Magnetic field detector in a medical implant

Abstract: The invention relates to a medical implant (1) containing a magnetic field detector (3) containing a sensor (4), whose a capacitance changes in the presence of a magnetic field with a predefined strength, and a capacitance-controlled unit (5, 13) connected to the sensor (4). The capacitance-controlled unit (5, 13) sends a measurement signal (9) to a detection unit (8). If a measurement signal (9) parameter affected by capacitance displays a specific change over a predefined period of time, a detection signal (11) is generated by the detection unit (8) to indicate the presence of a magnetic field. The sensor's (4) capacitance also changes when the sensor moves, and this is utilized for measuring activity in an activity measurement circuit (7) connected to the capacitance-controlled unit (5, 13) and for generating an activity signal (12) on the basis of the measurement signal (9).

Description: The present invention relates to a medical implant containing a magnetic field detector. In a medical implant such as a pacemaker, a magnetic field detector is used for non-invasive activation of different implant functions by means of e.g a permanent magnet which is moved adjacent to the implant on the exterior of the patient's body. With a pacemaker, for example, some of the functions which can be activated are e.g. disabling the pacemaker's demand function so the pacemaker adapts its operation to the battery's capacity, having the pacemaker operate according to some special temporary stimulation mode (e.g. in tachycardia) and programming the pacemaker. Detection of magnetic fields outside the implant art in a number of different ways, e.g. with the aid of reed switches, by changing the resonance frequency or inductance etc. is generally known. In the implant art, a conventional magnetic field detector consists of a reed switch. However, a reed switch is a sensitive and rather expensive component which also take up a relatively large amount of space in the implant. In order to eliminate the need for reed switches, utilization of the implant's telemetry unit for detecting the presence of magnetic fields, in addition to providing telemetric functions, has been proposed in recent years. US-4 541 431 shows one such proposal with a combined telemetry and magnetic field detector unit. It contains a conventional resonance circuit, with e.g. a coil, employed in telemetry for transmitting and receiving data. The resonance circuit is also used here for sensing the presence of magnetic fields whose strength exceeds a predefined value. The resonance frequency of the resonance circuit varies with the strength of the magnetic field. The resonance circuit is activated periodically, and the number of zero crossings for the resonance circuit's output signal are 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 predefined valued. Most contemporary pacemakers have some form of sensor-activated control of the pacemaker's stimulation rate and often use an activity sensor. One example of such a sensor-controlled pacemaker is provided in US-4 140 132 which contains a sensor, e.g. a piezoelectric element, which changes the pacemaker's stimulation rate on the basis of the element's vibrations. One object of the present invention is to set forth an improved device for detecting magnetic fields near an implant. According to the invention, this object is achieved with an implant of the above-described kind and with the features set forth in the characterizing part of the independent claim. According to the invention, therefore, the need for a reed switch as a magnetic field detector is eliminated by utilization instead of a capacitive sensor already in the pacemaker. At least part of the sensor is coated with a material, sensitive to magnetic fields, which causes a change in the sensor's capacitance in the presence of a magnetic field, a detection signal thereupon being generated on the basis of this change in capacitance in the presence of the magnetic field. According to one preferred embodiment, a capacitance-controlled amplifier is connected to the sensor, said amplifier emitting a measurement signal, gain then being the measurement signal parameter affected by capacitance. According to this first preferred embodiment, the magnetic field detector is also employed for activity measurement. According to a second preferred embodiment, a capacitance-controlled oscillator is connected to the sensor, said oscillator emitting a measurement signal, frequency then being the measurement signal parameter affected by capacitance. According to this second preferred embodiment, the magnetic field detector is also employed for activity measurement. According to a third and a fourth preferred embodiment, the measurement signal parameters affected by capacitance are gain and frequency respectively, exactly as in the first two embodiments. The difference between these embodiment is that there is no activity measurement in the third and fourth embodiments. The sensor used consists of a capacitive accelerometer containing at least two electrode plates, moveable in relation to each other, at least one of the electrode plates being coated with a material sensitive to magnetic fields, e.g. a ferromagnetic material or a permanent magnet material, whereby the sensor's capacitance depends on the distance between the electrode plates. The invention