No assignee for this patent application has been made.
News editors obtained the following quote from the background information supplied by the inventors: "Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (
"Each of these implantable neurostimulation systems typically includes at least one stimulation lead implanted at the desired stimulation site and an Implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the stimulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The neurostimulation system may further comprise a handheld Remote Control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
"Significant to the present inventions described herein, a typical IPG may be manually inactivated by the patient, e.g., to cease stimulation of the IPG during an emergency, by placing a magnet over the implanted IPG. This can be accomplished using any one of several different types of magnetically induced shut-down circuits.
"For example, referring to FIG. 1, one implementation of magnetically induced shut-down circuitry 2 generally comprises a magnetic field sensing device 4, such as a reed switch or a Hall sensor, a microcontroller 6, which controls and manages the operations of the IPG, and a delay circuit 8, which introduces a delay into an input of, e.g., 200-400 .mu.s. The output of the magnetic field sensing device 4 is coupled to an interrupt pin of the microcontroller 6, and further coupled to a reset pin of the microcontroller 6 via the delay circuit 8. Thus, when the magnetic field sensing device 4 senses a magnetic field, such as that emitted by a magnet passed over the IPG, a switch within the magnetic field sensing device 4 closes, thereby outputting a signal indicating the desire of the patient or user to cease stimulation. The signal is conveyed to the interrupt pin of the microcontroller 6, which responds by instantaneously shutting down power to the stimulation circuitry (now shown) of the IPG, thereby ceasing stimulation of the patient, as well as performing housekeeping functions, such as storing data. The signal is also conveyed to the reset pin of the microcontroller 6, which responds by rebooting itself. Significantly, the delay introduced by the delay circuit 8 into the signal output by the magnetic field sensing device 4 allows the microcontroller 6 to perform the aforementioned housekeeping functions prior to rebooting.
"IPGs are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic forces generated by MRI scanners, which may potentially cause damage to the neurostimulator as well as discomfort to the patient.
"In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the present of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of
"Despite the fact that a conventional IPG implanted within a patient undergoing an MRI will be automatically deactivated (i.e., the magnetic field present in the MRI scanner will be sensed by the magnetic field sensing device, thereby automatically deactivating the IPG), the strength of the gradient magnetic field may be high enough to induce voltages (5-10 Volts depending on the orientation of the lead inside the body with respect to the MRI scanner) on to the stimulation lead(s), which in turn, are seen by the IPG electronics. If these induced voltages are higher than the voltage supply rails of the IPG electronics, there could exist paths within the IPG that could induce current through the electrodes on the stimulation lead(s), which in turn, could cause unwanted stimulation to the patient due to the similar frequency band, between the MRI-generated gradient field and intended stimulation energy frequencies for therapy, as well as potentially damaging the electronics within the IPG. To elaborate further, the gradient (magnetic) field may induce electrical energy within the wires of the stimulation lead(s), which may be conveyed into the circuitry of the IPG and then out to the electrodes of the stimulation leads. For example, in a conventional neurostimulation system, an induced voltage at the connector of the IPG that is higher than IPG battery voltage (.about.4-5V), may induce such unwanted stimulation currents. RF energy generated by the MRI scanner may induce electrical currents of even higher voltages within the IPG.
"In one novel technique described in U.S. Provisional Patent Application Ser. No. 61/612,214, entitled 'Neurostimulation System for Preventing Magnetically Induced Currents in Electronic Circuitry,' which is expressly incorporated herein by reference, voltage supply rails of the IPG electronics are increased in response to an external signal from the RC or CP that places the IPG in an MRI-mode. In order to increase the voltage supply rails of the IPG electronics, it is necessary that the IPG not be deactivated in the presence of the magnetic field generated by the MRI scanner. In one proposed method, this can be accomplished by disabling the magnetic field sensing device to prevent deactivation of the IPG. However, it may be desirable to continue to monitor the magnetic field generated by the MRI, e.g., to determine when the MRI has been initiated and/or terminated. If the magnetic field sensing device is disabled during the MRI, this function cannot be accomplished.
"There, thus, remains a need to prevent an IPG from being deactivated during an MRI, while monitoring the magnetic field during the MRI."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "In accordance with a first aspect of the present inventions, an implantable medical device capable of being placed between a first operational mode (e.g., a normal mode) and a second operational mode (an MRI-mode) is provided. The implantable medical device comprises a magnetic field sensing device (e.g., a reed switch or a Hall sensor) configured for outputting a signal in response to sensing a magnetic field.
