An anatomic coordinate system of the femoral neck for highly reproducible BMD measurements using 3D QCT.

In this paper, a procedure for the determination of an anatomically oriented coordinate system of the femoral neck (NCS) in 3D spiral CT datasets is described. The origin of the NCS is centered in the minimal cross-sectional area of the neck. Its three axes are defined as follows: the so called neck axis is perpendicular to this area and points towards the femoral head, the second axis is the principal axis of the minimal cross-sectional area and the third axis is perpendicular to the other two. After a semi-automatic 3D segmentation of the proximal femur the NCS is automatically determined in a two-step minimization procedure. Relative to the coordinate system volumes of interest (VOIs) are positioned in which bone mineral density (BMD) and cortical thickness are analyzed. We investigated intra- and inter-operator precision of the position of the NCS, the BMD in cortical and trabecular VOIs, and cortical thickness in nine pelvic CT datasets obtained from clinical routine examinations. We further investigated the effect of increased noise by adding Gaussian distributed noise to measured projections before tomographic reconstruction. The mean precision error (averaged form the results of the nine datasets) of the NCS position was less than 0.5 mm and smaller than 2.25 degrees . There were no significant differences between inter- and intra-operator analyses. Precision errors in trabecular BMD were smaller than 3% in a stack of five 1 mm thin slices cut perpendicularly to the neck axis and smaller than 1% in a spherical VOI encompassing the neck. Relative precision errors for cortical BMD were smaller than 3% for both VOIs. An increase of noise up to a factor of 5 caused a maximal displacement of the NCS origin position by less than 1mm and a rotation by less than 2 degrees .