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268 V. Dini et al.
Daeschlein et al. 2017). Hyperspectral sensors (detectors) evaluate the reectance information of an object and provide reectance spectra for each pixel in the image. The data thus acquired can be used to reconstruct three-dimensional models. The spatial resolution depends on the distance between the sensor and the measured object. The acquired images are reprocessed using tabulated values for tissue chromophores of interest (oxygenated and deoxygenated haemoglobin, melanin, etc.) by estimating their concentrations (Xu et al. 2012).
This imaging method at wavelengths between 500 and 700 nm assesses the concentration of oxyhaemoglobin and deoxyhaemoglobin in the tissue providing maps of tissue O
level that allow for the assessment of wound healing potential
2
with use for example in diabetic foot and burn scars. These values were used to predict diabetic feet at risk of ulceration, the potential to heal or the risk of the development of chronic wounds (Nouvong et al. 2009). Compared with other optical systems (such as earlier RGBs, laser-based systems or magnetic resonance imaging), HSI has several advantages such as high achievable spatial resolution and spectral resolution beyond the visible range.
This method, however, is not without disadvantages: the methods that acquire the most data correspond to those with the highest cost, and numerous standardized databases are needed for the proper acquisition of data in a standardized and reproducible manner (Sen et al. 2016). Several studies have been conducted to examine the potential appli cations of HSI for medical diagnosis of tissue condition and for monitoring wound healing in clinical settings.
Yudovsky et al. in (2010) used HSI to detect and predict ulceration in the diabetic foot concluding that this method can optimize wound care by predicting the risk of ulceration and predicting healing outcomes (Yudovsky et al. 2010). Data on skin oxygenation and perfusion can also be used to dene the characteristics of irradiated skin by providing prognostic data on skin reaction immediately after irradiation (Chin et al. 2012). Calin et al. demonstrated in burn injuries how HSI can identify subcu­taneous edema much more rapidly than laser Doppler scanning (Calin et al. 2015).
Khaodhiar et al. developed a healing index based on the measurement of oxygen-haemoglobin and deoxyhaemoglobin at the wound site and in areas adja­cent to the ulcer (Khaodhiar et al. 2007). They found signicant differences between tissue oxygenation of healed and unhealed foot ulcers. Spectrophotometric analysis can also provide information about the layer of tissue that sustained the injury, as hematomas near the surface have a different colour signature than those in deeper tissues (Bohnert et al. 2000; Paul et al. 2015).
HSI has also been used to study ischemic wounds. However, because of its limitations, it has not yet been widely proposed in clinical practice. Many HSI even in this case the cheapest ones have poor reproducibility and low accuracy. We also have inherent limitations due to the method. The acquired data are vulnerable to variations in illumination, detection, and tissue surface conditions. Second, the acquired data may depend on inter-patient variations in skin colour and adipose tissue content. Clear correlations on tissue spectral properties and pathophysio­logical characteristics have not yet been made and adequate calibration tools are lacking (Sen et al. 2016). Ultimately through the study of hyperspectral images it
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will be possible to better study tissue healing mechanisms by investigating other chromophores such as mitochondrial cytochromes to establish cellular metabolism and clustering of broblast populations. It can be possible to determine the spectral signature of the cytochromes b/c, which are located on the membranes of mito­chondria and involved in the ATP-synthesis (Wahabzada et al. 2017). Current advances in sensors and machine learning technologies are also expected to provide powerful new tools for human medicine.
Laser Doppler Imaging (LDI)
Laser Doppler perfusion imaging (LDI or LDPI) allows you to quantify microcir­culatory ow in a well-de ned region of interest (ROI) of the skin. It is based on the Doppler effect principle by analysing the changes in the wavelength of electro­magnetic radiation reected by erythrocytes moving in blood vessels. The magni­tude and frequency distribution of the Doppler-shifted light are directly related to the number density and velocity of blood cells. Tissue penetrating laser light generates scattering and absorption with scattered light returning to tissue surface being registered by a photo detector and calculated as blood ow in real time (Kloppenberg et al. 2001).
