EP0382802A1

EP0382802A1 - Measurement probe for locating metallic objects in the human or animal body

Info

Publication number: EP0382802A1
Application number: EP19890906014
Authority: EP
Inventors: Pekka Nyberg; Thomas Weber; Rolf Bruhin
Original assignee: Koch Elektronik and Apparatebau AG
Current assignee: Koch Elektronik and Apparatebau AG
Priority date: 1988-07-01
Filing date: 1989-06-08
Publication date: 1990-08-22
Also published as: AU3696389A; WO1990000030A1

Abstract

Une sonde de mesure (1) comprend un circuit oscillateur auto-ajustable avec une bobine de mesure (5) et une bobine de rétroaction (6). La bobine de mesure forme une partie d'un circuit résonnant LC. Selon le procédé de mesure, l'oscillateur de la sonde de mesure est réglé de sorte que l'oscillation s'interrompt périodiquement et ne recommence qu'après un temps de retard déterminé. La (basse) fréquence des périodes d'oscillation forme la valeur de mesure. Lorsque la sonde (1) se rapproche d'une pièce métallique, le circuit résonnant est amorti. Le décalage des fréquences de périodes d'oscillation qui s'ensuit est mesuré par un circuit d'évaluation et affiché optiquement et/ou acoustiquement, ou utilisé par un dispositif de commande. On peut ainsi localiser de manière rapide et sûre une pièce métallique au moyen d'une sonde de dimensions réduites.A measurement probe (1) includes a self-adjusting oscillator circuit with a measurement coil (5) and a feedback coil (6). The measuring coil forms part of an LC resonant circuit. According to the measuring method, the oscillator of the measuring probe is adjusted so that the oscillation is periodically stopped and does not start again until after a determined delay time. The (low) frequency of the oscillation periods forms the measurement value. When the probe (1) approaches a metal part, the resonant circuit is damped. The ensuing oscillation period frequency offset is measured by an evaluation circuit and displayed optically and / or acoustically, or used by a control device. It is thus possible to quickly and safely locate a metal part by means of a probe of reduced dimensions.

Description

M E S S O N D E Z U R L O K A L I S I E R U N G

V O N M E T A L L I S C H E N T E I L E N

I M M E N S C H L I C H E N O D E R

T I E R I S C H E N K Ö R P E R

The invention relates to a measuring probe for locating metallic parts or particles in the human or animal body, and a method for locating metallic objects according to the preambles of claims 1 and 7, respectively.

Various devices are known for localizing metallic objects which are intended to enter the human or animal body by medical measures or accidentally. In order to make the previously necessary, complex X-ray or ultrasound recordings superfluous, devices have been developed which enable the objects to be localized by means of inductive measurement methods. At the same time, such measurement methods have the advantage that they allow direct location on the body and do not have to be done indirectly through image viewing.

Such a device is known for example from GB-PS 994,400, which contains a measuring probe with a measuring coil which is connected to an evaluation circuit. Further devices of this type are known from PCT application WO 86/02539 and US Pat. No. 4,416,289.

In order to enable practical use, predominantly in the operating room, various general conditions and requirements are specified for the devices of the type mentioned. The devices should be able to be operated independently of the mains, external 10 interference such as temperature, metallic objects, vibrations etc. should not have any influence on the measurement

^{~~~ The} device and especially the probe should be handy and a large measuring range should be possible. In addition, manufacturing factors must also be taken into account, for example

15 simple series production, great adjustment accuracy ex works and precise function over a longer period of time, ease of maintenance, etc.

Known devices show respect these requirements 20 problems by example have ^~~ insufficient sensitivity, must be tet umgeschal¬ for large measuring ranges manually on the device and especially unwieldy probes aufwei¬ sen. In principle, various measuring methods for the detection of metal pieces are possible for the applications mentioned, * 25 but it turns out that measuring methods, probe geometry and suitability of the device for practical use are directly related. Although known devices partly take these difficulties into account, they all have the disadvantage in common that the probe is too large and therefore unsuitable for practical use. Furthermore, the measuring probes of these devices must each be adjusted and matched to the actual measuring device so that there is no interchangeability of the probes, which is particularly problematic in the medical sector, for example during sterilization brings. In addition, the known measuring methods with permanently oscillating oscillators require a relatively large energy consumption, so that the devices are either network-dependent or the batteries have to be replaced frequently. Because of these disadvantages, inductive measuring devices for the exact localization of metallic objects are therefore used to this day in the field of medical applications to no or only limited extent.

