Patent Application
20090024021
The present invention generally relates to the use of transcranial magnetic stimulation in conjunction with functional magnetic resonance imaging. More particularly, the present invention relates to the use of transcranial magnetic stimulation (TMS) interleaved with fMRI to measure cortical sensitivity, brain communication, and to determine efficacy of medications, such as central nervous system active compounds.
For over a century, it has been recognized that electricity and magnetism are interdependent (Maxwell's equations) (Bohning, 2000). Passing current through a coil of wire generates a magnetic field perpendicular to the current flow in the coil. If a conducting medium, such as the brain, is adjacent to the magnetic field, current will be induced in the conducting medium. The flow of the induced current will be parallel, but opposite in direction, to the current in the coil (Cohen et al., 1990; Brasil-Neto et al., 1992; Saypol et al., 1991; Roth et al., 1991). Thus, transcranial magnetic stimulation (hereinafter “TMS”) has been referred to as “electrodeless” electrical stimulation to emphasize that the magnetic field acts as the medium between electricity in the coil and induced electrical currents in the brain.
TMS involves placing an electromagnetic coil on the scalp. Subjects are awake and alert. There is some discomfort, in proportion to the muscles that are under the coil, and to the intensity and frequency of stimulation. Subjects usually notice no adverse effects except for occasional mild headache and discomfort at the site of the stimulation. High intensity current is rapidly turned on and off in the coil through the discharge of capacitors. This produces a time-varying magnetic field that lasts for about 100-300 microseconds. The magnetic field typically has a strength of about 2 Tesla (or 40,000 times the earth's magnetic field, or about the same intensity as the static magnetic field used in clinical MRI). The proximity of the brain to the time-varying magnetic field results in current flow in neural tissue.
The technological advances made in the last 15 years led to the development of magnetic stimulators that produce sufficient current in brain to result in neuronal depolarization.
Neuronal depolarization can also be produced by electrical stimulation, with electrodes placed on the scalp (referred to as transcranial electric stimulation (“TES”)). Importantly, unlike electrical stimulation, where the skull acts as a massive resistor, magnetic fields are not deflected or attenuated by intervening tissue. This means that TMS can be more focal than TES. Furthermore, for electrical stimulation to achieve sufficient current density in brain to result in neuronal depolarization, pain receptors in the scalp must be stimulated (Saypol et al., 1991).
A striking effect of TMS occurs when one places the coil on the scalp over primary motor cortex. A single TMS pulse of sufficient intensity causes involuntary movement. The magnetic field intensity needed to produce motor movement varies considerably across individuals and is known as the motor threshold (Kozel et al., 2000; Pridmore et al., 1998). Placing the coil over different areas of the motor cortex causes contralateral movement in different distal muscles, corresponding to the well-known homunculus. TMS can be used to map the representation of body parts in the motor cortex on an individual basis. Subjectively, this stimulation feels much like a tendon reflex movement. Thus, a TMS pulse produces a powerful but brief magnetic field which passes through the skin, soft tissue, and skull and induces electrical current in neurons, causing depolarization which then has behavioral effects (body movement).
Single TMS over the motor cortex can produce simple movements. Over the primary visual cortex, TMS can produce the perception of flashes of light or phosphenes (Amassian et al., 1995). To date, these are the ‘positive’ behavioral effects of single pulse TMS. Other immediate behavioral effects are generally disruptive. Interference with, and perhaps augmentation of, information processing and behavior is especially likely when TMS pulses are delivered rapidly and repetitively. Repeated rhythmic TMS is called repetitive TMS (rTMS). If the stimulation occurs faster than once per second (1 Hz), it is modified as fast rTMS.
rTMS at frequencies of around 1 Hz has been shown to produce inhibition of the motor cortex. rTMS at higher frequencies of several minutes has been shown to excite underlying cortex for several minutes. Manipulations of frequency and intensity may produce distinct patterns of facilitation (fast rTMS) and inhibition (slow rTMS) of motor responses with distinct time courses. These effects may last beyond the duration of the rTMS trains with enduring effects on spontaneous neuronal firing rates. Determining whether, in fact, lasting increases and decreases in cortical excitability can be produced as a function of rTMS parameters, and whether such effects can be obtained in areas outside of the motor cortex, are of key importance.
TMS is generally safe with no side effects except mild headache in about 5% of subjects. Higher frequency TMS can produce seizures. With the publication of safety tables in 1998, there have been no unintended seizures produced in the world (Wassermann et al., 1996b; Wassermann, 1997; Wassermann et al., 1996a). Animal studies, along with human post-mortem and brain imaging studies (Nahas et al., 2000a), have all failed to find any pathological effects of TMS (Lorberbaum & Wassermann, 2000).
TMS evoked motor responses result from direct excitation of corticospinal neurons at or close to the axon hillock. It is thought that the TMS magnetic field induces an electrical current in the superficial cortex. The TMS magnetic field declines exponentially with distance from the coil. This limits the area of depolarization with current technology to a depth of about 2-cm below the brain's surface. Nerve fibers that are parallel to the TMS coil (perpendicular to the magnetic field) are more likely to depolarize than those perpendicular to the coil. It is thought, as well, that bending nerve fibers are more susceptible to TMS effects than straight fibers (Amassian et al., 1995.
Conventional TMS coils are either round or in the shape of a figure eight (Cohen et al., 1990). The figure eight designs are more focal than the round coils. Most coils are mere copper wire either alone or wrapped around a solid metal core. Because most coils are inefficient, they produce heat as a byproduct. The solid coils are more efficient, without a heating problem. Other manufacturers have used water cooling (Cadwell), or air cooling (Magstim) to deal with this issue. DARPA materials science research might drastically improve the current technology.
The peak effect of TMS can be localized to within less than a millimeter in terms of functional location. More work is needed in terms of actually understanding the exact location of TMS effects (Bohning et al., 2001; Bohning et al., 1997). There is much debate about whether one could devise an array of coils in such a way as to stimulate deep in the brain without overwhelming the superficial cortex.
