population-based study

Determination of change in bone mineral density (BMD) requires high-precision densitometry techniques. The purpose of the study is to investigate to what degree different densitometer phantoms reflect observed changes in human BMD and to investigate to what degree fluctuations in densitometers' measurement level influence bone loss estimates. Densitometer influence was assessed using the aluminum forearm phantom (AFP) provided by the manufacturer, the European forearm phantom (EFP) of semi-anthropomorphic calcium-hydroxyapatite, and repeated population measurements on different densitometer combinations. The mean follow-up time was 6.4 years (SD 0.6). Measured population bone loss varied from 4.6%/year to 3.2%/year, depending on densitometer combinations. These variations could not be explained by differences in sex, age, height, weight and baseline BMD. They were predicted by EFP measurements, but not AFP measurements. The EFP measurements indicate that X-ray tube replacement changed the densitometers' measurement level in one of three instances, whereas "wear and tear" did not. We used the EFP data for adjustment of the densitometers' measurement levels. After adjustment, the overall crude bone loss was reduced from 4.14% to 3.92%. Mean annual loss was reduced from 0.64% or 0.61%. We conclude that densitometer performance might influence the accuracy of bone loss estimates. Changes in performance are not detected by aluminum phantoms. Quality control of BMD measurements in longitudinal studies should be performed with anthropomorphic calcium-hydroxyapatite phantoms in order to detect possible differences between the participating densitometers' measurement levels.

Higher Bone Mineral Density in Rural Compared with Urban Dwellers: The NOREPOS Study

Norway has a very high incidence of osteoporotic fractures, with substantial regional differences in fracture incidence. The present study evaluated whether there are differences in bone mineral density (BMD) between regions in Norway with differences in fracture incidence. The authors used data collected in four large, population-based, multipurpose studies performed in four regions of Norway during 1994-2001. Distal forearm BMD was measured by single energy x-ray absorptiometry in 10,667 participants aged 40-75 years. Cross-calibration was performed by using the European Forearm Phantom. Mean distal forearm BMD was lower in the urban populations of Tromso, Oslo, and Bergen compared with the rural county of Nord-Trondelag, whereas there was no difference between the rural part of Tromso and Nord-Trondelag. For women, body mass index explained some of these differences. The prevalence of low BMD (z score < or = -1) in Oslo, Bergen, and urban Tromso, compared with Nord-Trondelag, was 1.6-1.7 times higher in men and 1.5-2.0 times higher in women, whereas no significant difference was found between rural Tromso and Nord-Trondelag. In this study, higher BMD was found in rural compared with urban areas of Norway, which might help explain the differences in fracture incidence. There was no apparent north-south gradient in BMD.

Accuracy of cortical and trabecular bone measurements with peripheral quantitative computed tomography (pQCT)

In order to assess the accuracy of peripheral QCT (Stratec XCT 960) we analysed scans of the European Forearm Phantom and another phantom consisting of HPO encased in aluminium tubes to simulate cortical walls. Additionally 14 cadaveric forearm specimen scans were compared to CT scans acquired on a GE9800Q. The accuracy for density assessment of the European Forearm Phantom was better than 3%. A small increase in density was observed with increasing thickness of the aluminium wall (10% for each mm). Density measurements within the wall were confounded by limited spatial resolution. For a thickness of less than 4 mm, the density within the wall was underestimated by up to 40%. The measurement of mineral content was not influenced by this effect and showed an accuracy error of less than 6%. The agreement of density measurements on the different CT systems was very strong (; RMSE<6.2%). Our findings suggest that the Stratec pQCT scanner very accurately measures volumetric trabecular and total bone mineral densities at the distal radius while the assessment of cortical density is associated with considerable inaccuracies due to limited spatial resolution.

Clinical evaluation of a high-resolution new peripheral quantitative computerized tomography (pQCT) scanner for the bone densitometry at the lower limbs

Precision, long-term stability, linearity and accuracy of the x-ray peripheral quantitative computerized tomographic (pQCT) bone scanner XCT 3000 (Norland-Stratec Medical Sys.) were evaluated using the European Forearm Phantom (EFP). In vivo measurements were assessed using a standardized procedure at the distal femur and the distal tibia. In the patient-scan mode, the spatial resolution of the system was lp/mm as measured at the 10% level of the modulation transfer function (MTF). The contrast-detail diagram (CDD) yielded a minimal difference in attenuation coefficient (AC) of 0.07 at an object size of 0.5 mm. The effective dose for humans was calculated to be less than 1.5 Sv per scan. Short-term precision in vivo was expressed as root mean square standard deviation of paired measurements of 20 healthy volunteers (%). At the distal femur total volumetric density (ToD) and total cross-sectional area (ToA) were found to be less sensitive to positioning errors than at the distal tibia. Structural parameters like the polar cross-sectional moment of inertia or the polar cross-sectional moment of resistance showed a good short-term precision at the distal femur ( and 1.4%). The relation between the two skeletal sites with respect to or showed a high coefficient of determination ( and 0.74).

Radial Cortical and Trabecular Bone Densities of Men and Women Standardized with the European Forearm Phantom

Radial Cortical and Trabecular Bone Densities of Men and Women Standardized with the European Forearm Phantom