The device according to GB-PS 994,400 takes external disturbances into account and partially adjusts itself to such disturbances with regard to sensitivity. However, the adjustment takes place with a relatively large time constant, so that the location may take a lot of time. Furthermore, in this device too, a probe adapted to the measuring device can no longer be replaced without requiring a new adjustment. At the same time, the measuring principle used requires a relatively large measuring coil with a ferrite core and thus leads to a probe that is difficult or impossible to use in the field of surgery. Another disadvantage of these and other known probes is that the measuring line and usually also the probe necessarily require shielding.

It is an object of the invention to provide a device which avoids the disadvantages mentioned above, is simple to manufacture, enables a very sensitive probe with small dimensions and has low energy consumption during operation, the probe on the measuring device being exchangeable at any time.

Furthermore, it is an object of the invention to provide a measuring method which allows the sensitivity to be adjusted automatically and manually over a large measuring range, takes into account interference influences during the location and permits rapid location. This object is achieved by the features mentioned in the characterizing part of patent claim 1 or 7.

The method according to the invention uses an oscillator oscillation which, however, is not permanently maintained, but regularly tears off in whole or in part, so that the energy consumption is kept as low as possible. At the same time, a very high sensitivity of the probe is achieved. It is possible to localize not only larger objects, but also small metal particles. The oscillator circuit is contained in the probe itself, so that the probe is calibrated and can be used together with various measuring devices. The high sensitivity of the measuring probe created by the measuring method permits a correspondingly small dimensioning of the components and thus of the entire probe.

Exemplary embodiments of the method according to the invention and the device are explained in more detail with reference to the following figures.

1 shows an exemplary embodiment of a measuring probe

Fig. 2 shows a circuit diagram of the oscillator circuit

Fig. 3 shows schematically in a block diagram the structure of a device according to the invention.

Fig. 4 shows a possible waveform of the oscillator in operation.

5 shows a circuit example for the measuring probe and the evaluation position. FIG. 1 shows an embodiment of a measuring probe 1, which contains an oscillator circuit, which is preferably provided on a circuit board 2 (printed circuit). At the tapered end of the probe 1, a measuring coil 5 is wound around a relatively short ferrite core 4. The ferrite core has a relatively small diameter of approx. 4 mm. A feedback coil 6 is wound around the same ferrite core 4. In addition to the circuit elements SE of the oscillator circuit, which are only indicated schematically, the printed circuit board 2 contains contacts or contact rows 8, 9 which on the one hand serve to connect these two coils 5, 6 and on the other hand are connected to a measuring line 3. This measuring line 3 connects the probe 1 to the evaluation circuit of the device and at the same time serves to supply power to the probe 1. A button 7 is preferably provided at an ergonomically suitable location of the measuring probe, which switches the probe and the measuring circuit into operation and in initialized the adjustment of the evaluation circuit in a manner to be explained later. Of course, the geometry of the probe, in particular also the arrangement of the two coils 5, 6, can vary.

Known devices cannot simply be reduced in size due to the inherent measuring principles, since then the actual goal, namely locating even small metal parts, is no longer possible due to the associated reduced sensitivity. In order to be able to meet the requirements for sensitivity, low susceptibility to faults, small geometry and simple interchangeability of the probe, etc., both the electronic circuit, in particular that of the oscillator, and the measurement method are important. The inventive idea is based on the fact that the Tasks in their entirety can only be solved if the measuring method and structure of the probe are ideally coordinated. If, for the sake of clarity, the explanations regarding the measuring probe and the measuring method are presented separately, they must always be understood in the common context of the method and the device.

Figure 2 shows the oscillator circuit contained in the probe in a possible embodiment. Contrary to conventional probes, not only the measuring track but also the entire oscillator circuit is accommodated in the probe itself. The measuring coil 5 and the feedback coil 6, the pushbutton 7 and the connections 18, 19 for the measuring line 3 [FIG. 1]. A first capacitor 20 forms a resonant circuit together with the measuring coil 5. An RC element 22, 24 is used to stabilize the operating point of a transistor 25. This transistor 25 in an emitter circuit is coupled at its base to the measuring coil 5 via the feedback coil 6. In order to ^" fulfill the phase condition of the oscillator circuit for the resonance frequency ^" , the two coils 5, 6 are preferably wound in the same direction. The winding connections with the same polarity are identified by the dots in the drawing. The transistor is emitter-side via a second resistor 23 is connected to the terminal 19, which is preferably grounded, and the connection of the transistor base via the capacitor 22 to the supply causes an additional, slight feedback, which increases the sensitivity of the circuit, and a second capacitor 21 stabilizes or decouples the circuit The circuit can be connected to the supply connection 18 or the supply not visible here by means of the button 7. The degree of feedback can be determined by the ratio of the two coils 5, 6, the mutual arrangement of the Spuien on the ferrite core and its dimensioning varies become. Good results are achieved, for example, with a turns ratio of 25:10 between the measuring coil 5 and the feedback coil 6, a capacitor 20 of 1 nF, a capacitor 22 of 100 nF and resistors R24 = 150 kΛ or R23 = 100 Ω.

