System for controlling transcranial magnetic stimulation (TMS) pulse delivery and EEG measurement

Abstract

Described is a system for controlling TMS pulse delivery and EEG measurement The system comprises a control system for triggering delivery of a TMS energy pulse from a TMS stimulator to a TMS coil arrangement; an EEG measurement system configured to receive EEG signals from a plurality of EEG electrodes; and a switching arrangement intermediate the plurality of EEG electrodes and the EEG measurement system. The control system is arranged to activate the switching arrangement to temporarily electrically isolate the EEG electrodes from the EEG measurement system at the time of triggering delivery of the TMS energy pulse. By electrically isolating the measurement system from the EEG electrodes as the TMS pulse is triggered and thus emitted, saturation of the amplifiers is prevented.

Description

[0001] TITLE

[0002] System for Controlling Transcranial Magnetic Stimulation (TMS) Pulse Delivery and EEG Measurement

[0003] BACKGROUND

[0004] Transcranial magnetic stimulation (TMS) is a form of brain stimulation in which a changing magnetic field is used to induce an electric current at a specific area of the brain through electromagnetic induction. An electric pulse generator, or stimulator, is connected to a coil which in turn is positioned above the scalp of an individual. The stimulator generates a changing electric current within the coil which creates a varying magnetic field, inducing a current within a region in the brain itself.

[0005] TMS is used to treat a variety of conditions, including depression, obsessive-compulsive disorder (OCD), and anxiety. Treatment is generally performed without any feedback as to the effect that the treatment has on the central nervous system. Accordingly, it is beneficial to monitor the effect of the treatment using Electroencephalography (EEG) signals and associated processor allowing direct observation of the brain's electrical response to the TMS pulse. This provides insights into the neural network activity within a specific brain region, its connectivity with other areas, and how this activity changes during treatment, with the goal of leading to a better understanding of therapeutic treatment efficacy and individual response to TMS therapy. Furthermore, signals from specific and targeted brain pathways can be monitored, meaning that a TMS may be triggered when a specific pathway in the brain is active.

[0006] It is therefore critical that the effect of the TMS pulse is accurately measured. When a TMS pulse is therefore applied to the brain, it is desirable that the effect on the brain is measured by the EEG electrodes immediately as otherwise there is a time delay when the effect is unknown. A problem exists in that application of a TMS pulse induces a voltage across the EEG electrodes of up to a few volts, whereas the EEG electrodes are designed to measure brain activity where the measurable signals are in the region of microvolts. This causes a significant problem, as the EEG measurement system becomes saturated by the TMS pulse, creating a TMS artifact. Consequently, the brain signals are immeasurable for a fewmilliseconds after the pulse is delivered. This is illustrated in Figure 1 which shows the single TMS pulse artifacts recorded by the EEG measurement system across 30 EEG channels after the TMS pulse is emitted at time t=0. The saturation and the recovery period of the EEG measurement system can be seen to last around 20ms.

[0007] A partial solution to this would be to increase the dynamic range of the EEG measurement system (i.e., by reducing the gain in the amplification stage before the analogue signal is converted into digital values). While this may prevent amplifier saturation, it would increase the input-referred noise, making low-level bioelectrical signals harder to distinguish from noise and worsening the signal-to-noise ratio (SNR). Therefore, this approach cannot mitigate the saturation effect without degrading the quality of the EEG signals.

[0008] A solution is therefore required as the period of brain activity after the TMS pulse is emitted is particularly important to obtain a beneficial insight into the effect that the TMS pulse is having on the brain activity.SUMMARY

[0009] Aspects of the present disclosure are set out in the accompanying independent and dependent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims.

[0010] According to an aspect of the present disclosure, there is provided a system for controlling TMS pulse delivery and EEG measurement, the system comprising:

[0011] a control system for triggering delivery of a TMS energy pulse from a TMS stimulator to a TMS coil arrangement;

[0012] an EEG measurement system configured to receive EEG signals from a plurality of EEG electrodes;

[0013] a switching arrangement intermediate the plurality of EEG electrodes and the EEG measurement system;

[0014] wherein the control system is arranged to activate the switching arrangement to temporarily electrically isolate the EEG electrodes from the EEG measurement system at the time of triggering delivery of the TMS energy pulse.

[0015] By electrically isolating the measurement system from the EEG electrodes as the TMS pulse is triggered and thus emitted, saturation of the amplifiers is prevented. The EEG measurement system and EEG electrodes are in electrical communication unless the switching arrangement temporarily isolates the EEG electrodes and measurement system. This means that the status of the measurement system is not activated and deactivated by turning off and back on again. The measurement system can remain ‘on’, and the switching arrangement temporarily isolates the measurement system from the EEG electrodes at the time of triggering delivery of the TMS energy pulse. This ensures that data is not lost through switching the measurement system itself between an on and off state, which can take 10ms for example.

