A company that uses computer imaging claims baseballs had a larger rubberized core and a synthetic rubber ring in 1998, including the ball Mark McGwire hit for his 70th homer. Universal Medical Systems Inc. said Wednesday that with the assistance of Drs. Avrami S. Grader and Dr. Philip M. Halleck from The Center for Quantitative Imaging at Penn State, it took images of 1998 baseballs.