Characterization of subcutaneous edema by mechanical impedance measurements

The mechanical behavior of gels of various water content, normal and edematous tissues have been studied. Static deformations and vibrations at low frequencies were applied to an area of each of these structures. Softer structures, e.g., gel containing a high percentage of water and pitting edema, have lower mechanical impedance (MI), while harder structures, e.g., nonpitting edema have higher MI. A comparison of the static force measured at bilateral sites on normal subjects show lower differences in impedance than do similar comparisons made in subjects having unilateral edema. This difference is more pronounced with greater static compression. The differences in the MI between different structures are greater at the lower frequencies. At the lower frequencies up to around 30 Hz the ratio of the magnitude of MI of edematous skin and that of contralateral normal skin can indicate the degree of edema. The MI of edematous tissues depends on the type of edema and varies with its degree. Thus this study introduces a quantitative method for the diagnostic and therapeutic assessment of unilateral subcutaneous edema.