Magnetic field detector unit

Abstract: The invention relates to a magnetic field detector (2) and a combined telemetry and magnetic field detector unit (52) in a medical implant (50). The magnetic field detector (2) and the combined unit (52) contain a magnetic field sensor (6) with a coil (18) and a diode (16). In determinations of the presence of a magnetic field, the coil (18) is charged by a source of voltage (V+) for a defined period of time (22), the time for the coil's (18) discharge (24) through the diode (16) depending on whether a magnetic field is present. A detection signal (30) indicating the presence of a magnetic field is generated if the discharge time (24) is less than a defined time threshold value (32).

Description: The present invention relates to a magnetic field detector and a medical implant comprising a combined telemetry and magnetic field detector unit. In a medical implant, e.g. a pacemaker, a magnetic field detector is used for non-invasive activation of different implant functions with the aid of e.g. a permanent magnet brought close to the implant on the outside of the patient's body. Some of the functions in e.g. a pacemaker which can be activated with a magnet are, for example: having the pacemaker disable the demand function so the pacemaker adapts its operation to the capacity of the battery, having the pacemaker operate according to a special, temporary stimulation mode in e.g. tachycardia, and in conjunction with pacemaker programming. A magnetic field detector intended for a medical implant should be small (the latest generation of pacemakers only weighs about 14 grams), insensitive to bumps and suit the assembly technique used for other components in the pacemaker. The detection of magnetic fields outside the implant art in a number of different ways is obviously generally known, e.g. with the aid of reed switches and Hall generators utilizing the Hall effect. The disadvantage of the Hall generator is its relatively heavy energy consumption. An electronic sensor for static magnetic fields is described in US-4 611 127. The sensor comprises a conventional resonance circuit consisting of e.g. a coil used to sense the presence of a magnetic field whose strength exceeds a defined value. The resonance frequency for the resonance circuit varies according to the strength of the magnetic field. The resonance circuit is activated periodically, and the number of zero crossings for the output signal from the resonance circuit is studied in a sensing window with a defined duration. If a given number of zero crossings occurs in the sensing window, this means that the strength of the magnetic field exceeds the defined value. A magnetic field detector of the conventional kind in the implant art consists of a reed switch. However, reed switches are relatively insensitive and rather expensive components which also take up a relatively large amount of space in the implant. To eliminate the need for a reed switch, therefore, utilization of the implant's telemetry unit even for detecting the presence of a magnetic field, in addition to performing telemetry functions, has been proposed in recent years. US-4 541 431 shows one such proposal with a combined telemetry and magnetic field detector in which magnetic field sensing is performed in the same way as described in US-4 611 127 cited above. The combined unit comprises a conventional resonance circuit consisting of e.g. a coil used in telemetry for transmitting and receiving data. The resonance circuit is also used here for sensing, in the above-described manner, the presence of a magnetic field whose strength exceeds a defined value. The object of the present invention is to achieve an improved device, employing fewer components than in the prior art, for detecting magnetic fields in general and for detecting magnetic fields near an implant in particular. This object is achieved with a magnetic field detector and a medical implant comprising a combined telemetry and magnetic field detector of the aforementioned type with the features set forth in patent claims 1 and 5. The invention achieves magnetic field detection in which a detection signal is generated in the presence of magnetic fields. The invention utilizes the fact that the inductance of a coil with a core changes in the presence of a magnetic field, because the magnetic field affects the permeability of the core. When a voltage is applied to the coil for a specific period of time and the time it takes the coil to discharge through a diode is measured, the presence of a magnetic field with a defined strength can be detected. This is possible, since the coil discharge time is a function of the coil's inductance which changes in the presence of a magnetic field. Detection