(Hα+[NII])/[OIII] ratio images have been used to search for low-ionization small-scale structures in a sample of 258 planetary nebulae. Radially symmetrical knots or jet-like structures, which are enhanced in the ratio images compared to the neighbouring regions of the nebulae, have been identified in 23 objects. Some of these features are barely detected or invisible in the Hα+[NII] and [OIII] images. Empirical evidence, as well as some simple modelling, demonstrate that the selected features are characterized by having a lower ionization than the surrounding gas. For the above properties, most of them resemble the low-ionization microstructures discussed recently by Balick et al. (1993, 1994), and named FLIERs (Fast Low-Ionization Emission Regions). The present work shows that the image-division technique is a very useful tool to look for ionization structures in planetary nebulae and other HII regions. Its main advantage is that the division removes the morphological components which are pure density enhancements, so that the output image remains mostly sensitive to excitation and abundance variations through the nebulae.