# Correlation Mapping of Perfusion Patterns in Cutaneous Tissue
## 4 Discussion and Conclusions  
Correlation mapping of tissue perfusion using PPGI contributes to the possibilities for assessing the microcirculation of subjects with DM. The proposed method allows quantitative assessment of local tissue perfusion with spatial resolution. On the one hand, this method can be considered relatively simple and computationally inexpensive. However, on the other hand, to make the results relevant, stable measurement conditions must be ensured. In our case, the need for synchronous recording of the contact PPG may be a drawback. Another possibility to overcome this is generating a reference signal from a larger ROI size [38] or using low-cost imaging systems such as a smartphone or consumer digital cameras to use their microphone input [39]. These solutions thus lead to a fully contactless measurement modality. The method was applied to a sample of two DM and two non-DM subjects, and we could observe different perfusion patterns for these two groups of subjects. In particular, a different response to DBT can be observed in DM subjects, where a low response is evident in subject 1 (Figure 6) compared to subject 2, which may be due to different ages and different degrees of impairment in the regulation of vasomotor mechanisms. The results show that respiratory activity also influences the peripheral hemodynamics of DM and healthy subjects. These changes in tissue perfusion are induced by sympathetic or parasympathetic activity. The effect of sympathetic activity reduces blood flow through the cutaneous tissue, and thus respiratory activity can also be observed to some extent in signals obtained by photoplethysmography [40; 41; 42]. The DC component of the photoplethysmography signal, which is unrelated to cardiac activity, can change its level just according to respiratory activity [43; 44; 21; 45]. Changes in the PPG signal conditioned particularly by respiratory activity may be caused, for example, by changes in arterial blood pressure transmitted mechanically in the thoracic region from large arteries to smaller ones or by oscillations in the activity of the autonomic nervous system [46]. In our case, choosing the DBT, we confirmed this effect of peripheral changes in cutaneous perfusion by a unique non-invasive and non-contact method of photoplethysmography imaging. Thus, tissue perfusion can be mapped spatially [4; 5; 7; 15; 39], allowing comprehensive monitoring of the subject's cardiovascular system responses [47]. This method could be used even in extensive skin damage where contact methods cannot be used for measurement, such as when examining ulcerations [15; 16]. Since the probands did not have an indicated diabetic foot syndrome, we did not focus to the choice of leg. However, for further study, it is suggested to record both legs simultaneously, as was used in the case of thermography measurements in [48]. Such data enable the detection of differential perfusion patterns [49], especially in comparing the feet with and without ulceration or skin damage. Although our sample of subjects did not include subjects with visible skin damage, this group of patients may still benefit from PPGI in screening perfusion or assessing the stage of development of their DM when there is a gradual decline in the  
Figure 7: (**a**) correlation map with marked regions (Upper, Middle, Lower); (**b**) Bar graphs with normalized values of correlation coefficients from marked regions for comparison of the perfusion changes in different stages of the experiment. regulatory capacity of vasoactive mechanisms [50], the efficacy of which could be evaluated using DBT. Although this work contains a small number of subjects with an extensive age range, we consider it a pioneering step toward developing a non-contact and non-intrusive monitoring system that would allow the investigating physician to view the microcirculatory damage in patients with diabetes mellitus while focusing on the spatial distribution of damaged tissue.