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Shedding light on fluorescent imaging

Updated: Feb 25, 2022

There is a lot of excitement in the lymphoedema community about the use of indocyanine green (ICG) dye and fluorescent imaging to observe lymph flow through superficial lymphatics. I have provided a brief summary of two recent articles and offer some thoughts in the future potential uses of this exciting new imaging technique.

Near Infrared fluoroscopy (NIRF) uses a fluorescent dye injected into the subcutaneous compartment. As all dyes are lymph obligatory load this is taken up by the superficial lymphatics and can be seen by a specialised camera. B Aldrich M, Marshall M, Sevick E, Lanza G, Kotyk J, Culver J, et al. Seeing it through: Translational validation of new medical imaging modalities 2012. 764-76 p.

What is ICG fluoroscopy (near infra-red imaging, NIR)

Also called near-infrared fluorescence imaging (NIRFI) or near-infrared fluorescence lymphatic imaging (NIRFLI) the technique uses a very small subdermal injection of the ICG dye (usually used for cardiac imaging). As all dyes are lymph-obligatory-load, whatever dye enters the subdermal spaces will be taken up by the initial lymph vessels (ILV) and transported along the superficial collector vessels. In a darkened room, a specialised near infra-red camera can be used to visualise the fluorescing dye as it is passed along by the lymph collector vessels. Once the dye enters the deeper lymphatics it can no longer be visualised using this method.

Study 1: Using NIR to measure the effects of MLD

Lopera C., Worsley P.R., Bader D.L. & Fenlon D. Investigating the Short-Term Effects of Manual Lymphatic Drainage and Compression Garment Therapies on Lymphatic Function Using Near-Infrared Imaging. Lymphatic Research and Biology 15.3 (2017): 235-240. doi:10.1089/lrb.2017.0001

This study measured the effect of manual lymph drainage (MLD) and compression on lymph flow in the upper limb of nine people without lymphoedema. Video images of lymph vessels in the forearm were collected before and after 15-minutes of MLD and again after 10 minutes of compression with a class 2 off-the-shelf compression sleeve (Juzo). MLD techniques were as described by Vodder and included: stationary circles at the upper arm close to the axillary lymph nodes, pump and scoop technique at the upper arm, stationary circles over the cubital lymph nodes, pump and scoop at the forearm, and pump and scoop progressing over the entire arm toward the axilla.

The effect of MLD was measured as the speed (millimetres/second) at which 'parcels' of lymph were passed along the vessel. The group median was 6.7 mm/s before MLD and doubled to 13.3 mm/s after 15 minutes of treatment. After a further 10 minutes of garment use, the median was 10.5 mm/s. These results were statistically significant (p<0.05) which reduces the likelihood that they occurred by chance.

Study 2: Using NIRFLI to determine how much pressure is needed to stop lymph flow

Belgrado, J. P., Vandermeeren, L., Vankerckhove, S., Valsamis, J. B., Malloizel-Delaunay, J., Moraine, J. J., & Liebens, F. (2016). Near-Infrared Fluorescence Lymphatic Imaging to Reconsider Occlusion Pressure of Superficial Lymphatic Collectors in Upper Extremities of Healthy Volunteers. Lymphatic Research and Biology, 14(2), 70-77. doi:10.1089/lrb.2015.0040

Image from Belgrado et al (2016). doi:10.1089/lrb.2015.0040 View online here

This study used a clear blood pressure cuff so the superficial lymph vessels could still be visualised under the cuff. The pressure in the cuff was gradually increased until the flow of lymph underneath was stopped. In the upper arm of 30 healthy volunteers without lymphoedema it took 80 mmHg to stop the flow in most people. For a few, it took up to 100 mmHg to stop the lymph altogether. They also did some lymphatic stimulating techniques in this group so its a bit hard to tease out what the occlusion pressure would be in an arm that had no simultaneous lymphatic stimulation.

What the results mean for lymphoedema

Since neither study was on people with lymphoedema, we can't draw direct conclusions about ICG for lymphoedema from these two studies. But they did show that;

  • there is a high degree of variability in normal lymph flow between individuals

  • stimulating the lymphatic system using MLD increases lymph flow

  • lymphatic pumping can be maintained under external pressures up to 80mmHg

What is the future of fluorescent imaging in lymphoedema

Such clear imaging of any functioning lymph vessels under the skin has many potential applications and is already used pre-operatively in micro-vascular surgery such as lympho-venous anastamosis.

It may also provide insights in non-surgical settings for difficult cases, but how practical it will be to guide conservative treatment for existing lymphoedema remains to be seen. From a clinical perspective, we already have bio-impedance spectroscopy (BIS) to detect and monitor sub-clinical oedema and I don't see an invasive technique such as the ICG injection replacing this, other than in difficult cases.

The technique does have huge potential in research - but first we need some reliability studies to see if it delivers the same results with different operators. We also need to establish standardised methodologies re the way the injection is performed and how the imaging is captured. Once we know it is a valid method for measuring lymphatic function in people with lymphoedema then we can use it to literally 'show' the effectiveness of different interventions.

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