Method and apparatus for monitoring a magnetic resonance image during transcranial magnetic stimulation

A method and apparatus for monitoring a magnetic resonance image of a patient during administration of transcranial magnetic stimulation. The method includes the steps of applying transcranial magnetic stimulation (TMS) to a patient using a probe that is substantially constructed of non-ferromagnetic material, monitoring a magnetic resonance image (MRI) of the patient during TMS wherein application of the TMS does not need to by synchronized to monitoring of the MRI.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to medical diagnostic and treatment methods
 and apparatus.
 2. Discussion of the Related Art
 Transcranial magnetic stimulation (TMS) is a technique for stimulating the
 human brain non-invasively. TMS uses the principle of inductance to get
 electrical energy across the scalp and skull without the pain of direct
 percutaneous electrical stimulation. It involves placing a coil of wire on
 the scalp and passing a powerful and rapidly changing current through it.
 This produces a magnetic field which passes unimpeded and relatively
 painlessly through the tissues of the head. The peak strength of the
 magnetic field is related to the magnitude of the current and the number
 of turns of wire in the coil. This magnetic field, in turn, induces a much
 weaker electrical current in the brain. The strength of the induced
 current is a function of the rate of change of the magnetic field, which
 is determined by the rate of change of the current in the coil. In order
 to induce enough current to depolarize neurons in the brain, the current
 passed through the stimulating coil must start and stop or reverse its
 direction within a few hundred microseconds.
 TMS is currently used in several different forms. In a first form, called
 single-pulse TMS, a single pulse of magnetic energy is delivered from the
 coil to the patient. Repetitive TMS or rTMS, refers to the delivery of a
 train of pulses delivered over a particular time period. An example of
 rTMS could be a train of pulses having a 10 Hz repetition rate that lasts
 for approximately 8 to 10 seconds. In a typical application, this train of
 pulses is repeated every 30 seconds for up to 20 or 30 minutes.
 Magnetic resonance imaging (MRI) is a technique for non-invasive imaging
 and diagnosis of body organs that uses the interaction between a magnetic
 field and protons in the body to provide images of body tissues.
 Functional MRI or fMRI is a subset of this technology and produces images
 of activated brain regions by detecting the indirect effects of neural
 activity on local blood volume, flow, and oxygen saturation. MRI systems
 have been commercially available for a number of years.
 The inventors have realized that it would be desirable to combine TMS and
 MRI technologies in order to provide diagnostic and therapeutic benefits.
 Conventionally, however, these two technologies have not been combined for
 a variety of reasons. First, it has been thought that there may be
 interactions between the TMS equipment and the MRI equipment due to the
 fact that both types of equipment generate and use magnetic fields.
 Therefore, the instantaneous magnetic field associated with the discharge
 of the TMS coil, which may be on the order of more than two TESLA might
 interact with the 1.5 TESLA static magnetic field of the MRI system in
 some unpredictable manner. Second, the discharge of the TMS coil near the
 sensitive imaging coil of the MRI system might disable or destroy the
 receiving circuitry within the imaging coil. Third, the mere presence of
 the TMS coil near the patient's head might contribute artifacts into any
 images provided by the MRI system. Fourth, the TMS electronics alone might
 produce artifacts on the images produced by the MRI system.
 SUMMARY OF THE INVENTION
 In broad terms, one aspect of the present invention provides a method and
 apparatus for monitoring a patient's MRI during TMS that does not require
 a time synchronization of the operation of the TMS device and the MRI
 system.
 This aspect of the invention is provided by a method and apparatus for
 monitoring a magnetic resonance image of a patient during administration
 of transcranial magnetic stimulation, including a transcranial magnetic
 stimulation (TMS) device and a magnetic resonance imaging (MRI) system.
 The system also includes a probe, coupled to the TMS device, the probe
 being constructed and arranged to deliver transcranial magnetic
 stimulation, wherein the probe is substantially constructed of
 non-ferromagnetic material, wherein timing of operation of a TMS device
 does not need to be synchronized to timing of operation of the MRI system.
 In accordance with another aspect of the invention, a probe is provided for
 delivering the magnetic pulse provided by a transcranial stimulation (TMS)
 device to a patient wherein the probe is substantially constructed of
 non-ferromagnetic material.
 Within this disclosure the term transcranial magnetic stimulation (TMS) is
 meant to include both single-pulse TMS and repetitive TMS. With this
 disclosure, the term magnetic resonance imaging (MRI) is meant to include
 all types of MRI and functional MRI.

