An apparatus includes a transmission x-ray source having a window including a target layer of at least one x-ray generating material and an internal aperture configured to allow a first portion of an electron beam to bombard the target layer and to block a second portion of the electron beam from bombarding the target. The first portion of the electron beam has a full-width-at-half-maximum width at the target less than or equal to 1 micron. The window is spaced from the internal aperture by a first distance D1. The apparatus further includes an x-ray detector system having a scintillator, an optical assembly, at least one image sensor configured to receive and respond to visible light by generating electrical signals, and a motorized stage configured to controllably adjust a position of the scintillator such that the scintillator is spaced from the window by a second distance D2, wherein D 2 2 / ( D 1 + D 2 ) 2 is less than 0.2.