A source-side radiation detector (SSRD) includes a detector module assembly, and a monitoring lens coupled to the detector module assembly, the detector module assembly and the monitoring lens being positioned proximate to an x-ray source, the monitoring lens including a plurality of slits configured to receive x-rays therethrough from the x-ray source, the detector module assembly being configured detect the x-rays transmitted through the slits and to generate information to track a position of a focal spot of the x-ray source.