Decoupling of parallel transmission arrays in magnetic resonance imaging
A magnetic resonance imaging (MRI) system includes a plurality of transmitters to generate a parallel transmission radio frequency (RF) pulse, an array of coils coupled to the plurality of transmitters to apply the parallel transmission RF pulse to a subject, and a decoupling system connected to the plurality of transmitters and the array of coils. The decoupling system includes a plurality of hybrid couplers, each hybrid coupler of the plurality of hybrid couplers being coupled to a respective pair of the plurality of transmitters and to a respective pair of the array of coils. The plurality of hybrid couplers are configured to diagonalize an impedance matrix of the plurality of coils.
1. A magnetic resonance imaging (MM) system comprising:
a plurality of transmitters to generate a parallel transmission radio frequency (RF) pulse;
an array of coils coupled to the plurality of transmitters to apply the parallel transmission RF pulse to a subject; and
a decoupling system connected to the plurality of transmitters and the array of coils;
wherein the decoupling system comprises a plurality of hybrid couplers, each hybrid coupler of the plurality of hybrid couplers being coupled to a respective pair of the plurality of transmitters and to a respective pair of the array of coils; and
wherein the plurality of hybrid couplers are configured to diagonalize an impedance matrix of the plurality of coils.
2. The MM system of claim 1 , wherein each hybrid coupler of the plurality of hybrid couplers is configured as an unequal division hybrid coupler.
3. The MM system of claim 1 , wherein each hybrid coupler of the plurality of hybrid couplers is configured as a 180 degree hybrid coupler.
4. The MM system of claim 1 , wherein the plurality of hybrid couplers comprises:
a first set of hybrid couplers configured to diagonalize a reactive component of the impedance matrix; and
a second set of hybrid couplers configured to diagonalize a resistive component of the impedance matrix after diagonalization of the reactive component.
5. The MM system of claim 4 , wherein:
the decoupling system further comprises a plurality of reactive elements;
the plurality of reactive elements are connected in series with the first and second sets of hybrid couplers;
the plurality of reactive elements are configured to present a respective reactance for each coil of the array of coils;
the respective reactance is opposite a reactance remaining for each coil after diagonalization via the first set of hybrid couplers.
6. The MM system of claim 4 , wherein:
the decoupling system further comprises a plurality of single port impedance matching networks; and
each single port impedance matching network of the plurality of single port impedance matching networks is configured to match an impedance remaining for each coil of the array of coils after diagonalization via the second set of hybrid couplers to an impedance of each transmitter of the plurality of transmitters.
7. The MM system of claim 1 , wherein:
the decoupling system further comprises a plurality of transmission lines;
each transmission line of the plurality of transmission lines is coupled to at least one of the plurality of hybrid couplers; and
each transmission line has a length that corresponds with three-fourths of a wavelength of the parallel transmission RF pulse.
8. The MM system of claim 1 , wherein the decoupling system is configured such that at least one pair of the plurality of transmitters is not coupled by one of the plurality of hybrid couplers.
9. A method of decoupling an array of coils of a parallel transmission magnetic resonance imaging (MM) system, the method comprising:
producing an excitation pulse for each coil of the array of coils;
amplifying each excitation pulse with a respective amplifier of a plurality of amplifiers; and
passing each amplified excitation pulse through a decoupling system to decouple the array of coils, the decoupling system comprising a plurality of hybrid couplers, each hybrid coupler of the plurality of hybrid couplers being coupled to a respective pair of the plurality of amplifiers and to a respective pair of the array of coils;
wherein the plurality of hybrid couplers are configured to diagonalize an impedance matrix of the plurality of coils.
10. The method of claim 9 , wherein each hybrid coupler of the plurality of hybrid couplers is configured as an unequal division hybrid coupler.
11. The method of claim 9 , wherein each hybrid coupler of the plurality of hybrid couplers is configured as a 180 degree hybrid coupler.
12. The method of claim 9 , wherein:
the plurality of hybrid couplers comprises first and second sets of hybrid couplers; and
diagonalizing the impedance matrix comprises diagonalizing a reactive component of the impedance matrix with the first set of hybrid couplers; and
passing each amplified excitation pulse further comprises diagonalizing a resistive component of the impedance matrix remaining after diagonalization of the reactive component.
13. The method of claim 12 , wherein passing each amplified excitation pulse further comprises cancelling a reactance remaining for each coil after diagonalization via the first set of hybrid couplers with a plurality of reactive elements of the decoupling system connected in series with the first and second sets of hybrid couplers.
14. The method of claim 12 , wherein passing each amplified excitation pulse further comprises adjusting an impedance remaining for each coil of the array of coils after diagonalization via the second set of hybrid couplers to match an impedance of each transmitter of the plurality of transmitters.
15. The method of claim 9 , wherein at least one pair of the plurality of transmitters is not coupled by one of the plurality of hybrid couplers.
16. A magnetic resonance imaging (MRI) system comprising:
a plurality of transmitters to generate a parallel transmission radio frequency (RF) pulse;
an array of coils coupled to the plurality of transmitters to apply the parallel transmission RF pulse to a subject; and
a decoupling system connected to the plurality of transmitters and the array of coils;
wherein the decoupling system comprises:
first and second sets of hybrid couplers, each hybrid coupler of the first and second sets of hybrid couplers being coupled to a respective pair of the plurality of transmitters and to a respective pair of the array of coils; and
a plurality of reactive elements coupling the first set of hybrid couplers to the second set of hybrid couplers;
wherein the first set of hybrid couplers are configured to diagonalize a reactive part of an impedance matrix of the plurality of coils,
wherein the plurality of reactive elements are configured to cancel reactance remaining after diagonalization by the first set of hybrid couplers, and
wherein the second set of hybrid couplers are configured to diagonalize resistance remaining after cancellation of the reactance.
17. The MM system of claim 16 , wherein each hybrid coupler of the plurality of hybrid couplers is configured as a 180 degree hybrid coupler.
18. The MM system of claim 16 , wherein the decoupling system is configured such that at least one pair of the plurality of transmitters is not coupled by the first and second sets of hybrid couplers.
19. The method of claim 9 , wherein:
the plurality of hybrid couplers comprises a first set of hybrid couplers configured to diagonalize a reactive component of the impedance matrix via rotation angles of the first set of hybrid couplers; and
the plurality of hybrid couplers comprises a second set of hybrid couplers configured to diagonalize a resistive component of the impedance matrix after diagonalization of the reactive component via rotation angles of the second set of hybrid couplers.
20. The method of claim 19 , wherein the decoupling system comprises a set of reactive elements to cancel the reactive component remaining after the diagonalization of the reactive component.