Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-10-07
2001-03-06
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S318000, C324S309000
Reexamination Certificate
active
06198282
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to a method and means for producing magnetic field gradients in MRI systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field along the z-axis. Additionally, there is a wobbling or precession occurring about this magnetic field, the rate of precession being the Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
, G
y
, and G
z
) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The magnetic field gradient subsystem of an MRI system is perhaps the most critical subsystem in defining the utility of a scanner. In general, more powerful gradient subsystems give greater applications capability. The power of a gradient subsystem is roughly equivalent to the product of the gradient amplitude and the gradient slew-rate. Gradient amplitude is determined by the current which the gradient amplifiers produce in the gradient coils, and gradient slew rate is the rate at which the gradient amplifiers can change the gradient amplitude.
In many circumstances, the only factor of importance in the generation of a gradient field pulse is the integral of gradient amplitude over the duration of the gradient pulse (i.e. the gradient pulse area). This is true, for example, with slice-select refocusing, phase-encoding, velocity or flow compensation, spoiling, rewinding and readout defocusing gradient pulses. Since the shortest duration gradient pulse of a given area provides the greatest flexibility in selecting pulse sequence echo time (TE) and pulse sequence repetition time (TR), it is highly desirable for the MRI system to produce these gradient pulses with the minimum pulse duration possible given the prescribed pulse area.
There is a need to provide minimum duration gradient pulses that do not operate beyond the physiological limits, for example as established by the Reilly equation.
SUMMARY OF THE INVENTION
The present invention relates to a method and system for production of gradient pulses for use in a MRI pulse sequence, and particularly, optimizing the production of the gradient pulses. Information to indicate the physiological limit on gradient slew rate as a function of gradient amplitude is calculated for the MRI system. An area of a prescribed magnetic field gradient pulse is calculated. The peak gradient amplitude and slew rate are then calculated for a optimal minimum duration pulse having the area of the prescribed magnetic pulse in which the slew rate does not exceed the physiological limit. An optimal magnetic field gradient pulse is produced having the area of the prescribed gradient, and the calculated peak gradient amplitude and gradient slew rate.
REFERENCES:
patent: 5378989 (1995-01-01), Barber et al.
patent: 5399969 (1995-03-01), Bernstein
patent: 5451878 (1995-09-01), Wirth et al.
patent: 5663647 (1997-09-01), Wirth et al.
patent: 5680046 (1997-10-01), Frederick et al.
General Electric Company
Ingraham Donald S.
Oda Christine
Shrivastav Brij B.
Testa Jean K.
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