We propose to test the feasibility of developing a novel instrument for transcranial magnetic stimulation (TMS) utilizing superconducting magnet coils instead of room-temperature coils. This superconducting TMS device (sTMS.) takes advantage of high current density that can be carried by type II superconducting wires made of niobium-titanium-copper. Such a wire can carry currents with a density on the order of 1 kA/mm2 of cross- sectional area compared to about 1 A/mm2 for conventional room-temperature copper-based TMS magnets. Since the magnetic field B generated by a TMS coil depends on the current in the wire, cross sectional area and number of turns of wire, it is possible to use relatively small TMS coils for eventually constructing a high- density multi-channel or even whole-head sTMS. No heat will be generated by the superconducting segment of the TMS circuit. Thus there is no need for a special heat sink as it is necessary for room-temperature TMSs, allowing us to construct a high-density sTMS. In our preliminary test we were able to construct a TMS system with a 2.2 cm diameter coil that could produce a magnetic field ramp (dB/dt) of 18,000 tesla/sec (T/s), which is comparable to 20-40 kT/s for conventional TMS systems having stimulator dimensions of 8-12 cm. The proof- of-principle device to be constructed in this Phase I project will employ capacitors that can operate at ~1000 V compared to 100V in the preliminary test. This will allow us to construct an sTMS device capable of delivering dB/dt of ~40 kT/s using a 2.0 cm diameter TMS coil with ~4 turns having an optimized inductance of 1-2 5H. We will measure the electrical field E in a bath of saline solution just below the sTMS coil to verify that this sTMS is capable of generating an electrical gradient of ~100 mV/mm at a distance of 2-3 cm comparable to the distance of the neocortex of a human brain. Once we establish the feasibility of constructing a single-channel sTMS, we propose to construct a 37-channel sTMS in Phase II to demonstrate that a high-density sTMS with a channel spacing of ~3 cm can be constructed. Compared to conventional TMS devices, this multichannel system is expected to provide many significant advantages. Four diagonal sTMS coils can be combined to produce a focal eddy current shaped like a line segment. The orientation of this current line can be adjusted continuously by varying the currents applied to the four sTMS coils in order to stimulate target neurons with a specific orientation. The position of the eddy current line can be adjusted continuously along the surface of the brain by varying the currents applied to all the channels. Today, TMS is the only technique capable of stimulating focal regions of the brain to study not only basic functions of the brain circuit, but also to serve as a useful treatment modality for depression and other neurological/psychiatric disorders. TMS devices based on the proposed design could significantly increase the usefulness of TMS devices in other areas including facilitation of post-injury reorganization in patients with stroke and other disorders.
The multichannel superconducting TMS (sTMS) to be developed during Phase II and a whole-head sTMS systems to be developed possibly during the commercialization stage (Phase III) would provide significant advances in applications of TMS to stimulate the human brain noninvasively. This will increase the effectiveness of TMS in treating patients with various neurological and psychiatric disorders including depression and mood disorder in general, and stroke and traumatic brain injury that require rehabilitation. ? ? ?