Since it was first developed (1 (Citation List 1)), TMS has been used to test nerve connections (2-6), nerve excitability (7-9), and nerve conduction times (10) in peripheral nerves (for review, see Ref. 11). One might think of this as testing a circuit with two anatomically separate active areas with a single connection. Paus et al (12) demonstrated that TMS might be combined with neuroimaging to explore the connectivity of more complex three dimensional networks in the brain, allowing the direct assessment of neural connectivity without requiring the subject to engage in any specific behavior.
Recently, TMS interleaved with functional neuroimaging has been successfully implemented by a small but growing number of research groups. In 1997, the first TMS/PET results were reported by Paus et al (12) and Fox et al (13). Also, in 1997, Ilmoniemi et al. (14) reported the first success with TMS/EEG. In 1998, Bohning et al (15) described the first successful interleaving of TMS and fMRI at 1.5 T. Interleaved TMS/fMRI has been shown to be effective in applications such as deception detection and inhibition, as described, e.g., in commonly assigned U.S. Provisional Application No. 60/341,297 filed Dec. 13, 2001 entitled “System and Method of Detecting Deception by fMRI,” U.S. Provisional Patent Application No. 60/396,054 filed 15 Jul. 2002 entitled “Functional Magnetic Resonance Imaging Guided Transcranial Magnetic Stimulation Deception Inhibitors,” and U.S. Provisional Patent Application Ser. No. 60/341,137 filed Dec. 13, 2001 entitled “fMRI-Compatible Skin Conductance Response (SCR) Monitor”, and PCT Application No. PCT/US02/40142 filed Dec. 13, 2002. These applications are incorporated herein in their entirety by this reference.
fMRI has better spatial and temporal resolution than PET, and because it does not use ionizing radiation, it is more suitable for repeated and long-term studies. It is also readily available, with 1.5 T MR scanners installed in medical centers around the world. Hence, TMS/fMRI is potentially the most promising of the three.
Despite all the advances made using TMS/fMRI, there still exists a need for a technique and system for adequately examining brain communication and cortical sensitivity. There also exists a need for examining effects of medication on the brain.
According to exemplary embodiments, methods and systems are provided for using transcranial magnetic stimulation in conjunction with functional magnetic resonance imaging to open up a whole new area of noninvasive in-vivo research into brain cortex excitability and connectivity and an objective means for applying and measuring the efficacy of therapeutic intervention.
According to a first aspect, the TMS paired-pulse technique can be combined with BOLD-fMRI neuroimaging, both for testing cortical sensitivity in areas other than motor cortex, and for using the BOLD response amplitude dependence on TMS ISI to investigate brain communication at high time resolution.
According to another aspect, interleaved TMS/fMRI may be used to examine medication effects (a process we now refer to as interleaved TMS/pharmacological MRI-phMRI).
These and other aspects will become apparent from the following description of various embodiments taken in conjunction with the Appendices, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
In one aspect, the present invention relates to a method of using TMS interleaved with functional brain mapping to test and measure cortical sensitivity and brain communication. In one embodiment, functional brain mapping using fMRI is used in conjunction with specific methods of placing the TMS device over the identified region(s) of the brain, and paired pulse TMS is applied to measure or test sensitivity of the identified region(s).
Embodiments of the present invention, however, are designed to extend beyond these specific technical methods, and cover as well any method of functional brain imaging (including but not limited to PET, SPECT), as well as any method for positioning the TMS device, within or outside of the actual scanner. Moreover, the ability of TMS to produce focal lesions is not specific to any one form of TMS device (figure eight, round, etc), or any one TMS manufacturer.
The capability to perform PP-TMS/fMRI provides a powerful new methodology for noninvasive in-vivo neurophysiology. The present invention provides tools and methods to transform coordinates of target site chosen in MR volume image of subject's brain to settings on TMS coil holder/positioner required to stimulate over that site in a TMS/fMRI study and, conversely, to transform the settings on the TMS coil holder/positioner into the line of peak magnetic field through the MR image volume.
Though fMRI has far better spatial resolution than EEG, its temporal resolution is relatively low, seconds as opposed to milliseconds. The goal of the Paired-Pulse TMS/fMRI work proposed here is an effort to bridge that gap by making use of the observation that TMS applied after a precisely timed delay can be used to modulate responses.
Beckers and Zeki (16) showed that the stimulation of primary visual cortex impairs visual acuity only when delivered at a latency of 60-90 msec following stimulus delivery. In rats, Ogawa et al (17) looked at the fMRI signal as a function of the interval between paired electrical stimulations of the rat forepaws and found a significant suppression of the BOLD signal when the interpulse interval was between 30-40 ms. Chen et al (18) performed a study in human volunteers, in which they plotted the level of BOLD response as a function of the interval between short visual stimuli and found a dip in that response at about 300 ms. It is clear that by using fMRI to observe the time dependence of TMS interference with previous stimulations or cognitive tasks, it is possible to investigate brain communications at time resolutions far greater than that of the hemodynamic response, and approaching that of EEG but with far greater spatial resolution (19, 20).
In general, this technique will make a major contribution to brain research, opening up a whole new area of noninvasive in-vivo research into brain cortex excitability and connectivity and providing an objective means for applying and measuring the efficacy of therapeutic intervention. For example, there is indirect evidence that intracortical inhibition and facilitation (21-24) are caused by separate mechanisms, as opposed to intracortical facilitation being a rebound of the preceding inhibition. However, this evidence has all been acquired through MEPs measured remotely at the target muscle group. With this new PP-TMS/fMRI technique, we will be able to 1) position the coil accurately and repeatable relative to brain anatomy, 2) measure the exact magnetic field distribution of the TMS coil stimulation relative to the brain cortex (25), and 3) observe the local response with millimeter resolution (20). This will provide a significant step forward in the ability to do noninvasive in-vivo neurophysiology.