Blood ow analysis can also be integrated with other classical imaging methods such as digital photography has been applied to the assessment of burns, scars, pressure ulcers, diabetic foot ulcers and venous leg ulcers. Providing prognostic data on burn wound outcomes and healing times. Several studies have shown greater accuracy in predicting the prognosis of various burn wounds than clinical data alone (Bohnert et al. 2000). LDI was used to study the microcirculation of diabetic wounds. pressure ulcers have also been studied using LDI. it has been shown that in stage 1 pressure ulcers there is a difference in blood perfusion between the skin areas of the pressure ulcers and the periwound undam aged skin. By means of the combined methods of LDI and digital photography they were monitored in the postoperative period.
LDI mainly is used in patients with burn wounds. The benet of LDI consists of an objective real time evaluation of burn wound depth but requires sophisticated knowledge for evaluation and interpretation (Droog et al. 2001).
Laser Speckle Imaging (LSI)
Laser Speckle Contrast Imaging (LSCI or LSI) is a strictly qualitative laser-based imaging technique used to study perfusion. Unlike the LDI technique described above, this imaging technique is used to provide quality data on a larger surface. Its principle of operation is based on the reection of light waves by the moving blood cells. When they are hit by photons, they generate some photons in phase and in anti-phase (in a constructive and destructive way) that create shadow areas that
270 V. Dini et al.
change continuously. In contrast to stable particles objects in movement like red cells in blood vessels show a speckle pattern changing over time in dependence of the degree of movement which is recorded as blurring signal with a CCD camera. In LSI, these randomly changing patterns are averaged over time to produce what is referred to as blurwhich is related to blood ow. The advantage of this type of acquisition is that it can be done quickly almost instantaneously. He Ne laser source at 633 nm have been used on animal models and to study the performance of scars (Paul et al. 2015).
Near-Infrared Spectroscopy (NIRS)
The operation of NIRS is based on the different degree of ligh t absorption in the near-infrared spectrum of tissue components with different oxygenation states. NIRS employs light emitting LEDs or laser diodes operating between 650 and 900 nm that illuminate the skin tissue, and the light is absorbed by various chro­mophores in the skin, particularly water, oxyhaemoglobin, and deoxyhaemoglobin. Deoxyhaemoglobin has a maximum absorption at 760 nm, while oxyhaemoglobin absorption is maximum at 900 nm. Water, useful for identication and quantization of edema, has an absorption peak at about 980 nm contained in this range. The reected NIR light is collected by a detector and analysed to provide measurements of chromophore content. Subsequent analysis of this data can be applied to the assessment of wound severity and healing. NIR has potential use in wound assessment because through it the degree of wound depth and quantication of edema can be classied. The increased severity studied for burn wounds is accompanied by decreased lesional vascularization.
Preclinical studies have shown that measurements of oxygen saturation and total haemoglobin alone can differentiate a supercial burn wound from a full thickness burn wound. This method has been seen to have a distinction between supercial, intermediate partial-thickness, deep partial-thickness, and full-thickness wounds. In detail, identifying a quantitative measure of water loss especially in the eld of burns is denitely a great potential for the use of NIR (Cross et al. 2009).
In diabetic wounds in animal models, it has been shown to differentiate diabetic ulcers, based on reduced perfusion and greater tissue disorganization than in non­diabetic controls. NRI methods have also been used to analyse the process of lesional neovascularization (Daeschlein et al. 2017).
Thermography
Due to the relationship between temperature and emissivity of infrared radiation (a property intrinsic to all objects), thermography methods have been developed and have found wide use in the eld of dermatology and particularly wound imaging. This correlation allows thermographic measurements of the skin to be analysed and
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Fig. 2 Thermographic image of a vascular ulcer of the right calf (lateral region)
processed to create color-coded images. This imaging method is known as IR thermography or static IR thermography (Fig. 2).
Leveraging temperature recording by static thermography and thermal excitation provided Dynamic active thermography (ADT) is an advanced thermographic technique that involves recording steady-state temperatures using a standard IR camera following thermal excitation of the tissue by a light source, usually halogen lamps. A second set of temperature measurements is taken after thermal excitation, and subsequent analysis allows quantication of the thermal diffusivity of tissue components and wound regions (Renkielska et al. 2006).
