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]