Hyperspectral imaging (HSI)
Hyperspectral imaging (HSI) is a technique for capturing more spectral information from an object than a colour camera with typically three colour channels (RGB) can provide. Ideally, HSI can capture a complete spectrum in each pixel. HSI has the potential to provide information about biomarkers as well as distinguish between arterial (oxygenated) and venous blood or different tissue types in real-time. Moreover, it allows for easy, fast, and uncomplicated non-invasive handling.[38, 39] By capturing and analyzing spectral data, HSI enables both quantitative and qualitative evaluation of flaps in real time. Hyperspectral imaging (HSI) facilitates noninvasive and easily recordable assessment of tissue perfusion, with each measurement taking just 5 seconds. Depending on the commercially available system some have to maintain a fixed distance of 50 cm between the camera and the tissue, enabling evaluation of a standardized area of 30 × 30 cm. To ensure high-quality data acquisition, precautions such as minimizing light interference from sources like sunlight and room lighting are advised. The fundamental principle of HSI involves conducting spectroscopic remission measurements within the visible and near-infrared spectral range of 500–995 nm. During imaging, tissues are illuminated, and the resultant altered light reflected from the tissue is analyzed. Haemoglobin, being a principal optical component of tissues, undergoes variations in its oxygenated and reduced forms, particularly between vital and ischemic tissues. Visible light penetrates tissues to a depth of around 0.5 mm, whereas the near-infrared spectral range delves deeper, reaching depths of 3–5 mm, thus influencing microcirculation and blood flow parameters within upper tissue layers.
The software utilized from different systems for analysis extracts four key parameters: superficial skin haemoglobin oxygenation (StO2), superficial haemoglobin concentration (THI), near-infrared spectroscopy (NIR), and tissue water index (TWI). Thresholds for these parameters serve as indicators of local perfusion impairment, where THI values ≥53%, NIR values ≤25%, and TWI values ≤43% suggest compromised perfusion. Specifically, a THI ≥53% at the flap centre may indicate venous congestion. [40] So in conclusion, this technology utilizes the reflection of light at various wavelengths from illuminated tissues to evaluate parameters such as perfusion, oxygen saturation, and haemoglobin content. There is no direct physical contact between the device and the patient, and the findings are presented through colour-coded images displayed on a computer interface. [41] HSI has shown promise in providing valuable information about flap health and perfusion status and has shown superiority in detecting malperfusion in flaps compared with clinical examination within the first 24 hours postoperatively. [41] Furthermore, a systematic review comparing near-infrared spectroscopy (NIRS) and HSI emphasized the reliability, accuracy, and user-friendliness of HSI. [39] Another study focused specifically on HSI’s role in perfusion monitoring of free and pedicled flaps[35]. With HSI, signs of deterioration can be detected hours before clinical diagnosis.[39]