If a metallic object is brought into the area of the tapered end of the probe 1 or the latter in the vicinity of a metallic object, the magnetic field of the measuring coil 5 (and also of the feedback coil 6) is known to be influenced and subsequently damped the resonant circuit. Subsequently, the change in the current consumption by the measuring probe serves as the measuring signal (see below). This frequency shift for the detection of such objects is preferably evaluated by means of an evaluation circuit downstream of the probe, to which the probe is connected via a measuring line. Since the entire oscillator circuit is built into the measuring probe itself, each probe can be compared individually and the degree of feedback can be determined. In this way, the "standardized" probes can be used together with various evaluation circuits. At the same time, this measure eliminates the need to shield the measuring line 3. Preferably, the measuring line between the probe and the device is decoupled from the evaluation circuit with regard to high-frequency signals. Since the oscillator circuit is balanced within the probe, long measuring lines can also be used, since inductive and capacitive interference from the measuring line is largely switched off.

FIG. 3 shows an embodiment of the device including the evaluation circuit in a block diagram. The described measuring probe (S) 1 and the evaluation circuit are supplied by a supply (B) 15. A current sensor (IS) 28 determines whether a current is being drawn through the measuring probe 1 and switches on the supply of the evaluation circuit only under this condition (indicated by arrow V). It is thereby achieved that both the supply of the probe and that of the evaluation circuit only by holding down the button 7 [Fig. 2] is switched on. A voltage controlled oscillator (VCO) 14 is connected to a calibration circuit (A) 13. An interference detector (ID) 12 is connected on the input side to the measuring probe 1 on the one hand via a high pass (HP) 27 and on the other hand to this VCO 14. On the output side there is a connection to a display circuit (D) 17. This interference detector generates, for example, one pulse per second between the VCO signal and the useful signal during the measurement. A frequency increase of these impulses thus indicates a greater frequency difference between the useful and reference signal and thus an approach to the metal part. The frequency comparator is in turn also connected on the input side via the high-pass filter (HP) 27 to the measuring probe 1 and the VCO 14. The high pass 27 serves to filter out the low-frequency useful signal. On the output side, there is a connection to the VCO 14 and to the balancing circuit 13 via a low-pass filter (TP) 11. This is an extreme low-pass filter with a time constant on the order of one to several minutes. When switched on, this low-pass filter is bridged by the equalization circuit 13 in order to permit the rapid ab then desired.

In order to be able to evaluate the damping or detuning of the resonant circuit mentioned by approaching metallic objects without problems, the oscillator circuit is operated in accordance with the method according to the invention. In conventional induction measurement methods, feedback is provided such that the feedback loop gain is greater than one, ie the oscillation amplitude remains constant after being switched on without being influenced. The measurement is directly evaluated during the measurement Change in the resonance frequency or the amplitude of the resonant circuit. In order to keep the energy consumption of the circuit as low as possible on the one hand and to obtain measurement results that are as clear as possible, on the other hand, according to the invention, the feedback is selected such that the oscillation periodically breaks off at least partially. This is achieved by mutually positioning the two coils 5, 6 on the ferrite core 4 and thus adjusting the feedback. As measurement results show, compared to conventional operating modes, a significantly more pronounced influence on the resonant circuit behavior is achieved, which allows a relatively simple evaluation circuit, leads to more precise localization and also allows a small dimensioning of the measuring probe.

In contrast to conventional measuring methods, the method according to the invention does not use the direct frequency shift or change in amplitude of the oscillating circuit, but an indirect useful signal, namely the low-frequency oscillation periods which are brought about by a corresponding oscillator circuit. In other words, the resonant circuit or the oscillation amplitude is damped by the probe approaching a metal part. This change in amplitude influences the feedback of the resonant circuit, as is shown, for example, in FIG. 2, and thus the low-frequency signal. As a result, the low frequency is shifted, which makes it available as a secondary (dependent) measurement variable. By using this measurement variable, greater sensitivity and significantly more precise results can be achieved compared to known measurement methods.

