Indocyanine green as a near-infrared fluorescent agent for identifying parathyroid glands during thyroid surgery in dogs

Background: Surgical procedures involving the thyroid gland require identification of the parathyroid glands. Indocyanine green (ICG) is a near-infrared (NIR) fluorescent contrast agent used for a variety of procedures such as intraoperative angiography, extrahepatic cholangiography, and lymph node mapping. In this study, we used a canine model to evaluate ICG for NIR fluorescent imaging of the parathyroid gland. Methods: Three dogs were used for the study. The dogs were administered general anesthesia, and after surgical dissection, each dog received a series of intravenous ICG doses ranging from 12.5 to 100 µg/kg ICG. The excitation light source used to illuminate the operating field was a NIR laser (λ = 785 nm). Intravascular ICG fluorescence (λ = 835/45 nm) was recorded using a charge-coupled device that employed optical filtering to block ambient and laser light. Fluorescent imaging was assessed after injection of each dose of ICG. Results: NIR fluorescent imaging visualized the parathyroid glands. The intensity curves showing the peak and plateau of fluorescence are similar regardless of the concentration of ICG. The time to peak fluorescent intensity was 50.2 ± 2.0 s after injection of ICG. Taking into consideration background fluorescent intensity, the estimated optimal dose of ICG was 18.75 µg/kg. At 106.7 ± 5.8 s, the parathyroid glands lost much of their fluorescence, although they remained sufficiently fluorescent to be distinguishable. There was a positive correlation of fluorescent intensity with ICG dose escalation up to 25 µg/kg. Conclusions: ICG NIR fluorescent imaging was useful in detecting the parathyroid glands of dogs. By allowing detection of parathyroid glands, the current technique shows promise for use by endocrine surgeons performing thyroidectomies.