It is often difficult to come up with a well-principled approach to the
selection of a spatial indexing mechanism for medical image databases.
Spatial information about lesions in medical images is critically
important in disease diagnosis and plays an important role in image
retrieval. Unfortunately, the images are rarely indexed properly for
clinically useful retrieval. One example is the well-known R-tree and
its variants which index image objects based on their physical
locations in an "absolute" way. However, such information is not
meaningful in medical content-based image retrieval systems, and the
approaches above suffer from problems caused by variations in object
size and shape, imprecise image centering, etc. A more appropriate
approach, which does not require object registration, is to model the
spatial relationships between the lesions and anatomical landmarks. To
convey diagnostic information, lesions must exist in certain locations
with regard to the landmarks. In this paper, we show that the histogram
of forces (which represents the relative position between two objects)
provides an efficient spatial indexing mechanism in the medical domain.
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