Liquid crystal thermography (LCT) uses a plate of thermochromic liquid crystals to measure tissue temperature distribution (Roback 2010; Benbow et al. 1994). Liquid crystals absorb heat radiated from the tissue, and the plate emits a spectrum of colours that correlates with temperature readings. The potential of thermography as an aid in the evaluation of burn wounds was identied in the early 1960s by Lawson (Lawson et al. 1961), who used infrared scanning to predict burn depth with 90 percent accuracy, conrmed by histology. These predictions were based on the simple principle that supercial burns may be warmer than healthy skin for the inammatory process, while deeper burns would be colder than healthy skin due to structural damage done to vascularization.
Subsequent studies have been aimed at predicting the outcome of burn wound healing, discriminating between wounds that heal spontaneously in three weeks or
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requires excision and grafting (Cole et al. 1990). A bias of the method lies in the evaporation and loss of water from the wound bed especially that which occurs in the early stages. By applying a nonpermeable cover to the wound bed (thus closing off the system), the problem of evaporative cooling can be circumvented thermo­graphic measurements to be taken even in the early stages. This expediency is useful since burns are best assessed within 3 days because of the subsequent heating of deeper burns (Liddington and Shakespeare 1996). Recent studies have used high-resolution digital infrared cameras with increased specicity, distinguishing between full-thickness, deep partial-thickness, and supercial partial-thickness burns (Hardwicke et al. 2013; Medina-Preciado et al. 2013). Studies comparing static thermography methods with dynamic methods have demonstrated the supe­riority of the latter in terms of accuracy, specicity, and sensitivity in determining the degree of depth of burn wounds (Renkielska et al. 2006).
Thermographic devices are cheaper and easier to use than laser methods (LDI), and are much more accurate than visual assessment, however, further studies are needed. In the evaluation of the diabetic foot, it has been established that increased temperature can be considered a reliable marker of inammation by providing predictive data on the risk of ulceration at the different sites explored and has also been correlated with the risk of amputation and infection (Bharara et al. 2012).
In 1994, Benbow et al. found that mean foot temperature, determined from eight standard sites on the plantar surface, could be used to assess the risk of ulceration and ischemic foot disease (Benbow et al. 1994). Diabetic patients with high mean foot temperature were at incre ased risk of neuropathic foot ulceration, while those with normal or low mean foot temperature were at risk of ischemic foot disease. Another method of temperature assessment is to perform an analysis of temperature variation between feet of the same patient.
About pressure ulcers, slow-healing and normal-healing pressure ulcers were identied by thermography in a 1973 study. This study showed that slow-healing ulcers were characterized by a temperature of less than 1 °C compared with the surrounding skin, whereas normal-healing ulcers had a temperature difference of
2.5 °C (Barton and Barton 1973). Newman and Davis studied patients admitted to a geriatric unit and succeeded in providing an estimate of the risk of insurgence of sacral pressure injuries (Newman and Davis 1981).
A higher temperature at the wound site than in the surrounding skin implies the presence of factors that may inhibit wound healing providing an instrumental nding that anticipates traditional clinical signs. With a view to preventing the risk of press ure injury occurrence, thermographic imaging of pressure injuries has been compared to the Braden Scale, the most common tool for predicting press ure injuries.
Thermography has also been used to monitor wound healing of venous ulcer aetiology (Judy et al. 2011; Mercer et al. 2008). In a study we recruited 18 patients affected by venous insufciency and lower leg ulcers. A total of 24 chronic wound bed and perilesional skin ulcers were assessed using an infrared camera and from this was showed a relationship between the wound bed score and the wound bed
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temperature according to several studies that have demonstrated 33 °C is the critical temperature level required for normal cellular activity (Dini et al. 2015).
In portable thermography, including using devices such as smartphones and tablets is gaining momentum and will bring major changes not least in terms of costs of devices.
However, this method, which is gaining momentum with promising results, is not without its difculties. The main limitations are due to technical difculties such as proper positioning of the patient and acquisition instrumentation and lack of correlations with validated quantitative scales (Paul et al. 2015).
