During operation of an x-ray source, an electron source emits a beam of electrons. Moreover, a repositioning mechanism selectively repositions the beam of electrons on a surface of a target based on a feedback parameter, where a location of the beam of electrons on the surface of the target defines a spot size of x-rays output by the x-ray source. In response to receiving the beam of electrons, the target provides a transmission source of the x-rays. Furthermore, a beam-parameter detector provides the feedback parameter based on a physical characteristic associated with the beam of electrons and/or the x-rays output by the x-ray source. This physical characteristic may include: at least a portion of an optical spectrum emitted by the target, secondary electrons emitted by the target based on a cross-sectional shape of the beam of electrons; an intensity of the x-rays output by the target; and/or a current from the target.