of the presence of a magnetic field is obtained when the discharge time is compared to a time threshold value corresponding to the presence of a magnetic field with a defined strength, and a detection signal is generated if the time is less than the threshold value. A first preferred embodiment of the invention relates to a magnetic field detector in which the coil's charging time is preset. A second preferred embodiment of the invention relates to a magnetic field detector in which the coil's charging time is the time elapsing from the start of charging until the intensity of current through the coil exceeds a preset value. A third preferred embodiment of the invention relates to a combined telemetry and magnetic field detector unit in which the coil's charging time is preset. A fourth embodiment of the invention relates to a telemetry and magnetic field detector in which the coil's charging time is the time elapsing from the start of charging until the intensity of current through the coil exceeds a preset value. The invention will now be described in greater detail, referring to the FIGURES in the attached drawings in which FIG. 1 is a schematic block diagram of a magnetic field detector according to the invention; FIG. 2 is a magnetic field sensor according to a first preferred embodiment; FIG. 3 is a magnetic field sensor according to a second preferred embodiment; FIG. 4 shows waveforms for voltage at two points in the magnetic field sensor; FIG. 5 is a block diagram of a medical implant and a programmer; FIG. 6 is a schematic block diagram of a medical implant according to the invention; FIG. 7 shows telemetry circuits according to a third preferred embodiment; FIG. 8 shows telemetry circuits according to a fourth preferred embodiment. In the FIGURES, the same reference designations are used for the same or similar elements. FIG. 1 shows a magnetic field detector 2 connected to a programmable control device 4. The magnetic field detector 2 comprises a magnetic field detector 6, control logic 8 and a magnetic field indicator 10. The magnetic field detector 10 comprises a voltage comparator 12 and a time measurement circuit 14 connected to the voltage comparator 12. In FIG. 2 is shown a magnetic field sensor 6 according to a first preferred embodiment. The magnetic field sensor 6 has a source of voltage V+, a diode 16, a coil 18 with a core and a switch S1. The discharge means 16 and the coil 18 are connected in parallel between the voltage V+ and the switch S1 which is connected, in turn, to ground. A measurement signal 20 is collected between the switch S1 and coil 18 and diode 16, the latter two connected in parallel. The opening and closing of the switch S1 are controlled by the control logic 8. FIG. 4 shows waveforms for voltage across the coil 18 (upper tracing) and for the measurement signal 20 (lower tracing). The vertical axis is for voltage in volts, and the horizontal axis is for time in µs. Referring to FIGURES 1, 2 and 4, the function of the magnetic field detector 2 will now be described according to the first preferred embodiment. In its normal state, the switch S1 is open. There is then no voltage across the coil 18, and the measurement signal 20 therefore has the same potential as the voltage V+. In a determination of the presence of a magnetic field with a defined strength, the coil 18 is charged by the source of voltage V+ for a defined period of time (the coil's charging time) 22. This is achieved when the switch S1 is closed for the defined period of time 22. During the defined period of time 22, voltage across the coil 18 is equal to the voltage V+. When the switch S1 re-opens, the voltage drops, and the coil 18 discharges through the diode 16. During this discharge period 24, voltage across the coil 18 is equal to minus the diode's 16 voltage drop in the conduction direction. After the discharge time 24, voltage returns to its normal level, i.e. there is no voltage across the coil 18. This return is performed in an oscillatory process. In measurement of the discharge time, the potential of the measurement signal 20 (the lower tracing in FIG. 4) is compared in the voltage comparator 12 to an adjustable voltage threshold value 26. The voltage comparator 12 generates a voltage signal 28 as long as the voltage exceeds the voltage threshold value 26. This voltage signal 28 is sent to the time measurement circuit 14 which emits a detection signal 30 if the duration of the voltage signal 28 is less than an adjustable time threshold value 32. The time threshold value 32 is selected so the presence of a magnetic field with at least one defined strength is detected if the duration of the voltage signal 28 is less than the time threshold value 32. The time measurement circle 14 can