We demonstrate that nanoparticle x-ray fluorescence computed tomography in mouse-sized objects can be performed with very high spatial resolution at acceptable dose and exposure times with a compact laboratory system. The method relies on the combination of the 24 keV line-emission from a high-brightness liquid-metal-jet x-ray source, pencil-beam-forming x-ray optics, photon-counting energy-dispersive detection, and carefully matched (Mo) nanoparticles. Phantom experiments and simulations show that the arrangement significantly reduces Compton background and allows 100 μm detail imaging at dose and exposure times compatible with small-animal experiments. The method provides a possible path to in vivo molecular x-ray imaging at sub-100 μm resolution in mice.