Electrical impedance tomography based image reconstruction and feto-maternal monitoring in pregnancy

doi:10.4236/health.2011.38079

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Vol.3, No.8, 482-486 (2011)

doi:10.4236/health.2011.38079

Health

Electrical impedance tomography based image

reconstruction and feto-maternal monitoring in

pregnancy

Sharvan Kumar Pahuja1*, Sneh Anand1, Amit Sengupt a1,2

1Centre for Biomedical Engineering, Indian Institute of Technology Delhi; Hauz Khas, New Delhi, India; *[email protected];

[email protected];

2OBGYN, Mumbai, India; [email protected]

Received 20 January 2011; revised 5 April 2011; accepted 15 June 2011.

ABSTRACT

Standard methods of monitoring the fetus and

maternal health during labor are cardioto-

cogram, tocography, ultrasound and magneto-

cardiograpghy. These methods have some limi-

tations in real time continuous monitoring and

cause some degree of inconvenience to the pa-

tient and demand special attendance of the ob-

stetrician also these methods cannot be used

for continuous monitoring of the fetal well being.

To overcome the limitations of above techniques ,

a non-invasive bioimped ance measuring method

is proposed. This technique helps in monitoring

and recording of the electrical field distribution

of a closed object. The output variation on the

outer surface is likely to provide information

because of fetal movements and related physio-

logical parameters. It will also help in the de-

velopment of Electrical Impedance Tomography

based imaging technique for a closed body

system with special reference to fetal monitor-

ing in-utero during pregnancy and labor. Also

we have developed the data acquisition system

of 16 electrodes with software for image recon-

struction.

Keywords: Impedance Plethysmography; Electrical

Impedance Tomography; Phantom

1. INTRODUCTION

Feto-maternal monitoring has become a very importa nt

tool to control a fetal health throughout the pregnancy and

during labor. The vital parameters like fetal ECG, uterine

contraction, fetal movements, and growth of the fetus,

amniotic fl uid vol um e and position of placenta ha ve to be

in good conditio n for the fetus health. Fetus in situ initi-

ates compensatory protective mechanism which enables

it to survive from any altered state/ abnormalities (such as

cerebral palsy, placenta previa, asphyxia, respiratory and

the growth) [12, 13] t hat m ay occur both during norm al as

well as abnormal pregnancy. These abnormalities could

be avoided by the timely diagnosis by continuously

monitoring the fetus and mother by the use of the in-

struments. It is imperative to monitor vital parameters at a

regular interval throughout pregnancy and during labor

for feto-maternal well being and for better outcome [14].

Techniques and Instrumentations have been developed to

monitor various parameters and for feto-maternal well-

being.

Cardiotocogram (CTG) is a standard method for

monitoring of fetal heart rate (FHR) and the uterine con-

tractions (UC) in the later stage of the pregnancy. By

analysis and appropriate interpretation of changes in the

CTG obstetrician are able to prevent still birth or as-

phyxia. Other available techniques such as external and

internal tocography (TOCO) [17], Ultrasound [16] and

Magnetocardiograpghy [17] have also been in used for

monitoring of vital parameters of the fetus and mother

[18]. The growth of the fetus and in understanding of the

anatomy and functions of an organ or a physiological

process, various invasive and non-invasive imagining

techniques are available. All medical modalities rely on

ionizing radiation to produce images of structure and

function through a variety of mechanism but the main

biological risk is cancer when imaging in utero. Ultra-

sound is widely used for imaging of the fetus which has

some limitations like long term monitoring is not possible

and demands well trained experience ultrasonographer

for data acquisi t i on .

Despite their scientific perfection all these methods

cause some degree of inconvenience to the patient and

demand special attendance of the obstetrician. Also these

methods cannot be used for continuously monitoring of

S. K. Pahuja et al. / Health 3 (2011) 482- 486

483

the feto-maternal parameters because they are bulky,

expensive, time consuming and n e ed skilled personnel.

To overcome the limitations, here electrical impe dance

monitoring based instrument is proposed. This technique

helps in continuously recording and monitoring non-

invasively of inside electrical field distribution in term of

voltage for a given current. The output voltage on the

abdomen surface of a pregnant lady is likely to provide

information regarding fetal movement and its growth.

The method has already been used in measurement of

bioimpedance. An electrical impedance plethysmograph

based instrum ents have been using in moni toring of bl ood

flow in t he heart [Nyboer]. Nicander et al. a nd Skour ou et

al. have s hown that a ti ssue’ s im pedance signature may be

more sensitive to presence of tumor tissues than co nven-

tional imaging technique of computed tomography and

ultrasound. Also the technique has already been used for

monitoring respiration [Bake], measurement of cardiac

output [Kubicek]. Further the concept of Electrical Re-

sistive Tomography was proposed independently by

Henderson and Webster for medical imaging. Kim has

applied impedance technique for measurem ent of various

physiological e vents. Eac h organ is an aggre gate of many

different cells held together by intracellular supporting

structures. The conductivity of human tissues varies from

cerebrospinal fluid to bone in very large difference. The

range is 15 mS/cm to 0.067 mS/cm [4]. The Table 1

shows the conductivities of various tissues assumed for

50 Hz [Cech]. As shown in table the distribution of con-

ductivity inside the body shows good contrast hence good

electrical conductivity images can be produced of these

distributions.

2. THEORETICAL MODEL AND

MEASURING METHODOLOGY

Figure 1 shows p ictorial representation of current dis-

tribution and equi-potential lines on the surface of the

Table 1. show s the conductivities of various tissues assumed

for 50 Hz.