In some embodiments, real-time blood oxygen level dependent (BOLD) functional MRI (fMRI) analysis offers one approach to functional brain imaging. This approach enables the rapid interpretation of functional imaging results, even while the subject is still in the scanner performing the task. This method is very useful in the pre-surgical mapping of language areas within the brain. In one such embodiment, the subject is next placed in a fMRI scanner such as 1.5 Tesla Philips or Picker Edge 1.5 T scanner and a structural picture of the brain is acquired.
Though the TMS stimulators to be used may not differ from the standard product, the multiplexing unit required to channel the bi-phasic output of these two units through a single TMS coil is an improvement that can be custom built. This improvement will increase both the signal-to-noise and improve the timing control of the interleaving of TMS with fMRI.
A novel holder/positioner greatly increasing the accuracy of coil positioning and allowing the TMS coils position to be referenced to brain anatomy via MR images can be used in some embodiments. In one preferred embodiment, a positioning system is used such as described in copending, commonly assigned U.S. Provisional Application No. 60/381,411 (Bohning et al.), filed May 17, 2002 entitled “A TMS Coil Positioner System” and PCT/US03/15300. These application are hereby incorporated by reference herein for all purposes.
Other embodiments can incorporate other positioning technology. The paired-pulse multiplexer and control circuitry may be integrated with the TMS/fMRI hardware.
There are two main parts to the software development enhancing existing TMS/fMRI software to perform Paired-Pulse TMS/fMRI. The first is a module, which will send the appropriate signals to the multiplexer control circuitry to create a pair of TMS pulses (S1 and S2) with any desired amplitudes (A1 and A2) and interstimulus interval (ISI). The second is the integration of the paired-pulse module into the software used to interleave TMS with fMRI. In general, this is the additional parameterization needed to specify the paired-pulse, and a generalization of the capabilities of the software for handling cyclic and randomized averaged single trial (AST) fMRI experiments. It is recommended that the hardware and software according to exemplary embodiments be tested for timing accuracy and fail-safe prevention of TMS pulse overlap prior to actual use.
Two studies on healthy volunteers will demonstrate the potential of this noninvasive “paired-pulse” TMS/fMRI technique. In the first study, the modulation in BOLD fMRI response associated with thumb movement induced by a series of paired TMS pulses as a function of the interval between the pulses (ISI) will be used to show that the modulation of the BOLD response is sensitive to variations in ISI of the order of milliseconds. In the second study, the relative response versus ISI of “primary” and “secondary” sites activated by TMS applied over prefrontal cortex will be measured as a means of determining the functional dependence of the two sites.
Under the control of an independent computer, the multiplexing unit channels the pulsed output of two bi-polar TMS stimulators through a single coil in switched alternation so as to create a series of paired TMS pulses with a precisely controlled variable interpulse interval (IPI). In addition, the TMS pulse multiplexing circuitry may have a very low inductance to handle the very brief (≈250 μs) and very high currents (10,000 A) used to generate the TMS pulses and to protect each stimulator from the pulses generated by the other, since they will both be firing through a single coil.
The multiplexer circuitry may include blocking sub-circuitry both in the control lines (signal control) from the computer and in the output of the stimulators (pulse control and multiplexing) to eliminate the possibility of simultaneously firing both stimulators and overlapping the TMS pulses. The S1 and S2 lines from the computer will be fed into the signal control circuit. When a pulse comes down either of the control lines (S1 or S2), the other line will be effectively cut for 1 ms to prevent a spurious computer pulse or noise from triggering the other stimulator for 1 ms. The S1 and S2 outputs from this circuit will then be sent to the Trigger Input Ports of the two Magstim units. (Note: The Trigger Input Port accepts TTL compatible signals via its BNC connector; input polarity and whether leading or trailing edge triggered are switch selected.)
Similarly, in the pulse control and multiplexing circuitry, a second protection circuit can, in some embodiments, be combined with the TMS pulse multiplexing circuitry to make it impossible for two TMS pulses to be combined and accidentally raising the stimulation level even if the stimulators should fire without control signals. This downstream blocking control can be initiated by the synchronization pulses available from the TRIGGER OUTPUT port of the Magstim. (Note: These are TTL level pulses which are, typically, used to drive external recording equipment. Polarity and pulse duration, either 50 μs or 50 ms, are switch selectable.)
Previous work (15, 26, 27) has shown that imaging can be performed without significant problems from RF interference if the TMS stimulator is kept outside of MR scanner's RF shielded room, the TMS coil cable is brought into the rear of the MR magnet through a custom-built RF filter box (Lindgren, Inc) grounded to the RF room, and the stimulation/response signal cables are brought into the RF room with the appropriate filters (28). Ancillary control cables from the MR scanner console and experimental control Macintosh are routed through the ceiling over the RF room to the MR scanner electronics room and the appropriate cabinets and TMS stimulator.
The timing control for defining protocols and for interleaving the TMS and fMRI image acquisition has also been improved. A G4 Macintosh is used in one preferred embodiment along with the required Input/Output boards. Timing accuracy has been improved from 11.4±3.4 ms to −0.2±0.3 ms.
PP-TMS/fMRI uses positioning technology for accurately positioning the TMS coil over a selected area of cerebral cortex. The TMS coil mounting system provides flexible coverage of the scalp to stimulate over any desired area of cerebral cortex yet hold the coil firmly in position during the experiment. This also allows repeatedly positioning the subject with respect to the TMS coil holder and of relating the coil's position to the anatomy of the brain. Schematic drawings of systems to accomplish these goals are seen in
Designed with six degrees of freedom, this holder can be used to position the TMS coil over a selected point on the cerebral cortex and then orient the coil so that the plane of the coil is tangent to the skull at that point. In one embodiment, the holder's movements are orthogonal to each other to simplify both the positioning and the computation of the coil's position relative to the isocenter of the MR magnet. Personal computer software allows transformation between coil settings and MR image volumes acquired on the MR scanner while the subject is in position for the PP-TMS/fMRI study.
