Sakamoto (1987), Some Physical Results From an Impedance Camera

8/9/2019 Sakamoto (1987), Some Physical Results From an Impedance Camera

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Clin. Phys. Physiol. Meas., 1987, Vol. 8, Suppl. A , 71-76. Printed in Gr ea t Britain

Some physical results from an impedance cameraK Sakamoto*, T J Yorkey and J G WebsterDepar tment of Electrical and Computer Engineering, University of Wisconsin, Madison, WI,U SAAbstract. W e discuss some physical results from an imp edance camera. We placed agar blocksof various resistivities and sizes in saline. We showed th at when the aga r block was small or closein resistivity to the saline solution, we were u nable to reconstruct it. We attempted to reconstructan image of a fist, but were unable to achieve this due to curre nt flowing in the arm. W e concludedthat three-dimensional analysis may be needed, along with more sensitive reconstructionalgorithms an d measurements.

1. IntroductionX-ray CT, ultrasonic imaging, NM R imaging and other imaging systems have maderemarkable progress and contributed to an advancement of clinical diagnosis in medicine.These imaging techniques, however, cannot provide satisfying answers to all demands. Forexample, it is very difficult to get useful information from the lung by these techniques, andthese imaging systems are very expensive. For these reasons, many researchers haveconsidered impedance tomography. Barber and Brown (1983) presented impedance imagesof the arm and body using applied potential tomography but, as they stated, many problemsremain unsolved.

We examined some physical results from an impedance camera, based on the iterativealgorithm proposed by Kim et a l ( l 9 8 3 ) and modified by Yorkey er aI (1985) . We used thefinite-element method to calculate the electrical field and perturbation matrix needed in theiterative procedure. We measured the data experimentally using a physical phantom andreconstructed the image.

2. Experimental procedure2.1. Physical phanto m and m easurement systemFigure 1 shows the physical phantom we used. The physical phantom consists of a1 2 c m x 12 cm tank with 16 electrodes which are 2 mm wide and 3 cm high arrangedequidistantly on the tanks wall. Figure 2 shows the frequency characteristics of the electrodeimpedance and electrolyte ( 1 20 R cm saline solution). From these results, the electrodeimpedance at 50 kHz is less than 26 R and the spread impedance is 564 0.We used agarblocks with various resistivities, glass bottles, and glass beakers as test objects.

Figure 3 shows the block diagram of the measurement system. We used driven shields forthe current electrode pair, and buffers connected directly onto the voltage-measuringelectrodes to reduce the effect of stray capacitance to less than 1 pF. The input capacitance* Permanen t address: Departmen t of Electr ical Engineering, Sophia University, Tokyo, Japa n. This work was donewhile on sabbatical in the University of Wisconsin.

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of the buff er was 1 pF ,an d the analogue switch had a resistance of 2 MR and a capacitance of1 p F when the swi tch was off, therefore the total capacitance between the electrode andbuffer was about 3pF. The buffer input impedance was larger than 1 M R and the


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Some physical results fro m an impedance camera 73measurement error was about 0.08% at the buffers. Th e buffer ou tpu t was connected to thedifferential amplifier throu gh the analogue switches. Th e CMRR of the differential amplifierwas mo re than 90 dB a t 50 kHz, but the m easurem ent system CMRR was between 80 dB a nd90 dB because of the switch-on resistance. Th e signal went to the multiplier a n d the low-passfilter and was connected to the AD converter. Th e noise level of the amplifier was about10 pV a t 50 kHz. We used two differential constant-current sources at 50 k H z as adifferential co nstan t-curre nt source of 1 mA p-p so th at th e effects of a common mode inputsignal and a 60 Hz in pu t noise signal are less tha n th e amplifier noise. Since the m inim alinput signal level in our experiment was about 300pV, here was about 3% measurementerror in the worst case. Th e amplifier had a gain control circuit which w as controlled by th ecomputer automatically to get a better signal-to-noise ratio.2 .2 . Measurement of transfer impedanceAt first, we measured the voltage differences for all electrode combinations with only thesaline solution in th e physical phanto m. W e comp ared these with the theoretical values todeterm ine the shap e factors. Th en we measured th e voltage differences when some objectswere p ut inside the physical ph antom filled with the sam e saline solution as before and wecalculated the transfer imp edan ce by multiplying these measurements by the shap e factors.The resistivity of the saline solutions was 120C2 cm . Th e measurement t ime was about100 m s an d the calculation time for one reconstruction iteration was about 15 s.3. Results and dicussionFigure 4 shows one example of the sensitivity distribution (Geselowitz 1971) inside thephysical pha ntom . We calculated the sensitivity analytically using the imag e m ethod when

