Bioelectricity and its measurement

Bioelectrical impedance (BI) measures the opposition (impedance) of biological tissues to the flow of alternating electric current. This opposition is highly correlated both with the concentrations of electrolyte ions and water in the tissues, as well as with their structure cell sizes, density, and spacing [1]. In the field of body composition, the technique is known as Bioelectrical Impedance Analysis (BIA), and it has been used for the estimation of total body water (TBW), fat-free mass (FFM) and fat mass (FM) [2]. Impedance is measured using a tetrapolar approach as described in [3]. 

One of the main determinants of the number of subjects required to reach the desired statistical power is the precision of the measurement tool utilized. More precise measurements will reduce the number of subjects required. As an example, if a study is being conducted to assess the influence of a dietary supplement on body fat, several measurement tools could be used to assess this outcome. These tools range from low levels of cost and precision (e.g. skinfold measurements) to moderate levels (e.g. bioelectrical impedance) to high levels of cost and precision (dual x-ray absorptiometry - DXA). A study that uses skinfold measurements to measure the outcome will require many more subjects than one which employs DXA. Therefore, it is often less expensive in total to utilize a more expensive measurement tool, because the more precise tool will allow the study to have sufficient power with a smaller number of subjects. 

Several methods are available to evaluate a patient s actual body composition rather than total body mass. Skin-fold measurement may be of value in evaluating subcutaneous adiposity (adipose tissue accumulation) proper technique is required for reliable results. Other anthropomorphic measurements such as bioelectrical impedance, dual-energy x-ray absorptiometry, and total body water immersion are also available.These last techniques are often of value in research studies, but it is clinically impractical to use them routinely (Elberg et al., 2004). 

The first is the Dirichlet condition, which says that one has a set of discrete measurements of the voltage of a subset of the outer surface. The second is the natural Neumann condition. While it does not look much different from the formulation of the direct problem, the inverse formulations are ill-posed. The bioelectric inverse problem in terms of primary current sources does not have a unique solution, and the solution does not depend continuously on the data. Thus, to obtain useful solutions, one must try to... 

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