Coordinates obtained from anatomical locations within the brain on MR images are translated into coil holder settings for accurate and repeatable placement of the TMS coil over those locations. Alternatively, when the coil has been positioned functionally, the settings can be read off and fed into the personal computer software to obtain the coordinates of the coil in the MR scanner's imaging frame of reference.
Though its dimensions and characteristics may be similar to a standard figure-8 TMS coil, e.g., two 70 mm loops and an average inductance of 16.35 pH and maximum field of about 2.2 T, the coil according to exemplary embodiments may be constructed without the normal handle used for handheld applications and have a short stub mounted in the center of the back of the coil for mounting in the holder's radial spar.
The paired-pulse timing control module can be implemented in one embodiment on a Macintosh G4 equipped with a set of input/output (I/O) boards and Labview (National Instruments, Inc.). This module is parameterized and coded in such a way that it can be executed at any desired time in the PP-TMS/fMRI experimental protocol to generate the two stimuli (S1 and S2) with any desired amplitudes (A1 and A2) with any desired interstimulus interval (ISI).
This software can have the same basic structure as that used for the averaged single trial (AST) TMS/fMRI study we did to detect and measure the BOLD signal time course for a single TMS pulse (23), but will be generalized to handle a wider range of fMRI protocols, and the paired-pulse software module can be inserted as an alternative to the single pulse triggering facility.
Numbered citations in the description above correspond to the citations listed in Citation List 1 below.
In the TMS paired-pulse technique, two TMS pulses, separated by a variable interstimulus interval (ISI) are applied to motor cortex while electromyographic (EMG) recordings are made of the motor evoked potentials (MEPs) induced. It is a well characterized physiological tool for testing intracortical inhibition and facilitation, in health and disease, as well as the influence of CNS-active drugs. We have combined the TMS paired-pulse technique with BOLD-fMRI neuroimaging both for testing cortical sensitivity in areas other than motor cortex, and for using the BOLD response amplitude dependence on TMS ISI to investigate brain communication at high time resolution.
For our study, after obtaining informed consent, interleaved paired-pulse TMS/fMRI (1) was performed (to-date) on four healthy volunteers in a whole body 1.5 T MR system (Philips Intera, Rel.8.1.1, Philips Medical Systems, Best, The Netherlands) using a 20 cm diameter circular phased array coil pair and a single shot gradient-echo EPI pulse sequence (TR=1500 ms, TE=40 ms □=80°, matrix 64×64, FOV 256 mm, 11 slices, slice thickness 4 mm, gap 1 mm). A Macintosh G3 laptop with NI DAQCard-AI-16E-4 general purpose I/O board and custom Labview software controlled the firing of two Magstim 220 Stimulators through a BiStim Multiplexer synchronously interleaved with the fMRI acquisition. Using Mathematica, a list of paired-pulse events with ISI of 50, 100, 150, 200, 250, 300, and 1000 ms, pseudo-randomly ordered and spaced, was generated so that the TMS pulses would minimally affect the MR pulse sequence RF pulses. The same event list was later used both to remove TMS compromised images and as the paradigm event list for data analysis with SPM to find areas of BOLD activation.
One data set was discarded due to excess movement. Analysis of the other two data sets revealed clusters of pixels with locally high t-values in motor and auditory cortex. Time curves of BOLD response were extracted from the clusters, cycle-averaged and, finally, averaged across the two sets of data. This is shown in
In
In
The data analyzed to date demonstrate the feasibility of combining paired-pulse TMS (2) with fMRI. They also demonstrate that the modulation of the BOLD response amplitude as a function of the ISI between pairs of TMS pulses may be used to test intracortical inhibition and facilitation over the entire brain cortex in health and disease (3), as well as to investigate brain communication at time resolutions an order of magnitude greater than that of the hemodynamic response itself (4, 5). Additional subjects are being recruited for study; their data will be presented as well.
This work was funded in part by a South Carolina Research Initiative Grant.
Citations in the preceding section correspond to those listed in Citation List 2 below.
According to another aspect of the invention, TMS and functional brain mapping may be used to determine efficacy of medications, such as central nervous system (hereinafter “CNS”) active compounds. In one embodiment, functional brain mapping, such as fMRI or BOLD fMRI, is used in conjunction with specific methods of placing the TMS device over the identified regions of the brain. Embodiments of the present invention, however, are designed to extend beyond these specific technical methods, and cover as well any method of functional brain imaging (including but not limited to PET, SPECT, qEEG, MEG), as well as any method for positioning the TMS device, within or outside of the actual scanner. Moreover, the ability of TMS to produce focal lesions is not specific to any one form of TMS device (figure eight, round, etc), or any one TMS manufacturer.
In one embodiment, fMRI is used to determine the brain region or regions that shows activation and/or inhibition while the person is using the CNS-active compound of interest or a particular dosage of such a compound. Once this area is identified using fMRI (or other brain imaging methods), TMS is applied over this region to determine the level of excitation or inhibition relative to excitation or inhibition levels of these areas when the subject is not using the CNS-active compound or is using a differing dosage of such a compound. In some embodiments, measurement of excitation and/or inhibition use paired-pulse TMS as described above.
According to some embodiments, functional brain imaging is applied to a subject to determine brain regions that experience activation and/or inhibition during periods when the subject has taken a CNS-active compound, or a particular dosage thereof.
In some embodiments, the functional brain imaging occurs during both a calibration phase and an analysis phase. In such embodiments, real-time functional brain imaging data is initially gathered during the calibration phase and used during an analysis phase; further real-time data accumulated during the analysis phase can in certain embodiments then be used as feedback to further tune the calibration phase data and enhance the ability to measure efficacy. In yet further embodiments, no calibration phase is required; rather, real-time functional brain imaging data is accumulated during analysis. This imaging data is refined during analysis so that the efficacy measurement improves over the course of analysis. Any suitable functional brain imaging technique can be used including without limitation, including fMRI, PET, SPECT, qEEG and MEG.
