Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1994-12-01
2001-04-10
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000
Reexamination Certificate
active
06215309
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to producing modulated gradient fields in magnetic resonance imaging (MRI) systems.
A variety of modulated gradient field waveforms are known to be useful in MRI systems. Some MRI systems, such as those disclosed in Rzedzian U.S. Pat. No. 4,628,264, produce a sinusoidally varying gradient field by sinusoidally driving a circuit comprising the gradient coil connected in parallel with a capacitor. The desired current is realized by selecting a capacitor/coil combination with a resonant frequency corresponding to the desired gradient field frequency, and driving the coil at that frequency with a sinusoidal voltage source. In this way, the voltage source supplies only a small fraction (1/Q, where Q is the quality factor of the resonant capacitor/coil circuit) of the coil current. The majority of the coil current comes from the resonant discharge of the capacitor.
Other MRI systems produce a steady gradient field by driving the gradient coil with a pulse of constant amplitude. The leading and trailing edges of these pulses rise and fall gradually, consistent with limits on the rate at which the current through the gradient coil can be changed. For example, trapezoidal pulses of coil current are often used.
SUMMARY OF THE INVENTION
The invention features a series topology for the coil and storage capacitor in the circuitry driving the coil of an MRI system. The coil and capacitor thus form a series resonant circuit that can deliver a sinusoidal current through the coil at a resonant frequency. A power source and switch are connected in series with the coil and capacitor, and current flow to the coil is initiated and interrupted by closing and opening the switch.
In preferred embodiments, a second switch is connected in parallel with the capacitor, to shunt current around the capacitor, thus permitting the coil to be driven directly by the power source; a precharging circuit (which preferably receives its power from the same power source as used for driving the coil) is connected to the capacitor; run controller circuitry opens and closes both switches to produce varying types of pulses; the pulses have at least one sinusoidal segment comprising a portion of a sinusoid of the resonant frequency; a nonsinusoidal segment (e.g., one of constant amplitude) preferably follows the first sinusoidal segment and a second sinusoidal segment follows the nonsinusoidal one; the sinusoidal segments consist of an integral number (i.e., 1, 2, . . . N) of quarter cycles of a sinusoid of the resonant frequency.
The circuitry can thus advantageously be used to generate both sinusoidally varying and steady gradient fields in an MRI system. Sinusoidally varying gradient fields can be generated by closing the switch in series with the capacitor to initate a sinusoidal resonance. Steady gradient fields can be generated in three segments: (1) a first sinusoidal segment produced by allowing the series resonant circuit to resonate for a quarter cycle, during which time current rises from zero to a maximum value; (2) a constant amplitude segment produced by shunting across the capacitor so that the power source drives the coil directly, during which time the current remains at the maximum value; and (3) a second sinusoidal segment produced by allowing the series resonant circuit to resonate for another quarter cycle, during which time current falls from a maximum to zero.
Other features and advantages of the invention will be apparent from the description of the preferred embodiment and from the claims.
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Crump Stephen
Rzedzian Richard
Arana Louis
Aurora Imaging Technology, Inc.
Fish & Richardson P.C.
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