1 Introduction
Electrical impedance tomography (EIT) technology is an advanced process detection technology with the characteristics of invasive, safety, real-time, and economy. EIT has been widely used in industry
[1, 2], as well has a very promising application prospect in the fields of organism physiological function research, disease diagnosis and bedside monitoring
[3, 4]. Medical diagnosis has always been valued
[5], and EIT technology is desired to make great progress, because the ideal EIT can reconstruct real-time dynamic images of the distribution of impedance changes of biological tissues under different frequency and biological organs physiological activities (such as breath, heart beat). Moreover, it can also compensate for the existing defects of medical imaging technology in the field of functional imaging to achieve the purpose of early diagnosis and treatment of diseases. EIT technology has the ability to solve the problem of effective real-time visualization monitoring
[6, 7].
In order to achieve the above goals, the following key problems of EIT technology need to be solved
[8]. Firstly, there is the problem of dynamic measurement. In the human respiratory process, the measuring electrodes have to be attached to the skin surface of the human chest. Thus, the locations of these electrodes are moving dynamically. Secondly, as the measuring electrodes move, the size and shape of the measuring field will change dynamically. However, in the existing industrial detection applications, the measuring electrodes are attached on the boundary. In order to be adapted to the traditional measuring electrodes, the existing method assumes that the boundary of the measuring field is invariable. At present, in the human chest two-dimensional measurement of EIT technology, all of the tests and measuring electrodes (usually 16 or 32) must artificially be fixed and adjusted on the surface of the human thorax to keep them in a cross-section. In general, these electrodes are separately fixed on the surface of the chest, orientate the position of each electrode through the geometric measurement or other means of distance sensors, and then connect the measured field through all electrode positions. In this way, the cross section of the chest can be visualized by EIT technology.
There are three problems with the above existing measurement of the human thorax
[9, 10]. Firstly, when these measurements and adjustments are repeatedly and continuously, all electrodes cannot distribute evenly among the same cross-section. Secondly, the shape and the size of the measuring field determined by electrodes are dynamic in the human respiration process. Therefore, the existing measuring methods cannot adapt to the dynamic changes of the measurement points, and it is difficult to pick up real-time dynamic data. In addition, electrode adjustment is also easy to break the connection points between electrodes and conductors, scuff electrodes, and break off the electrodes from the skin and cause other issues
[11-13]. These problems seriously affect the service life of the electrodes and the accuracy of the measured signal.
2 Design of the New Sensor System
This section illustrates the structure of the new EIT sensor system and the measuring principles.
2.1 Structure of the New EIT Sensor System
The new EIT system, which consists of a 16-electrode array of capacitance sensors, is fixed in a steel plane with an ellipsoidal measuring area, as shown in Figure 1.
Figure 1 Manufactured sensor system |
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Each electrode is equipped on a sliding capacitance rod and its end is connected to the surface of measuring human chest, and the other end is connected to the EIT measuring and controlling system. One sliding capacitance sensor, including a metallic cylinder, a hollow non-metallic rod, a metallic rod, and lubricate centric mediums, as shown in Figure 2.
Figure 2 The structure of the new sensor |
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A pair of metal cylinders and metal rod serves as fixed and movable capacitance electrodes to measure the capacitance values. The two ends of the metal rod are connected to the metal cylinder and the nonmetal hollow rod respectively. A signal wire is connected to the external measuring circuit through the hollow non-metallic rod. The press rid of the connecting lines is fixed on the metallic rod. In addition, the hollow metallic rod and the metallic rod are also used to adjust the length of the entire sliding rod. In order to ensure the accuracy of the permittivity, centric mediums apply the Teflon, whose friction coefficient is the smallest of all plastic materials, then the metal rod can be moved freely into the metallic cylinder. The press rod is used to fix the metallic cylinder and these centric media and then can be updated easily. All the above components are placed in the same plane. The sliding rods can freely slide along a sliding way. The movement of the sliding rod can generate different capacitance values corresponding to the position of the rod.
2.2 Measuring Principle of the New System
As illustrated in Figure 3, the initial capacitance of the concentric cylindrical capacitance displacement sensor is given by
Figure 3 Concentric cylindrical capacitance sensor |
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The capacitance changes when the entire metallic rod moves relative to the fixed metallic cylinder in the human respiratory process. Suppose the displacement of the metallic rods is x, and the generating capacitance value between the metallic rod and the metal cylinder is given by
With reference to Equation (1) and Equation (2)
where is the product of the permittivity of vacuum and relative dielectric constant of the medium between metallic rod and metallic cylinder; x is the displacement of the metallic rods; is the half of the metal cylinder length; is the inner diameter of the metallic cylinder; is the external diameter of the metallic cylinder.