will now be described in greater detail, referring to attached drawing in which FIG. 1 is a schematic block diagram of a medical implant with the invention implemented; FIG. 2 is a schematic block diagram of a first preferred embodiment of the magnetic field detector according to the invention; FIG. 3 is a detailed circuit diagram of the capacitance-controlled amplifier according to the first embodiment; FIG. 4 is a schematic block diagram of a second preferred embodiment of the magnetic field detector according to the invention; FIG. 5 is a schematic block diagram of a third preferred embodiment of the magnetic field detector; and FIG. 6 is a schematic block diagram of a fourth preferred embodiment of the magnetic field detector. Identical or similar parts in the FIGURES have the same reference designations. In FIG. 1 is shown a medical implant 1 containing a control device 2 to which a magnetic field detector 3 is connected. The implant 1 can be a pacemaker, a defibrillator, a nerve stimulator or some other implantable device. In FIG. 2 is shown the magnetic field detector 3 according to a first preferred embodiment, the magnetic field detector 3 containing a capacitive sensor 4, a capacitance-controlled amplifier 5, a low-pass filter 6, an activity measurement circuit 7 and a detection unit 8. The employed type of sensor 4 is a capacitive accelerometer containing at least two electrode plates, a mass element being attached to one of the plates. When the sensor moves, the plate with the mass element moves towards or away from the other plate, causing a change in the sensor's capacitance. When the plate with the mass element is coated with a material sensitive to magnetic fields, such as a ferromagnetic material or a permanent magnet material, the presence of a magnetic field will change the distance between the plates, thereby changing the capacitance of the sensor 4. The capacitance change caused by the presence of a magnetic field is much greater than the change induced by acceleration due to physical activity. The magnitude of the change in capacitance depends on the thickness of the material, sensitive to magnetic fields, with which the electrode plate is coated and can be established during the sensor's fabrication. In order to achieve a magnetic field with a predefined strength, an ordinary permanent magnet, placed on the patient's skin above the implant, is used. A requirement for a modern pacemaker is that the pacemaker must be able to detect a magnet at a distance of 40 cm. The sensor 4 is connected to the amplifier 5 in such a way that the sensor's capacitance affects the amplifier's gain. The amplifier 5 has a direct current input signal. The amplifier's 5 output signal is filtered in the low-pass filter 6, a measurement signal 9 then ensuing. The measurement signal 9 is sent to the activity measurement circuit 7 and the detection unit 8. The measurement signal 9 is compared in the detection circuit 8 to an adjustable voltage threshold value 10. If the measurement signal 9 exceeds this threshold value 10 for a predefined period of time, a detection signal 11 is generated and sent to the control device 2. The voltage threshold 10 is set at a value corresponding to a measurement signal level when a magnetic field with a predefined strength is present. The detection signal 11 is sent to the control device 2 but also used as a control signal for the activity measurement circuit 7. The activity measurement circuit 7 sends an activity signal 12 to the control device 2 when no detection signal 11 is sent to the activity circuit 7. The activity signal 12 is used in the control device 2 for e.g. changing a pacemaker's stimulation rate. In order to prevent transient increases in the magnetic field from causing erroneous detection of a magnetic field, two conditions must be met for the detection signal 11 to be generated. Firstly, the measurement signal 9 must exceed the voltage threshold value 10. Secondly, the measurement signal 9 must exceed the threshold value 10 for a predefined period of time. So a detection signal 11 is only generated if these two conditions are met. The detection unit also includes a time measuring circuit (not shown), in addition to a voltage comparison circuit (not shown), for determining whether the measurement signal 9 exceeds the threshold value 10 for the predefined period of time. The voltage comparison circuit and the time measurement circuit will not be described here, since they are constructive components familiar to anyone well-versed in the art. The duration of the predefined interval is selected at the time the implant is programmed and set at a value about the same as a normal stimulation interval (857 ms). A normal stimulation interval of 857 ms corresponds to a stimulation rate of 70 beats a minute. The interval is set at a value of this magnitude because it is assumed that any movement-induced change in capacitance cannot remain constant that long, thereby preventing movement from being erroneously interpreted as the presence of a magnetic field. If a detection signal 11 is sent to the control device 2, the control device 2 performs the aforementioned activities to be carried out when the implant is the magnet mode. These will not be described here, since they are measures familiar to anyone well-versed in the art. If no magnetic field is detected, the control device 2 exits from the magnet mode, and