ACT3: A high-speed, high-precision electrical impedance tomograph

Electrical impedance imaging (EIT) is an imaging technique in which the electrical properties of the interior of a body are assessed by measurements made at the surface of the body. As an imaging technique, EIT may have several advantages over techniques such as computed tomography and magnetic resonance imaging. The primary advantage of EIT is the relatively low cost of the instrumentation. Also, since EIT assesses the electrical impedance inside a body, it may be well suited for several biological applications: early stage detection of pulmonary edema, noninvasive cardiac output determination, and noninvasive tissue temperature measurement.; This thesis describes the design and implementation of Rensselaer's third generation electrical impedance tomography, ACT3. The purpose of this instrument is to collect the surface current and voltage measurements for subsequent reconstruction of the internal impedance image. ACT3 applies digitally synthesized sinusoidal currents with 16-bit amplitude precision and a maximum peak value of 1mA and frequency of 30kHz to each of 32 electrodes. For a given applied current, the maximum likelihood estimates of the resulting real and quadrature voltages at each electrode are computed in parallel in 4.3mS with a nominal signal to noise ratio of 104dB. To attain the large signal to noise ratio and fast measurement time while keeping the system cost low, an oversampling discrete quadrature demodulator was implemented using a digital signal processor (ADSP2101) and a nonuniform sampling scheme. In order to assure 16-bit accuracy of the measured voltages, the current source output impedance is maintained at 64M{dollar}Omega{dollar} by an automatic trimming arrangement. System voltmeter and current source calibration are also automated.