DEVICE FOR DETECTING FOREIGN METALLIC OBJECTS IN THE TISSUES OF A LIVING ORGANISM

D. K. AVDEEVA, V. E. DREIZIN, I. G. LESHCHENKO

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

The operation of removing metallic foreign objects from the human body often presents great difficulties for the surgeon. Of the possible methods for detecting foreign objects in the tissues of a living organism (X-ray, electromagnetic, ultrasonic, radiation), the first two methods (X-ray and electromagnetic) have found practical application in medicine. Determining the location of a foreign object by means of X-rays is the most reliable method, but, unfortunately, it does not always prove sufficient. The principal disadvantage of the X-ray method is the impossibility of using it during a surgical operation. This was the main reason for developing a new instrument for detecting foreign metallic objects in the tissues of a living organism, making it possible to localize foreign metallic objects both before and during an operation.

On the other hand, foreign bodies located in the soft tissues of the human body move; as a result, the X-ray method, which permits preliminary localization of foreign objects in the tissues of a living organism, can lead to a large error in determining their position at the time of the operation. From this point of view, electromagnetic methods have an advantage over the other methods. This fact was noted as early as 1942 in an article by Prof. I. G. Kochergin [1], which emphasizes the great importance, for the successful performance of operations, of the instrument “Electromagnetic Probe,” created at the Tomsk Polytechnic Institute during the Great Patriotic War.

The “Electromagnetic Probe” instrument has the following disadvantages:

1. The presence of two manual adjustments.
2. A low operating frequency, as a result of which small nonferromagnetic metallic objects were not localized by the sensor.

The main problem in designing a new instrument was the task of creating a highly sensitive sensor that would make it possible to detect foreign metallic objects with great accuracy.

Of the various sensors operating on the principle of eddy currents, in the present case the most suitable is a sensor with a surface-mounted coil, which makes it possible to detect foreign objects conveniently. To obtain the greatest sensitivity of the sensor, experimental studies were first carried out to determine the optimal confi-

configurations of the sensor-coil cores and the choice of the optimal operating frequency.

To select the optimal operating frequency, a sensor with the following parameters was investigated: core diameter—1.5 mm, sensor length—30 mm, number of turns—40, inductance—5 µH.

The experiment was carried out with the distance from the end face of the sensor to the object being detected equal to zero, since under these conditions the greatest sensitivity of the sensor is achieved.

The choice of the core diameter is based on obtaining the smallest error in detecting the location of the foreign object, since this error is at least no smaller than the diameter of the sensor coil.

As the foreign body, a steel needle 15 mm long with a maximum diameter of 1 mm and a drop of oval-shaped tin-lead solder with a transverse diameter of 6 mm were studied. Investigations of the sensor for the optimal frequency were carried out using a KV-1 Q-meter. The results of the experiment are presented graphically in Fig. 1, a, b. On the basis of the experimental data, the optimal frequency of 9 MHz was selected.

Fig. 1. a—dependence of the sensitivity of the sensor to a steel needle on the frequency of the supply voltage for three different positions of the needle relative to the sensor with the distance between them equal to zero: 1—with the tip toward the sensor, 2—with the eye toward the sensor, 3—transverse to the sensor; b—dependence of the sensitivity of the sensor to a drop of oval-shaped tin-lead solder on the frequency of the supply voltage; c—dependence of the sensor sensitivity on the distance of the foreign body from the end face of the sensor; d—dependence of the change in the magnitude of the magnetic-field intensity when moving away from the center of the core along its axis for different configurations of the core end face: 1—concave spherical, 2—flat, 3—convex spherical.

Further experiments were carried out in order to determine the change in the sensitivity of the sensor as the foreign object was moved away from the end face of the sensor.

A graphical representation of the experimental results is presented in Fig. 1, в.

As the sensor approaches a foreign object, the sensitivity increases according to a hyperbolic law; a sharp increase in sensitivity is observed at distances of less than 0.5 cm.

The choice of the optimal core configuration was carried out on a model by experimentally studying the electromagnetic field of a coil with dimensions \(l = 110\) mm, \(d = 25\) mm, and with cores having different tip configurations: flat, spherical, and concave spherical.

Figure 1, г shows dependences illustrating the change in the magnetic-field strength when moving away from the center of the core along its axis for various configurations of the core end face.

Thus, it was experimentally shown that the greatest field strength at the same points of the field is achieved with a concave spherical surface of the core end face.

The design of the sensor is shown in Fig. 2.

Structurally, the sensor is made in the form of a tube 2 with an overall length of 150 mm, ending in a metal handle, on the surface of which a switching button 4 is brought out. The tube is made of plastic with an internal cylindrical opening. The outer surface of the tube has a slight taper.

The search coil 1 is located over the plastic tube; the ends of the coil leads are soldered to the leads from the generator unit. After the leads are soldered, the coil is potted with styracryl.

