An optical coherence tomographic (OCT) system includes a sample arm, a reference arm and an OCT probe. The probe irradiates a bodily lumen and a fluid contained within the bodily lumen with light of a sample beam transmitted through a double-clad fiber (DCF). A first detector detects light of a reference beam and light reflected from the bodily lumen and propagated through the core of the DCF to generate OCT interference signals. Light backscattered by the bodily lumen and by the fluid contained in the bodily lumen is propagated through a cladding of the DCF and detected by a second detector to generate an intensity signal. A processor analyzes the intensity signal, and triggers a pullback of the probe and initiates recording of OCT images of the bodily lumen in response to the intensity the backscattered light reaching a predetermined threshold value.