Bioimpedance Measurement

Electrical impedance is the opposition to the flow of an electrical current, being the ratio between an alternating sinusoidal voltage and an alternating sinusoidal flow. In consequence impedance is a passive magnitude that does not irradiate energy, therefore energy must be provided in two ways: by exciting the tissue with current or with voltage. In this case, the EBI measurement method used consists on injecting an electrical signal with a known current and measuring the reciprocal complementary voltage magnitude the value is calculated using the Ohm’s Law, in the frequency domain. Z = V / I

Where Z is the impedance, V the voltage and I the current. The diagram block of a typical impedance measurement system is the following: Typical diagram block of a impedance measurement system

Measurement types

Depending on the type of information that we are looking for analyse several types of measurements can be done. Non-phase BIA measurement: This method measures the total resistance of the body (Z). This does not give determination of the phase angle and, as such, a subdivision of impedance into water and cellular resistance, so that no judgement can be made about the body cell mass or the extra-cellular mass with non-phase measurements Phase BIA measurement: The phase sensitive technique of measuring allows the impedance Z to be differentiated into its two components resistance (R), that shows the water resistance, and reactance (Xc), that show the cell resistance. This let us differentiate between the body cell mass and extra-cellular mass. Phase BIA multi-frequency measurement: BIA is frequency dependent, therefore for a more intensive study we can study the resistance and reactance in a different single low frequencies measurements (1-5 KHz). With this type of measurement it is possible to achieve a subdivision of the total body water into intra-cellular and extra-cellular body water.

Measurement Techniques

There are several approaches to measure Bioimpedance that should be chosen depending on the desired characteristics of the system built. Null Techniques: Detection with this technique is very simple and it is based in a simple ampere meter. The most common method used is a Wheatstone bridge. This is a high accurate method with the inconvenient of needing a large number of electronic components and not being time efficient in some applications because of its iterating process. Deflection Techniques: The impedance estimation is done by measuring the voltage drop or the current through the load as a response to an alternative known current. This method is based on simple electronics using complex operations. Due to Measurements of Bioimpedance & Instrumentation this an application specific integrated circuit or microcontroller is needed to carry these operations. Its main characteristic is the time efficient, being able to do short time accuracy measurements. For the impedance estimation we are going to focus only in the deflection techniques that are those that are commonly used in bioimpedance measurements. In impedance estimation is it possible to do single frequency and multy-frequency analysis using different techniques, basing the study in the known excitation provided and the measured obtained. So for single frequency or sweep frequency measurements usually Sine Correlation is used and for multi-frequency measurements it is common used the Fourier Transform.