DETAILED DESCRIPTION
 FIG. 1 illustrates a typical TMS probe 10. The probe includes a housing 12,
 typically constructed of molded plastic, having a handle region 14.
 Conductor 16 is coupled from a TMS device to handle region 14. Typically,
 probe 10 is held within close proximity to the patient's head in a region
 where magnetic stimulation is desired. The operator typically holds probe
 10 by using handle region 14.
 Within probe 10 are two coils 18, 20 typically constructed of copper. When
 a high current short time duration signal is provided to coils 18, 20 via
 conductor 16, a large magnetic pulse is generated. Coils 18 and 20 are
 constructed and arranged so that the magnetic pulse provided by each coil
 constructively combines to deliver a magnetic pulse wave.
 In monitoring an MRI during TMS, the inventors discovered that using a
 probe such as probe 10 presented two main problems. First, the shape of
 probe 10 with handle region 14 protruding from the region of the junction
 between coils 18 and 20 makes the probe bulky and difficult to insert into
 the imaging coil of an MRI system. In addition, the inventors discovered
 that when the probe was located in the imaging coil of an MRI system,
 significant distortion of the MRI image as well as voids in the MRI image
 occurred, due to the significant amount of ferromagnetic material
 incorporated into probe 10. This ferromagnetic material came primarily
 from various epoxies used to hold the probe together and to hold coils 18,
 20 in position, as well as various metal interconnects used to connect
 coils 18 and 20 to conductor 16.
 To overcome the deficiencies of probe 10, the probe of FIG. 2 was
 developed. As illustrated in FIG. 2, probe 30 includes a housing 32 that
 may be constructed of plastic or a suitable material similar to housing
 12. However, unlike probe 10, probe 30 has a handle region 34 that is
 disposed away from the intersection of coils 18, 20. In one embodiment,
 handle region 34 is disposed in line with axis 36 so that handle region
 34, coil 18, and coil 20 are substantially in line.
 The purpose of locating coil 18, coil 20, and handle region 34
 substantially in line is that this configuration is easier to place inside
 the imaging coil of the MRI system. In the MRI system, the imaging coil
 must be located in close proximity to the patient's head and there is a
 relatively limited amount of space available between the patient's head
 and the imaging coil. The configuration of FIG. 2 allows the probe to fit
 within the imaging coil and allows conductor 16 to exit and be connected
 to the TMS device in a manner that is comfortable for the patient.
 A second feature of probe 30 is that substantially all ferromagnetic
 material including interconnects and epoxies has been removed. Any
 interconnects necessary between coils 18, 20 and conductor 16 are provided
 by non-ferromagnetic material such as copper. Probe 30 is thus constructed
 of substantially non-ferromagnetic materials. The use of non-ferromagnetic
 materials significantly reduces the magnitude of any distortion in the MRI
 image introduced by the presence of probe 30 within the imaging coil of
 the MRI system. Within this disclosure, the term non-ferromagnetic is
 meant to refer to materials having very low magnetic permeablities and
 relatively low residual magnetism and hystersis. One example of
 non-ferromagnetic materials is copper.
 Reference is now made to FIG. 3, which figure illustrates how a TMS device
 can be used in conjunction with an MRI system. In FIG. 3, an MRI system 40
 and a TMS device 42 are provided. MRI system 40 may be a Siemens Vision
 System magnetic resonance imaging system and typically includes a
 cylindrical magnet 44 having a bore 46 in which a patient 50 is placed in
 order to undergo magnetic resonance imaging. The MRI system includes a
 sliding platform 48 that the patient 50 lies on. An imaging coil 52
 typically including a number of pickup coils is provided. Imaging coil 52
 detects the magnetic field generated as a result of exposure of patient 50
 to the large magnetic field provided by magnet 44.
 The signal detected by imaging coil 52 is carried by cable 54 to processing
 electronics 56. Processing electronics 56 process the signals provided by
 imaging coil 52 to provide a magnetic resonance image of the brain of
 patient 50.
