Multi-spectral Imaging to Assess Wounds in Peripheral Vascular Disease Patients

Peripheral vascular disease (PVD) is a common disease of the elderly, resulting in the compromise of blood flow to the lower limbs. As a consequence of impaired tissue perfusion, PAD patients can experience pain, diminished exercise capacity, and tissue loss, with some ultimately requiring amputation. Globally, lower extremity peripheral artery disease is the third leading cause of atherosclerotic cardiovascular morbidity, affecting over 200 million individuals worldwide. Peripheral vascular disease is becoming more prevalent with the increase of incidence with age and a globally aging population. In 2010, it was estimated that the number of people living with peripheral artery disease increased by 23.5% in the last decade. The economic burden of PVD is significant. In the United States alone, PVD accounts for over $20 billion in annual healthcare related costs. Specifically, non-healing wounds represent a significant portion of this expenditure at more than $3 billion per year.

The demand for the development of an effective method to characterize the viability of PVD wounds has resulted in the emergence of several innovative techniques. Commonly used diagnostic methods are ankle-brachial index (ABI), pulse volume recordings, duplex ultrasonography, venous plethysmography, angiography by X-ray, computed tomography, and magnetic resonance imaging. However, many of these techniques lack the ability to triage and adequately determine the viability of the wound. Ankle brachial index (ABI) is one of the most common screening techniques used to establish the presence of PVD, but it lacks sensitivity in creating sensitive and specific categorization of wounds as viable or non-viable. ABI gives an indication of a hemodynamically significant obstruction, but does not measure blood flow directly. Doppler ultrasound is another common method that can measure arterial blood flow, however some groups suggest that the femoral artery blood flow may not be an accurate hemodynamic indication of muscle ischemia. Magnetic resonance spectroscopy is also a new technique that has been validated in its application to assess PVD. Novel magnetic imaging sequences such as PIVOT (Perfusion, Intravascular Venous Oxygen Saturation) combine Blood Oxygen Level Dependent (BOLD) MRI and Arterial Spin Labeling (ASL) to comprehensively assess the tissue. Despite the strengths that many MRI techniques promise, clinical use limitations remain due to the high costs and availability of resources.

Peripheral vascular disease-induced lower extremity wounds have inadequate perfusion. Currently, there are three major approaches to improve vascular perfusion to the area: medical management, open surgery, and endovascular surgery. Medical management consists of lifestyle modification and management of associated risk factors such as smoking, diabetes mellitus, hyperlipidemia, hypertension, and hypercoagulability. While advances have been made in the medical management of lower extremity ischemia-induced wounds, large arterial revascularization remains the current standard of care. Open bypass surgery is often employed to bypass stenotic arteries and improve perfusion to lower extremity wounds. Over the last two decades, endovascular intervention has emerged as an additional therapy as it is minimally invasive and may have lower associated morbidity and mortality. In addition to improved perfusion of lower extremity wounds by medical management of surgical intervention, local wound care is required to expedite the healing process and prevent infection.

However, there remains a need for effective triage technologies to help clinicians decide whether surgical management is needed. This would allow for early determination of surgical versus conservative management and inpatient versus outpatient management. If surgery is required, patient functional outcomes are improved if it takes place earlier. As well, inpatient management of surgical patients is extremely costly. Outpatient management of patients with peripheral vascular wounds that do not require surgery would significantly reduce the costs associated with surgical intervention.

Near-infrared point spectroscopy (NIRS) non-invasively measures flow, concentration, and oxygenation of hemoglobin in arterioles, capillaries, and venules several centimeters deep in tissue. NIRS is safe and comfortable for patients and is well-suited to measure markers of viability in peripheral vascular disease wounds, including tissue perfusion, oxygenation and hemoglobin. To date, studies have demonstrated the validity of NIRS technology in patients with peripheral arterial disease. The Multi spectrum infrared device (MSID) represents an important evolution of existing NIRS imaging devices, which were designed for the clinical environment. The MSID has become a commercial device produced by KENT imaging (Calgary, Canada) after several animal and clinical trials have validated its use. The MSID is a new generation of NIR imaging devices with enhanced portability and functionality. The MSID is able to monitor many variables, such as cytochrome oxidase as a marker of tissue oxygen utilization, oxygen saturation, perfusion and methemoglobin as a marker of free radical injury. The advantage of the MSID is its capacity to account for skin colour. Melanin absorbs light and attenuates the amount of NIR light received by the camera. The Kent imaging device has unique mathematical algorithms to account for melanin content of the skin. In many previous studies done with NIRS technology in PVD wounds, melanin content was not considered to be a factor as patients demographic was predominantly Caucasian. However, in many multicultural cities where there is a wide variability in patient melanin content, melanin corrections in NIRS imaging are very important.

Using this new and clinically applicable NIRS technology designed for PVD, this study seeks to identify viable from non-viable wounds and to determine the severity of wound in assessing for vascular intervention indication. This technology is well-suited for use in a wound patient population as the measurements times are short and movement of subjects is not an issue. As such, this project intends to apply novel technology that has the capacity to quickly assess wounds as a complication of PVD.