Hyperspectral Imaging (HSI) as a new diagnostic tool in free flap monitoring for soft tissue reconstruction: a proof of concept study

Abstract

Objectives: Free flap surgery is an essential procedure in soft tissue reconstruction. Complications due to vascular compromise often require revision surgery or flap removal. We present hyperspectral imaging (HSI) as a new tool in flap monitoring to improve sensitivity compared to established monitoring tools.

Methods: We performed a prospective observational cohort study including 22 patients. Flap perfusion was assessed by standard clinical parameters, Doppler ultrasound, and HSI on t0 (0 h), t1 (16-28 h postoperatively), and t2 (39-77 h postoperatively). HSI records light spectra from 500 to 1000 nm and provides information on tissue morphology, composition, and physiology. These parameters contain tissue oxygenation (StO2), near-infrared perfusion- (NIR PI), tissue hemoglobin- (THI), and tissue water index (TWI).

Results: Total flap loss was seen in n = 4 and partial loss in n = 2 cases. Every patient with StO2 or NIR PI below 40 at t1 had to be revised. No single patient with StO2 or NIR PI above 40 at t1 had to be revised. Significant differences between feasable (StO2 = 49; NIR PI = 45; THI = 16; TWI = 56) and flaps with revision surgery [StO2 = 28 (p < 0.001); NIR PI = 26 (p = 0.002); THI = 56 (p = 0.002); TWI = 47 (p = 0.045)] were present in all HSI parameters at t1 and even more significant at t2 (p < 0.0001).

Conclusion: HSI provides valuable data in free flap monitoring. The technique seems to be superior to the gold standard of flap monitoring. StO2 and NIR PI deliver the most valuable data and 40 could be used as a future threshold in surgical decision making. Clinical Trial Register This study is registered at the German Clinical Trials Register (DRKS) under the registration number DRKS00020926.

Keywords: Flap surgery; Hyperspectral imaging; Imaging; Monitoring; Reconstructive surgery.

Fig. 1

The Hyperspectral Imaging Camera System consisting of a mobile table stand, a swiveling hyperspectral camera including an illumination unit and objective lenses as well as the integrated evaluation software. Picture Copyright© Leipzig University Hospital, Department of Orthopedic, Trauma and Plastic Surgery; Leipzig Saxony, Germany

Fig. 2

Tissue oxygenation (StO2-) and NIR Perfusion Index of all measured areas at t1 (three areas per flap). The orange square is indicating the critical zone. All areas on viable flaps were outside and at least one area of each revised flap was in the critical zone

Fig. 3

Tissue oxygenation (StO2-) Index differences over time (t0-t2) between revised (n = 6) and non-revised flaps (n = 16)

Fig. 4

NIR Perfusion Index differences over time (t0–t2) between revised (n = 6) and non-revised flaps (n = 16)

Fig. 5

First line (a–c) showing a patients HSI results on t1 showing decreasing circulatory support with increasing distance from the A. tibialis anterior pedicle. The distal part had to be revised and covered by meshed skin graft. Second line (d–f) showing HSI results in a Latissimus Dorsi free flap tissue transfer on t0 indicating complete flap failure (both in StO2 and NIR PI)

Fig. 6

First column (a, d, g) = HSI color image; second column (b, e, h) = HSI StO2; third column (c, f, i) = HSI NIR Perfusion Index. First line (a–c) = t0, second row (d–f) = t1, third line (g–i) = t2. HSI photos and parameters over time. It becomes apparent that critical zones are already visible in the HSI, even though the clinical appearance is still regular (d–f). The critically areas both had to be removed and covered by meshed skin graft