 Reference is now made to FIG. 4, which figure is a cross sectional view
 along lines 4--4 of FIG. 3. In FIG. 4, the relationship among the various
 components is illustrated. As can be seen in FIG. 4, imaging coil 52
 surrounds the head of patient 50 and probe 30 is disposed between imaging
 coil 52 and the head of patient 50. The inventors have noted from
 experiments that when probe 30 generates a magnetic pulse, there is some
 interaction between the static magnetic field provided by magnet 44 and
 the magnetic pulse provided by probe 30. This interaction may in fact
 cause probe 30 to actually move or twist. To avoid any injury or
 discomfort to the patient as a result of this twisting or torquing motion,
 a pad 60 is disposed between the head of patient 50 and probe 30. In order
 to maximize the effect of the magnetic pulse provided by probe 30, pad 60
 should be as thin as possible while still providing appropriate protection
 and comfort for patient 50. The inventors have determined that pads having
 a material thickness in the range of 1/4" to 1/3" can make the patient
 comfortable without significantly diminishing the strength of the magnetic
 pulse delivered by probe 30.
 In their experiments, the inventors found that there were no adverse
 effects on processing electronics 56 as a result of the magnetic pulse
 generated by probe 30.
 TMS device 42 may be, in one embodiment, a Magstim Super Rapid device. The
 inventors have determined that as long as the TMS device 42 is kept at
 least ten feet away from MRI system 40, there are no adverse
 electromagnetic interactions between MRI system 40 and TMS device 42.
 Extension cables 62 are used to provide the connection between probe 30
 and TMS device 42.
 The system illustrated in FIGS. 3 and 4 may be operated in a number of
 ways. One way is to begin magnetic resonance imaging of a patient and then
 to apply TMS pulses and observe the effects of the TMS treatment on the
 MRI. Another way of operating the system is to apply a TMS treatment and
 then turn off the TMS device 42 and then observe the MRI. Monophasic or
 biphasic magnetic stimulation may be used as the mode of brain
 stimulation. Symmetrical biphasic stimulation, in which the pulses are of
 equal strength is useful because it can reduce the amount of movement of
 probe 30.
 The use of transcranial magnetic stimulation in combination with magnetic
 resonance imaging can provide a variety of diagnostic, research, and
 therapeutic benefits.
 The invention may be used to create a functional map of a patient's brain,
 by observing which areas of the brain become active (by observing, for
 example, the blood oxygen level on the MRI) in response to stimulation of
 a particular area of the brain. Thus, the effect of stimulation of one
 part of the brain upon other parts of the brain can be observed and
 recorded in order to create a map of functional connectivity within the
 brain of normal subjects as well as the brains of subjects suffering from
 some psychiatric impairment such as depression or schizophrenia.
 Development of functional brain maps, that is relationships between
 activation of one area of the brain and the effect that activation of that
 area has on other areas of the brain, can significantly improve
 understanding of the operation of the brain in both normal subjects and
 those with any type of cognitive disorder.
 The invention can also be used to identify pathway lesions or problems for
 any type of cognitive disorder by comparing, for example, the MRI of
 patients with a cognitive disorder with a normal MRI.
 The invention can also be used to determine the efficacy of various
 treatments. For example, if the patient's brain was in an undesired state,
 TMS could be applied and the effect on the MRI (in particular, the effect
 on the blood oxygen level) could be monitored and the TMS could be applied
 until the MRI indicates that the patient's brain is no longer in an
 undesired state. That is, the blood oxygen levels or other parameters
 being monitored by the MRI are no longer abnormal. In the same way, the
 invention can be used to determine when a patient's brain has reached a
 desired state. For example, TMS can be applied and the MRI can be
 monitored and the treatment continued until the particular parameters
 being tracked, such as blood oxygen level, have reached a desired state in
 the MRI.
 Furthermore, the invention can be used to make TMS applications more
 efficient. For example, if it is desired to stimulate a specific area of
 the brain, TMS can be applied, the MRI can be observed, and if the MRI
 indicates that a different region of the brain was activated rather than
 the target region, the position of probe 30 can be adjusted and the MRI
 observed once again. This process may be continued until probe 30 is
 stimulating the precise area of the brain that is desired.
 In addition, the present invention allows the results of a TMS treatment to
 be seen immediately after stimulation. Thus, the immediate and longer term
 effects of TMS stimulation can be observed.
 Having thus described at least one illustrative embodiment of the
 invention, various alterations, modifications, and improvements will
 readily occur to those skilled in the art. Such alterations,
 modifications, and improvements are intended to be within the spirit and
 scope of the invention. Accordingly, the foregoing description is by way
 of example only and is not intended as limiting. The invention is limited
 only as defined in the following claims and the equivalents thereto.