During the breathing process, as the thoracic cage expanding and contracting, the sliding rods closely stick with the chest, and the position between the metal rods and the metallic cylinder changes relatively. With reference to Equation (3), the linear relationship of and can be obtained, the displacement of the metal rods and the capacitance between metallic rods and metallic cylinder. Hence, the change of displacement can be obtained by the change in capacitance.
The magnitude of the output voltage in the actual measurement process reflects the change of the measured capacitance. Measured capacitance and voltage converting circuit are shown in Figure 4, where represents excitation source, and represent the grounded capacitances distributed on both ends of the measured capacitance, represents the capacitance under the testing process, represents the feedback capacitance, represents the feedback resistance, represents the output voltage of the operational amplifier.
Figure 4 C/V converting circuit |
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When , the output voltage of C/V converting circuit is given by
The C/V converting circuit is shown in Figure 4. The system generates a voltage signal proportional to the measured capacitance. As long as the output voltage amplitude is known, the value of measured capacitance can be derived. With reference to Equation (3) and Equation (4):
(the initial capacity of concentric cylindrical capacitance displacement sensor) and (the feedback capacitance of the C/V converting circuit) are both invariable. Putting into Equation (5) results in
It is obvious that the displacement and the output voltage have a linear relationship when excitation voltage is fixed. Then the real-time changes of any chest contour through the location information can be detected by the displacements of 16 concentric metallic rods.
In the human respiratory process, the electrodes attached to the human chest drive the sliding rod moving in the corresponding sliding approach, and then connect each sliding rod end in the ellipsoidal shell to determine the boundary of human chest at any time. In fact, the scale length of 32 sliding rods on an ellipsoidal shell can be solved by capacitance of each capacitor sliding rod. Ellipsoidal shell length of the long axis and short axis is known, and each sliding rod is ellipsoidal circle evenly distributed. Therefore, when the ellipsoidal boundary points are known in prior, the chest boundary can be determined by the relative position of each electrode, as shown in Figure 5. Then the locations of these measured boundary construct a new boundary to surround an investigating EIT field, and the imaging field can be repartitioned to approximate the actual variant field.
Figure 5 The chest boundary diagram |
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3 Experiment
In the finite element method of the EIT imaging field, the investigated imaging area is divided into a number of pixel units. In this paper, the imaging area is partitioned into 812 pixels. In the existing EIT sensors, the thoracic imaging cannot correspond to the actual boundary contour of the human body, since the thoracic profile is arbitrary when breathing. Consequently, the real chest boundary is assumed to be round. With the help of the new sensor system, the real-time variant positions of irregular chest contour have been measured 32 electrodes. The system was evaluated to be stable
[14, 15].
Figure 6 shows a data measuring process on a volunteer through the newly designed EIT sensor system.
Figure 6 A real-time measurement to a volunteer |
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For the exhalation and inhalation states, measured data is considered to be empty-field data in the EIT imaging process when the inhaling air quantity reaches the maximum. Thus the static measured data is processed as a series of full-field data. The most commonly used in EIT imaging algorithm, linear back projection (LBP)
[13], was applied to visually test the imaging effect on the volunteer's chest. In LBP algorithm, the conductivity distribution is assumed to be a number of discrete pixels in the measurement space, so the conductivity in the same pixel region is constant. In fact, the grey value of any pixel can be calculated using a weighted form in LBP algorithm. In this paper, LBP algorithm is used to reconstruct images of actual objectives.
Figure 7 shows a group of reconstructed EIT images of LBP algorithm, where the changing air contents and positions can be observed. In particular, all the reconstructed EIT images are stable and real-time, correctly corresponding to the inhaling and exhaling process. These results validate the effectiveness of the designed sensor system.
Figure 7 A real-time measurement to a volunteer |
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4 Summary
The overall system architecture of a new EIT sensor system has been designed and discussed. The purpose of this system is to deal with real-time measurement problems in the case of dynamic change of position in measuring sensors and the investigated fields. Then, some important modules are presented in detail. This method enables the system to adapt to any irregular boundary and shape. The experimental results show the new sensor system is effective. The proposed designing method is simple, general, and efficient, and provides the basis for further application of medical diagnosis and bedside monitoring.
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