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Electrical Impedance Tomography _EIT_ and Magnetic Resonanse

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Electrical Impedance Tomography _EIT_ and Magnetic Resonanse Powered By Docstoc
					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 4above 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. 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 Figure 1. A. Picture with EIT electrode levels and with calculated. two RIP-electrode bands. B. Photograph of EIT For RIP, the rib cage gauge is placed at level 2, 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.
EIT level

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. We normalized dI(Glob) with dRC to account for differences in tidal volume (Fig. 2).

Figure 2. dI(Glob) (from EIT data) normalized with dRC.

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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-raterelated 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 Figure 3. Distribution of ventilation in the abdominal circumference changes with respiration. thorax: A. Dorsal part of lung, B. Ventral part of We conclude that best level for electrode 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 respiratoryrate-related impedance changes can also be found in areas with no lungs.

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REFERENCES Brown BH. “Electrical impedance tomography (EIT): a review” J Med Eng Technol. 2003:27:97-108. 2. Brown BH, Seagar AD “The Sheffield data collection system” Clin Phys Physiol Meas 1987:8:A91-97. 3. Boone K, Barber D, Brown B. “Imaging with electricity: report of the European Concerted Action on Impedance Tomography” Chest 1997:111:1222-1228. 4. Harris ND, Suggett AJ, Barber DC, Brown BH “Applications of applied potential tomography (APT) in respiratory medicine” Clin Phys Physiol Meas 1987:8:A155-65. 5. Hahn G, Spinova I, Baisch F, Hellige G. “Changes in the thoracic impedance under different ventilatory conditions” Physiol Meas 1995:16:A161-A173. 6. van Genderingen HR, van Vught AJ, Jansen JR. “Estimation of regional lung volume changes by electrical impedance pressures tomography during a pressure-volume maneuver”Intensive Care Med. 2003 Feb;29(2):233-40. Epub 2002 Dec 14. 7. Victorino JA, Borges JB, Okamoto VN, Matos GF, Tucci MR, Caramez MP, Tanaka H, Sipmann FS, Santos DC, Barbas CS, Carvalho CR Amato MB “Imbalances in regional lung ventilation: a validation study on electrical impedance tomography” Am J Respir Crit Care Med 2004:169:791-800. 8. Frerichs I, Braun P, Dudykevych T, Hahn G, Genee D, Hellige G. “Distribution of ventilation in young and elderly adults determined by electrical impedance tomography” Respir Physiol Neurobiol. 2004 Oct 12;143(1):63-75. 9. Konno K, Mead J. “Measurement of the separate volume changes of rib cage and abdomen during breathing” J Appl Physiol. 1967 Mar:22(3):407-22. 10. Strömberg NO,Dahlbäck GO, Gustafsson PM. “Evaluation of various models for respiratory inductance plethysmography calibration” J Appl Physiol. 1993 Mar:74(3):1206-11. 1.

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