  We describe a novel technique to characterize ultrashort electron bunches in
X-ray Free-Electron Lasers. Namely, we propose to use coherent Optical
Transition Radiation to measure three-dimensional (3D) electron density
distributions. Our method relies on the combination of two known diagnostics
setups, an Optical Replica Synthesizer (ORS) and an Optical Transition
Radiation (OTR) imager. Electron bunches are modulated at optical wavelengths
in the ORS setup. When these electron bunches pass through a metal foil target,
coherent radiation pulses of tens MW power are generated. It is thereafter
possible to exploit advantages of coherent imaging techniques, such as direct
imaging, diffractive imaging, Fourier holography and their combinations. The
proposed method opens up the possibility of real-time, wavelength-limited,
single-shot 3D imaging of an ultrashort electron bunch.
