METAL-DETECTOR SENSOR FOR DETECTING FOREIGN METALLIC INCLUSIONS IN THE TISSUES OF A LIVING ORGANISM

D. K. Avdeeva, I. G. Leshchenko

(Presented by the scientific and technical seminar of the Department of Information and Measurement Technology)

Metal detectors intended for detecting foreign metallic bodies in the tissues of a living organism are described in [1, 2, 3, 4]. The problem of accurately determining the location of foreign metallic inclusions in the tissues of a living organism, especially during surgical operations, as well as the problem of increasing the sensitivity of a metal-detector sensor, has remained relevant up to the present time. In [5], results are reported from studies of the sensitivity of an applied eddy-current sensor to bodies whose entry into the organism is most probable (steel needles, shot, bullets, etc.). The optimal geometry was selected for a coil with and without a core, and the maximum sensitivity of the sensor to the objects being detected was obtained. On the basis of applying a sensor with optimal parameters, the device “Medical Eddy-Current Metal Detector—MITM-1” was developed and has found practical application in surgical clinics.

In the present work, results are reported from experimental studies of sensors of a new design, which have high accuracy in determining the location of a foreign metallic body while retaining a sufficiently high sensitivity and operating range. In an ordinary sensor, which is a single-layer coil with a ferrite core, increasing the accuracy of determining the location of a foreign metallic body, associated with reducing the diameter of the sensor, leads to a decrease in the operating range of the metal-detector sensor.

The design of the proposed sensor is shown in Fig. 1,a. The metal-detector sensor consists of two coaxial coils I and II, equal in length, with aligned end faces, arranged so that the turns of the inner coil I are laid directly on ferrite core III, whose length is considerably greater than its diameter. The outer coil II is placed over the inner one, forming an annular air gap. The coils are connected in series, in phase, into the circuit of a high-frequency generator.

The axial characteristics of the sensors are determined by the dependences of the relative sensitivity of the sensors

$$ \frac{\Delta L}{L_0} $$

($\Delta L$ is the absolute change—)

… in the inductance of the sensor when a metallic ball is introduced into its field along the sensor axis, \(L_0\) is the initial inductance of the sensor) on the distance \(l\) (mm) between the end face of the sensor and the ball. The experiments were carried out using an E 12-1 capacitance and inductance meter.

Figure labels and inscriptions:

• \(\Delta L/L_0\), \(\times 10^{-3}\)
• \(l\)
• \(a)\)
• \(b)\)
• Sensor No. 1
• Sensor No. 2
• Sensor No. 3
• \(\varnothing_{\text{sh.}} = 5\) mm
• \(\varnothing_{\text{sh.}} = 10\) mm
• Sensor No. 3 (Table 1)
• I, II, III
• \(D_0\), \(D_1\), \(D_2\)
• \(l_1\), \(l_2\)

Fig. 1

As an example, Fig. 1 shows several characteristic dependences of the sensitivity of the sensors to copper balls with diameters of 5 mm (Fig. 1,a) and 10 mm (Fig. 1,b), the geometrical dimensions of which are presented in Table 1.

On the basis of the studies carried out (Fig. 1a, b), it was concluded that sensor 3 has high sensitivity to both small and large metallic bodies in comparison with sensor 1, which has the same core diameter, and a somewhat lower sensitivity to large bodies in comparison with sensor 2.

Table 1

In order to determine the locality of detection by the sensor of metallic bodies of various sizes, studies were carried out of the directional diagrams of the sensors. Figure 2 gives an example for three sensors as applied to a copper sphere 5 mm in diameter, located at a distance of 5 mm from the end face of the sensor.

Fig. 2

at a distance of 5 mm from the end face of the sensor. In this case sensor 3 has better directivity than sensor 2. In this experiment the diagrams were obtained by shifting the copper sphere relative to the axis of symmetry of the sensor along the radius at every millimeter.