In some embodiments, real-time blood oxygen level dependent (BOLD) functional MRI (fMRI) analysis offers one approach to functional brain imaging. This approach enables the rapid interpretation of functional imaging results, even while the subject is still in the scanner performing the task. This method is very useful in the pre-surgical mapping of language areas within the brain. In its current implementations, fMRI appears sensitive enough to detect brain regions impacted by CNS-active compounds and varying dosages thereof.
In one such embodiment, the subject is placed in an fMRI scanner, such as 1.5 Tesla Philips or Picker Edge 1.5 T scanner, and a structural picture of the brain is acquired as depicted in
The structural images acquired are transferred to a translational system that allows targeting specific regions in the brain based on MRI or other functional brain scans. In one preferred embodiment, the translational system can be referred to as Brainsight (Rogue Research Inc.). Brainsight is an image analysis and frameless stereotaxy software system that enables the use of landmarks on the face and head (that are also identifiable on the MRI) to localize very specific areas of the brain. Other translational systems can be used within the scope of the present invention.
Using the fMRI analysis, the brain regions that show significant activation during deception are identified on the structural brain images. Using Brainsight, the location on the scalp over these brain regions are identified and marked. The distance from skull to cortex over the motor and prefrontal cortex is measured using Brainsight; a particular embodiment of this apparatus is depicted in
The TMS coil is positioned directly over the brain region identified as being activated during deception. Various coil positioning technology can be used. In one preferred embodiment, a positioning system is used such as described in the copending applications mentioned above in the previous section.
Lamotrigine (LTG) is a use-dependent sodium channel inhibitor with broad-spectrum anti-convulsant efficacy against a range of epilepsy syndromes1 (superscript notations throughout this section refer to Citation List 3 below). Recently, several double-blind, placebo-controlled trials have demonstrated the acute and prophylactic antidepressant activity of LTG in bipolar disorder2-4. Anticonvulsant mood stabilizers may work through the same mechanisms needed for seizure control, but in different brain regions. Thus, some have suggested that LTG stabilizes mood by reducing cortical excitability in areas relevant to the pathogenesis of mood disorder5.
As described above, transcranial magnetic stimulation (TMS) is a non-invasive means to stimulate the cerebral cortex, as well as to assess motor cortex excitability6, 7. TMS has been used to examine the pharmacologic effects of anticonvulsant drugs on the excitability of motor corticospinal pathways in both patients with epilepsy and normal subjects7,8. In volunteers or patients with complex partial seizures, LTG significantly increased the resting motor threshold (RMT)6,8,9. Thus, TMS combined with Motor Evoked Potential (MEP)'s can provide useful information about medication effects, but the information is limited to drug effects on motor circuits. TMS over all non-motor brain areas does not produce an easily observable behavioral response, so TMS alone cannot provide information about medication effects in these other important brain regions.
Combining TMS with non-invasive imaging techniques allows one to observe TMS effects throughout the brain. Initial studies used fluorodeoxyglucose (FDG)10,11 or oxygen (O15)12,13 positron emission tomography (PET). Our group at the Medical University of South Carolina (MUSC) pioneered and developed a technique for interleaving TMS with blood oxygen level dependent (BOLD)-functional magnetic resonance imaging (fMRI)14,15. TMS-induced brain activation does not depend on subject attention, skill or effort, which can influence the amount and location of brain activation in other activation tasks16. Thus interleaved TMS/fMRI is a non-invasive method to stimulate the cortex and connected brain regions reliably and repeatedly17.
To our knowledge, no one has yet used interleaved TMS/fMRI to examine medication effects (a process we now refer to as interleaved TMS/pharmacological MRI-phMRI).
In the present study we used interleaved TMS/phMRI to image brain activity during TMS over motor cortex and prefrontal cortex in healthy subjects after receiving a single oral dose of placebo or LTG. We sought to compare RMT and the BOLD TMS-induced pattern of brain activation after LTG or placebo. We hypothesized that, compared to placebo, a single oral dose of LTG would inhibit brain excitability. This LTG blunting would be seen in increased RMT and reduced TMS-induced BOLD activation over motor cortex. We further speculated that LTG would blunt TMS-induced brain activation during TMS over the prefrontal cortex as well as in associated limbic regions. This proof-of concept study sought to test specifically whether interleaved TMS/phMRI might prove a useful tool in understanding LTG's mood-stabilizing mechanisms of action. We also sought to understand whether the interleaved technique might be used to investigate, in general, pharmacological compounds.
Based on our study, interleaved transcranial magnetic stimulation/pharmacological MRI suggests that Lamotrigine inhibits cortical and enhances limbic excitability in healthy young men.
All subjects included in this study were given a detailed explanation of the procedure and signed a written informed consent form approved by the MUSC Investigational Research Board (IRB) and the Food and Drug Administration (FDA). Fourteen healthy young men (aged 18-30) were recruited by local advertisement and then had a screening history and physical examination, structured diagnostic interview18, baseline laboratory work (basic metabolic panel, liver panel, and hematology), and urine drug screen for drugs of abuse. All subjects were right-handed (as determined by the Annett Handedness Questionnaire19) and were non-smokers.
Study design: We performed a randomized, double-blind, crossover trial involving two visits at least one week apart (
General Procedure: After arriving at the laboratory in the early afternoon, baseline RMT was determined and baseline plasma levels of LTG were drawn. They were then given a single oral dose of 325 mg LTG or placebo. They then waited quietly for 3 hours. Three hours after taking the oral pill, RMT was determined and serum plasma levels were again drawn.
TMS: Focal TMS was delivered by a MAGSTIM Super Rapid stimulator (Magstim Co, Whitland, Dyfed, U.K) and applied through a focal figure-of-eight magnetic coil (each wing 70 mm in diameter). The optimal position of the magnetic coil for eliciting a MEP in the right abductor pollicis brevis (APB) was determined by holding the coil tangential to the scalp, and moving it in small steps over the presumed area of the left primary motor cortex at a slightly suprathreshold stimulus intensity. The coil was always held horizontally with the handle pointing backward and laterally at 45 degrees from the midline. This position was marked with a pen on a reusable latex swimming cap in order to assure constant placement of the coil throughout the visits. Stimulus intensity and threshold values were expressed as percent of the maximal stimulator output.
