Abstract:
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.

Description:
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&#39;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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which are incorporated herein by reference and in which like elements have been given like reference characters, 
     FIG. 1 is a plane view of a conventional TMS probe; 
     FIG. 2 is a plane view of a TMS probe in accordance with one aspect of the invention; 
     FIG. 3 illustrates a system in accordance with the present invention in which TMS may be used in conjunction with MRI; and 
     FIG. 4 is a cross section along line  4 — 4  of FIG. 3 illustrating the placement of various components within the system. 
    
    
     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&#39;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&#39;s head and there is a relatively limited amount of space available between the patient&#39;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 ¼″ to ⅓″ 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&#39;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&#39;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&#39;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&#39;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.