Photorefractive effects that are affected by x-rays are used to design an x-ray imaging system with outstanding spatial resolution. An elementary holographic reflection grating is recorded in a photorefractive iron-doped lithium niobate crystal. The x-rays yield a conductivity pattern that erases the hologram locally. The diffracted visible light now bears the x-ray image. In first experiments, structures of 25 μm size are easily resolved with a dose of 90 Gray and a signal-to-noise ratio of 6. The detector is fully reversible, has a logarithmic response, and is able to operate for hard x rays (>100 keV).