Acoustic emission testing is an effective technique to assess the condition of several types of structures, including electric power transformers and reactors. Traditional event location algorithms assume uniform propagation speed for the acoustic signals. This may not correspond to reality in case of inhomogeneous structures or equipments, as is the case with power transformers, since they are constituted of several materials, such as copper, steel, insulating oil, paper and cardboard, each one with a different acoustic propagation speed, limiting the accuracy of event location. A travel-time tomography algorithm can be used for the event location algorithm, giving a better precision to the results, as well as giving a mapping of acoustic propagation speeds inside the equipment under test. This mapping corresponds to an imaging of the internal components, allowing a better diagnostic, since the events can be better evaluated, correlating them with the position of critical components of the equipment. This represents a new way of using acoustic emission data. Basic principles of acoustic emission tomography are presented, as well as results obtained from electric power equipment tests.