be realized in a plurality of ways. According to one, the duration of the voltage signal 28 is measured, and this time is then compared to the time threshold value 32, whereupon the detection signal 32 is generated if the duration of the voltage signal 28 is less than the time threshold value 32. Another way is to have a counter count as long as the voltage signal 28 is present. If the count is interrupted before the counter reaches the time threshold value 32, the detection signal 30 is generated. Typical values are 2.8 V for the voltage V+ and 0.7 V for the voltage drop across the diode. The short time in which the coil 18 charges is on the order of some tens of microseconds, e.g. 30 µs. The coil's 18 discharge time 24 is about 135-145 µs in the absence of a magnetic field and about 120-130 µs in the presence of a magnetic field. An ordinary permanent magnet, placed at an appropriate distance from the magnetic field detector, is used to achieve a magnetic field with a defined strength. According to one application of the invention, the magnetic field detector 2 is located in a medical implant. The magnet is then applied to the patient's exterior skin above the implant. The implant can be a pacemaker, for example. The pacemaker must then be able to detect a magnet from a distance up to 40 mm. According to the above-described first preferred embodiment, the charging time of the coil is therefore predetermined. According to a second preferred embodiment, the time required for coil charging is the time elapsing from the start of charging until the intensity of current passing through the coil exceeds a preset level. This is achieved in a magnetic field sensor according to FIGURE 3. As in the first preferred embodiment, the magnetic field sensor 6 according to this second preferred embodiment has a source of voltage V+, a diode 16, a coil 18 with a core and a switch S1. This second embodiment also comprises a first resistor 34 connected between the switch S1 and ground, a D flip-flop 36 whose Q output terminal controls the switch S1 and a comparator 38 with one input terminal 40 connected to a reference voltage and one input terminal 42 connected between the switch S1 and the first resistor 34. The comparator's 38 output terminal 44 is connected to the D flip-flop's 36 clock input terminal. In a determination of the presence of a magnetic field with a defined strength, the control logic 8 sends a pulse to the pre-set input terminal 46 on the D flip-flop, causing the output terminal Q to go high, thereby closing the switch S1 so charging of the coil 18 starts. The coil continues to charge until the intensity of current through the coil 18 exceeds a defined level. Since current through the coil 18 is the same as the current through the first resistor 34, current intensity is determined by measuring the voltage across the resistor 34. When the potential above the resistor 34 but below the switch S1 exceeds the reference voltage 40, the comparator 38 sends a clock signal to the D flip-flop, causing the potential on the D input terminal, i.e. the ground potential, to be clocked over to the Q output terminal, and the switch S1 opens. The intensity of current through the coil 18 is on the order of several hundred µA, e.g. 200-400 µA. The resistance of the first resistor is 500-1000 õ. This produces a discharge time for the coil 18 of the same magnitude as in the first preferred embodiment. Measurement of the duration of the discharge time 16 is performed in the same way as for the first preferred embodiment. According to the first and second embodiments, the magnetic field detector 2 can be used for magnetic field detection in an implantable medical device which could be a pacemaker, a defibrillator, an insulin pump or some other medical device. A third and fourth embodiment of the invention relates to a combined telemetry and magnetic field detector unit in a medical implant. Here, the implant could also consist of a pacemaker, a defibrillator, an insulin pump or some other medical device. FIG. 5 shows a medical implant 50, inserted under the skin 48, comprising a programmable control device 4 for control and coordination of the units in the implant 50 and a combined telemetry and magnetic field detector 52 with e.g. duplex communications with a programmer 56 placed on the exterior of the skin 48. FIG. 6 is a block diagram of the combined telemetry and magnetic field detector unit 52, henceforth referred to as the combined unit 52, according to a third preferred embodiment of the invention. The combined unit 52 comprises telemetry circuits 54 used for transmitting and receiving signals to and from the programmer 56 and for sensing magnetic fields, a magnetic field indicator 10 which detects the presence of a