the activity measurement circuit 7 is reactivated. FIG. 3 is a more detailed circuit diagram showing the circuitry of the capacitance-controlled amplifier 5. The circuit contains eight switching transistors, T1-T8, and a switching capacitor, C1. The sensor's capacitance is designated C2. Vref designates a constant direct current constituting an appropriate part of the implant's battery voltage. The transistors T1-T8 serve as switches and are controlled by the control device 2 with a digital clock (not shown) created by reduction of the frequency of a crystal oscillator (not shown). The switching rate selected depends on the construction's design, but a frequency on the order of 500 Hz is selected. The transistors are switched in groups in which the transistors in an initial group, T1, T4, T5 and T7, are conductive simultaneously and in which the transistors in a second group, T2, T3 and T6, are non-conductive. The groups are enabled alternately, and the amplifier's output voltage, Uut, only supplies a measurement value when the second group is enabled and conductive. Uut is sampled with T8 which is enabled at the same time as the second group. The described activation of the amplifier 5 is a type referred to as "switch-capacitor" (SC) enablement. Switching is based on the circumstance that when a capacitor is alternately charged and discharged, a resistance is simulated which can approximately be calculated as R = 1/f s *C, in which f s is the clock rate. Output voltage from the amplifier 5 can now be written as Uut = 1/f s *C 2 /1/F s *C 1 * Vref, yielding through abbreviation Uut = C1/C2*Vref. FIG. 4 shows the magnetic field detector 3 according to a second preferred embodiment, the magnetic field detector 3 containing a capacitive sensor 4, a capacitance-controlled oscillator 13, a pulse-forming circuit 14, an activity measurement circuit 7 and a detection unit. Here, the sensor 4 consists of a capacitive accelerometer 4, which is partially coated with a material sensitive to magnetic fields, just as in the above-described preferred first embodiment. The sensor 4 is connected to the oscillator 13 in such a way that the sensor's 4 capacitance affects the oscillator's frequency. The oscillator 13 can be e.g. a conventional RC oscillator in which the frequency is set at several kHz. The oscillator's 13 output signal is processed in the pulse-forming circuit 14, a measurement signal 9 then being obtained. The pulse-forming circuit 14 consists e.g. of a Schmitt trigger which evens out the flanks so the measurement signal 9 becomes a square wave. The measurement signal 9 is sent to the activation circuit 7 and to the detection unit 8. In the detection unit 8, which contains e.g. a counter, the measurement signal's 9 frequency is compared to an adjustable frequency threshold value 15. If the frequency exceeds this frequency threshold for a predefined period of time, a detection signal 11 is generated and sent to the activity measurement circuit 7 and the control device 2. The predefined period of time is selected in the same way as previously described for the first preferred embodiment. If the material, sensitive to magnetic fields, with which the sensor 4 has been coated is a permanent magnet material, the absolute change in the oscillator's 13 frequency is studied. This is because the distance between the sensor's 4 electrode plates either increases or decreases, depending on the polarity of the external magnetic field, causing capacitance to increase or decrease, and causing, in turn, an increase or decrease in the oscillators 13 frequency. The threshold 10 is set at a value corresponding to a frequency for the measurement signal 9 when a magnetic field with a predefined strength is present. The change in the oscillator's 13 frequency in the presence of a magnetic field with a predefined strength is much greater than the changes found in activity measurement. As previously noted, the magnitude of the change can be established during the sensor's 5 fabrication by varying the thickness of the coating of the electrode plate with a material sensitive to magnetic fields. The detection signal 11 is therefore generated if a magnetic field with a predefined signal strength is present. In other respects this embodiment operates the same way as the above-described first preferred embodiment. The magnetic field detector 3 can obviously be used solely for magnetic field detection. Its design is then simpler than the design of the above-described detector, since it does not have any circuits for activity measurement. Two embodiments, solely for magnetic field detection, are described below. FIG. 5 shows a third preferred embodiment of the magnetic field detector 3 which is only used for detection of magnetic fields. The magnetic field detector 3 is connected to a control device 2 and contains a capacitive sensor 4, a capacitance-controlled amplifier 5, a low-pass filter 6 and a detection unit 8. The sensor 4 is a capacitive accelerometer containing two plates, which are partially coated with a material sensitive to magnetic fields, of the same kind previously described in this application. The sensor 4 is connected to the amplifier 5 in such a way that the sensor's capacitance affects the amplifier's gain. The amplifier 5 has a direct current input signal. The amplifier's 5 output signal is filtered in the low-pass filter 6, a measurement signal 9 then