FIG. 4 shows a typical signal form of the oscillator at the node designated K in FIG. 2 with "critical" feedback of the two coils 5, 6. The amplitude of the oscillator oscillation is in Dependence of the time t is shown. The maximum amplitude, corresponding to a supply voltage of 5 V, is of the order of 4 V. The resonant circuit frequency is approximately 1 MHz. As can be seen from the figure, the oscillation periodically ceases and only begins to rock again after a period of time T2 and to cease again after a period of time T1. It is not necessary within the scope of the inventive concept that the oscillation stops completely, but a decrease in the amplitude during the time period T2 is sufficient. The partial or total interruption of the oscillation caused by appropriate feedback leads to oscillation periods T1 which occur, for example, at a low frequency of approximately 4 kHz. The ratio of T2 to Tl can be influenced by the amount of feedback and the corresponding design of the oscillator circuit (adjustment during manufacture). The low-frequency vibration is influenced by the inductive influence of a metal part on the resonant circuit and subsequently on the degree of feedback, and is shifted in the order of magnitude from one to a few hundred Hertz. If the metal part is approached very closely, a frequency shift in the order of magnitude of one to several kilohertz is effected. As has been shown experimentally, it is possible to vary the operating frequencies over a wide range without adversely affecting the sensitivity of the circuit. The larger T2 selected, the lower the energy consumption of the oscillator. In order to achieve the best possible sensitivity of the probe, great importance must be attached to the low-frequency signal curve. Good results are achieved, for example, with a ratio T2-T1 equal to 0.1 to 0.3. In principle, a high sensitivity with regard to the inductive curve can also be achieved with other, non-continuous signals. Accordingly, the operating signal is to be optimized with regard to the shortest possible operating phases (energy consumption) and signal course (high sensitivity). For example, it can be provided that the oscillator oscillates only sporadically, in that T2 is greater than T1 - left

is scheduled. Favorable measuring frequencies for the high-frequency oscillation are in the range from 100 kHz to a few megahertz, for the low-frequency signal between 1 and 10 kHz.

Of course, the method according to the invention is not limited to the explained oscillator circuit or the signal curve shown. The circuit according to FIG. 2, however, offers the possibility of simple adjustment and easy adjustment of the desired degree of feedback.

An additional advantage is provided by the button 7 provided on the probe, which serves to interrupt the feed or measuring line 3 within the scope of the method according to the invention. The supply of the evaluation circuit is preferably also switched on and off by this button. For the time being, this ensures that the oscillator or evaluation circuit only consumes energy during actual operation. At the same time, by interrupting the line, a new adjustment of the sensitivity of the measuring circuit is achieved or interference from surrounding materials or other short-term disturbances are reduced.

In contrast to the device mentioned in the introduction according to GB 994 400, which aims at fully automatic adjustment, the button is used to initialize a semi-automatic adjustment device. By switching on the probe, ie by pressing the button, the adjustment circuit 13, which forms part of the evaluation circuit, is activated at the same time. The adjustment circuit 13 in turn effects the tuning of the VCO 14, which settles to the low frequency of the probe in less than one second, for example 300 msec. The control of the VCO 14 is switched in such a way that external influences are also corrected with a slow time constant. In accordance with the task of the invention, a brief release of the user-friendly button 7 on the measuring probe enables an immediate readjustment and thus the compensation of interference. This enables a much faster location than with conventional devices.

The probe is preferably provided with an optical display, for example an LED display, which is controlled by the evaluation circuit. Since the probe is intended to be in the user's field of vision, the location of metal particles, for example during an operation, can be followed optically, which also serves to quickly and precisely locate a foreign body in the human or animal body.

Figure 5 shows a detailed circuit example of the invention. The circuit parts shown in FIG. 3 are each indicated here by dash-dotted lines and have corresponding reference numerals. The power supply is preferably carried out via a combination of a battery accumulator (6V) and solar cells. A corresponding battery monitor 31 shows the state of charge of the batteries on the device by means of an LED display. As can easily be seen from the circuit diagram, the evaluation circuit is preferably made using integrated circuits. constructed, preferably using a PLL circuit. This PLL circuit can preferably be calibrated with a potentiometer in order to achieve an adaptation to the probes that are matched in terms of manufacture. Of course, it is possible to compare probes and devices during manufacture and thus achieve the desired interchangeability of the probes. The power consumption of the probe or the evaluation circuit is on the one hand through the use of CMOS technology and largely avoidance of analog elements and on the other hand through the The on / off principle of the process is kept low. The measuring signal delivered by the probe 1 is converted into a square-wave signal 30. The frequency shift caused by the inductive action can be digitally evaluated with a conventional frequency comparator. The square wave signal supplied by the voltage-controlled oscillator VCO 14 serves as the reference frequency and is tuned to the "quiescent frequency" of the balanced probe when the probe is switched on, ie by pressing the button 7. On the. Frequency comparator 16 and the low-pass filter 11 automatically adapt the VCO 14 to frequency shifts in the measurement signal, but only with a large time constant, so the VCO 14 basically lasts for minutes when switched on after the frequency has been compared. In other words, only slow changes, for example temperature fluctuations, are tracked.