magnetic field, a telemetry signal circuit 58 for signals received by the telemetry circuits 54 and control logic 8 for controlling the telemetry circuits 54, the magnetic field indicator 10 and the telemetry signal circuit 58. As in the first and second embodiments, the magnetic field indicator 10 comprises a voltage comparator 12 and a time measurement circuit 14 connected to the voltage comparator 12. FIG. 7 shows the telemetry circuits 54 according to the third preferred embodiment of the invention, the telemetry circuits comprising a source of voltage V+, a diode 16, a coil 18 with a core, a capacitor 60, a second resistor 62, a junction point 64 and a number of switches S1-S6. With the aid of the switches S1-S6, which are controlled by the control logic 18, the components in the telemetry circuits 54 are enabled according to the function the telemetry circuits 54 is to have. The voltage V+ is connected via a switch S3 to the junction point 64. The diode 16 and the coil 18 are connected in parallel between the junction point 64 and the switch S1 which, in turn, is connected to ground. Diode 16 is connected via a switch S2 to the junction point 64. A measurement signal 20 is collected between the switch S1 and the coil 18 and diode 16, the latter two connected in parallel, and the measurement signal 20 is connected to the magnetic field indicator 10 via a switch S4. The capacitor 60 is connected to the junction point 64 and, via a switch S5, to ground. The second resistor 62 is connected to the junction point 64 and ground via a switch S6. The telemetry signal circuit 58 is connected to the junction point 64. The combined unit 52 senses at a defined interval, for example once a second, whether a magnetic field is present. When such a determination is made, the switches S2, S3 and S4 are closed whereas S5 and S6 are open. Determination of the presence of a magnetic field with a defined strength is performed in the same way as described in conjunction with the first preferred embodiment. Thus, the coil 18 is charged by the source of voltage V+ for a defined period of time 22. This is achieved when the switch S1 is closed during the defined period of time 22. Voltage across the coil during the defined period of time 22 is equal to the voltage V+. When the switch S1 re-opens, the voltage drops, and the coil discharges through the diode 16. During the discharge time 24, the voltage across the coil 18 is equal to minus the diode's voltage drop in the direction of conduction. After the discharge time 18, voltage returns to its normal level, i.e. there is no voltage across the coil. This return is performed in an oscillatory process. When discharge time 24 is measured, the measurement signal's 20 (lower curve in FIG. 4) potential is compared in the voltage comparator 12 in the magnetic field indicator 10 to an adjustable voltage threshold value 26. As previously noted, the coil's discharge time 24 is also measured in the magnetic field indicator 10, a detection signal 30 then being generated if the discharge time 24 is less than a defined time threshold value 32. When the combined unit 52 is to be used for telemetry, the switches S2 and S4 are open and S1 is closed. In transmission, S5 is closed, the capacitor 62 thus being connected to ground, whereas S6 is open, i.e. the second resistor 60 is not connected to ground. The coil 18 and the capacitor 60 form a resonance circuit which, with an appropriate choice of component values, has a preferred resonance frequency of 8 khz and a high Q value. Switch S3 opens and closes rapidly, so brief pulses are sent to the resonance circuit in which the coil 18 serves as the transmission antenna for the electromagnetic waves. The generated train of pulses comprises the information which is to be transferred to the programmer 56. When signals are received from the programmer 56, S3 and S5 are open, whereas S6 is closed. The coil 18 picks up the signals transmitted by the programmer 56 and sends them, via the telemetry signal circuit 58, to the control device 4. The telemetry signal circuit 58 comprises e.g. a comparator circuit (not shown) in which the signal level for the received signal is compared to a defined threshold level, and only signals exceeding this level are processed. In reception, the second resistor 62 damps the signal so there is no residual voltage on the coil 18 at the start of transmission or reception. FIG. 8 shows a fourth preferred embodiment of the invention. This embodiment only differs from the above-described third embodiment in two respects. Firstly, the time period in which the switch S1 closes and the coil 18 charges is the same, as is the case for the second embodiment, as the time elapsing from the start of charging until the intensity