B. Ülker and N.G. Gençer Electrical and Electronics Engineering Department, Middle East Technical University, Ankara, Turkey
Implementation of a Data Acquisition System for Contactless Conductivity Imaging Editor’s Note: This article tied for third-place in the 2001 EMBS Student Paper Competition. here are number of techniques to obtain conductivity images of the human body [1]. In applied current electrical impedance t omogr aphy (ACEIT), current injection and voltage measurements are both performed by the surface electrodes. In induced current electrical impedance tomography (ICEIT), currents are induced by magnetic induction and voltage measurements are performed by the surface electrodes. Both of these techniques employ electrodes attached on the body, and there are a number of limitations related to electrodes and associate cabling [2]. In this paper, a new medical imaging system is implemented to image the conductivity distribution by magnetic coupling [3], [5]. The basic principle of the system is shown in Figure 1. The basic instrument is a three-coil differential transformer. Sinusoidal current excites the center coil (primary) and the two receiver coils (secondary) are connected in series phase opposition to cancel out the voltage induced by direct coupling. The timevarying magnetic field in the transmitter
Φ=
(1)
where AR is the magnetic vector potential crated by the reciprocal current IR in the detector coil. Here, J is equal to JT in the excitation coil and JI in the conductive body. Then flux in the detector coil can be obtained by taking the integrals in the corresponding volumes:
T
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1 A ⋅ JdV IR ∫ R
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Φ=
coil induces current in the receiver coils and conductive body. The receiver coil senses the magnetic fields of both, induced by the transmitter coil and the conductive body.
Theoretical Background
The theoretical formulation relating conductivity to magnetic measurements is given in [3]; however, it will be presented here briefly. Using the magnetic reciprocity theorem, it is possible to obtain the flux SYMBOL in the detector coil as follows (Figure 2): IEEE ENGINEERING IN MEDICINE AND BIOLOGY
+
1 A ⋅ J dV I R ∫ R T COIL
1 . A ⋅ J dV I R ∫ R I BODY (2)
The first term on the right-hand side (RHS) is the primary flux, directly coupled from the transmitter coil. The second term represents the flux caused by the induced currents. The electromotive force in the receiver coil can be expressed as v = − jwΦ wAR ⋅ dl v = − j I T ∫ IR wAR ⋅ σ(wAT + ∇φ )dv′. − ∫ IR (3) The two terms in the RHS represent the primary (vp) and (vs), respectively.
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Methodology Data Collection System
The block diagram of the data collection system is shown in Figure 3. The coils are coaxial and 15 mm in radius. Receiver coils are composed of 10,000 turns wound on a 3 cm diameter delrin rod using 0.06 mm copper wire. The transmitter coil is 100 turns at the same size as the receiver coils. Oscillator output of the lock-in amplifier feeds the power amplifier, which excites the transmitter coil. The induced primary voltage is cancelled out by the nulling circuitry so the residue signal becomes nearly 0 V when there is no conducting object. The output of the nulling circuitry is connected to the input of the lock-in amplifier for phase-sensitive detection (the lock-in amplifier’s reference signal is fed with the transmitter coil current). The instrument is controlled by a PC and the data are directly collected to the computer. D/A outputs of the lock-in amplifier are also controlled by PC, which feeds the XY scanner. The probe is connected to the XY scanner. Therefore, it is possible to collect fully computer-controlled data.
Choice of Excitation Frequency
The operating frequency should be lower than 100 kHz since the displacement currents are small and propagation terms are negligible in that frequency range. In most of the electrical impedance tomography studies, an operating frequency of 50 kHz is usually selected to avoid the stray capacitance effects in the measurements [4]. Similarly, in this study, although the secondary voltages increase with the square of the frequency, low frequency is desired to decrease the electric field pick-up. Low-frequency operation is also preferred to decrease the residue signal and minimize the efforts for nulling. In order to work with the minimum residue signal (when there is no conducting object near the transducer), the residue signal is recorded by sweeping the frequency of the excitation signal between 0 to 100 kHz. Due to the available electronics and coil characteristics, the minimum residue signal is achieved at 60 kHz and this frequency is chosen as the operating frequency.
Transmitter Coil Receiver Coils Induced Electric Field
Secondary Flux Lines Conductive Body
Conductivity σ
1. General principle of contactless conductivity imaging system.
Conductivity
Infinity
V σ
Ji Conductive Object Volume
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S
V’
2. The relation between magnetic flux densities between transmitter coil current IT and receiver coil IR.. Computer
Pow. Amp
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XY Scanner
RS232
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The aim is to measure the voltage caused by the conductive object. The problem is that the signal to be measured is usually too small compared to the resi-
Total Volume
IR
IT
Induced Current Density in Conductive Object
Receiver Coil 2 Transmitter Coil Receiver Coil 1
R
3. The structure of the data collection system. IEEE ENGINEERING IN MEDICINE AND BIOLOGY
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Monitor Resistance Voltage
0-90° Adjustable Phase Shifter
Gain Adjustable Amplifier P
A
Lock-In Input S
Residue Signal from Receiver Coils
Subtractor
due voltage (for example, 30 mV compared to 27 V). Consequently, there should be a nulling circuit to cancel the effects of the residue voltage (Figure 4). The signal on the monitor resistor is amplified and phase shifted to obtain a signal, which is equal in magnitude and phase with the residue signal. The amplification and shifting operations are performed with manually controlled potentiometer. The residue signal becomes 30 mV when the nulling circuit is employed.
