Coil array mode compression for parallel transmission magnetic resonance imaging
A method for target-dependent, sparsity-enforced selection for choosing a substantially optimal connection of radiofrequency (“RF”) transmitters to the elements of the RF coil array is provided. In particular, a method is provided that selects the linear combinations of the “N” spatial mode profiles of a transmission RF coil array, such that the k-space trajectory and pulse duration acceleration capabilities of the array are advantageously utilized. A sparsity-enforcement method that determines a subset of the available spatial modes for a parallel transmission RF coil array is employed to this end. In this manner, the utilization of the encoding power of a highly-parallel N-mode coil array in a system with only “P” available excitation channels is enabled.
1. A method for operating a radiofrequency (RF) coil array that includes a plurality of transmission channels and forms a part of a magnetic resonance imaging (MRI) system, the steps comprising:
a) selecting a desired pattern of RF excitation;
b) estimating an RF transmission profile map indicative of the transmission characteristics of the parallel transmission RF coil array;
c) determining, from the desired pattern of RF excitation and the estimated RF transmission profile map, a plurality of k-space locations at which RF energy is to be deposited;
d) calculating, from the determined k-space locations and the estimated RF transmission profile map, a sparse set of spatial modes indicative of a linear combination of transmission profiles of the RF coil array;
e) selecting, from the calculated set of spatial modes, a subset of spatial modes; and
f) producing, with the RF coil array and using the selected subset of spatial modes, an RF excitation field characterized by the desired pattern of RF excitation.
2. The method as recited in claim 1 in which step d) includes performing an iterative minimization that enforces sparsity on the calculated set of spatial modes.
3. The method as recited in claim 1 in which step d) includes performing an iterative minimization that penalizes nonzero mode energies.
4. The method as recited in claim 1 in which step e) includes selecting a preselected number of spatial modes having the largest mode energies.
5. The method as recited in claim 1 in which step f) includes:
f)i) calculating an RF excitation pulse waveform using the subset of spatial modes selected in step e); and
f)ii) producing the RF excitation field with the calculated RF excitation pulse waveform while driving the RF coil at the selected subset of spatial modes.
6. The method as recited in claim 5 in which k-space weightings defined by the remaining spatial modes are employed to calculate the RF excitation pulse waveform.
7. The method as recited in claim 1 in which the determined plurality of k-space locations is a sparse set of k-space locations.
8. The method as recited in claim 1 further comprising:
g) acquiring, with the MRI system, image data from a subject using the produced RF excitation field; and
h) reconstructing an image of the subject from the acquired image data.
9. A method for determining a set of spatial modes for a parallel transmission radiofrequency (RF) coil array that forms a part of a magnetic resonance imaging (MRI) system, the steps comprising:
a) selecting a desired pattern of RF excitation;
b) determining, from the desired pattern of RF excitation, a plurality of k-space locations at which RF energy is to be deposited;
c) estimating an RF transmission profile map indicative of the transmission characteristics of the parallel transmission RF coil array;
d) calculating, from the determined k-space locations and the estimated RF transmission profile map, a sparse set of spatial modes indicative of a linear combination of transmission profiles of the parallel transmission RF coil;
e) selecting, from the calculated set of spatial modes, a subset of spatial modes that are employed to produce the desired pattern of RF excitation
f) acquiring, with the MRI system, image data from a subject using the selected subset of spatial modes and calculated sparse set of spatial modes; and
g) reconstructing an image of the subject using the acquired image data.
10. The method as recited in claim 9 in which step d) includes performing an iterative minimization that penalizes nonzero mode energies.
11. The method as recited in claim 9 further comprising:
h) calculating an RF excitation pulse waveform that produces the desired pattern of RF excitation when driven by the subset of spatial modes calculated in step e).
12. The method as recited in claim 11 in which the calculation of the RF excitation pulse waveform in step h) includes using the subset of spatial modes selected in step e).
13. The method as recited in claim 12 in which k-space weightings defined by the remaining spatial modes are employed to calculate the RF excitation pulse waveform.
14. The method as recited in claim 9 in which the determined plurality of k-space locations is a sparse set of k-space locations.