of current through the coil 18 exceeds a defined level. This is achieved with a D flip-flop 36 and a comparator 38 in the same was as described in conjunction with the second preferred embodiment. Secondly, a switch S7 is connected across the resistor 34. This switch S7 is only closed during telemetry. For the combined unit 52 according to the third and fourth embodiments, no determination is made as to whether a magnetic field is present as long as telemetry transmission is in progress. When telemetry has been completed, magnetic field determination starts again, e.g. once a second. This control of the enablement of different functions is exercised by the programmable control device 4. FIGURE designations 1 Magnetic field detector 4 Programmable control device 6 Magnetic field detector 8 Control logic 10 Magnetic field indicator 12 Voltage comparator 14 Time measurement circuit 16 Diode 18 Coil 20 Measurement signal 22 Charging time 24 Discharge time 26 Voltage threshold value 28 Voltage signal 30 Detection signal 32 Time threshold value 34 First resistor 36 D flip-flop 38 Comparator 40 Reference voltage 42 Comparator input terminal 44 Comparator output terminal 46 D flip-flop's pre-set input terminal 48 The skin 50 Medical implant 52 Combined telemetry and magnetic field detector unit 54 Telemetry circuits 56 Programmer 58 Telemetry signal circuit 60 Capacitor 62 Second resistor 64 Junction point S1-S7 Switches V+ Voltage

Claims: A magnetic field detector (2) which has a magnetic field indicator (10) and a magnetic field sensor (6) comprising a source of voltage (V+) and a coil (18), characterized in that the magnetic field sensor (6) also comprises a diode (16), the coil (18) being charged by the source of voltage (V+) for a defined period of time (22) when the presence of a magnetic field is determined, and the magnetic field indicator (10) is arranged to measure the coil's discharge time (24) through the diode (16), the magnetic field indicator (10) generating a detection signal (30) for the presence of a magnetic field if this discharge time (24) is less than a defined time threshold value (32). A magnetic field detector (2) according to claim 1, characterized in that the defined time period (22) for the coil's (18) discharge is the time elapsing from the start of charging until the intensity of current through the coil (18) exceeds a defined value (40). Magnetic field detector (2) according to any of the preceding claims, characterized in that the magnetic field indicator (10) comprises a voltage comparator (12), which compares the voltage across the coil (18) to an adjustable voltage threshold value (26), and a time measurement circuit (14) holding the defined threshold value (32) for the coil's (18) discharge time (24), the voltage comparator (12) generating a voltage signal (28) as long as the voltage across the coil (18) exceeds the voltage threshold value (26), the voltage signal (28) thereafter being sent to the time measurement circuit (14) which emits the detection signal (30) if the voltage signal's (28) duration is less than the time threshold value (32). Magnetic field detector (2) according to any of the preceding claims, characterized in that said magnetic field detector (2) is arranged in a medical implant (50). Magnetic field detector (2) according to claim 4, characterized in that said magnetic field detector is included in a combined telemetry and magnetic field detector unit (52), the combined unit (52) comprising control logic (8), the magnetic field indicator (10) and a telemetry circuit (54) comprising the source of voltage (V+), a plurality of switches (S1-S7) and the coil (18), the control logic (8) being arranged to actuate the switches (S1-S7) so the combined unit (52) switches between telemetry and magnetic field detection, the coil (18) being used for transmitting telemetric signals to/from a programmer (56) and for magnetic field detection, respectively. Magnetic field detector (2) according to claim 5, characterized in that the telemetry circuit (54) comprises the diode (16) which is disconnected from the coil (18) during telemetry, the coil (18) being charged by the source of voltage (V+) for a defined period of time (22) in determinations of the presence of a magnetic field, and the magnetic field indicator (10) is arranged to measure the time for the coil's discharge (24) via the diode (16), the magnetic field indicator (10) then generating the detection signal (30) for the presence of a magnetic field if this discharge time (24) is less than the defined time threshold value (32).

Inventors: LIDMAN JOHAN, NILSSON KENTH, LIDMAN, JOHAN, NILSSON, KENTH
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1995
Main classification: A61N1
IPC codes: G01R33, A61N1