Electrical Impedance Spectroscopy imaging of the thigh using current excitation frequencies in the mid-ß frequency dispersion range

Electrical I mpedance Tomography ( EIT) i s a non -invasive i maging technique which consists on placing a number of electrodes (normally 8, 16 or 32) equidistantly a round the periphery of the object to be s tudied. One of the preferred methods for obtaining a projection for image reconstruction uses an alternating current signal injected between a pair of adjacent electrodes and the voltage developed in the periphery is measured using the rest of the electrodes. An image reconstruction algorithm is then used to produce an image or resistivity map of the cross-section. The non-invasive nature o f the t echnique has resulted i n numerous bi omedical a pplications r eported o ver t he past t hree d ecades. However, since EIT is a soft-field sensing technique most of the reported works use current excitation frequencies in the alpha of low-beta frequency dispersion ranges. In particular for breast cancer detection using biompedance information, i t h as been r eported, and g enerally accep ted, t hat t umors e xhibit a significant i mpedance ch ange i n t he m idand h igh-beta f requency di spersion ranges. T herefore, i t i s o f g reat i nterest t o increase t he cu rrent ex citation f requency. In addition, i t is common to find that the preferred image reconstruction methods are qualitative (i. e. Weigthed Backprojection), to benefit robustness and reconstruction speed, since quantitative methods are iterative, and may fail to converge when noise is present in the measured voltage signals. The authors pr esent a n E IT da ta a cquisition s ystem for obt aining da ta f rames us ing current excitation frequencies from the alphato mid-beta frequency dispersion ranges. The equipment generates the cu rrent excitation s ignal by d irect digital synthesis that allows current excitation frequency sweeping. Thus, in effect, the equipment is an impedance spectroscopy imaging system. The equipment was used to obtain images of a volunteer’s thigh to assess the performance o f t he equipment. Images were reconstructed using qualitative (Weighted Backprojection) and quantitative (Modified Newton-Raphson) image reconstruction methods, a t different f requencies r anging f rom 20 0 K Hz t o 20 M Hz. Q uantitative images were in a greement with th e qualitative im ages. A s frequency i s i nProceedings IWBBIO 2014. Granada 7-9 April, 2014 1755 creased it is possible to detect different anatomic features. At low frequencies, the similar conductivity of bone (0.01–0.06 S/m), muscle (0.04–0.14 S/m) and fat (0.02–0.04 S/m) impedes delimiting the internal organ boundaries. As frequency increases, the impedance properties of the different tissues begin to appear. A t 2 0 M Hz t he r econstructed i mages show t wo ot her r egions of hi gher conductivity consistent with blood flow at the location of the femoral artery and femoral vein and the great saphenous vein, which suggest that the equipment can be used to provide qualitative and quantitative images at the mid-beta frequency dispersion range.