Detection of Perforators Using Smartphone Thermal Imaging

Sir: We read with interest the article by Hardwicke et al. entitled “Detection of Perforators Using Smartphone Thermal Imaging.”1 Since Itoh and Arai published their first English article on the use of thermography in perforator mapping,2 a number of articles have followed. This is nicely referenced in the book Imaging for Plastic Surgery.3 Over the past decade, we have routinely used thermal imaging in perforator flap surgery. In addition to systematic comparison with intraoperative findings, we have evaluated thermal imaging in conjunction with ultrasound Doppler imaging, computed tomography, and indocyanine green fluorescence angiography.4,5 Hardwicke et al. used static thermal imaging. In our experience, perforator mapping is most reliably performed with dynamic infrared thermography. After a mild cold challenge, rewarming of the skin proceeds by means of perforators. After cooling, perforators compete with each other in skin rewarming. By analyzing the rate and pattern of rewarming at the hotspots, a suitable perforator can be reliably selected. (See Video, Supplemental Digital Content 1, which shows dynamic infrared thermography in preoperative perforator mapping, https://links.lww.com/PRS/B898.) First-appearing hotspots are always associated with arterial Doppler signal and related to clearly visible perforators on computed tomographic angiography. Hotspots emerging simultaneously can be differentiated by the size of the rewarmed area at the hotspot; this difference is difficult to visualize with static thermography (Fig. 1). [See Figure, Supplemental Digital Content 2, which shows dynamic infrared thermography recorded with FLIR ONE in the preoperative phase. After 2 minutes’ cooling with a desktop fan, recording of the rewarming phase visualizes the difference between the bright hotspot on the right and left sides. With static thermal imaging (above, right), this difference would be more difficult to visualize, even after a period of acclimatization, https://links.lww.com/PRS/B899.]Fig. 1.: Static thermal imaging with FLIR ONE after acclimatization but no cooling (above) and dynamic infrared thermography recorded with FLIR ONE in the preoperative phase (below). After 2 minutes’ cooling with a desktop fan, recording of the rewarming phase visualizes the difference between the bright hotspot on the right and left sides. In contrast to Hardwicke et al., we prefer using a rainbow palette to highlight small differences in temperature.Video.: Supplemental Digital Content 1 shows dynamic infrared thermography in preoperative perforator mapping, https://links.lww.com/PRS/B898.We have documented that intraoperative dynamic infrared thermography can confirm that perforator dissection has been uneventful before flap transfer. Evaluation of the anastomosis is easily performed with dynamic infrared thermography and one can simply differentiate between arterial and venous perfusion problems.4 [See Figure, Supplemental Digital Content 3, which shows FLIR ONE images visualizing the appearance of hotspots located over the selected perforator (cross) after opening of the anastomosis. Within 3 minutes, the number and brightness of hotspots increase, https://links.lww.com/PRS/B900.] After flap inset, dynamic infrared thermography can be used to confirm adequate perfusion of the flap. Hardwicke et al. used static thermography to differentiate between well-perfused and inadequately perfused areas of the flap. We argue that this is inappropriate. Our dynamic infrared thermography study on perfusion dynamics of deep inferior epigastric artery and superficial inferior epigastric artery flaps during the first postoperative week clearly demonstrated that perfusion of these flaps is a dynamic process. There is a stepwise progression of perfusion that proceeds faster at the level of the subdermal plexus than at the subcutaneous layer. Thermography during the first postoperative week reveals the appearance of hotspots that were not visible intraoperatively. Resection based on intraoperative thermography may therefore result in discarding of viable tissue. Curran and colleagues have concluded that FLIR ONE (FLIR Systems, Inc., Wilsonville, Ore.) does not appear suitable for collecting absolute temperature data.6 However, in perforator mapping, only relative temperature differences are used. Dynamic infrared thermography can increase the reliability of FLIR ONE for perfusion imaging in the preoperative, intraoperative, and postoperative phases of perforator flap surgery. We congratulate Hardwicke et al. for bringing attention to the use of inexpensive smartphone infrared cameras in perforator flap surgery. FLIR ONE has considerable technical limitations. We would recommend using cameras with higher resolution for diagnostics, evaluation of treatment, and research purposes. Nevertheless, smartphone infrared cameras may have interesting potential as an alternative to expensive thermal cameras in certain clinical situations. More research is necessary to evaluate their utility in clinical and research practice. DISCLOSURE The authors have no financial interest in any of the products or devices mentioned in this communication. Sven Weum, M.D., Ph.D.Alexander Lott, M.D.Medical Imaging Research GroupDepartment of Clinical MedicineUiT The Arctic University of Norway, andDepartment of RadiologyUniversity Hospital of North Norway Louis de Weerd, M.D., Ph.D.