Method for reducing maximum local specific absorption rate in magnetic resonance imaging
First Claim
1. A method for producing a radio frequency excitation field with an RF coil that forms a part of an MRI system, the steps comprising:
- a) estimating an electric and a magnetic field produced by the RF coil;
b) estimating an RF transmission profile of the RF coil using the estimated magnetic field;
c) selecting locations in k-space to which RF energy is to be deposited;
d) determining, using the estimated RF transmission profile and selected locations in k-space, a plurality of candidate RF pulse waveforms corresponding to a plurality of candidate RF pulses that have substantially similar excitation profiles but different specific absorption ratio (SAR) profiles;
e) calculating a local SAR value at each spatial location within a selected portion of the subject for each candidate RF pulse waveform;
f) calculating a plurality of weighting factors, using the determined plurality of candidate RF pulse waveforms and calculated local SAR values for each candidate RF pulse waveform, the plurality of weighting factors defining a number of times to apply RF pulses corresponding to selected ones of the plurality of candidate RF pulse waveforms in order to substantially minimize maximum local SAR in the selected portion of the subject; and
g) directing the MRI system, using the calculated plurality of weighting factors and the associated selected ones of the plurality of candidate RF pulse waveforms, to produce a plurality of RF excitation pulses during a selected time period in order to produce an RF excitation field.
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Abstract
A method for reducing maximum local specific absorption rate (“SAR”) in a magnetic resonance imaging (“MRI”) system is disclosed. More specifically, a plurality of candidate radio frequency (“RF”) pulses are designed and the manner in which they are applied to a subject is determined such that the maximum local SAR is substantially reduced relative to applying the candidate RF pulse that produces the lowest maximum local SAR alone. Put another way, this “time-multiplexing” of a set of RF pulses that each produce approximately the same excitation pattern yields a lower maximum local SAR than does transmitting the individual RF pulse having the lowest local SAR over many repetition times (“TRs”). A convex optimization method is utilized to determine the manner in which the RF pulses are multiplexed in time such that a substantially lower maximum local SAR is achieved.
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Citations
17 Claims
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1. A method for producing a radio frequency excitation field with an RF coil that forms a part of an MRI system, the steps comprising:
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a) estimating an electric and a magnetic field produced by the RF coil; b) estimating an RF transmission profile of the RF coil using the estimated magnetic field; c) selecting locations in k-space to which RF energy is to be deposited; d) determining, using the estimated RF transmission profile and selected locations in k-space, a plurality of candidate RF pulse waveforms corresponding to a plurality of candidate RF pulses that have substantially similar excitation profiles but different specific absorption ratio (SAR) profiles; e) calculating a local SAR value at each spatial location within a selected portion of the subject for each candidate RF pulse waveform; f) calculating a plurality of weighting factors, using the determined plurality of candidate RF pulse waveforms and calculated local SAR values for each candidate RF pulse waveform, the plurality of weighting factors defining a number of times to apply RF pulses corresponding to selected ones of the plurality of candidate RF pulse waveforms in order to substantially minimize maximum local SAR in the selected portion of the subject; and g) directing the MRI system, using the calculated plurality of weighting factors and the associated selected ones of the plurality of candidate RF pulse waveforms, to produce a plurality of RF excitation pulses during a selected time period in order to produce an RF excitation field. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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Specification