The ornamental design for an arm of an x-ray imaging device, as shown and described.
FIG. 1 is a perspective view of an arm of an X-ray imaging device showing my new design;
FIG. 2 is a front elevation view of thereof;
FIG. 3 is a rear elevation view of thereof;
FIG. 4 is a right side view of thereof;
FIG. 5 is a left side view of thereof;
FIG. 6 is a top plan view of thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a top view of one member of the arm of an X-ray imaging device in a detached and flat state.
The broken lines in the drawing views are included for the purpose of illustrating portions of the X-ray imaging device that form no part of the claimed design.