Recent studies are increasingly using brain-imaging techniques in the technology-enhanced learning (TEL) context to understand students' cognitive processes during technology-assisted learning, with the ultimate goal to improve students' outcomes in these environments. Given the importance of the promising impact of brain-imaging techniques in the technology-enhanced learning context, it is of utmost importance to analyze and investigate studies published in the intersection of these research areas. However, despite the growing interest in this promising research field, there is a lack of systematic literature review investigating how brain-imaging techniques have been applied in technology-enhanced learning environments. Therefore, this article presents a systematic literature review (SLR) using a well-accepted guideline to perform a rigorous study of the current literature to investigate, analyze, and understand how brain-imaging techniques have been applied in technology-enhanced learning contexts. This SLR considered studies published in nine academic databases that resulted in a total of 3910 studies that, after the selection process, were reduced to 37 studies (published during 2001-2019) for the final analysis. The studies' content analysis was used to classify the studies according to their objectives, brain-imaging techniques and devices used, educational levels, study domains, and studies outcomes. The main results indicate that (i) most studies aim to use brain-imaging methods to measure/detect students' psychological processes while using educational technology to provide a personalized experience, (ii) the most used brain-imaging technique in the studies is electroencephalogram, and (iii) the most investigated study domain was Mathematics and Biology. Moreover, this article highlights some gaps found in the state-of-the-art and provides insights that can be used for future research.