Resting Motor Threshold (RMT): Surface electromyographic (EMG) was recorded from the APB using 9-mm Ag—AgCl electrodes in a belly-tendon montage. The placement of electrodes on the thumb and hand was marked with a pen for exact re-placement in consecutive visits on the same day. The raw EMG signal was amplified by a factor of 100 gain and band-pass filtered, 2.0 kHz (low) to 70 kHz (high) with a High Performance Band pass Filter Model V-7548 (LAB Linc. Co). The EMG was recorded on a G3 Macintosh with MacCRO (version 2.1).
RMT was determined in the resting APB in 4 steps: In step one and step three, thresholds were approached from a slightly suprathreshold intensity by reducing the stimulus intensity in 1% steps with a 5 sec interval between pulses, whereas in steps two and four, thresholds were approached from a slightly subthreshold intensity by increasing the stimulus intensity. RMT was defined as the first intensity that produced a MEP of greater than 50 μV in 3 out of 6 trials in the resting target muscle. A mean RMT for baseline or after medication was calculated by averaging the four values. Determination of the RMT using this technique usually lasted 30 minutes.
Combined TMS and MRI: Immediately following RMT determination, interleaved TMS/phMRI acquisitions were performed in a Picker EDGE 1.5 T MR scanner with actively shielded magnet and high-performance gradients (27 mT/m, 72 T/m/sec) using a typical gradient echo, echo planar imaging (EPI) fMRI sequence (tip angle=90, TR=3 sec, FOV 27.0 cm, fifteen 7 mm thick slices, 1 mm gap). TMS was delivered using a Dantec MagPro with a special nonferromagnetic TMS coil of figure-8 design with an 8-meter cable (Dantec Medical A/S, Skovlunde, Denmark) and a room set up identical to prior TMS/fMRI studies from our group. TMS pulses and the fMRI sequence were interleaved as described before21. Each cycle, illustrated in
TMS Coil Placement in the MRI scanner: Motor cortex: Before being placed into the MRI scanner, subjects had their resting motor threshold (RMT) quickly determined with the Dantec TMS while sitting on the MRI gantry. For many reasons (different capacitors, coil design, length of cable, MRI filter), the RMT determined with the Magstim in the BSL was not the same RMT needed inside the MRI scanner with the Dantec. After this new MRI RMT was determined, the TMS coil was rigidly mounted in the MR head coil with a specially designed TMS coil-holder, adjustable in six dimensions22. Subjects wore swim caps and special earplugs. With the head coil on the gantry outside the scanner bore, subjects inserted their head into the head coil and adjusted their position while the TMS coil was intermittently pulsed with 100% RMT. Subjects adjusted their head until pulsing the coil caused visible movement of the contralateral (right) hand APB (3 out of 6). As soon as a subject's new MRI RMT-correct scalp location was determined, the holder's six dimensions and earplugs were locked. These head holder settings and RMT were recorded and used with the second visit. During the second MRI visit a week later, the headholder was set with the previous week's coordinates for that subject, and the previous RMT was used for the second visit.
On each visit, immediately after the Motor Cortex MRI study, subjects were removed from the scanner and the TMS device was moved to position it over the left prefrontal cortex. The left prefrontal cortex stimulation site was defined as a location 5-cm rostral and in a parasagittal plane from the site of maximal APB stimulation. Subjects then reentered the scanner for the prefrontal TMS scan, which was identical to the Motor study described above except for the TMS coil location.
Individual fMRI Data Analyses: MR scans were transferred into ANALYZE format and then further processed on Sun workstations (Sun Microsystems, Palo Alto, Calif.). Scans were checked using MEDx3.3 (Sensor Systems Inc, Sterling, Va.) for movement across runs, and then were coregistered to a mean image using automatic image registration. For all subjects, movement across the 14.7-minute study was less than 2 mm in all 3 axes. After correction of motion, we used a delayed boxcar model, employed a high-pass filter to remove signal drift, cardiac and respiratory effects, and other low frequency artifacts. Then, we spatially transformed each subject's data into the Talairach Atlas (input voxel dimensions, 2.1×2.1×8 mm, to output voxel dimensions, 4×4×4 mm), smoothed (4×2 mm) the data and generated z map with the Statistical Parametric Mapping (SPM) 96 module in MEDx3.3. We assumed an uncorrected F threshold UF P>0.99 to preserve as many voxels as possible for the cluster analysis. Only clusters showing a statistical weight of P<0.05 were considered to be significantly activated.
Group fMRI Data Analyses: All subject's unthresholded z maps were combined based on comparison of condition (TMS vs Rest), intensity (100% RMT-TMS vs 120% RMT-TMS), visit (LTG vs Placebo). The combined group z maps were thresholded using z≧3.09 (p≦0.001) and cluster statistical weight (spatial extent threshold) of p<0.05. We used either paired or unpaired t-tests in MEDx3.3 for all comparisons of interest and both areas of stimulation.
Magnitude of BOLD time course response: To compare the magnitudes of BOLD signal changes, two types of data were recorded. The different maps of LTG and placebo were used to make a mask of left motor cortex (82 voxels) and a mask of left hippocampus (19 voxels) (
The percent change of RMT=100[(post-dose RMT−pre-dose RMT)/pre-dose RMT]. Paired Student's t tests (two tailed) were performed for the percent change of RMT between LTG and placebo. Wilcoxon nonparametric tests were performed for the number of active voxels in the region of interests (ROI) between LTG and placebo. We performed Pearson correlations between the percent change of RMT and the change of active voxel number. Two-way analysis of variance (ANOVA) was performed for % BOLD signal change in the different intensity stimulation and the different medication conditions. All statistical analyses were performed using SPSS 10.0 (Statistical Product and Service Solutions Inc, Chicago Ill.).
