Physiol. Meas. 19 (1998) 501{516. Printed in the UK PII: S0967-3334(98)94029-1
Quanti cation of blood volume by electrical impedance
tomography using a tissue-equivalent phantom
Rosalind Sadleir† and Richard Fox‡
† Department of Physics, University of Western Australia, Nedlands 6907, Western Australia
‡ Department of Medical Physics, Royal Perth Hospital, Box X2216 GPO Perth, Western
Australia
Received 12 May 1998, in nal form 26 August 1998
Abstract. An in vivo electrical impedance tomography (EIT) system was designed to accurately
estimate quantities of intra-peritoneal blood in the abdominal cavity. For this it is essential that
the response is relatively independent of the position of the high conductivity anomaly (blood)
in the body. The sensitivity of the system to the introduction of blood-equivalent resistivity
anomalies was assessed by using a cylindrical tissue-equivalent phantom. It was found that a
satisfactorily uniform response of the system in both radial (transverse) and axial (longitudinal)
directions in the phantom could be achieved by ltering resistivity pro le images obtained by
EIT measurement, and by using extended electrodes to collect data. Post-processing of single
impedance images gave rise to a quantity denoted the resistivity index. A lter was then used
to remove the remaining radial variation of the resistivity index. It was calculated by evaluating
the resistivity index of a number of theoretically calculated images, and constructing a correction
lter similar to those used to remove lens imperfections, such as coma, in optical components.
The 30% increase in the resistivity index observed when an anomaly was moved to the maximum
extent allowed by the lter calculation (0.75 of the phantom radius) was reduced by the lter to
6%. A study of the axial dependence observed in the resistivity index using electrodes extended
in the axial direction by ±5 cm found that the variation in resistivity index with axial position
was about half of that observed using small circular electrodes similar to those used in the
Shef eld mark I system.
Keywords: electrical impedance tomography, volume estimation, linearized backprojection,
intra-peritoneal bleeding
1. Introduction
Electrical impedance tomography (EIT) (Barber et al 1984, Barber and Brown 1986, Gisser
et al 1988) may be used to obtain cross-sectional representations of the electrical impedance
distribution inside the body. The method usually involves application of low frequency
(∼50 kHz) current to the body surface and measurement of the electrical eld pattern
that results. This is achieved by attaching a set of electrodes, equally spaced about a
single transverse plane, to the perimeter of the body. The linearized backprojection method
described by Barber (Barber and Brown 1986), and implemented in the Shef eld mark I
system (Brown and Seagar 1988), involves making an approximation to the solution to
the non-linear inverse problem relating impedance distribution to electrical eld patterns
and is best adapted to continuous monitoring of changes of resistivity inside the human
body. This EIT system was made with the aim of obtaining dynamic studies of uid
distribution in the peritoneal cavity. Speci cally, we wished to detect and quantify the
0967-3334/98/040501+16$19.50 c© 1998 IOP Publishing Ltd 501