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Correlating motion of internal organs with the displacements of fiducial markers during respiration

The purpose of the presented work is to establish quantitative correlations between displacements of markers on the human torso with displacement of internal anatomical points. Two sets of 5-15 points are selected. The external set {Rext} consists of points on the human chest and abdomen defined with respect to external anatomic features such as nipples, navel, floating ribs, abdominal muscles or bone joints. These points are used as pre-defined locations for attaching fiducial markers. The internal set {Rint} consists of points uniquely identifiable using MR images: coordinates of heart apex and heart center, diaphragm lobes, organ boundaries and other distinct features of internal anatomy. Short breathhold MR scans of the torso are acquired for various respiratory stages: full exhalation, full inhalation and several intermediate positions. For each stage, both {Rint} and {Rext} are determined from the MR images - the internal set from examining the anatomy and the external one from vitamin-E markers attached to the skin. Displacements of each of the points in the two sets with respect to the reference frame are computed in three dimensions. Correlations in the displacements are established from a series of patient studies. The information is used for two primary applications: motion correction in tomographic reconstruction and mechanical modeling of human anatomy. The motion correction application uses the established mapping for reconstruction of projections acquired during motion on a moving grid. Grid motion is deduced from optical tracing of fiducial markers placed at {Rext}. In constructing mechanical models of the human anatomy, the mapping between {Rint} and {Rext} can be used either both as an independent experimental test or explicitly as an extra set of spatiotemporal constraints.