42: Coil: 3T coil bench testing
Here are steps to test coil(s) on the bench to see if they are properly tuned and matched. Please refer to the schematics for components.
Changing the capacitor value on the coil with switches SW1 and SW2 off can change the resonance frequency. Here we use the double probe to measure S12.
Turning on switches SW1 and SW2 connects the coil with the pre-amplifier via the impedance matching network. A characteristic "M" shape waveform will be shown in the S21 plot when the coil is matched to the low input-impedance pre-amplifier.
Appropriate overlapping between the neighboring coils is the most common approach to reduce the mutual inductance of neighboaring coils. In this demo, we first turned off SW1 and SW2. Then the distance between two coils was adjusted until the S21 plot shows a nice peak.
With separately tuned and matched coils, connect both of them to the pre-amplifier. Turn on and off SW1 and SW2 switches to make sure that both coils are tuned (SW1 and SW2 off; single peak S12), decoupled between each other (SW1 and SW2 off; single peak S12), and matched (SW1 and SW2 on; M-shape S12).
Detuning is an important safe feature for receiver coils. During MRI excitation, the receiver coils should be "detuned": their resonant frequency should be away from the Larmor frequency in order to avoid the interference to the MR signal excitation and aberrant E-fields. We used "active detuning" here: The detuning signal was enabled by an external DC current to forward bias a L-C resonant circuit, which interrupted the resonant frequency of the receiver coils.
The S12 measurement using a double problem shows that: during the "tuned" state, there was a characteristic "M" shape with the central dip matched the Larmor frequency. As we turned on the external signal to "detune" the coil, the S12 measurement dropped significantly. We particularly tuned the inductor in the detuning circuit such that the receiver coil has the most detuned performance (most significant S12 drop).
