Geometrical Transformations Of Density Images

With proper calibration several image acquisition modalities produce images whose intensity is proportional to the density of some conserved quantity or to the projection of such a density. Such "density" images may be obtained for example in computer assisted tomography or magnetic resonance imaging (MRI); projected density images may be obtained via radiography or scintigraphy. Within a given imaging modality, density images of a given set of objects are related through one-to-one geometrical transformations. Differences among the images may arise from object motion between acquisitions or from changes in the acquisition device itself. In this paper techniques are presented for finding approximate geometrical transformations between density images of a set of objects acquired via the same modality. Applications are discussed with respect to motion artifacts that obscure artery images in digital subtraction angiography (DSA) and field inhomogeniety artifacts in MRI. Experimental results in DSA are presented to demonstrate that the principle difficulty lies not in finding an effective transformation for removing motion artifacts, but in selecting one that distinguishes between the artifacts and the artery.

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