BACKGROUND: Near-infrared fluorescence (NIRF) imaging is a new imaging technique that is used to visualize lymphatic vessels in humans. It has a high spatial and temporal resolution, allowing real-time visualization of lymphatic flow.

METHODS AND RESULTS: The current study investigated the intra- and inter-individual variability of the technique, and how local hypo- and hyperthermia (20°C, 40°C), as well as exercise affect lymph transport. In this study, 10 healthy volunteers were studied twice, with 2 weeks between. NIRF imaging was conducted by using intradermal Indocyanine green injections and a custom-built camera setup. All data were blinded before analysis and presented as mean ± standard deviation. Mean contraction frequency and lymph propulsion velocity were 0.59 ± 0.13 minutes-1 and 1.51 ± 0.24 cm/s, respectively, with no significant difference during each 4 hours examination or between the two visits. The maximal pressure that the lymphatic flow in the vessels could overcome on test day 1 and 2 was 56 ± 9 mmHg and 57 ± 9 mmHg, respectively (p = 0.496). Local hyperthermia increased contraction frequency from 0.62 ± 0.4 minutes to 1.46 ± 0.5 minutes-1 (p < 0.05). Hypothermia caused no significant changes. Immediately after exercise (exercising at a simulated distance of 1.4 km on a cycle ergometer), an increase in lymph propulsion velocity from 1.5 ± 0.49 to 2.2 ± 0.63 cm/s was observed (p < 0.05); whereas contraction frequency was unaltered. A decrease in contraction frequency from 0.68 ± 0.25 minutes to 0.35 ± 0.19 minutes-1 was observed 10 minutes after exercise, without a change in velocity.

CONCLUSIONS: NIRF imaging can be conducted for 4 hours without a change in lymphatic activity. Furthermore, it has the sensitivity to detect changes in lymphatic activity by local hyperthermia and exercise. No changes were seen after local hypothermia. Pumping pressure shows good repeatability, whereas the other parameters show poor repeatability.