Fourteen subjects were enrolled and were studied. Technical problems with the fMRI scanner or TMS machines meant that not all subjects provided complete data sets. One subject had a baseline RMT greater than the Magstim machine output. After we determined this, the subject was not studied further. Of the 13 subjects studied on two days, 12 subjects (age 25.31±2.70 years) had usable paired TMS RMT data, and of these, two subjects completed the protocol, but their MRI data on at least one of the visits was not usable because of MRI scanner problems. Thus, 10 subjects had complete placebo and LTG interleaved TMS/phMRI data as well as complete RMT data.
None of the subjects reported experiencing adverse effects of the drug treatment or the stimulation.
Consistent with our pre-study hypothesis, LTG inhibited the motor cortex and elevated mean motor RMT significantly by 14.9% (SD 9.6) from the same day baseline compared with a placebo increase of 0.6% (SD 10.9) from the same day baseline (Paired Student's t-test, t=3.41, df=11, p<0.01) (see Table 1 and
Correlation analyses were performed on the RMT data between visits to assess for the repeatability of the RMT, and the natural variation. The baseline RMT on visit one correlated with the baseline RMT on visit 2, indicating good reliability of the RMT within subjects across visits one week apart (r=0.84, n=12, p<0.01). On both visits, the pre-dose RMT correlated well with the post-dose RMT. On the LTG day, the correlation was shifted, with higher RMT following LTG (placebo visit: r=0.89, n=12, p<0.01; LTG visit: r=0.86, n=12, p<0.01). However, we failed to find a correlation between the serum levels of LTG and post-dose RMT (r=0.34, n=12 p=0.33).
Motor cortex TMS after either placebo or LTG (within day analysis) at both 100% RMT and 120% RMT resulted in diffuse activation in the brain (see Table 2). 120% RMT stimulation caused more activation than did 100% RMT stimulation in the motor cortex underneath the coil on the placebo day (see Table 2 and
A formal between-day analysis revealed that, compared to placebo, on the day subjects were taking LTG, they had significantly less TMS-induced activation in the motor cortex (underneath the coil) and other regions. (see
The number of active voxels (120% RMT stimulation minus rest over motor cortex) for placebo and LTG in 10 subjects is shown in
As a further check on the whole brain imaging analysis described above, we examined the timecourse of activation of voxel clusters in the motor cortex directly underneath the coil. For this region, the cycle-averaged time-activity curve was plotted and an estimate obtained of the level of activity in the 120% RMT TMS sub-cycle relative to the preceding rest sub-cycle.
Eight subjects provided usable data from the prefrontal interleaved TMS/phMRI visits. Two subjects whose results were included in the motor cortex analysis were not able to be used in the prefrontal analysis because their prefrontal TMS scans showed more than 2 mm of movement.
Prefrontal cortex stimulation compared to rest, after either placebo or LTG at both 100% RMT and 120% stimulation, induced activation in diffuse brain regions. On either day, unlike with the motor cortex stimulation, there were no statistically significant differences in the pattern of activation between 100% RMT and 120% RMT. Of particular note, brain activity was not significantly increased from rest at the site of stimulation immediately underneath the coil with either 100% RMT or 120% RMT stimulation. (see Table 4 and
A formal between-day analysis showed that, with respect to the rest condition, there was increased brain activity in the hippocampus and the orbital frontal gyrus during 100% RMT stimulation when in the presence of LTG compared to placebo. (
The number of active voxels (100% RMT stimulation minus rest over prefrontal cortex) after placebo or LTG in 8 subjects are shown in
We examined the timecourse of activation of the cluster of voxels in the left hippocampus.
To our knowledge, this is the first report to use the interleaved TMS/fMRI technique to investigate the regional brain effects of a central nervous system (CNS)-active compound (referred to as interleaved TMS/phMRI). We found, consistent with our hypothesis, that LTG inhibited the motor cortex when we applied TMS over the motor area for thumb movement. This LTG inhibition was evident both in the electrophysiological measurements, and the regional brain activity. Over the motor cortex, the brain imaging and electrophysiological domains as well were highly correlated. Surprisingly, we found that LTG had a different effect when we applied TMS over the prefrontal cortex. Not only did LTG not inhibit the BOLD response, it actually increased activity in the limbic system.
The results demonstrate that it is possible to combine TMS and phMRI to evaluate both decreasing and increasing regional brain effects of CNS compounds. We thus conclude that interleaved TMS/phMRI is feasible as a new neuroscience tool, and may have several important uses.
Analysis of the group fMRI data of TMS over motor cortex on the placebo day revealed robust EMS-induced activation of the ipsilateral motor cortex14,23 as well as bilateral activation of the auditory cortex. Interestedly, the present data also showed that TMS caused activation of the contralateral (right) motor cortex as well. Although the control of movement is one of the clearest hemispherically-lateralized functions in the brain24, human functional neuroimaging studies of hand motor control commonly report bilateral activation in primary motor cortex25,26. We also compared BOLD-fMRI responses at two different stimulation intensities, and found that high intensity motor cortex stimulation (120% RMT) was associated with significantly increased activation compared to lower intensity (100% RMT) stimulation14, on the placebo day only. These results on the placebo medication day replicate our previous studies of motor cortex TMS/fMRI, all of which have shown dose-dependent TMS effects15,27 Finally, we also analysed the timecourse of activation in motor cortex and found a 1% BOLD activation relative to baseline could be observed at 120% RMT stimulation.
Several prior TMS studies have shown that LTG increases the threshold of MEPs elicited by TMS6,8,28. In the present study, we confirmed the inhibitory effect of LTG on MEPs. LTG caused a 14.9% increase in RMT in healthy young adults, which agrees with previous TMS studies with the compound6,8,29. A region of interest analysis of the fMRI data showed that LTG reduced activation in the motor cortex, directly under the coil, and in other diffuse areas of the cortex. As one might predict, the increase in MEP threshold correlated with the decrease in BOLD-fMRI measures in the presence of LTG.
