Atomic resolution imaging using secondary electrons (emitted as a result of the interaction of incident fast electrons with a specimen) was only achieved recently. There has been considerable speculation as to the physical mechanisms underpinning the imaging. In this paper we use a quantum mechanical model to show that the image contrast is due to electrons ejected in inner-shell ionization events initiated by the primary beam, an atomic scale, and focused coherent electron probe. The angular probability distribution of the ejected electrons is key in understanding the (relative) contrast from different atomic species within the specimen. For a given species of atom, this angular probability distribution is predominantly determined by the angular momentum quantum number of the bound electron prior to ionization. The model is compared to experiment and reproduces the essential features in the data.