Videomicroscopy
Videomicroscopy captures photons from a bre optic light source through a magnifying glass, enabling direct skin micro-imaging. It has a certain degree of invasiveness as it requires direct skin contact. Advances in microscopy allow for visualization of skin tissue at the cellular level and can be used for non-invasive histological analyses. Traditionally, the use of light microscopy in conjunction with histological analysis is the gold standard for determining burn wound depth. Since the relationship between skin capillary plexus integrity and burn depth has been established, videomicroscopy provides the information needed to determine burn depth with accuracy comparable to LDI and clinical evaluation. It measures directly, without artifacts caused by histological sampling, the microstructures of great importance in the study of wound and periwound micro-vascularization. McGill et al. claim that the advantages of videomicroscopy over LDI are easier portability, reduced cost, non-variation due to patient movement, skin curvature or ambient light. In videomicroscopy there is little chance for the patient to have discomfort or infection but is instead a very well tolerated method. There are still numerous studies that validate it (McGill et al. 2007).
Optical Coherence Tomography
OCT generates high-resolution images of tissue microstructure in 2D images by exploiting low-coherence interferometry which works by optical scattering from tissues that measures the echo delay of these waves. A laser light beam is split into two separate paths: one is focused into the tissue and the other is guided into a reference point. The reected light beams from both targets are recombined to generate an interference pattern (Paul et al. 2015). The reections obtained from the target area are correl ated with the reference light with distinct tissue propagation, thus enabling structural information of even larger tissue areas to be obtained. OCT systems do not require contact between probe and skin making the method mini­mally invasive (Li et al. 2020). The eld of application of OCT in skin wounds lies more in monitoring structural changes by allowing quantication of newly formed skin tissues.
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A variant of OCT that takes advantage of the bi-refringence characteristics of tissue elements (e.g., collagen) is polarization-sensitive OCT (PS-OCT), using a polarizer we can determine the depth of lesions with an accuracy comparable to histological analysis (Srinivas et al. 2004). This assessment is achieved by corre­lating burn depth OCT is further improved for burn imaging by an extension of PS-OCT, Mueller matrix OCT (Todorović et al. 2008). Jiao et al. demonstrated that Mueller matrix OCT, which separates phase-based from amplitude-based polar­ization contr ast, can collect more detailed information from tissues, which is also applicable to burn depth assessment (Jiao et al. 2003). OCT angiog raphy has been used to quantify wound area and rate of wound contraction.
Using PS-OCT, an analysis of the micro-vascularization of burn wounds can also be performed. Kim et al. used a combination of microvascular measurements and birefringence data to characterize burn depth claiming that PS-OCT can allow more immediate and cost-effective detection than current standards (Kim et al.
2012). Data obtained from OCT can be used clinically to monitor scar progression
after a burn (Liew et al. 2013). Ultra-high-resolution OCT allows identication of wound size, epidermal migration, dermal-epidermal junction formation, and wound composition, lending itself as a detailed and non-invasive metric of wound healing. OCT can be used to monitor wound re-epithelialization, as it accurately differen­tiates epidermal and dermal layers. In addition, measurements of wound size, inammation, re-epithelialization, and early resorption aid in monitoring assisted wound healing. OCT may also prove to be a useful tool for the study of chronic wounds, where epidermal and dermal structure may be compromised, and re-epithelialization is problematic. Quantitative data on epithelialization, collagen deposition, and inammation can be obtained noninvasively with accuracy com­parable to histology, which is preferable to invasive biopsies that may interfere with the healing processes being studied.
Dynamic OCT (D-OCT)is a new methodology to visualize the skin vascu­lature through motion detection from speckle variance (Mariampillai et al. 2008; Themstrup et al. 2016). Initial imaging of blood vessels in burns and wounds is promising; the development of D-OCT allows visualization of capillaries up to 20 lm in diameter and perfusion of the mid/upper dermis, which could allow under­standing of the contribution of blood substances to wounds (Schuh et al. 2017). This offers potential advantages over photoplethysmographic techniques that detect changes in venous volumes in deeper tissues but without interpreting capillary function.
