CSEM Scientific and Technical Report 2008

The CSEM Electrical Impedance Tomography (EIT) Demonstrator
J. X. Brunner, S. H. Böhm, P. O. Gaggero
EIT has been known for more than 20 years but, in spite of early successes, has not found its way into clinical practice. The main reasons are
cumbersome use and the associated poor signal to noise ratio. Both issues are being addressed and feasibility is shown in a demonstrator.
Mechanically ventilated intensive care patients with acute lung
failure (75 per 100’000 population) have a mortality rate of 40
to 50%. The lives of many of these patients could be saved by
appropriately adjusting the ventilator if adequate monitoring
were available. Electrical Impedance Tomography (EIT) could
become the key for the dynamic determination of these
optimal settings, as has been shown in first clinical studies [1] .
EIT allows continuous creation of cross-sectional images of
the human body, providing for the monitoring of key body
functions non-invasively and without side-effects in real-time.
In contrast to existing technology such as X-ray, CT, MRI or
PET, EIT neither needs ionizing radiation nor bulky and
expensive equipment. Potentially, an EIT system costs at
least 100 times less than conventional 2D imaging systems.
EIT provides images of body functions such as the local flow
of blood or air rather than just snapshots of body anatomy and
composition. EIT images can be used in the Intensive Care
Unit (ICU), in emergency medicine and at home.
EIT uses 32 electrodes around the chest to inject small
amounts of electrical current and to measure the result of
these injections as voltage distributions at the body surface.
The current is very small and safe (below 10 mA at around
100 kHz), and cannot be felt by the patient. The measured
values are converted into pictures of local conductivities,
which are representative of body tissues and fluids, allowing
the real-time monitoring of blood perfusion and breathing.
Unfortunately, the EIT electrode assembly is cumbersome and
difficult to use (Figure 1). CSEM is therefore developing an
electrode belt assembly and analysis system with the
following components: 32 skin-electrodes with integrated
impedance converters (E-Nodes), chest band with integrated
E-Nodes, data bus around the chest to communicate among
the E-Nodes, a Field Programmable Gate Array FPGA to drive
the E-Nodes and demodulate the signals for transfer via
ETHERNET to a connected PC, and a PC based image
construction and display unit. The E-Nodes apply an AC
current (50 kHz up to 200 kHz), measure voltages in the range
of 0.1 mV to 5 V (absolute and relative), provide a program to
switch between current source and voltage measurement, and
apply this program relative to the position around the chest.
The intended use is for intensive care and emergency patients
who need mechanical ventilation. Thus, the following
requirements apply:
• Suitable for female & male ICU patients, independent of
body position (supine, prone, upright)
• Permit EIT monitoring for hours up to weeks, will not
impede nursing and physiotherapy
• Will not create skin lesions (decubitus), will not restrict
breathing movements
• Applicable in