Multi-frequency electrical impedance tomography (EIT) systems require stable voltage controlled current generators that will work over a wide frequency range and with a large variation in load impedance. In this paper we compare the performance of two commonly used designs: the first is a modified Howland circuit whilst the second is based on a current mirror. The output current and the output impedance of both circuits were determined through PSPICE simulation and through measurement. Both circuits were stable over the frequency ranges 1 kHz to 1 MHz. The maximum variation of output current with frequency for the modified Howland circuit was 2.0% and for the circuit based on a current mirror 1.6%. The output impedance for both circuits was greater than 100 kohms for frequencies up to 100 kHz. However, neither circuit achieved this output impedance at 1 MHz. Comparing the results from the two circuits suggests that there is little to choose between them in terms of a practical implementation.

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Howland circuits have been widely used as powerful source for exciting tissue over a wide frequency range. When a Howland source is designed, the components are chosen so that the designed source has the desired characteristics. However, the operational amplifier limitations and resistor tolerances cause undesired behaviors. This work proposes to take into account the influence of the random distribution of the resistors in the modified Howland circuit over the frequency range of 10 Hz to 10 MHz. Both output current and impedance of the circuit are deduced either considering or the operational amplifiers parameters. The probability density function due to small changes in the resistors of the circuit was calculated by using the analytical modeling. Results showed that both output current and impedance are very sensitive to the resistors variations. In order to get higher output impedances, high operational amplifier gains are required. The operational amplifier open-loop gain increases as increasing the sensitivity of the output impedance. The analysis done in this work can be used as a powerful co-adjuvant tool when projecting this type of circuit in Spice simulators. This might improve the implementations of practical current sources used in electrical bioimpedance.

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Introduction: Hemiparesis post-stroke usually results in locomotor limitations. As conventional rehabilitation is monotonous, the Serious Games (SG) represents an excellent treatment strategy, allowing to perform physical training in an interesting and enjoyable way. Objective: To evaluate the effects of an exercise program using the SG developed for hemiparetic stroke patients’ locomotor rehabilitation. Method: Non-Randomized Controlled Clinical Trial. Twenty-four hemiparetic stroke patients with subacute or chronic stroke (twelve men), mean age of 57.8 ± 10.4 years (injury time of 16.8 ± 19.6 months) participated in the study. The experimental group (n = 16) participated in an exercise program with the SG for lower limb rehabilitation. The control group (n = 8) received conventional treatment (kinesiotherapy). The intervention consisted of sessions twice a week for ten weeks. The following parameters were assessed: muscle strength (dynamometry), spasticity (Modified Ashworth Scale), functional mobility (Timed Up and Go Test - TUGT), and the gait speed (GS). Results: Both groups showed improvements, but the experimental group was better in all the studied variables, the muscular strength of the lower limb paresis and of the quadriceps femoris (p = 0.002; d = 0.7); and for the hamstrings (p < 0.001; d = 1.3), TUGT (p < 0.001; d = 0.4), and GS (p = 0.001; d = 0.4). Conclusion: The exercise program with the SG was useful for the patients treated in this study. The results showed a superiority of the SG regarding the conventional treatment in all the controlled variables. This was probably because of the greater repeatability of the exercises and the increased attention and motivation.

For Electrical Impedance Spectroscopy (EIS) applications, the Enhanced Howland Current Source (EHCS) is a popular choice as an excitation circuit due to its simplicity, reliability, and safety. However, its output impedance degradation at high frequency leads to errors that are unacceptable for high load impedance applications, such as the ones which use dry or microelectrodes. Based on a proposed mathematical model, this work designed an EHCS circuit which includes an output current buffer and frequency compensation. PSpice simulations were performed as proof of concept, and then the measured data were collected for comparison. For the proposed circuit, called here Load-in-the-Loop Compensated Enhanced Howland Source (LLC-EHCS), the results showed that the output current errors are lower than 1% up to 3.7 MHz over the load range of 560-2200 Ω and 1.2 MHz with 5.6 kΩ. On the other hand, for the case of the standard EHCS circuit, these frequencies are 170 and 80 kHz, respectively. Also, the output linear swing was found to be 3 times higher than the EHCS. It can be concluded that the proposed LLC-EHCS may be widely used as an excitation circuit for high load and wide bandwidth EIS applications.

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