Psychophysical responses to needling depth using ultrasound imaging.

De-qi, a characteristic sensory phenomenon associated with acupuncture, is considered essential for its therapeutic efficacy. Although needling depth is widely recognized as a key factor influencing de-qi, few studies have quantitatively examined this relationship within anatomically defined safety boundaries. In this study, we investigated psychophysical responses to acupuncture stimulation at two distinct acupoints, GB21 and ST36, using ultrasound-guided depth control and standardized de-qi assessment. For each participant, three reference depths were defined: a superficial depth just below the epimysium, a hazardous depth in proximity to critical anatomical structures, and an individualized target depth corresponding to the onset of an adequate de-qi sensation. Needling at the individualized target depth elicited significantly stronger de-qi sensations compared to superficial needling. Moreover, individualized target depth stimulation at GB21 produced more intense de-qi sensations than at ST36 and was associated with a wider safety margin. In contrast, the individualized target depth at ST36 was located closer to its hazardous depth, indicating a narrower safety window. Deeper insertions were associated with de-qi-specific sensations, such as soreness, heaviness, and aching, whereas sharp pain was more commonly reported at superficial depths. These findings demonstrated that de-qi sensation is influenced by both needling depth and anatomical context, highlighting the importance of personalized depth control. The integration of real-time imaging with psychophysical feedback may enhance the efficacy and safety of acupuncture interventions.