Digital image subtraction of temporally sequential chest images by rib image elimination

We studied image subtraction of temporally sequential chest images by rib image elimination. The image subtraction presents clinically useful information related to interval changes in chest images. Clinical chest images are not usually reproducible in terms of patient positioning, X-ray projection, inspiration, cardiac pulsation, etc. A proper image registration is needed prior to the subtraction. An image registration method by nonlinear warping was suggested [A. Kano et al., 1994]. In the method, template regions of interest and the corresponding search area regions of interest are selected in the lung areas on the two images. The method provides an improper image registration for some pairs of temporally sequential images. One of the reasons for the improper image registration is the difference between the shifts of lung and rib by inspiration. Some pairs of temporally sequential chest images show large difference of lung position in the pairs in spite of small difference of rib position in the pairs. In this case, the method does not provide a proper image registration. To get the proper image registration of the temporally sequential images with different position changes for the rib and lung, we eliminated rib images from the chest images assuming that the contribution of rib to the chest image is uniform. The nonlinear warping was then applied to the rib eliminating images leading to the properly registered images and enhanced interval changes.