Hybrid coupler for magnetic resonance imaging
A transmit/receive switch for magnetic resonance imaging (MRI) of a 1 H atomic nucleus at 3T (tesla) and 7T magnetic field strengths includes a first and second dielectric substrate and a first and a second microstripline hybrid coupler formed on the first and second dielectric substrates respectively. A first port and a second port are connected to a first junction and a second junction respectively of the first microstripline hybrid coupler, a third port and a fourth port are connected to a third junction and a fourth junction of the second microstripline hybrid coupler respectively. Each port is connected by a matching network embodied by a microstripline wire to a pair of parallel resonant circuits. The transmit/receive switch is configured to operate at a fundamental frequency of about 127.8 MHz at 3T and at a third harmonic frequency of about 298 MHz at 7T.
1 . A hybrid coupler for magnetic resonance imaging (MRI) of a 1 H atomic nucleus at 3T (tesla) and 7T magnetic field strengths, comprising:
a dielectric substrate having a first edge opposite a second edge and a third edge opposite a fourth edge, wherein a first central axis of the dielectric substrate is configured to bisect the first edge and the second edge and to extend from the first edge to the second edge and wherein a second central axis of the dielectric substrate is configured to bisect the third edge and the fourth edge and to extend from the third edge to the fourth edge;
a multi-bended microstripline configured to have a first arm, a second arm, a third arm and a fourth arm;
a first port connected by a microstripline wire to a junction between the first arm and the fourth arm;
a first pair of parallel resonant circuits connected to the junction between the first arm and the fourth arm;
a second port connected by a microstripline wire to a junction between the third arm and the fourth arm;
a second pair of parallel resonant circuits connected to the junction between the third arm and the fourth arm;
a third port connected by a microstripline wire to a junction between the third arm and the second arm, wherein a 50Ω terminator is connected to the third port;
a third pair of parallel resonant circuits connected to the junction between the third arm and the second arm;
a fourth port connected by a microstripline wire to a junction between the first arm and the second arm;
a fourth pair of parallel resonant circuits connected to the junction between the first arm and the second arm,
wherein the first arm includes a first angled leg, an S-shaped loop connected to the straight leg, an inverted S-shaped loop connected to the S-shaped loop, and a second angled leg, wherein the S-shaped loop and the inverted S-shaped loop have mirror image symmetry about the second central axis,
wherein the second arm includes a first angled leg, a first straight leg parallel to the second edge, a second straight leg perpendicular to the first straight leg, a first loop which extends towards the third edge, a second loop which extends towards the second central axis, a third loop which extends towards the fourth edge, a third straight leg parallel to the second straight leg, a fourth straight leg parallel to the second edge and which extends towards the fourth edge, and a second angled leg,
wherein the third arm is located opposite the first arm and is a mirror image of the first arm about the first central axis,
wherein the fourth arm is located opposite the second arm and is a mirror image of the second arm about the second central axis, and
wherein the first arm and the third arm each have a microstripline width Wa, the second arm and the fourth arm each have a microstripline width Wb, wherein Wb is about seven times Wa;
wherein the multi-bended microstripline is configured to receive RF electrical signals at the first port and transmit RF electrical signals at a resonance frequency of about 127.8 MHz and at a third harmonic frequency of about 298 MHz from each of the second port and at the fourth port, wherein the RF electrical signals transmitted from the fourth port are orthogonal to the RF electrical signals transmitted from the second port,
wherein the third port is isolated,
wherein the first arm and the third arm have a same width,
the first arm and the third arm each have mirror image symmetry about the second central axis; and
the first arm and the third arm each have mirror image symmetry about the first central axis.
2 . The hybrid coupler of claim 1 , wherein:
the first arm and the third arm are each configured to have a resistance of about 50Ω and a width of about 1.18 mm; and
the second arm and the fourth arm are each configured to have a resistance of about 35.35Ω and a width of about 2.25 mm.
3 . The hybrid coupler of claim 1 , wherein each parallel resonant circuit of the first, second, third and fourth pair of parallel resonant circuits includes a tuning inductor in parallel with a tuning capacitor, wherein the inductance of the tuning inductor is about 85.89 nH and the capacitance of the tuning capacitor is about 18 pF.
4 . The hybrid coupler of claim 1 , wherein the width Wa equals about 0.53 mm and the width Wb equals about 3.51 mm.
