The human body consists of various biological tissues with frequency-dependent electrical conductivities. These properties change over time, for example due to cardio-respiratory activity. Electrical impedance tomography (EIT) injects a harmless current into the body and measures the resulting surface potential to recover regional conductivity distributions inside the human body. Time-differential EIT (tdEIT) provides information about conductivity changes over time. tdEIT assumes that the change in conductivity is small compared to the reference measurement and is only local, so that the background conductivity remains constant. This assumption allows a linear reconstruction, e.g. using the GREIT algorithm (Adler et al 2009). This technique can be used to monitor ventilation of critically ill patients, so that most EIT-applications address breath-by-breath lung monitoring using tdEIT approaches. However, many lung pathologies develop over a long period of time and thus can not provide a reference image for tdEIT. An edema, for example, impairs the gas exchange due to fluid accumulation in the airspace of the lung. Similar symptoms arise due to a pneumonia, which is an inflammatory infection of the alveoli. However, multi-frequency EIT (mfEIT) might have the potential to monitor lung pathologies without a baseline (Packham et al 2012). The main fundamental characteristic of multi-frequency EIT (mfEIT) is the combination of tissue properties and regional information to reconstruct the location of various tissues inside the body. The reconstruction of mfEIT is much more complex than tdEIT, as mfEIT performs simultaneous measurements at multiple frequencies, which contain the regional frequency-dependent conductivity. This was utilized in Mayer et al (2006), who reported a method for direct reconstruction of tissue-parameter. A similar principle was later specified by Malone et al (2013). This spectral constraints algorithm (SCA) uses multiple frequencies to reconstruct a tissue fraction inside the image, which can be described as a tissue-probability map. The number of constraints in the reconstruction is increased due to prior knowledge about the underlying

Bioimpedance measurements use current or voltage sources to inject an excitation signal into the body. These sources require a high bandwidth, typically from 1 kHz to 1 MHz. Besides a low common mode, current limitation is necessary for patient safety. In this paper, we compare a symmetric enhanced Howland current source (EHCS) and a symmetric voltage source (VS) based on a non-inverting amplifier between 1 kHz and 1 MHz. A common mode reduction circuit has been implemented in both sources. The bandwidth of each source was optimized in simulations and achieved a stable output impedance over the whole frequency range. In laboratory measurements, the output impedance of the EHCS had its -3 dB point at 400 kHz. In contrast, the VS reached the +3 dB point at 600 kHz. On average over the observed frequency range, the active common mode compensation achieved a common mode rejection of -57.7 dB and -71.8 dB for the EHCS and VS, respectively. Our modifications to classical EHCS and VS circuits achieved a low common mode signal between 1 kHz and 1 MHz without the addition of complex circuitry, like general impedance converters. As a conclusion we found VSs to be superior to EHCSs for bioimpedance spectroscopy due to the higher bandwidth performance. However, this only applies if the injected current of the VS can be measured.