To get an initial estimate of the antenna efficiency, you can use a pick-up coil placed inside your solenoid RF coil.

Important Notes:

• The pick-up loop area and number of turns must be known.
• In this example, a 2-turn pick-up loop with a diameter of 1.6 cm is used (see illustration below).

1. Tune and match your solenoid RF coil to 50 Ω at the desired frequency. Ensure the coil is inside your scanner.
2. Connect the solenoid to Port 1 of the Vector Network Analyzer (VNA).
3. Connect the pick-up loop to Port 2 of the VNA.
4. Measure the S21 parameter.

The setup is illustrated below:

The RF solenoid efficiency ($\varepsilon$) can be estimated as:

$$ \varepsilon = \frac{\sqrt{2} \cdot R_L}{2\pi \cdot f \cdot n \cdot S} \cdot S_{21} $$

Where:

• $R_L$ is the load resistance (50 Ω)
• $n$ is the number of turns of the pick-up loop
• $S$ is the surface area of the pick-up loop
• $S_{21}$ is the transmission coefficient (linear scale)

Once you have your first rough estimation of the RF coil efficiency, add a new coil to the RF tab of the session window.

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In the Main window, choose the RabiFlops sequence from the sequence list under the Custom tab.

If you already have an estimate of the antenna efficiency, it can help you select appropriate input parameters for the sequence.

The figure below shows an example configuration of the RabiFlops sequence used to determine coil efficiency:

1. Select the excitation and refocusing amplitudes.
2. Set the refocusing time to 0.
3. Choose the excitation time interval and specify the number of excitation time steps.
4. Set the Rephasing method to 1, indicating that the refocusing pulse time will be twice the excitation pulse.
5. Adjust other sequence parameters, such as the repetition time (TR), long enough to avoid T1-related effects.
6. Choose the calibration method: ECHO or FID.
   • ECHO is more robust in highly inhomogeneous magnetic fields.
   • FID is sensitive to ring-down signal if dead time is too short, and produces noisy results if dead time is too long.
7. Select the calibration point: min or max.
   • In theory, the minimum of the Rabi oscillation is independent of TR, allowing the use of shorter TR values.

Then run the sequence and the time required to get $\pi$ flip angle will be shown in the console. Then, calculate the efficiency and update the value in the RF tab of the session window.