In addition to TMS over the motor cortex, we then applied the interleaved TMS/phMRI technique over the prefrontal cortex, using a probabilistic positioning method. In this case we were limited to examining the fMRI measurements alone, since there is no overt behavioral response, like an MEP, to prefrontal cortical stimulation. We have shown previously that unilateral TMS applied over the prefrontal cortex (left) has bilateral effects, and that higher intensity stimulation produces greater ipsi- and contralateral activation30. In addition, other PET and SPECT studies have shown that increases and decreases in blood flow or metabolism occur during and shortly after repetitive TMS (rTMS) applied over the prefrontal cortex11;31;32. The present study confirmed the bilateral cortical effects of TMS when applied to the prefrontal cortex. However, on the placebo day, we found no significant difference between the activation at 100% RMT and 120% RMT with prefrontal TMS. Although we failed to find prefrontal cortex induced activation underneath the prefrontal coil, the results showed significant bilateral activation of the limbic system only when the subjects were taking LTG. These paradoxical prefrontal results, where LTG is not inhibiting but rather increasing limbic activation, softly suggest that LTG may have a unique relationship with limbic activation, that differs from its effects in the motor circuit. This may be due to differential regional effects of LTG, or due to some interaction of cortical-limbic loops and relative governance. Although these are intriguing results, they are highly speculative given the non-hypothesized nature of these findings and the small sample size. An additional study attempting replication is recommended.
LTG's anticonvulsant activity has generally been attributed to its ability to stabilize the inactive form of brain sodium channels33,34, though this alone may not account for its broad efficacy35. Indeed, LTG has also been shown to have activity at other ion channels36-39. Although the molecular target or targets through which LTG exerts its therapeutic effect may not be known precisely, evidence suggests that reduction in glutamate release and enhancement of GABA release may be important downstream effects40-42. Of particular interest is a recent study from Calabresi et al (1999)43, which found that LTG reduced cortico-striatal excitatory transmission in the rat via a pre-synaptic mechanism that may be independent of sodium channel blockade.
A key finding of the present study was that BOLD responses induced by TMS in the motor cortex could be inhibited by LTG (a BOLD signal decreased of 50% relative to baseline). Furthermore, the effect of LTG was stronger on TMS at 120% RMT than at 100% TMS and the timecourse analysis (
What does this Tell Us about LTG's Mechanism of Action in Bipolar Disorder?
Double-blind, placebo-controlled trials have demonstrated the acute and prophylactic antidepressant activity of LTG in bipolar disorder5,45,46. Various hypotheses have been proposed regarding its mechanism of action on mood. One may speculate that the efficacy of LTG in bipolar disorder is related to its anticonvulsant efficacy, and so also to its anticonvulsant mechanisms of action. However, the clinical profile of LTG in bipolar disorder is different from that of either valproate or carbamazepine, and in fact its spectrum of anticonvulsant efficacy is also somewhat different, notably its efficacy versus absence seizures47.
The present study in healthy volunteers may not be relevant to drug effects in patients with bipolar disorder, but the surprising limbic activation obtained in the presence of LTG when TMS was applied to the prefrontal cortex is worth considering with the clinical situation in mind. Studies of the neuropathology in familial Major Depressive Disorder have reported changes in morphology and metabolism in selected areas of the limbic system, such as the hippocampus48-50 so orbital frontal lobe51,52 and amygdala53. Frodl54 reported smaller hippocampal gray matter volumes in patients with a first episode of major depression compared with healthy subjects. Furthermore, recent data suggests55 that bipolar disorder is associated with a significant decrease of glutamic acid decarboxylase (GAD) mRNA-positive neurons and of GAD65 mRNA expression in the hippocampus. Regarding pharmacotherapy, several studies have reported increased regional activation (the left prefrontal cortex, thalamus, and medial frontal gyrus56-60) post-treatment in depressed patients. These findings provide soft evidence of limbic system abnormality in bipolar disorder. The present study showed that LTG could induce increased activity in hippocampus in normal subjects compared with placebo. This leads to speculation that the antidepressant effect of LTG could be mediated by increasing activity in hippocampus or other limbic structures.
Like all studies, this initial proof-of-concept study suffers from limitations that bear on the interpretation of the results. The prefrontal cortex data failed to show our earlier finding of activation underneath the coil30. Although the present data over motor cortex showed dose-dependent TMS effects, we failed to find the same dose-dependent TMS effects over prefrontal cortex (
In conclusion, this current study suggests that the interleaved TMS/phMRI technique has utility in understanding the regional brain effects of LTG, and likely other CNS-active compounds. Using the technique, we found as hypothesized that LTG has an inhibitory effect on motor cortical neuronal excitability measured both by RMT and interleaved TMS/phMRI. On the other hand, LTG may have a complex effect on prefrontal TMS, with cortical inhibition and limbic facilitation. It is unclear if these effects may be relevant to the efficacy of LTG in mood disorders. Further studies are warranted with this promising new technique.
Some embodiments can include a precursor step to functional brain imaging and/or application of TMS that involves evaluating the subject for potential risk. If potential risk is greater than a predetermined level with respect to a particular functional brain imaging technique, or particular parameter set associated therewith, and/or TMS configuration, or particular parameter set associated therewith, an alternative technique, configuration and/or parameter set can be used. Such an alternative technique, configuration and/or parameter set can, in certain embodiments, be subject to its own potential risk evaluation with respect to the subject.
Citations in the preceding section correspond to those listed in Citation List 3 below.
Throughout this application, including the citation lists, various publications have been referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
While various embodiments of the invention are described above and illustrated in the drawings, it is to be understood that certain changes can be made in the form and arrangement of the elements of each system and steps of each method according to the present invention as would be known to one skilled in the art without departing from the underlying scope of the invention as is particularly described above. Furthermore, the embodiments described above are only intended to illustrate the principles of the present invention and are not intended to limit the invention to the disclosed elements.
This application claims priority to U.S. Provisional Applications No. 60/377,692 and No. 60/431,820, herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60431820 | Dec 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10535775 | Mar 2006 | US |
| Child | 12035997 | US |
