Field-frequency lock system for magnetic resonance system
First Claim
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1. In an MRI system having a magnet for producing a polarizing magnetic field B0 in a region, and a gradient coil assembly for producing magnetic field gradients in a subject to be imaged located in the region, a field-frequency lock system which comprises:
- a microcoil located outside the gradient coil assembly and in the region in which the polarizing magnetic field B0 is produced;
a resonant sample disposed adjacent the microcoil and containing a material that produces a signal when the microcoil is energized with an rf pulse;
transmit means for producing an rf pulse that is applied to the microcoil;
receive means for acquiring a signal from the microcoil that is produced by the resonant sample material;
a frequency comparator for measuring the difference frequency (Δ
ω
) between the frequency of the acquired signal and a reference frequency which corresponds to a frequency produced by the resonant sample material when the polarizing magnetic field B0 is at a nominal value; and
means responsive to the difference frequency Δ
ω
for compensating the operation of the MRI system as it performs a scan on the subject for variations in the magnitude of the polarizing magnetic field B0.
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Abstract
A field-frequency lock system for an MRI system includes a microcoil and resonant sample located to sense changes in the polarizing magnetic field. Changes are detected as a shift in frequency of the NMR signal produced by the resonant sample, and the frequency shift is used to compensate the MRI system. Compensation is achieved by adjusting the RF reference signal employed in the MRI system transceiver.
106 Citations
12 Claims
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1. In an MRI system having a magnet for producing a polarizing magnetic field B0 in a region, and a gradient coil assembly for producing magnetic field gradients in a subject to be imaged located in the region, a field-frequency lock system which comprises:
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a microcoil located outside the gradient coil assembly and in the region in which the polarizing magnetic field B0 is produced;
a resonant sample disposed adjacent the microcoil and containing a material that produces a signal when the microcoil is energized with an rf pulse;
transmit means for producing an rf pulse that is applied to the microcoil;
receive means for acquiring a signal from the microcoil that is produced by the resonant sample material;
a frequency comparator for measuring the difference frequency (Δ
ω
) between the frequency of the acquired signal and a reference frequency which corresponds to a frequency produced by the resonant sample material when the polarizing magnetic field B0 is at a nominal value; and
means responsive to the difference frequency Δ
ω
for compensating the operation of the MRI system as it performs a scan on the subject for variations in the magnitude of the polarizing magnetic field B0.- View Dependent Claims (2, 3, 4, 5, 6)
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7. A method for compensating an MRI system for changes in the polarizing magnetic field of the MRI system, the steps comprising:
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monitoring a microcoil positioned in the polarizing magnetic field at a location in which magnetic field gradients are minimal;
acquiring an NMR signal from a resonant sample disposed adjacent the microcoil;
producing a difference signal by comparing the frequency of the acquired NMR signal with the frequency of a reference signal that is indicative of the NMR signal produced by the resonant sample when the polarizing magnetic field has a nominal value; and
producing a compensating signal for the MRI system from the difference signal which offsets changes in the polarizing magnetic field. - View Dependent Claims (8, 9, 10, 11, 12)
adjusting the frequency of a reference signal in the MRI system with the compensating signal.
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11. The system as recited in claim 10 in which the adjusted reference signal is employed during a scan to demodulate NMR signals produced when performing a scan.
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12. The system as recited in claim 10 in which the adjusted reference signal is employed to produce rf excitation pulses when performing a scan.
Specification