A COMPARISON OF SIMILARITY MEASURES FOR DIGITAL SUBTRACTION RADIOGRAPHY

Subtraction is useful in detecting small changes in sequentially acqui red radiographs. Even if the imaging geometry is constant, radiographs must be registered after their digitization. To compare different alg orithms for image registration and to register digital X-rays themselv es, various similarity measures have been proposed. This study compare s eight mathematical similarity standards using 172 radiographs acquir ed in different, but exactly known projection. Whenever the computatio n time is a critical factor, e.g. registering images using methods sim ilar to correlation techniques, the entropy of the subtraction image's histogram function (EHDI) is found to be the best similarity standard . If not, e.g. comparative assessing different image registration tech niques, the cross covariance coefficient (CCC) is appropriate. (C) 199 7 Elsevier Science Ltd.