The evaluation circuit is constructed in a largely customary manner and translates the measurement variable into an acoustic and / or optical warning or control signal. The location is carried out, for example, by increasing the frequency of the acoustic signal emitted via a loudspeaker, but can also be used directly for controlling automatic location devices.

To locate a metallic foreign body in the body, the measuring probe is passed over the area in question. By pressing the button 7, the probe 1 and the evaluation circuit are switched on, ie the oscillator is made to vibrate, and the VCO 14 is simultaneously tuned to the same frequency. While the VCO 14 maintains this frequency, the frequency of the useful signal is changed by the probe approaching the foreign body, which is indicated acoustically and optically by the evaluation circuit. The evaluation circuit preferably contains a direction detection 10 [FIG. 5], which it allows, by means of an optical display distinguish between approaching or removing the probe from the metal part in certain areas. An optical display 26 also takes place on the probe, for example by means of an LED or a numerical display which contains information about the size or distance of the foreign body. In order to achieve a very precise location, the button is released and pressed again at certain intervals, which leads to a new adjustment of the VCO frequency and thus compensates for interference or adapts the required measurement sensitivity. Of course, it is also possible to provide a setting option for certain measuring sensitivities directly on the evaluation circuit.

For special applications, in addition to the low-frequency signal, the change in the high-frequency oscillation frequency caused by the detuning of the oscillating circuit can also be measured and used for the evaluation. The measuring line must then also transmit the RF signal, and of course the oscillator circuit must not have RF decoupling.

Claims

PATENT CLAIMS

1. Measuring probe for localizing metallic parts in the human or animal body with an induction measuring coil, characterized in that the measuring probe (1) contains a self-aligning oscillator circuit with an oscillating circuit (5, 20), which via a measuring or 5 supply line (18, 19) can be connected to an evaluation circuit.

2. Measuring probe according to claim 1, characterized in that the oscillator circuit comprises a measuring coil (5) and a feedback coil (6) which are wound around a common ferrite core (4).

3. Measuring probe according to one of claims 1 or 2, characterized in that the measuring coil (5) is part of an LC-15 resonant circuit (5, 20) whose input voltage by means of a transistor (25) in an emitter circuit via a Feedback coil (6) is reinforced.

4. Measuring probe according to one of the preceding claims, 20 characterized in that the measuring probe (1) contains a button (7) for interrupting the measuring or supply line (18, 19).

5. Measuring probe according to claim 4, characterized in that a RC element (22, 24) connected between the collector and the supply fixes the operating point of the transistor (25).

- - lo -

6. Measuring probe according to one of the preceding claims, characterized in that the measuring probe (1) contains an optical display which can be connected to the evaluation circuit. 5

7. Method for localizing metallic parts in the human or animal body, characterized by the following method steps: a) An oscillator circuit is switched and operated in such a way that the oscillation periodically breaks off in whole or in part and only after a delay time (T2 [Fig. 4]) starts to oscillate again. b) The frequency of the occurring oscillation periods

(Tl [FIG. 4]) is stored by an evaluation circuit. c) The frequency shift of the oscillation periods (T1) caused by a relative movement of the oscillator circuit relative to a metallic particle compared to the stored value is determined and displayed by the evaluation circuit.

8. The method according to claim 7, characterized in that the oscillator frequency between 500 kHz and 5 MHz and 25, the frequency of the oscillation periods (Tl) is between 1 kHz and 10 kHz.

Method according to one of claims 7 or 8, characterized in that the stored value of the frequency is automatically adapted to slow frequency changes by the evaluation circuit. _ _1? _

10. The method according to any one of claims 7 to 9, characterized in that a switch (7) is actuated at intervals during the approach to the metallic part and the current frequency of the oscillation periods occurring (Tl ) is saved again by the evaluation circuit.

11. The method according to any one of claims 7 to 10, characterized in that both the frequency of the 10 oscillation periods (Tl) occurring and the frequency of the oscillator oscillating circuit are evaluated.