Electrical Impedance Tomography (EIT) and Magnetic Resonanse

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					            Electrical Impedance Tomography (EIT) in healthy subjects
                I. Kadzinska MSc, H.R. van Genderingen PhD, R.M. Heethaar PhD
   Dept. of Physics and Medical Technology, Vrije Universiteit Medical Centre, Amsterdam, The Netherlands

Topic area: Clinical applications

ABSTRACT: The aim of the present study was to investigate with Electrical Impedance Tomography (EIT) the
distribution of respiratory-induced impedance changes in healthy subjects by Electrical Impedance Tomography
(EIT), using different electrode positions. We applied EIT in 9 healthy volunteers (up to this moment) who were
breathing spontaneously in regular rate. Simultaneously to EIT, we applied Respiratory Inductive
Plethysmography (RIP) as the reference method to measure tidal volume. The EIT electrodes were placed on four
different levels (level 1-under the arm pit, level 2-between the arm pit and sternum, level 3-sternum, level 4-
above the navel) and 16 electrodes per level were placed equidistantially on the body. Subjects were placed in
supine position. From the EIT recordings, the global impedance change dI(Glob) and regional impedance change
dI in a number of regions (ventral, dorsal, left, right) were determined. From the RIP recordings we measured
dRC (amplitude change of the Rib cage band ) and dAB (amplitude change of the Abdominal band) and
calculated tidal volume. We normalized dI(Glob) with dRC to account for differences in tidal volume, and
investigated the dependency of impedance amplitude of the electrode level. In addition, we calculated the
distribution of ventilation over the ventral-dorsal and right- left side of the lungs. In both cases the distribution
was between 40% and 60%. We conclude that the ventilation is distributed nearly homogeneously in healthy
volunteers.


1. INTRODUCTION

Electrical impedance tomography (EIT) is an imaging technique with potential powerful
applications in medicine. The principle of EIT is based on the measurement of voltages
resulting from rotating injection of known small alternating electrical currents through
electrodes attached on the circumference of an object.1,2,3
The relative impedance change assessed with EIT is proportional to changes in lung volume
(that was shown by Harris 1987, Hahn 1995 and van Genderingen 2003).4,5,6 In a recent study
Victorino & Amato (Victorino, 2004)7 demonstrated in patients a close relationship between
regional change in lung aeration determined by CT and regional relative impedance change.
Recently it was suggested that homogeneity of ventilation could be predicted or assessed with
EIT in order to prevent further augmentation of lung injury (Van Genderingen 2003, Frerichs
2004).6,8 To assess abnormal ventilation distribution in the lungs, it is essential to know the
normal ventilation distribution over the lung area and if the results of the EIT measurements
depend on the cranial-caudal electrode level.
The aim of the present study is to investigate the distribution of respiratory-induced
impedance changes in healthy subjects by Electrical Impedance Tomography (EIT), using
different electrode positions.

2. METHODS

In 9 healthy volunteers we applied EIT to determine regional impedance changes.
Subjects were breathing spontaneously in regular rate.

The EIT electrodes were placed on four different levels (Fig. 1):


Electrical Impedance Tomography (EIT) in healthy subjects;         i.kadzinska@vumc.nl           Page 1/4
- level 1: just under the arm pit
- level 2: between arm pit and sternum
- level 3: sternum
- level 4: on the lower abdomen, just above the
navel.
                                                                                                        EIT level
Sixteen electrodes per level were placed                                                           1-4
equidistantially around the thorax. One earth
electrode was placed on the lower abdomen.
Subjects were placed in supine position.
Simultaneously to EIT, we applied Respiratory                                                    Respitrace
Inductive Plethysmography (RIP) as the                                                   A.
reference method to measure tidal volume.
RIP is a validated technology to assess
(spontaneous) respiration. The respiratory system
consists of two compartments, the rib cage and
abdominal compartment (Konno and Mead
1967).9 Their motion can be recorded by two
gauges, which are integrated in flexible bands
positioned around the rib cage and the abdomen.
By application of a calibration technique
(isovolume manoeuvre or QDC, Strömberg                                                            B.
1993)10 the changes in lung volume can be
calculated.                                          Figure 1. A. Picture with EIT electrode levels and with
For RIP, the rib cage gauge is placed at level 2,    two RIP-electrode bands. B. Photograph of EIT
                                                     electrodes (four levels, 16 electrodes per level).
the abdominal gauge on the lower abdomen.
Calibration data for RIP was acquired according
to the isovolumic manoeuver: The subject closes
his/her mouth and nose by hand and takes a few breaths without any air moving in or out, i.e.
the person moves air between the rib cage and abdominal compartment. The actual RIP
calibration was done off line.


3. RESULTS

From the EIT recordings, the global impedance
change dI(Glob) and regional impedance change dI
in a number of regions (ventral, dorsal, left, right)
were determined. From the RIP recordings we
measured dRC (amplitude change of the Rib cage
band) and dAB (amplitude change of the
Abdominal band) and calculated tidal volume.                  Figure 2. dI(Glob) (from EIT data) normalized
We normalized dI(Glob) with dRC to account for                with dRC.
differences in tidal volume (Fig. 2).




Electrical Impedance Tomography (EIT) in healthy subjects;   i.kadzinska@vumc.nl        Page 2/4
Tidal impedance amplitude was similar on 1st and 2nd level, and was significantly lower on the
3rd level. There was still a strong respiratory-rate-related impedance change in the abdomen
(4th level). In addition, we calculated the
distribution of ventilation over the ventral-dorsal
and right-left side of the lungs (Fig. 3). On the
gravitational     axis,     ventilation   is    most
                         nd
homogeneous on the 2 electrode level (Fig. 3A
and 3B), approximately 50% in the ventral and
dorsal areas. However, on the 1st electrode level,
around 60% of the tidal variation is detected in the
ventral part, whereas on the 3rd electrode level
40% was detected in the ventral part.
In the lateral direction ventilation was distributed
more homogeneously: with electrode levels 1-3,
both the left and right lungs received
approximately half of the tidal volume, as
indicated in figure 3C and 3D.


4. DISCUSION AND CONCLUSION

Main results of this study is that global impedance
amplitude as well as ventilation distribution are
strongly dependent on electrode level and that
even in areas without lungs a respiratory-rate-
related impedance changes can be found.
Ventilation was distributed homogeneously over
the dorsal and ventral parts on the 2nd electrode
level. The differences in ventilation on the other
levels (1st, 3rd) may be explained by the presence
of other organs.
On the 1st electrode level the heart may influence
homogeneity in that level. On the 3rd electrode
level the movements of diaphragm may influence
the EIT images. Interestingly, these imbalances
were not found in the left-right direction.
Global impedance changes were dependent on the
electrode level. On the 4th electrode level the
impedance changes may have be induced by
abdominal circumference changes with respiration.       Figure 3. Distribution of ventilation in the
We conclude that best level for electrode               thorax: A. Dorsal part of lung, B. Ventral part of
                                                        lung, C. Left lung and D. right lung.
placement in Electrical Impedance Tomography is
the 2nd level, half way between the armpit and the
xyphoid. Care should be taken to carefully place the electrodes on the lung area as respiratory-
rate-related impedance changes can also be found in areas with no lungs.




Electrical Impedance Tomography (EIT) in healthy subjects;   i.kadzinska@vumc.nl      Page 3/4
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