[0016] The EEG measurement system may be electrically connected to EEG electrodes through a plurality of conductive channels made of conductive elements. The EEG measurement system is beneficially positioned such that it will not receive the TMS energypulse. It is therefore outside the effect of the TMS energy pulse. This means that protection circuitry for the EEG measurement system is not required.

[0017] The EEG system comprises a plurality of EEG electrodes. The EEG electrodes may be termed EEG sensing electrodes.

[0018] The control system may be arranged to trigger delivery of the TMS pulse through a first trigger signal, and the control system is arranged to deliver a second trigger signal to temporarily activate the switching arrangement. The first and second signal are beneficially emitted simultaneously and concurrently. It will be understood that a typical switch in a TMS stimulator is a thyristor, meaning that the first trigger signal causes switching of the thyristor and there is a first time period between transmission of the first trigger signal to actual delivery of the current to then deliver the electromagnetic field. The switching arrangement between the plurality of EEG electrodes and the measurement system may comprise faster acting switches acting in a second time period less than the first time period. Such switches may comprise a CMOS switch in each of the EEG channels, meaning the EEG electrodes are electrically isolated from the measurement system fractionally earlier than the electromagnetic field is delivered thereby ensuring that the signals from the EEG amplifiers are not saturated.

[0019] A separate channel may extend between each EEG electrode and the measurement system, wherein the switching arrangement comprises a switch in each channel. CMOS switches are typically utilised as they enable fast switching meaning the delay between emission of the TMS pulse and switching back to a measuring capability is minimised. This time frame may be less than 1 microsecond.

[0020] The typical period the switching arrangement is activated (meaning the EEG electrode are electrically isolated from the measurement system) is at least the period over which a TMS pulse is delivered. A typical TMS pulse is delivered in 300 microseconds. Accordingly, the switching arrangement may be activated for a time period slightly greater than the time of a TMS pulse delivery to ensure there is no saturation effect caused by any transient magnetic fields in the TMS coil.The EEG measurement system may include a processor, biopotential amplifier and an Analogue to Digital Converter (ADC). The measurement system receives the EEG signals from each of the EEG electrodes, amplifies them, converts them to the digital values and processes those signals.

[0021] There are typically between 32 and 256 individual EEG electrodes. The switching arrangement may comprise a switch intermediate every individual EEG electrode and measurement system. The switches may be activated concurrently.

[0022] Typical pulse parameters of the TMS pulse are peak to peak voltage of 3kV, current of lOkA and time period of 300 microseconds.

[0023] The present disclosure may extend to a TMS pulse delivery system and EEG measurement system, where the TMS pulse delivery system comprises a TMS coil arrangement and a TMS stimulator for delivering a TMS energy pulse from the TMS stimulator to the TMS coil arrangement, and where the EEG measurement system comprises a plurality of EEG electrodes for emitting a plurality of EEG signals to the measurement system.

[0024] The measurement system may be configured to receive EEG signals and control delivery of the TMS pulse dependent upon the EEG signals received.

[0025] The measurement system may be configured to identify a property associated with one or more of the EEG signals and control delivery of a TMS pulse dependent upon the property identified. Accordingly, the measurement system may process the EEG signals and identify a property such as EEG signal frequency bands and control the delivery of the TMS pulse to coincide with the identified band.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of this disclosure will be described hereinafter, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which:Figure 1 shows a graphical representation of the saturation effect measured by EEG signals upon delivery of a TMS pulse according to known measurement EEG measurement to TMS pulse delivery;

[0028] Figure 2 is a schematic representation of a system for controlling and monitoring TMS pulse delivery and EEG measurement according to an illustrative embodiment of this disclosure;

[0029] Figure 3 is schematic representation of the switching arrangement for a single EEG measurement channel according to an embodiment of this disclosure;

[0030] Figure 4 is a schematic representation of the measured signal from an EEG electrode according to an embodiment of this disclosure.DETAILED DESCRIPTION

[0031] Embodiments of this disclosure are described in the following with reference to the accompanying drawings.

[0032] Referring to Figure 2, there is schematic representation of a system for controlling and monitoring TMS pulse delivery and EEG measurement according to an illustrative embodiment of this disclosure. Presented is a TMS coil 2 under the control of a stimulator 4, where the stimulator includes a control system arranged to control delivery of a current pulse to the coil 2, which in turn then delivers an electromagnetic pulse to the scalp of a subject.

[0033] Also shown in Figure 2 is a schematic net 6 that carries a plurality of EEG sensors 8 (typically between 32 and 256) and is designed to seat over the scalp of a subject so that the scalp is received into the recess 7. The sensors 8 carried by the net 6 are utilized for measurement of electrical activity at multiple sites in the brain. Each of the individual channels from each EEG sensor 8 are connected to an EEG measurement system 10 which has been schematically presented adjacent to the stimulator 4. The configuration shown therefore allows the effect on the brain of TMS pulses that are subjected to the scalp of a subject to be measured by the plurality of sensors 8.

