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The prevalence of heart failure (HF) has increased worldwide. Excessive fluid retention often causes leg edema in patients with HF, resulting in disease progression and readmissions. It is important that the severity of leg edema is regularly assessed in patients with HF. Although edema is assessed by indentation depth and leg circumference, these data may vary depending on the measurement procedure. Therefore, these methods are currently not accurate enough to evaluate edema. Diffuse reflectance spectroscopy (DRS) can detect conditions such as subcutaneous edema. Some studies have evaluated edema using DRS, but those studies have not been considered detailed parameters and where measurements should be taken. The objective of this study was to optimize DRS in evaluating leg edema. First, we investigated the ideal location for evaluating leg edema by characterizing the subcutaneous structures in various regions using ultrasound imaging. The medial part of the tibia was found to be appropriate for DRS measurement due to its simple, layered structure. Second, optical penetration depth was examined with different source-detector (SD) distances at wavelengths near the absorption peak of water. We found an appropriate SD distance was 10 mm, for measurement of water content in subcutaneous fat. Furthermore, a DRS system optimized using our findings was able to obtain good correlation between water content and absorbance in a leg edema phantom. These findings suggest that leg edema can be evaluated quantitatively in a DRS system. We anticipate that this system will aid management of fluid retention in patients with HF.
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