5 . The hybrid coupler of claim 1 , wherein each microstripline wire has an impedance characteristic of 40Ω and a 90° electrical length at about 298 MHz.
6 . A transmit/receive switch for magnetic resonance imaging (MRI) of a 1 H atomic nucleus at 3T (tesla) and 7T magnetic field strengths, comprising:
a first dielectric substrate and a second dielectric substrate, wherein each dielectric substrate includes a top side, a bottom side, a first edge opposite to a second edge, a third edge opposite to a fourth edge, wherein a first central axis of each dielectric substrate is configured to bisect the first edge and the second edge and to extend from the first edge to the second edge and wherein a second central axis of the dielectric substrate is configured to bisect the third edge and the fourth edge and to extend from the third edge to the fourth edge;
a first microstripline hybrid coupler formed on the first dielectric substrate, wherein the first microstripline hybrid coupler includes a multi-bended microstripline;
a second microstripline hybrid coupler formed on the second dielectric substrate, wherein the second microstripline hybrid coupler includes a multi-bended microstripline, wherein the multi-bended microstripline of the second microstripline hybrid coupler is identical in construction to the multi-bended microstripline of the first microstripline hybrid coupler;
wherein each multi-bended microstripline includes:
a first arm, a second arm, a third arm and a fourth arm;
a first junction located between the first arm and the fourth arm;
a second junction located between the first arm and the second arm;
a third junction located between the second arm and the third arm;
a fourth junction located between the third arm and the fourth arm;
a plurality of pairs of parallel resonant circuits, wherein a pair of parallel resonant circuits is connected to each of the first junction, the second junction, the third junction and the fourth junction of the first microstripline hybrid coupler and the second microstripline hybrid coupler;
a first port connected to the first junction of the first microstripline hybrid coupler, wherein the first port is configured to transmit radio frequency electromagnetic signals into the first junction of the first microstripline hybrid coupler;
a second port connected to the second junction of the first microstripline hybrid coupler;
a third port connected to the third junction of the second microstripline hybrid coupler;
a fourth port connected to the fourth junction of the second microstripline hybrid coupler; and
a switching circuit connected between the first microstripline hybrid coupler and the second microstripline hybrid coupler, wherein the switching circuit includes:
a first PIN diode having a first terminal connected to the fourth junction of the first microstripline hybrid coupler and to the first junction of the second microstripline hybrid coupler and a second terminal connected to a ground plane located on the bottom side of the first dielectric substrate,
a second PIN diode having a first terminal connected to the third junction of the first microstripline hybrid coupler and to the second junction of the second microstripline hybrid coupler and a second terminal connected to a ground plane located on the bottom side of the first dielectric substrate, and
a power amplifier connected to the first port, wherein the power amplifier is configured to transmit the RF electromagnetic signals to the first port,
wherein, in a transmission state, the first PIN diode and the second PIN diode are configured to be forward biased and RF electromagnetic signals applied to the first port radiate from the second port, and
wherein, in a reception state, the first PIN diode and the second PIN diode are configured to be reverse biased, and 3T and 7T magnetic field strength electromagnetic signals generated by the MRI imaging of the 1 H atomic nucleus are received by the first microstripline hybrid coupler and the second microstripline magnetic coupler and are radiated from the fourth port,
wherein the first arm and the third arm have a same width,
the first arm and the third arm each have mirror image symmetry about the second central axis; and
the first arm and the third arm each have mirror image symmetry about the first central axis.
7 . The transmit/receive switch of claim 6 , wherein each parallel resonant circuit includes an inductor having a value of about 85.89 nH and a capacitor having a value of about 18 pF.
8 . The transmit/receive switch of claim 6 , wherein, for each multi-bended microstripline coupler, the width Wa equals about 0.53 mm and the width Wb equals about 3.51 mm.
9 . The transmit/receive switch of claim 6 , further comprising a 50Ω terminator connected to the third port.
10 . The transmit/receive switch of claim 6 , further comprising:
a plurality of microstripline wires, wherein a microstripline wire is connected between each port and each junction of each of the first microstripline coupler and the second microstripline coupler.
11 . The transmit/receive switch of claim 10 , wherein each microstripline wire has an impedance characteristic of 40Ω and a 90° electrical length at about 298 MHz.