According to some embodiments, an image processor includes an image generator, a region acquirer, and a label applicator. The region acquirer acquires at least one two-dimensional region designated on at least one first perspective image generated from three-dimensional volume data of a target. The label applicator applies a label on at least one first three-dimensional region. The at least one first three-dimensional region is a part of a second three-dimensional region. The second three-dimensional region is defined by the at least one two-dimensional region, a point and a surface which is defined by a set of straight lines between the point and the boundary of the two-dimensional region. The first three-dimensional region is defined to be a first overlapping region where the three-dimensional volume data and the second three-dimensional region overlap.