  Megaelectronvolt ultrafast electron diffraction (UED) is a promising
detection tool for ultrafast processes. The quality of diffraction image is
determined by the transverse evolution of the probe bunch. In this paper, we
study the contributing terms of the emittance and space charge effects to the
bunch evolution in MeV UED scheme, employing a mean-field model with an
ellipsoidal distribution as well as particle tracking simulation. Small
transverse dimension of the drive laser is found critical to improve the
reciprocal resolution, exploiting both smaller emittance and larger transverse
bunch size before the solenoid. The degradation of reciprocal spatial
resolution caused by the space charge effects should be carefully controlled.
