Recently, several theoretical studies have shown that it is possible to obtain global images of the Earth's inner magnetosphere through detection of Energetic Neutral Atoms (ENA). These ENA, when precipitating toward the Earth, interact with the ionospheric and atmospheric particles through both elastic and inelastic collisions. These various interactions, which cause energy loss and deflection of the original particles, put critical constraints on the possibility of obtaining reliable magnetospheric images by ENA detection from low-altitude orbits. It follows that the estimate of a lower-altitude limit for the ENA imaging technique is crucial for this kind of studies. In the present study the ENA interaction processes are analyzed, and the rate of ENA destruction is estimated as a function of altitude for different energies and solar activities, along a radial line of sight on the magnetic equator, at MLT=0000.¿ 1997 American Geophysical Union