In order to determine the optimal geometrical parameters of sensors with series-aiding connected windings, Figs. 3 and 4a, b present the results of experimental studies.

with sensor 4. Figure 3 shows the directional patterns for different diameters of the outer coil \((D_2)\), with a constant length of the inner \((l_1)\) and outer \((l_2)\) coils equal to 10 mm. As can be seen from Fig. 3, the sensor with an outer-coil diameter of 30 mm has the narrowest directional pattern. Directional pattern 6 shows the role of the inner coil and was obtained for a sensor with an outer diameter of 30 mm in the absence of the inner coil. The results of the studies show that, owing to the presence of coil 1, the field is redistributed at the end face of the sensor, as a result of which the locality of the sensor increases and its sensitivity to small foreign metallic bodies is enhanced.

Fig. 3

Figure 4 presents the directional patterns of the sensor and the dependence of its sensitivity, at a given distance \((d)\) between the end face of the sensor and the ball, on the length of the inner and outer coils for the optimum diameter of the outer coil, equal to 30 mm. The sensor with \(l_1 = l_2 \ne 10\) mm has the narrowest directional pattern; at the same time (Fig. 4, b) the sensitivity of the sensor differs little from the maximum. Displacement of the end faces of the sensor coils relative to each other leads to a decrease in sensitivity and to a broadening of the directional pattern.

On the basis of the obtained optimum geometric dimensions of sensors with windings connected in series opposition on a ferrite core, a universal metal detector has been designed that has a narrow directional pattern and sufficiently high sensitivity to both small and large metallic bodies located in the tissues of a living organism. As a result, during surgical operations there will be no need to change sensors or additionally adjust the equipment. In addition, increasing the accuracy with which the location of the detected object is determined significantly reduces the degree of tissue trauma during opera-

…operations for removing foreign metallic bodies from the tissues of a living organism.

Fig. 4

a)

Labels in the diagram:

• $\varphi_{\mathrm{sh}} = 10 \text{ mm}$
• $d = 10 \text{ mm}$
• $\Delta$ — displacement of the center of the ball (mm)
• $1 - \ell_1 = \ell_2 = 1 \text{ mm}$
• $2 - \ell_1 = \ell_2 = 3 \text{ mm}$
• $3 - \ell_1 = \ell_2 = 5 \text{ mm}$
• $4 - \ell_1 = \ell_2 = 7 \text{ mm}$
• $5 - \ell_1 = \ell_2 = 10 \text{ mm}$
• $6 - \ell_1 = \ell_2 = 20 \text{ mm}$

Labels in the graph:

• $\dfrac{\Delta L}{L_1} \cdot 10^3$
• $\varphi_{\mathrm{sh}} = 10 \text{ mm}$
• $d = 1 \text{ mm}$
• $d = 2 \text{ mm}$
• $d = 3 \text{ mm}$
• $d = 5 \text{ mm}$
• $d = 7 \text{ mm}$
• $d = 10 \text{ mm}$
• $\ell_1 = \ell_2 \;(\text{mm})$

REFERENCES

1. R. D. Miroshnichenko, A. N. Golovin. Inventor’s Certificate No. 103504. Bulletin “Inventions, Industrial Designs, and Trademarks”, 8, 1956.

2. A. I. Derkach, A. G. Laburg, I. V. Filippov. Inventor’s Certificate No. 65664. Bulletin “Inventions, Industrial Designs, and Trademarks”, 1, 1946.

3. A. I. Portyagin, V. N. Shuketov, V. P. Leman. Inventor’s Certificate No. 186081. Bulletin “Inventions, Industrial Designs, and Trademarks”, 18, 1966.

4. R. I. Yanus et al. Inventor’s Certificate No. 200114. Bulletin “Inventions, Industrial Designs, and Trademarks”, 16, 1967.

5. D. K. Avdeeva, I. G. Leshchenko, V. K. Zhukov, Yu. K. Zemlyakov. Eddy-current sensors and an instrument for detecting foreign metallic objects in the tissues of a living organism. In: Proceedings of the All-Union Scientific and Technical Conference “Modern Methods and Means of Nondestructive Testing of Materials and Products,” November 18–20, 1970, Minsk.

6. Order 2609.