as mat is used in an x - ray diagnostic apparatus such as an x - ray ct apparatus . this mat comprises : a foaming plastic plate having at least major surfaces and edge portions , a biological body under medical examination being positioned above one of the major surfaces ; and a waterproof sheet having an x - ray absorption coefficient higher than that of the foaming plastic plate , for covering the foaming plastic plate in such a manner that planes of the waterproof sheet at said edge portions of the foaming plastic plate are not positioned in parallel with parallel planes of the major surfaces .