D-OCT has been extended to the examination of perilesional skin. Since 2015, software-based methods have been developed to detect blood ow from OCT images by detecting microscopic changes between OCT images acquired in a very short time interval
D-OCT is signicantly different from Doppler-OCT because it uses intensity changes rather to detect motion. Although Doppler-OCT has been shown to be able to detect blood ow in the skin, it loses sensitivity due to high background noise and because most of the blood ow is perpendicular to the laser beam (Zhao et al.
2000; Ren et al. 2002). D-OCT has the additional capability of being able to assess
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with sufcient resolution the size and morphology of blood capillaries even at depth (Boas and Dunn 2010). D-OCT detects blood motion, on the order of 0.1 to 1.0 mm/s. Therefore, it lters blood vessels where blood is stationary or ows very slowly or in lymphatic vessels.
Holmes et al used D-OCT, to study capillaries in periwound skin in 11 patients with chronic wounds (of venous or arteriovenous etiology) of long duration. Specically, in a non-invasive way, they studied capillaries at the 4 poles along the wound edge, the center of the wound, the skin proximal to the wounds placed at a xed distance and at a control site. In their study, they provi ded the anatomical and functional characteristics of the explored sites providing useful clinical-instrumental information to enable more scientic advances in this promising wound imaging method (Holmes et al. 2019).
Fluorescence Imaging
Fluorescence imaging (FLIM) takes advantage of the endogenous uorescence emitted by the skin's natural uorophores (such as collagen and elastin) and can complement the uorescence analysis of exogenous uorescent materials. The skin is irradiated by a laser source with specic excitation wavelengths and captures and lters the light at the wavelength of interest. Injection into the bloodstream of indocyanine was used as exogenous uorescence and its uorescence levels cor­related with vascularity and burn wound depth (Still et al. 2001).
In FLIM, on the other hand, the uorescence of, nicotinam ide adenine dinu­cleotide (NADH) in the wound bed is measured. NADH is involved in oxidative phosphorylation and signals c ellular metabolism that can be used as a marker for skin wound healing. The main drawback of this technique is that it requires a dye injection to provide blood ow measurements.
FLIM and Second Harmonic Imaging (SHG) were combined and used in the study of wound healing. Through FLIM, cell metabolism rates can be monitored by measuring NADH levels while SHG can assess collagen deposition at the wound bed site. These methods taken together are comparable to histochemical analysis and may be suitable for the study of skin wound healing (Deka et al. 2013). FLIM correlating with cellular metabolism can prove to be a useful marker of tissue vitality in the study of the healing process in skin wounds (Queen and Harding
2020). Devices that assess the uorescence of endogenous bacterial porphyrins
noninvasively and in real-time have also been devised.
Gram-positive emit a red uorescence, while pyoverdine of Pseudom onas aeruginosa and other Gram-negative bacteria emit a cyan uorescence.
Light green uorescence corresponds to slough, dark green uorescence to granulation tissue, black color for necrotic tissue and dark red-purple-black for vascularized areas (Janowska et al. 2021). Le et al. used this technology on 350 patients. They found that 82% had bacterial loads >104 CFU/g with failure to detect infection in 85% of cases using Clinical Signs and Symptoms Checklist (CSSC) assessment alone (Le et al. 2021). In a retrospective study of 229 foot ulcers,
276 V. Dini et al.
uorescence imaging was correlated with a 49% reduction of antimicrobial dress­ings, a 33% in systemic antibiotic, and a 23% increase in wound healing rate (Price
2020).
In a study, a handheld autouorescence device was used to evaluate the reduction of bacterial load associated with 3 different therapeutic approaches by correlating images acquired with NRS pain scale, Wound Bed Score (WBS), and quality of life (QoL) assessment demonstrating a superiority of zinc oxide bandage over dressings and multic omponent bandages or sharp debridement, dressings, and multicomponent bandages (Janowska et al. 2022).
Confocal Microscopy
Confocal microscopy or confocal laser scanning microscopy (CLSM) has recently become commonly used in the eld of dermatology. It is a high-resolution optical detection technique using a light source and lens that allows images to be acquired at multiple depth levels. A detector array collects re-emitted signals from the focal plane, eliminating all backscattered photons from the surrounding material that would appear blurred. The greatest advantage of this technique is based on the ability to evaluate the skin in vivo by applying a kind of real-time cyto-histology.
