External x-ray beam flattening filter

Claim 1

An external filter for flattening a plurality of large x-ray fields of an x-ray beam produced by a linear accelerator of the type having means for producing x-rays, means for collimating the x-rays into an x-ray beam having a longitudinal axis, and first filter means in said collimating means for roughly shaping said fields of the beam, said external filter comprising: A. a single solid disc-shaped member semi-permeable to said xrays and peripherally symmetrically shaped in the configuration of said x-ray field about a central axis, said disc-shaped member comprising: i. a planar lower surface lying in a plane perpendicular to the central axis; and ii. a geometrically shaped upper surface defined relative said lower surface by: a. a first ring-shaped surface parallel to said lower surface, defining a washer shaped volume therebetween about said central axis; b. a second surface linearly radially extending from said lower surfaCe to an outwardly directed peripheral edge of said first surface, defining a ring having a geometrical volume of triangular cross-sectional area about said central axis; c. a third surface radially extending from an internally directed edge of said first surface toward said central axis and terminating at a circle thereabout, defining a ring having a geometrical volume of polygon shaped cross-sectional area about said central axis; and d. a fourth surface forming a radially directed extension of said third surface in the direction of said central axis and terminating at said lower surface, defining a ring having a geometrical volume of triangular cross-sectional area about said central axis; and B. mounting means of x-ray permeable material, having a planar mounting surface to receive the lower surface of said discshaped member for mounting said disc-shaped member thereon, said mounting means being adapted for mounting alignment with said linear accelerator such that said longitudinal beam axis and said disc central axis are colinear.