## 1 Introduction  
Cutaneous wounds are very common [1] and can be differentiated into acute and chronic wounds. In healthy subjects, acute wounds can be related to incisions, abrasions, or burns and usually heal uneventfully and without complications. In patients with underlying pathologies, for example, peripheral arterial and venous diseases or diabetes, these acute wounds can exhibit an impaired healing pattern, possibly leading to a chronic wound [2; 3; 4; 5; 6]. In developed countries, it has been estimated that 1-6% of the general population will experience such a chronic wound during their lifetime [7; 8; 9; 10]. Normal wound healing is a well-organized, dynamic process with four overlapping phases: hemostasis, inflammation, proliferation and tissue formation, and tissue remodeling. A fundamental step in normal wound healing is the rapid re-establishment of a functional microcirculation in hypoxic tissue [11], realized by the process of angiogenesis, which refers to the growth of new capillaries from pre-existing blood vessels via sprouting [12]. The vasculature does not only provide the blood supply for the affected tissue but is also the site for the initiation of inflammatory events [13; 14], which are a prerequisite for the healing of the wound. Multiple physiological and biochemical factors can lead to a deregulation or interruption of one or more of the normal healing phases and thus result in a chronic wound. Clinical assessment of cutaneous wounds uses dermatoscopy, wound photography, or measuring tape. In preclinical research, histology and immunohistochemistry (IHC) can provide an insight into morphological changes and both cellular and molecular events [15; 16]. However, for visualization of larger sections of the vascular network and, in particular, for the evaluation of the dynamic vascular remodeling during wound healing and the investigation of the pathophysioc mechanisms of non-healing wounds, accurate, quantitative, and non-invasive imaging techniques are needed. Here, various imaging modalities for investigation of the cutaneous perfusion have been employed. Non-optical methods like magnetic resonance imaging [17; 18], micro-computed tomography (uCT) [19], or uCT angiography [20] can be employed for the assessment of angiogenesis but lack the spatial resolution for the visualization of capillary remodeling during wound healing. Additionally, the latter requires the injection of a contrast medium. In recent years, ultrasound-based techniques like contrast-enhanced ultrasound [21] and acoustic angiography [22; 23] have been used for vascular imaging. Based on the injection of a non-ionizing contrast agent into the blood stream and detection of the ultrasound signal with high-frequency receiver elements, the latter provides resolutions of a few hundred microns and allows characterization of the vascular architecture. Also, non-invasive optical methods have been widely used for microvascular imaging. Photocaustic (PA) imaging provides favorable penetration depth and sufficient resolution for visualizing the cutaneous vasculature in animals [24] and humans [25]. Beyond pure imaging of the vascular network, the technology has been employed for the assessment of quantitative vascular [26; 27; 28] and hematologic [29] parameters. Optical coherence tomography (OCT), based on the principle of low coherence interferometry, provides label-free, non-contact, noninvasive, high spatial and temporal resolution cross-sectional imaging of biological tissues. Analyzing intrinsic dynamic changes in the OCT signal due to moving red blood cells, OCT angiography (OCTA) as a functional extension of OCT enables visualization of the vascular network in the probed tissue without the need for a contrast agent [30; 31; 32]. The technology's high resolution provided an insight into vascular changes related to ocular diseases like diabetic retinopathy [33; 34] or age-related macular degeneration [35; 36] and allowed for the analysis of the vasculature in lesions of the skin [37; 38; 39].