It can in fact provide such detailed images that clear images of cells in the epidermal layersbe discerned, and at the dermal level it is possible to distinguish cellular blood populations from the prole. Skin appendages such as sebaceous glands, hair shafts and sweat-gland ducts can also be seen.
To cover the entire sample, manual scanning of the sample is always performed, but because only a small region is illuminated at each acquisition step, confocal microscopy is not ideal for large elds of view, although modern instruments have been developed to cover larger areas. The quality of images obtained by CM can depend on the technology used and the ability of the operator.
By changing the focal depth, this technique acquires a series of horizontal section images, which can be reproc essed to provide a nal image. CLSM can be used to collect high-resolution images in different spatial planes at each stage of the skin wound repair process. Because of its very high resolution, it can provide information on histologic cellular and architectural morphology. With this tech­nique, blood ow, new tissue formation, and tissue remodelling can be visualized completely noninvasively (Paul et al. 2015).
At present, confocal microscopic technology is under continuous development in almost all elds of dermatology. However, because of its excessive cost, the use of confocal microscopy is for now only conned to research centres. With the miniaturization of devices and the containment of costs it proves to be of immense contribution in wound healing. it is one of the most promising methods.
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Ultrasound
Ultrasound (US) imaging, since its rst use in 1979, It has been widely used and has become more is very important in dermatology for a wide range of purposes, from skin lesion localization and classication to pre- and post-treatment evaluation, outcome assessment, and follow-up studies (Firooz et al. 2017; Kavros and Coronado 2018). US is a versatile, painless, low-risk, and non-invasive procedure that can be easily and repeatedly performed (Bhatta et al. 2018).
The most used probes in dermatology range from less than 10 MHz (traditional US), 10–30 MHz (HFUS: High-Frequency US) and 30-100MHz (UHFUS: Ultra-High Frequencies US), the choice depends on the clinical query (higher US frequencies have better image resolution but limited depth of penetration compared to lower frequencies). Another advantage that US allows is the ability to assess the degree of vascularity through Doppler US evaluation.
Conventional US (frequency <10 MHz) and Doppler US evaluation are fre­quently used as an aid in diagnostic and therapeutic algorithms of chronic lower extremity, in order to evalua te the nature of the lesions and identify the presence of alterations in the venous and supercial circulation (Izzetti et al. 2021; Kirsner and Vivas 2015). US have also been used to study the depth of wounds and dene the involvement of various skin structures (Flanagan 2003). When evaluating a skin wound with US, it is recommended that highly trained personnel is recruited, because this technology is operator dependent (Mani et al. 2016).
HFUS is used for wound healing from 1990. HFUS has been used to evaluate the efciency of therapeuti c intervention on healing scars (Kerckhove et al. 2003). Hoffman K. et al have described by means of the HFUS the phases of healing of a cutaneous wound comparing through the follow-up of cryotherapy wounds but the differentiation between necrotic tissue, brous tissue, neo-epithelium, and granu­lation tissue was very difcult (Hoffmann et al. 1993).
Tissue analysis by HFUS has been proposed as an aid method in the prevention of pressure injuries in bedridden patients (Dyson et al. 2003). The potential of HFUS in wound monitoring with a wound dressing in situ was proposed (Ke nney and Delves 1997).
The use of UHFUS in the study of ulcerated lesions has shown encouraging results. UHFUS modern machinery can provide ima ges with resolution in the 30-micron range (Izzetti et al. 2020a, b; Oranges et al. 2019). For its higher reso­lution UHFUS can provide establish an estimated amount of granulation tissue, depth of necrotic tissue, physical dimensions of the wound, wound volumes and structural components (Fig. 3)
UHFUS could also quantify the degree of edema of the lower limbs, providing objective parameters on its decrease according to the type of bandage.
The new machines for skin ultrasound can also provide in a single frame an entire scan of the surface of the lesion. By means of the software often supplied with the machines it is also possible to reconstruct the ultrasound structure in 3D post acquisition providing a wound growth tracking US not only in 2D.