[0034] In operation the stimulator 4 emits a first trigger signal to the coil 2 to effect delivery of an electromagnetic pulse 12 to the brain of the subject. In the present invention, a simultaneous second trigger signal 13 is emitted by the control system in the stimulator which triggers switching of an electrical isolator 14 intermediate the EEG sensors 8 and the measurement system 10. The measurement system may comprise amplifiers and an Analogue to Digital Converter (ADC). This means that as the electromagnetic pulse from the TMS device is generated, the EEG sensors are electrically isolated from the amplifiers. This occurs substantially concurrently, and as such there is no measurement at the point of triggering. The switching arrangement that causes the electrical isolation comprises a CMOS switch in each channel that individually connects each sensor to the ADC. The provision of a CMOS switch in each channel under the control of the control system of the stimulator means that switching can occur very quickly (in the order of Imicrosond) meaning in turn that the amplifiers are not saturated by the TMS pulse itself but record brain activity almost immediately after emission of the TMS pulse.The period of time when the switching arrangement 14 is activated can be termed the blanking period. This means that no EEG data is recorded during this period of time which importantly covers the period of time that the pulse is delivered. As described above this blanking period is minimized as using fast switching switches such as CMOS switches, and the blanking time period is therefore restricted to approximately Imillisecond per pulse.

[0035] The trigger signal line 13 can act as a bidirectional communication line, allowing the EEG measurement system to send commands to the TMS device. Two-way communication ensures that the EEG device can provide control to the TMS device.

[0036] Referring to Figure 3, a schematic representation of the switching arrangement for a single EEG measurement channel according to an embodiment of this disclosure is presented. The trigger signal 13 is output from the TMS device (control system of the stimulator 4) to a CMOS switch 14 electrically isolating the EEG electrode from the measurement system at the time of triggering delivery of the TMS energy pulse and during the period of delivery of the pulse.

[0037] Referring to Figure 4, shown is the measured artifacts taken either from an individual EEG electrode or a processed signal from multiple EEG electrodes showing the effect of an embodiment of this disclosure. Referring to the 10 Hz sine wave, which models the EEG signals, the artifacts caused by the delivered TMS pulse is minimally visible (effectively by switching the CMOS switches 14), as compared to Figure 1. Accordingly, a zoomed in portion of the 1ms time period when the CMOS switches are activated (open) shows the minimal effect on the on the recorded data from the EEG electrodes.

[0038] Although particular embodiments of this disclosure have been described, it will be appreciated that many modifications / additions and / or substitutions may be made within the scope of the claims.

Claims

CLAIMS1. A system for controlling TMS pulse delivery and EEG measurement, the system comprising:- a control system for triggering delivery of a TMS energy pulse from a TMS stimulator to a TMS coil arrangement;- a measurement system configured to receive EEG signals from a plurality of EEG electrodes;- a switching arrangement intermediate the plurality of EEG electrodes and the measurement system;wherein the control system is arranged to activate the switching arrangement to temporarily electrically isolate the EEG electrodes from the measurement system at the time of triggering delivery of the TMS energy pulse.

2. A system according to claim 1 wherein the control system is arranged to trigger delivery of the TMS pulse through a first trigger signal, and the control system is arranged to deliver a second trigger signal to temporarily activate the switching arrangement.

3. The system according to any preceding claim wherein the first and second signal are emitted simultaneously.

4. The system according to any preceding claim comprising a separate channel extending between each EEG electrode and the measurement system, wherein the switching arrangement comprises a switch in each channel.

5. The system according to claim 4 wherein each of the switches comprises a CMOS switch.

6. The system according to any preceding claim wherein the switching arrangement is arranged to be activated for less than 1 microsec.

7. The system according to any preceding claim wherein the period the switching arrangement is activated is at least the period over which a TMS pulse is delivered.

8. The system according to any preceding claim wherein the measurement system comprises a processor.

9. The system according to any preceding claim wherein the measurement system comprises an Analogue to Digital Converter (ADC).

10. The system according to any of claims 8-9, wherein the measurement system includes a biopotential amplifier.

11. A TMS pulse delivery system and EEG measurement system comprising a system for controlling TMS pulse delivery and EEG measurement according to any preceding claim, where the TMS pulse delivery system comprises a TMS coil arrangement and a TMS stimulator for delivering a TMS energy pulse from the TMS stimulator to the TMS coil arrangement, and where the EEG measurement system comprises a plurality of EEG electrodes for emitting a plurality of EEG signals to the processing system.

12. A TMS pulse system according to claim 11 wherein the processing system is configured to receive EEG signals and control delivery of the TMS pulse dependent upon the EEG signals received.

13. A TMS system according to claim 12 wherein the processing system is configured to identify a property associated with one or more of the EEG signals and control delivery of a TMS pulse dependent upon the property identified.

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