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Photon imaging for MeV gammas has serious difficulties due to hugebackgrounds and unclearness in images, which are originated from incompletenessin determining the physical parameters of Compton scattering in detection,e.g., lack of the directional information of the recoil electrons. The recentmajor mission/instrument in the MeV band, Compton Gamma RayObservatory/COMPTEL, which was Compton Camera (CC), detected mere $\sim30$persistent sources. It is in stark contrast with $\sim$2000 sources in the GeVband. Here we report the performance of an Electron-Tracking Compton Camera(ETCC), and prove that it has a good potential to break through this stagnationin MeV gamma-ray astronomy. The ETCC provides all the parameters ofCompton-scattering by measuring 3-D recoil electron tracks; then the ScatterPlane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx),which CCs cannot measure, is also obtained, and is found to be indeed helpfulto reduce the background under conditions similar to space. Accordingly thesignificance in gamma detection is improved severalfold. On the other hand, SPDis essential to determine the point-spread function (PSF) quantitatively. TheSPD resolution is improved close to the theoretical limit for multiplescattering of recoil electrons. With such a well-determined PSF, we demonstratefor the first time that it is possible to provide reliable sensitivity inCompton imaging without utilizing an optimization algorithm. As such, thisstudy highlights the fundamental weak-points of CCs. In contrast we demonstratethe possibility of ETCC reaching the sensitivity below $1\times10^{-12}$ ergcm$^{-2}$ s$^{-1}$ at 1 MeV.

An Electron-Tracking Compton Telescope for a Survey of the Deep Universe by MeV gamma-rays