"The medical device further comprises a logic circuit configured for continuously asserting the signal when the neurostimulation device is in the first operational mode, and intermittently asserting the signal during at least one time period when the neurostimulation device is in the second operational mode. In one embodiment, the signal is intermittently asserted during a plurality of successive time periods, e.g., in accordance with a duty cycle. The medical device further comprises a delay circuit coupled between the magnetic field sensing device and the first input terminal of the control circuitry, the delay circuit configured for introducing a time delay (e.g., in the range of 200 ms-400 ms) into the signal. The time delay is less than the time period during which the signal is continuously asserted, but greater than each of the time period(s) during which the signal is intermittently asserted. In one embodiment, the logic circuit comprises a logic gate (e.g., an AND-gate or a NOR-gate) and a register configured for storing a signal assertion bit, with the logic gate having a first input coupled to the magnetic field sensing device, and a second input coupled to the register.
"The medical device further comprises control circuitry configured for performing a function (e.g., deactivating the medical device) in response to receiving the delayed signal at a first input terminal. The control circuitry may be configured for performing another function (e.g., recording information based on the sensed magnetic field) in response to receiving the signal at a second input terminal. An optional embodiment of the medical device comprises a plurality of electrical terminals configured for being respectively coupled to a plurality of stimulation electrodes, and stimulation output circuitry configured for outputting electrical stimulation energy to the plurality of electrical terminals.
"In accordance with a second aspect of the present inventions, a method of operating a medical device implanted within a patient is provided.
"The method comprises, when the medical device is in a first operational mode (e.g., when the patient is not undergoing an MRI), generating a first signal in response to sensing a magnetic field, continuously asserting the first signal, introducing a time delay into the first asserted signal, wherein the time delay is less than the time period, thereby prompting the neurostimulation device to perform a function (e.g., deactivating the medical device) in response to the first delayed signal.
"The method further comprises, when the medical device is in a second operational mode (when the patient is undergoing an MRI), generating a second signal in response to sensing a magnetic field, intermittently asserting the second signal during at least one time period, introducing a time delay into the second asserted signal, wherein the time delay (e.g., in the range of 200 ms-400 ms) is greater than each of the at least one time period, thereby preventing the neurostimulation device from performing the function in response to the second delayed signal. The second signal may be intermittently asserted during a plurality of successive time periods, e.g., in accordance with a duty cycle. The method may optionally further comprise performing another function (e.g., recording information based on the sensed magnetic field) in response to the second asserted signal when the medical device is in the second operational mode.
"Other and further aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
"The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate how the above-recited and other advantages and objects of the present inventions are obtained, a more particular description of the present inventions briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
"FIG. 1 is a prior art embodiment of magnetically induced shut-down circuitry used in an implantable pulse generator (IPG);
"FIG. 2 is a plan view of a Spinal Cord Stimulation (SCS) system constructed in accordance with one embodiment of the present inventions;
"FIG. 3 is a plan view of the SCS system of FIG. 2 in use within a patient;
"FIG. 4 is a plan view of an implantable pulse generator (IPG) and three percutaneous stimulation leads used in the SCS system of FIG. 2;
"FIG. 5 is a plan view of an implantable pulse generator (IPG) and a surgical paddle lead used in the SCS system of FIG. 2;
"FIG. 6 is a block diagram of the internal components of the IPG of FIGS. 4 and 5;
"FIG. 7 is a block diagram illustrating the components of one embodiment of a magnetic field sensing assembly used in the IPG of FIGS. 4 and 5;
"FIG. 8 is a block diagram illustrating the components of another embodiment of a magnetic field sensing assembly used in the IPG of FIGS. 4 and 5;
"FIG. 9 is a timing diagram of a signal assertion/de-assertion waveform used by a logic circuit in the magnetic field sensing assembly of FIG. 8;
"FIG. 10 is a flow diagram illustrating a technique used by the neurostimulation system of FIG. 2 to deactivate the IPG of FIGS. 4 and 5 when the IPG is in a normal mode; and
"FIG. 11 is a flow diagram illustrating a technique used by the neurostimulation system of FIG. 2 to monitor a magnetic field while preventing deactivation of the IPG of FIGS. 4 and 5 when the IPG is in an MRI-mode."
For additional information on this patent application, see: Feldman, Emanuel; Parramon, Jordi; Bocek, Joseph M.; Gururaj, Kiran. Neurostimulation System for Enabling Magnetic Field Sensing with a Shut-Down Hall Sensor. Filed
Keywords for this news article include: Patents, Therapy, Treatment, Electronics, Microcontroller.
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