Introduction and evaluation of a gray-value voxel conversion technique

In micro finite element analyses (mu FEA) of cancellous bone, the 3D-imagin g data that the FEA-models are based on, contain a range of gray-values. In the construction of the eventual FEA-model, these gray-values are commonly thresholded. Although thresholding is successful at small voxel sizes, at larger voxel sizes there is substantial loss of trabecular connectivity. We propose a new method: the gray-value method, where the mu FEA-models use t he information within the 3D-imaging data directly, without prior threshold ing. Our question was twofold. First, how does the gray-value method compar e to both plain and mass-compensated thresholding? Second, what is the effe ct of element size on the results obtained with the gray-value method? We u sed nine mu CT-scans of human vertebral cancellous bone. These were degrade d to represent different resolutions, and converted into mu FEA-models usin g plain thresholding, mass-compensated thresholding, and the gray-value met hod. The apparent elastic moduli of the specimens were determined using mu FEA. The different methods were compared on the basis of the apparent elast ic moduli, compared to those calculated for a 28 mum reference model. The r esults showed that the gray-value method greatly improves the results relat ive to other methods. The gray-value method gives accurate predictions of t he apparent elastic moduli, for voxel sizes up to one trabecular thickness (Tb.Th.). For voxel sizes greater than one Tb.Th. the accuracy, although st ill better than for both thresholding methods, becomes increasingly worse. (C) 2001 Elsevier Science Ltd. All rights reserved.