Effect of different CT scanners and settings on femoral failure loads calculated by finite element models

Abstract

In a multi-center patient study, using different CT scanners, CT-based finite element (FE) models are utilized to calculate failure loads of femora with metastases. Previous studies showed that using different CT scanners can result in different outcomes. This study aims to quantify the effects of (i) different CT scanners; (ii) different CT protocols with variations in slice thickness, field of view (FOV), and reconstruction kernel; and (iii) air between calibration phantom and patient, on Hounsfield Units (HU), bone mineral density (BMD), and FE failure load. Six cadaveric femora were scanned on four CT scanners. Scans were made with multiple CT protocols and with or without an air gap between the body model and calibration phantom. HU and calibrated BMD were determined in cortical and trabecular regions of interest. Non-linear isotropic FE models were constructed to calculate failure load. Mean differences between CT scanners varied up to 7% in cortical HU, 6% in trabecular HU, 6% in cortical BMD, 12% in trabecular BMD, and 17% in failure load. Changes in slice thickness and FOV had little effect (≤4%), while reconstruction kernels had a larger effect on HU (16%), BMD (17%), and failure load (9%). Air between the body model and calibration phantom slightly decreased the HU, BMD, and failure loads (≤8%). In conclusion, this study showed that quantitative analysis of CT images acquired with different CT scanners, and particularly reconstruction kernels, can induce relatively large differences in HU, BMD, and failure loads. Additionally, if possible, air artifacts should be avoided. © 2018 Orthopaedic Research Society. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res.

Keywords: CT protocol; CT scanner; failure load; femur; finite element model.

Figure 1

Axial, sagittal, and coronal slice and 3D plots of the trabecular (red) and cortical (green) ROIs.

Figure 2

One femur seemed to have lost bone marrow in the femoral head after the first compared to the second scanning session (A). For comparison, we show another femur after the first and the second scanning session (B). The red line depicts the edge of the trabecular ROIs.

Figure 3

Output of the scanners using the standard protocol (3 mm slices, FOV 480, standard kernel), in % relative to the average of all scanners (mean ± SD) for HU and BMD in the cortical and trabecular ROI, and simulated failure load. *significant difference.

Figure 4

Difference between standard scan (3 mm slices, FOV 480, standard kernel) and variation as percentage of the standard scan (mean ± SD) of variations in slice thickness (A), FOV (B), and reconstruction kernel (C) for HU and BMD in the cortical and trabecular ROI, and simulated failure load. * significant effect, holds for all of the CT scanners, as the interaction between CT scanner and slice thickness or FOV was not in the statistical model. +, significant effect.

Figure 5

Effect of an air gap (0, 5, and 10 cm) between calibration phantom and body model as percentage (mean ± SD) relative to the standard scan (0 cm air gap, 3 mm slices, FOV 480, standard kernel) for cortical HU and BMD, trabecular HU and BMD, and simulated failure load. a, significant difference on all CT scanners. b, significant difference on P1, P2, and To.