ensuing. The measurement signal 9 is sent to the detection unit 8. The measurement signal 9 is compared in the detection circuit 8 to an adjustable voltage threshold value 10, whereupon a detection signal 11 is generated and sent to the control device 2 if the measurement signal 9 exceeds this threshold value for a predefined period of time. The voltage threshold 10 is set at a value corresponding to the measurement signal 9 level when a magnetic field with a predefined strength is present. So the detection signal 11 is generated when a magnetic field with a predetermined strength is present and sent to the control device 2. The capacitance-controlled amplifier 5 is enabled in the same way as was previously described in conjunction with FIG. 3. FIG. 6 shows a fourth preferred embodiment of the magnetic field detector 3 which is only used for the detection of magnetic fields. The magnetic field detector 3 is connected to a control device 2 and contains a capacitive sensor 4, a capacitance-controlled oscillator 13, a pulse-forming circuit 14 and a detection unit 8. Here, the sensor 4 consists of a capacitive accelerometer, just as in the above-described embodiments, which is partially coated with a material sensitive to magnetic fields. The sensor 4 is connected to the oscillator 13 in such a way that the sensor's 4 capacitance affects the oscillator's frequency. The oscillator 13 can e.g. be a conventional RC oscillator in which the frequency is set at several kHz. The oscillator's 13 output signal is processed in the pulse-forming circuit 14, a measurement signal 9 then being obtained. The pulse-forming circuit 14 consists e.g. of a Schmitt trigger which evens out the flanks so the measurement signal 9 becomes a square wave. The measurement signal 9 is sent to the detection unit 8. In the detection unit 8, which contains e.g. a counter, the measurement signal's 9 frequency is compared to an adjustable frequency threshold value 15, whereupon a detection signal 11 is generated and sent to the control device 2 if the measurement signal 9 exceeds this threshold value for a predefined period of time. Even in this embodiment, as in the second preferred embodiment, the absolute change in frequency must be studied if the sensor 4 has been coated with a permanent magnet material. The threshold 10 is set at a value corresponding to the frequency of the measurement signal 9 when a magnetic field with a predefined strength is present. So the detection signal 11 is generated, when a magnetic field is present, and sent to the control device 2.

Claims: A medical implant (1) with a magnetic field detector (3), characterized in that the magnetic field detector (3) contains a capacitive sensor (4) which is at least partially coated with a material sensitive to magnetic fields, said material causing a change in the sensor's capacitance when a magnetic field is present, whereupon the magnetic field detector generates a detection signal (11) on the basis of this change in capacitance when a magnetic field with a predefined strength is present. A medical implant (1) according to claim 1, characterized in that the magnetic field detector (3) also contains a capacitance-controlled unit (5, 13) connected to the sensor (4) and sends a measurement signal (9) to a detection unit (8), whereupon a change in capacitance causes a change in at least one measurement signal (9) parameter when a magnetic field is present, whereupon the detection unit generates the detection signal (11) in the presence of the magnetic field if this parameter change amounts to a predefined value for a predefined period of time. A medical implant (1) according to claim 2, characterized in that the capacitance-controlled unit (5, 13) is a capacitance-controlled amplifier (5), the measurement signal (9) parameter affected by the capacitance then being gain. A medical implant (1) according to claim 2, characterized in that the capacitance-controlled unit (5, 13) is a capacitance-controlled oscillator (13), the measurement signal (9) parameter affected by the capacitance then being frequency. A medical implant (1) according to any of the preceding claims, characterized in that the sensor (4) also measures activity by changing its capacitance when the sensor (4) moves, whereupon an activity signal (12) is generated on the basis of this change in capacitance. A medical implant (1) according to claim 5 and any of claims 2-4, characterized in that the measurement signal (9) is sent by the magnetic field detector (3) to the detection unit (8), as well as to the activity measurement circuit (7), the activity measurement circuit (7) generating the activity signal (12) on the basis of the measurement signal (9) when the sensor (4) moves. A medical implant (1) according to any of claims 1-6, characterized in that the sensor (4) consists of a capacitive accelerometer which contains at least two electrode plates, moveable in relation to each other, the sensor's (4) capacitance depending on the distance between the electrode plates. A medical implant (1) according to claim 7, characterized in that at least one of the electrode plates is coated with a material sensitive to magnetic fields.

Inventors: BERGSTROEM INGA, BUDGIFVARS GOERAN, BERGSTROEM, INGA, BUDGIFVARS, GOERAN, Bergström, Inga, Budgifvars, Göran
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1995
Main classification: A61N1
IPC codes: G01R33, A61N1, G01V3