Tissue Conductivity (S m-1)

Bone 0.0201

Fat 0.0196

Heart 0.0827

Muscle 0.233

Skeleton of fetus 0.0201

Soft tissue of fetus 0.216

Spinal cord 0.0227

Uterus 0.229

cell contains homogenous electrolyte solution. The high

frequency low AC current is applied between electrodes

I1, I2 which gets uniformly distributed and the resulting

voltage is measured between electrodes V1 and V2. Fig-

ure 2 shows the complete experimental setup. The exci-

tation current is applied into the subjects between Q-S

electrodes in the range of (0.8 mA - 1 mA AC) at 20 -

100 KHz. The voltage produced between P-R electrodes

is amplified and fed to PC using data acqu isition system.

This set up is act as a bridge configuration as shown

Figure 3. Any variation inside (because of fetal move-

ment) will affect the abdomen surface of the pregnant

woman. This can be measured non-invasively in term

voltage. The current and voltage electrodes are switched

through different combinations so that the transfer im-

pedance is measured for different position of applied

currents.

The same principle is used in Electrical Impedance

Tomography (EIT) in which an image of the conductiv-

ity distribution within a cross-section of the body is ob-

tained. The technique exploits the electrical characteris-

tics of the tissues by measuring voltages induced on the

electrodes attached to the surface when the currents are

passed between them. The instrument is makes imped-

ance measurement using 16 electrodes on the surface of

the body. A current is passed between pairs of electrodes

(QS). The voltage developed between remaining pairs of

electrodes is than measured sequentially by an array of

instrument amplifiers and fed to data acquisition system

for recording and further analysis. The general schematic

diagram of the data acquisition is shown in Figure 4.

3. METHODS AND MATERIAL

REQUIRED

Presently some preliminary work is being carried out

Figure 1. Four-e lec t rode measurement system.

S. K. Pahuja et al. / Health 3 (2011) 482- 486

484

Figure 2. Block Diagram of impedance measuring system.

Figure 3. Equivalent bridge diagram in case of fetal move-

ments.

Figure 4. Block diagram of the real-time EIT configuration.

by conducting experiments on a phantom uterus and

fetus. Vital parameters have been recorded using exter-

nal tocography and impedance measurement system.

And images have been reconstructed by image recon-

struction algorithm.

Advanced experimental studies will be carried out to

develop and standardized the instrumentation. The actual

clinical applicability shall be checked at later stages

during clinical trials (on human and animals). The ex-

periments will be conducted on both pregnant women

(with gestation age 25 - 36 week s) and pregnant animals

for the clinical trials to establish the relationship between

fetal movement and immature fetal autonomic nervous

system from the recorded data.

4. MODEL BASED EXPERIMENTS

Presently some preliminary work is being carried out by

conducting experiments on a phantom uterus and fetus.

The work is validated by conducting some experiments

by taking papaya and plastic box as phantom. First ex-

periment is performed on phantom papaya. Papaya is

chosen as it imitates the properties of human’s uterus.

Second experiment is conducted on plastic box.

4.1. Case Study 1

Figure 5 shows papaya, watermelon, melon and co-

conut as phantom where we could observe the variation

when the ac curr ent was supplied through two electro des.

The electrode configuration is different in each case.

4.2. Case Study II

Second experiment was conducted on plastic container.

The set up is as shown in Figure 6. The input here is 4

volts at frequency of 40 KHz. The output was measured

between other pairs of electrode by opposite method of

data acquisition.

4.3. Case study II

The electrical conductivity distribution was measured

for a closed object using data acquisition methods. Data

was collected by applying ac current pattern in the range

of 1 - 5 mA at a frequency of 40 KHz to the phantom

papaya through the two conducting electrodes and then

inside conductivity is measured on the papaya surface in

term of voltage between remaining pairs of the elec-

trodes using op posi t e method.

The current-voltage distribution relationship of the

applied current and measured voltage is determined by

Poisson’s or Laplace’s equation with given boundary

conditions measured at the surface. An image of conduc-

tivity distribution was obtained for the closed object

using appropriate image reconstruction algorithm. Soft-

ware is also developed for creating meshes, nodes and

for solving the independent equations for finding the

pixel intensity of an image using iterative methods. Fig-

ure 7 shows the complete experiment set up and the im-

ages reproduced by using image reconstruction algo-

rithm.

S. K. Pahuja et al. / Health 3 (2011) 482- 486

485

Figure 5. Experimental Setup with different phantom.

Figure 6. Complete experimental setup with plastic

container as model.

(a)

Empty Papaya

(b)

Object 1

Object 2

Object 3

(c)

Figure 7 (a) Experiment setup (b) The 2-D image of an

empty papaya (c ) The 2-D image of the phantom papaya

with objects inside.

S. K. Pahuja et al. / Health 3 (2011) 482- 486

486

5. CONCLUSION [9] Smith, R.W., Ian, L. and Brian, H.B. (1995) A real-time

electrical impedance tomography system for clinical

use-design and preliminary results. Biomedical Engi-

neering, 42, 133-140.

The new technique being proposed in this work is non-

invasive, user friendly, economical and for mass health

care which can be used by the poor community and the

basic health work er. Also the nov el ap pro ach will h elp in

defining the morphological and physiological changes

occurring inside the body using EIT as a functional non

invasive imaging tool.

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Perinatal Medicine, 31, 275-280.

[11] Leman, H., Catherine, M. and Jean, G. (1999) Use of the

electrohysterogram signal for characterization of contrac-

tions during pregnancy. IEEE Transactions on Biomedi-

cal Engineering, 46, 672-680.

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