In order for the tip of the sensor to be thin, like a needle, the coil is not wound along the entire length of the core. The end of the core that is free of turns is covered with a thin layer of styracryl. Before use, the plastic tube is sterilized (either in alcohol or in formalin).

The handle is made of anodized duralumin and consists of a cylindrical hollow tube 6 and two caps (3, 7), which are screwed onto the tube; for this purpose, threads are provided on the tube and on the caps.

The upper cap has an opening into which the plastic tube is inserted.

The metal handle shields the generator from external electric fields and has a common point with the “ground” terminal of the instrument.

The sensor is made with a built-in generator in order to reduce the effect of interference on the stability of the generator frequency.

The generator is mounted between two textolite disks fastened by means of three textolite posts. After the generator has been adjusted, the entire assembly is potted with styracryl. Externally, the generator unit is a cylindrical tube from which the switching button and leads protrude.

Experimentally, the sensor was tested in the pediatric clinic at the Tomsk Medical Institute during operations to extract shot from the knee joint and to remove a needle fragment from the heel.

The use of the sensor for detecting foreign metallic objects during an operation gave positive results. With the aid of the sensor, bodies lying at a depth not exceeding 20 mm were detected.

The search coil is connected to the generator circuit and determines the circuit inductance. The fundamental frequency of the generator is a function of the inductance of the oscillatory circuit and, to a first approximation, depends only weakly on its resistance \(R\).

Fig. 2. Design of a sensor for detecting foreign metallic objects in the tissues of a living organism.

From the change in the generator frequency when a metallic object is introduced into the field of the sensor, one can judge the location of the foreign metallic object in the tissues of the human organism.

The block diagram of the device is presented in Fig. 3.

A sensor is included in the circuit of generator \(G_1\). On the basis of experiment it was found that, when the distance to the foreign object is equal to zero,

Fig. 3. Block diagram of a device for detecting foreign metallic objects in the tissues of a living organism

the generator frequency changes by an amount \(\Delta f_{\max}=+280\ \text{kHz}\) or \(\Delta f_{\max}=-120\ \text{kHz}\).

The sign of the frequency deviation depends on the material of the foreign object (ferromagnetic or non-ferromagnetic). Since the frequency deviation is large, the difference frequency was chosen equal to \(1\ \text{MHz}\).

Generator \(G_2\) generates oscillations with a frequency of \(8\ \text{MHz}\).

Oscillations with frequencies of 8 and \(9\ \text{MHz}\) are fed through the emitter followers \(EP\) to the mixer \(Sm\). The emitter followers serve as buffer stages, since they weaken the coupling between the generators, as a result of which the phenomenon of frequency pulling of the generators is eliminated.

At the output of the mixer, various combination frequencies are formed: \(n f_1\), \(n f_2\), \(n f_1 \pm n f_2\).

The difference frequency from the mixer is fed to the intermediate-frequency amplifier \(UPCh_1\), the circuit of whose final stage is coupled to the circuit of the frequency detector \(D_1\).

In the absence of a foreign object in the field of the sensor, the frequency of generator \(G_1\) is constant; therefore, an intermediate frequency of \(1\ \text{MHz}\) comes from the mixer.

The circuit of frequency detector \(D_1\) is tuned to a frequency of \(1\ \text{MHz}\); therefore, at the output of the frequency detector the voltage is equal to zero.

At the moment a foreign metallic object enters the field of the sensor, the frequency of generator \(G_1\) changes, since the parameters of the sensor change; as a result, the intermediate frequency also changes.

At the output of the frequency detector at this moment a constant voltage appears, proportional to the change \(\Delta f\) and having the same sign as the deviation \(\Delta f\).

The constant signal from the detector is modulated at a frequency of \(1000\ \text{Hz}\) by the modulator \(Mod\), which is controlled by a rectangular voltage \((f = 1000\ \text{Hz})\).

The rectangular voltage is formed by the multivibrator \(M\), whose pulse repetition frequency is \(1000\ \text{Hz}\).

The pulse emitter follower \(I\!E\!F\) eliminates the influence of the load on the operation of the multivibrator.

The modulated signal is amplified by a low-frequency amplifier and fed either to a loudspeaker or to earphones. The dead zone of the sensor is determined by the instability of the intermediate frequency \(f_1 - f_2\), which in turn is determined by the instability of the frequencies \(f_1, f_2\).

To reduce the instability of the intermediate frequency, the device uses an automatic frequency-control system \((AFC)\), which includes the following units: an intermediate-frequency amplifier \(IFA_2\), a frequency detector \(D_2\), and a control element \((CE)\).

As a result of the action of the \(AFC\), the stability of the intermediate frequency is increased by an order of magnitude and brought to \(10^{-4}\).

Thus, the developed device makes it possible to quickly and conveniently localize foreign metallic objects in the tissues of a living organism both before an operation and during the course of an operation.

REFERENCES

1. Military Field Surgery, Medgiz, 1942, pp. 113–117.