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74the electrolyte inside the physical pha nto m was homogeneous. Figu re 4 shows that ourmeasurement technique has a very low sensitivity in the centre of the computer model.Perhaps a different set of electrode combinations would have a higher sensitivity.W e set agar blocks whose resistivity was 500 R cm in the m iddle of the physical phantomfilled with s aline solution. T he reconstructed images are show n in figure 5 . The number ofi terations w as 10 for both images. Figure 5(a) shows the results when the object size is6 cm x 6 cm . Th e calculated values near the centre of the imag e were almost 500R cm. Wemig ht conclude that the im age size is almost the s am e size as the actual object size. Figure5(b) shows the result when the object size was 3 cm x 3 cm . The image is very poor in bothresistivity valu e and size. W hen the object size is 1 cm x 1 cm , it is impossible to ob tain animage.Figure 6 shows the reconstructed image when we set an agar block whose resistivity is170 SZ cm and s ize is 6 cm x 6 em in the middle of the physical phantom filled with salinesolution. Resistivity value and size of the image were slightly different from those of theobject even thoug h we can recognize the existence of the object from the image. When theobject size was less than 3 cm x 3 cm, it was impossible to reconstruct an image.Fr om these experim ental results, we conclude that w hen a n object size is large, the imagereconstruction is very good even though the difference between the resistivity of the salinesolution an d tha t of the object is small. For example, if the size of an object is 6 c m x 6 cm ,the resolution of resistivity is abou t 50R cm, a s shown in figures 5 and 6. O n the other hand,wh en the size of a n object is small, the reconstructed im age is very poor. Fo r example, if the

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(b)Figure 5. ( a )Reconstructed image when a 6 cm x 6 cm 500 R cm agar block is placed in t he centre of 120 R cm aline,( b ) Reconstructed image when the agar block was only 3 cm x 3 cm . No te the poor spatial resistivity resolution.


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Some physical results from an impedance camera 7 5

Figure 6. Reconstructed image when a 6 cm x 6 cm 170 R cm agar block is placed in the centre of 120 R cm saline. Itwas impossible to reconstruct a smaller blo ck.size of an ob ject is only 1 c m x 1 cm we found im age reconstruction impossible even thoughthe resistivity difference is infinite.Figure 7 ( a ) shows the reconstructed image when we arranged a glass bottle of size2 cm x 4 m and two 60 R cm a gar blocks whose sizes are 1.5 cm x 6 cm and 1.5cm x 3 cmin th e physical pha nto m filled with saline solution as sho wn in figure 7 (b ) . Th e reconstructedimage is good in bo th resistivity and size. On th e other han d, figure 7(c) shows thereconstructed image when three ag ar blocks whose sizes are 2 cm x 3 cm, 2 cm x 3 c m a n d6 cm x 2 cm and resistivities are 200 R cm , 160R cm and 300 R cm, respectively arearrang ed in th e physical phan tom filled with saline solution as shown in figure 7(d). We can

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Figure7. ( a )Reconstructed im age of (b). (b )True image of (a) . c )Reconstructed image of (d) , ote the lack of spatialand resistivity resolution. ( d ) True image of (c) .


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76 K Sakamoto, T J Yorkey and J G Webster

Figure 8. Reconstructed image of fist when placed in the centre of the phantom tank filled with 1200 m salinesolution.recognise the existence of both th e 200 SZ cm block an d the 300 Q cm block. T he resistivityvalue and size of th e 200 51 cm block image are almost the sam e as the actual block. But inspite of th e recognition of the existence of the 300R cm block from the image, th e size andresistivity of the image are quite different from those of the block. We can no t find the160 R cm block in the image.

Fro m these exp eriem ental results, if the resistivity of th e salin e solution was large r thantha t of the object, the resolutions in both size and resistivity seem to be better tha n those whenth e resistivity of saline solution was lower than th at of the object. Th is phenom eno n may becaused by the higher current density passing through the objects than that in the salines o h io n.One of the reconstructed images of the fist of a human being in the physical phantomfilled with saline solution is presented in figure 8. The image is very poor a nd in the middle ofthe image, th e resistivity is unreasonab ly low. Since th e curre nt density in the fist becomeslower du e to the curre nt passing through the arm , the app are nt resistivity a t the m iddle partof the fist becomes lower.From the experimental results, we conclude that for accurately imaging living tissuesthree-dimensional analysis or th e use of weighting functio ns are necessary, along w ith carefulselection of electrode combinations and appropriate electrode arrangement to give highsensitivity, and amplifier design to give high S/N ratio. Also, Yorkey (1986) showed thatbetter reconstruction algorith ms exist than the one we used. Per hap s the sam e data wouldhave resulted in better images with a different algorithm.AcknowledgementThis work was supported by the N ational Science Found ation under G ra nt ECS-8407402.ReferencesBarber D C and Brown B H 1983 Imped ance spatial distributions of resistivity using applied potential tomographyGeselowitz D B 1971 A n application of electrocard iographic lead theory to im pedanc e plethysmography IEEE T r mKim Y, Webster J G and Tom pkins W J 1983 Electrical Impedan ce imaging of thorax J . MicrowauePower 18245-57Yorkey T J 1986 A quantitative comparison of the reconstruction methods used in impedance tomography. PhDrhesis University of Wisconsin, MadisonYorkey, T J , Webster J G and Tompkins W J 1985 A n improved perturbation technique for impedance tomographyand some criticisms (Department of Electrical and Computer Engineering, University of Wisconsin, Madison,WI 53706)

Electron. Let t . 19 933-5Biomed. Eng. BME-18 3 8 4 1

Sakamoto (1987), Some Physical Results From an Impedance Camera

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