XY Scanning System
To scan the probe over the phantom a computer-controlled XY scanning system is used (Figure 5). The motion is controlled by two step motors, which are controlled by a step motor driving circuit that is fed by computer. The scanning system has a minimum step size of 0.4 mm.
4. The block diagram of the manually adjusted nulling circuitry.
Measurements
5. The picture of the XY scanning system with the probe attached. The system is made up of all plastic materials to avoid artifacts.
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Three different experiments are performed using different objects and/or experiment parameters. The result of each experiment is presented with grayscale image and contour plots. The actual object geometry is drawn on the grayscale images. The first experiment is performed with a syringe filled with 0.6 S/m saline and prepared with NaCl. The 6.9 cm × 10.1 cm pattern is scanned at 69 × 101 (with 1 mm step length). The data acquisition time is 572 min. The voltage profile is shown in Figure 6(a) and (b). The second experiment is performed with a syringe filled with 0.6 S/m saline solution. The 10.1 cm × 10.1 cm pattern is scanned at 25 × 25 (with 4 mm step length). To increase SNR, 50 measurements are obtained for each point. The total data acquisition time is 270 min. The voltage profile is shown in Figure 7(a) and (b). The third experiment is performed with a 3 cm diameter circle phantom filled with 0.2 S/m solution. The 10.1 cm × 10.1 cm pattern is scanned at 17 × 17 points (with 6 mm step length). To increase SNR, 100 measurements are obtained for each point. The total data acquisition time is 240 min. The voltage profile is shown in Figure 8(a) and (b).
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Results and Discussion
(a)
(b)
6. Experiment results of syringe filled with 0.6 S/m solution scanned with 1 mm accuracy. There is an artifact due to external effects at the left corner. (a) Grayscale image. (b) Contour plot. 154
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In this paper, a data acquisition system is realized for conductivity imaging via contactless measurements. It is observed that field profiles are good representatives of the conductive objects scanned by the September/October 2002
measurement system. Three different data sets are taken with different scanning resolutions. Note that although the scanning resolution is decreased 25 times, it is still possible to observe the conductive object. The data acquisition time is due to the data acquisition time limit of the lock-in amplifier. As might be expected, the secondary voltage profiles spreads. It is possible to employ an image reconstruction algorithm and obtain conductivity images with better resolution. In conclusion, the first results of a promising new medical imaging modality were presented. This system can provide necessary conductivity information for electromagnetic source imaging of the human brain. Moreover, this system can be applicable to brain breast and lung imaging [1]. Other application areas should further be explored. Basak Ülker was born in Ankara, Turkey, in 1978. She received the B.S and M.S. degrees in electrical engineering from the Middle East Technical University, Ankara, in 1999 and 2001, respectively. She is currently enrolled in the Ph.D. program of the electrical engineering department of Northeastern University, Boston, Massachusetts. Her research interests include physics-based wavefield inversion routines for medical and subsurface applications. Nevzat G. Gençer received the B.Sc. degree in 1985 from Bosphorus University, Istanbul, Turkey, and the M.Sc. and Ph.D. degrees from Middle East Technical University (METU), Ankara, Turkey, in 1988 and 1993, respectively, all in electrical and electronics engineering. He was a teaching assistant and instructor in the Electrical and Electronics Engineering Department, METU, from 1987 to 1994. He held a postdoctoral position in the Neuromagnetism Laboratory of the Physics Department at New York University, New York, during 1994-1995. Then he worked as a research assistant professor in the same department. In 1996, he joined the Electrical and Electrical and Electronics Engineering Department of METU as an assistant professor. Since 1997 he has been working as an associate professor in the same department. In 1999 he received the Research and Encour-
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8. Experiment results of phantom filled with 0.2 S/m solution, 3 cm in diameter, scanned with 6 mm accuracy. (a) Grayscale image. (b) Contour plot.
agement Award, given by Prof. Dr. Mustafa N. Parlar Education and Research Foundation. He is a member of IEEE/ EMBS. His research interests are in mathematical and computational aspects of medical imaging and application of numerical electromagnetics to biomedical problems, especially electrical impedance imaging and electro-magnetic source imaging. E-mail:
[email protected]. Address for Correspondence: Basak Ulker,
[email protected]
References [1] J.P. Morucci and B. Rigaud, “Bioelectrical impedance techniques in medicine,” Crit. Rev. Biomed. Eng., vol. 24, nos.4-6, pp. 655-677, 1996.
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[2] N.G. Gençer, Y.Z. Ider, and S.J. Williamson, “Electrical impedance tomography: Induced current imaging achieved with a multiple coil system,” IEEE Trans. Biomed. Eng., vol. 43, pp. 139-149, 1996. [3] N.G. Gençer and M.N. Tek, “Electrical conductivity imaging via contactless measurements,” IEEE Trans. Med. Imag., vol. 18, no. 7, pp. 617-627, 1999. [4] J.P. Morucci and B. Rigaud, “Bioelectrical impedance techniques in medicine,” Crit. Rev. Biomed. Eng., vol. 24, nos.4-6, pp. 467-597, 1996. [5] P.P. Tarjan, “Electrodeless measurements of resistivity fluctuations in the human torso and head,” Ph.D. dissertation, Syracuse University, Syracuse, NY, 1968.
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