Magnetic stimulator power and control circuit

Details Magnetic stimulator power and control circuit Magnetic stimulator power and control circuit

2001-08-08

2003-04-22

Hindenburg, Max F. (Department: 3736)

Surgery

Magnetic field applied to body for therapy

C324S657000, C324S673000

Reexamination Certificate

active

06551233

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
N/A
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
Magnetic stimulator systems have a wide range of medical applications, including transdermal nerve stimulation. In many cases a monophasic stimulus is desired, in which each magnetic pulse in a pulse train has a rapid rise and a slow fall. In contrast, a pulse having similar rise and decay rates is referred to as being biphasic.
The general prior art approach for generating such pulses is illustrated in
FIG. 1. A
stimulus coil
12
acts as an inductance in the circuit
10
, and the magnetic field at the coil
12
, which is proportional to the coil current, is the output stimulus. A pre-charging power supply
14
charges a capacitor C to a predetermined voltage. A switch S
1
is used to connect the coil
12
across the capacitor C, causing an LC oscillation with a fast current rise in the coil
12
. When the capacitor
16
voltage reaches zero, at which point the coil
12
current will be near its peak, a second switch S
2
is closed bringing a resistor R into the circuit, causing an LCR resonant decay with a slow fall time.
The magnitude of the current pulse in the coil
12
, and thus the resulting magnetic field, is determined by the capacitor C pre-charge voltage. The relative rate of rise and fall of the current, and the shape of the pulse, is determined by the values of the fixed circuit elements, such as the coil
12
inductance, the capacitance of capacitor C, and the resistance of resistor R. All of the capacitor C pre-charge energy is dissipated in the resistor R, coil
12
, and switches S
1
, S
2
.
One embodiment
20
of this prior art approach, used in the MagPro magnetic stimulator by Medtronic, Inc., Minneapolis, Minn. (formerly Dantec Medical A/S), is illustrated in FIG.
2
. Values for the circuit components are: C=180 &mgr;f; L
coil
≈11 &mgr;H; and R≈60-90 m&OHgr;. This system uses a thyristor
24
as the first switch, and a diode
22
as the second switch, providing simple, static control over the coil
28
pulse shape out of the pre-charge power supply
26
. Coil current and voltage waveforms with a 220 Vac input pre-charge power supply
26
are shown in FIG.
7
.
The prior art approach to generating the desired waveform, while being simple and straightforward, has two serious deficiencies. First, the capacitor pre-charge energy for each pulse is entirely dissipated in the circuit elements, thus requiring a pre-charging power supply that draws a large amount of power from the utility or other source. For many practical applications, the amount of power required for desired pulse magnitudes and repetition rates is greater than can be drawn from a conventional 15 A, 110 V outlet, thus necessitating either a higher-voltage or higher-current utility outlet. Furthermore, the dissipated power results in a large amount of heat loss into the environment which is undesirable and potentially unsafe.
A second deficiency associated with the prior art approach is that the pulse shape and duration at the coil is determined entirely by the values of the constituent circuit elements, values which cannot be adjusted dynamically. In order to enable flexible or adaptive control over the resulting waveform, either monophasic or biphasic, accurate dynamic adjustment of stimulator circuit characteristics such as the relative rise and fall rates of the circuit current must be enabled.
BRIEF SUMMARY OF THE INVENTION
A new stimulator circuit
100
that overcomes the limitations of the prior art is disclosed. The general structure of the present invention is illustrated in FIG.
3
. The system
100
has a pre-charge power supply
102
, a capacitor C, a set of switches S
1
, S
2
, S
3
, and S
4
, and a stimulator coil
104
. The switches, which can be implemented using a variety of devices as discussed in detail below, enable flexible control over the coil current waveform without requiring the physical reconfiguration of circuit elements. The switches and coil may be collectively referred to as a coil switching circuit
106
.
Certain common reference designators are used in multiple drawings, such as the legends “C” and “S
1
,” though this is merely for convenience and is not to imply that the devices so designated are necessarily the same in each illustrated embodiment.
In the presently disclosed invention, the shape of the output current pulse is controlled by the modulation of the switches S
1
through S
4
. This contrasts with the prior art, in which the current pulse shape is determined only by the values associated with constituent circuit elements and in which current rise time is not dynamically controllable. Furthermore, with the presently disclosed circuits and methods, much of the energy (limited by parasitic losses in the coil, switching devices, etc.) is returned from the coil to the capacitor for reuse on the succeeding pulse. The presently disclosed invention thus has lower power requirements and produces less heat as compared to the prior art.


REFERENCES:
patent: 4039939 (1977-08-01), Wagner
patent: 4638670 (1987-01-01), Moser
patent: 5285161 (1994-02-01), Rzedzian et al.
patent: 5479102 (1995-12-01), El-Hamamsy et al.
Dvorak, J. et al. “Motor Evoked Potentials by means of Magnetic Stimulation in Disorders of the Spine,” Methods in Clinical Neurophysiology, vol. 3 No. 3 (Jul. 1992), pp. 45-64.
Eisen, A. “Cortical and Peripheral Nerve Magnetic Stimulation,” Methods in Clinical Neurophysiology, vol. 3 No. 4 (Dec. 1992), pp. 65-84.
Pascual-Leone, A. et al. “Safety of rapid-rate transcranial magnetic stimulation in normal volunteers,” Electroencephalographyand Clinical Neurophysiology, vol. 89 (1993), pp. 120-130.
“MagPro Magnetic Stimulator Instruction Manual,” DANTEC Medical A/S, Nov. 1992, pp. 1-15.
“Magnetic Stimulator MagPro Service Manual & Circuit Diagrams,” DANTEC Medical A/S, Feb. 1993, pp. 1-38.
“Magnetic Stimulator Accessories for MagPro,” DANTEC Medical A/S, four pages.
“Magnetic Coil Transducer MC125 used with MagPro,” DANTEC Medical A/S, Aug. 1993, two pages.
“Magnetic Coil Transducer MC-B70 used with MagPro,” DANTEC Medical A/S, Aug. 1993, two pages.

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