Subsalt wave-equation based migration (WEM) scan is an effective way for subsalt velocity updating, and has become an industry standard practice; however, the cost of generating migration scan is still comparatively high. With the more expensive Reverse Time Migration (RTM) gradually becoming a routine migration tool, this standard subsalt scan approach, becomes impractical. In this paper, we propose a new methodology for updating subsalt velocity using RTM-based scan with a range of variable time imaging conditions (Delta-T or DT scan). The generation of the RTM-based DT scan requires only a single pass of RTM. We generate a set of RTM scan images by applying zero-time as well as non-zero time imaging condition. Synthetic and real data testing confirm the effectiveness of this new approach and suggest this DT scan approach could serve as an efficient alternative or even a replacement to the standard subsalt migration scan approach for RTM.