Pressure injury image analysis with machine learning techniques: A systematic review on previous and possible future methods

The average life expectancy of world population has steadily increased during the last decades, and it is expected to continue growing during the next century. At the same time, medical professionals have witnessed an increasing incidence rate of diseases related to sedentary lifestyles and unhealthy eating habits, such as diabetes, especially in the Western hemisphere [12]. The convergence of these two trends has increased the number of long time care patients subject to lengthy periods of immobilization who suffer pressure injuries: a type of chronic wounds resulting from damage caused by pressure over time causing an ischemia of underlying skin structures. These injuries appear most commonly at bony prominences like the sacral area and the heel [10]. Some factors which contribute to pressure injury formation are: skin contact with a bed or a chair without frequent position changes, contact with urine or stool, diseases like diabetes that affect blood flow, injuries which restrict your body positioning and your nutritional status or medications. There are many other factors than may put the skin at risk for pressure injuries, as presented in Fig. 1.

Despite all the prevention measures that are put in place, pressure injuries may still develop. More than 2.5 million people in the United States develop pressure injuries [27]. And the risk of developing them during hospital stays is 3 times greater than the risk of being involved in a car accident. Healthy skin, muscle or fat tissue starts to die when blood flow is slowed or interrupted by pressure. This pressure can be caused by the bones pushing down on a surface like a chair or a bed.

Pressure injury stages are defined based on the deepest parts of the ulcer and the type of tissue affected. Higher stage ulcer represents deeper tissue damage and more serious injury. There are four main stages of pressure injuries [30]:

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  Stage 1: Pressure injury often presents as intact skin with redness, and is usually over a bony area. The skin would stay red after the pressure is removed. This is because the first area affected by pressure is muscle tissue. In fact, due to its aerobic metabolism, muscle tissue requires more oxygen. Hence, when blood circulation is interrupted, lesions begin appearing in deep muscle levels before reaching the skin. The surface area may be painful, warm or softer compared to surrounding tissues.

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  Stage 2: Pressure injury presents as an area where the top layer of the skin is missing with the open area being pink or red.

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  Stage 3: Pressure injury is deeper and more severe than a stage 1 or 2 pressure injury. It goes into fat tissue, and dead tissue may also be present.

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  Stage 4: Pressure injury is the most severe form of this wound. It is the deepest ulcer possible and would reach muscle tissue and possibly bones. Dead tissue may also be present with stage 4 pressure injuries. Fig. 2 depicts the main tissue types that can be present in stage 3 and 4 pressure injuries.

There are also two additional stages of pressure injuries. One is called unstageable because there are such exceedingly dead tissues in the wound that physicians cannot examine its depth. The other type of pressure injuries is called deep tissue pressure injury (DTPI). It usually begins with skin that is not broken, and it is usually purple or dark purple in color. This type of ulcer can open quickly and become a deep pressure injury [29], as it appears in Fig. 3.

Pressure injuries can occur in different areas, including: back or sides of the head, rims of the ears, shoulders, hipbones lower back, backs or sides of the knees, heels, ankles and toes. The age of the patient, coupled with certain medical conditions such as diabetes, slows down the healing of pressure injuries, which often are not healed before the death of the patient [3], and represent a significant economic cost for medical care services [21]. Pressure injuries are both painful and prone to infection, which calls for a continuous monitoring of their evolution by medical staff.

Documenting the pressure injury once the patient is admitted to the hospital is the first essential step in the process of prevention and assessment. Firstly, caregivers need to check if the patient has already an existing pressure injury. In such case, documenting the stage, the size, the color, the drainage and other characteristics is the first step towards a properly done management of the pressure injury. In case the state of the wound is severe, wound care team is to be contacted to prescribe a personalized treatment. This includes reducing pressure and friction by repositioning every one to two hours and using support surfaces to protect sensitive skin and minimize shear, cleaning and dressing the wound depending on its stage, and removing damaged tissue. The latter intervention is crucial to keep the wound free of infected or dead tissue. Depending on the state and stage of the pressure injury, the treatment may differ. When the wound is in its first stages, a gentle cleaning and proper dressing is required to prevent it from getting infected. When the pressure injury is in stage 3 or 4, it may take several months to heal, or may never do, especially if other risk factors are permanently present, such as: poor blood circulation and vascular diseases, diabetes, poor nutrition, immobility. Hence, a user friendly system for a non-intrusive assessment of pressure injuries would represent a valuable tool for frequent assessment and monitoring of these wounds.

To avoid the discomfort of patients in the process, non-invasive wound monitoring techniques are preferred. In this sense, imaging techniques are called to play a key role, as they allow an accurate analysis of its features with no need of contact with the wound itself.

For this reason, this work aims at providing the reader with a comprehensive review of state-of-the-art imaging techniques applied on pressure injury image analysis, and the possibility for deep learning to outperform these techniques.

This study represents a systematic review which gathers the contributions of many studies on the assessment of skin wounds using image processing techniques. Since the number of papers dealing with pressure injuries in particular was limited, we have extended our research on skin wound analysis in general, including: chronic wounds, lower extremity wounds, diabetic ulcers, malignant ulcers, venous ulcers and skin lesions. The studies of skin wounds using image processing have skyrocketed during the beginning of the 21st century. Hence, the majority of the presented contributions were published during the last decade.

The remainder of this paper is organized as follows: Section 2 presents the methodology of paper selection (search terms, inclusion/exclusion criteria, statistical data about the number of papers per year, country and research problem). Then, different categories in pressure injury assessment using image processing are discussed in Section 3, including: wound segmentation, wound measurement, tissue classification and healing prediction. As mentioned previously, other types of skin wounds in the selected papers were addressed to broaden the techniques used for wound imaging analysis. Next, Section 4 outlines Deep Learning techniques used in biomedical image processing. Finally, in Section 5 we discuss the usability of Deep Learning to efficiently assess pressure injuries using image processing. Last but not least, the reader interested in the broader field of non-invasive chronic wound monitoring via imaging techniques is referred to the recent survey by Mukherjee et al. [15].