Add new RF coils - josalggui/MaRGE GitHub Wiki

Step-by-Step Guide to Adding New RF Coils to MaRGE

To add new RF coils to MaRGE, you need to configure some parameters in the hw_config.py file. This file includes two key parameters related to RF coils:

  1. b1Efficiency: Used for standalone sequence execution.
  2. antenna_dict: A dictionary containing RF coil names and their respective efficiencies,
  3. used by MaRGE to display available antennas in the session window.

Understanding Efficiency

Efficiency in MaRGE is a floating-point value with units of $((\text{rad}/(\text{a.u.} \cdot \mu \text{s}))$. It represents the time in $\mu \text{s}$ required to achieve a specific flip angle at maximum power (when RF amplitude is set to 1). This value serves as a starting estimate, but it is recommended to use the RabiFlops sequence for a more accurate measurement, as efficiency depends on the load.

1. Estimating Antenna Efficiency with a VNA

If no initial efficiency estimation is available, you can use a pick-up coil placed inside the solenoid RF coil to measure it.

Requirements:

  • Knowledge of the pick-up loop's area and number of turns.
  • Example setup:
    • A 2-turn pick-up loop with a diameter of 1.6 cm.
    • A solenoid coil with 44 turns, 1 mm wire diameter, 150 mm length, and 150 mm diameter.

pickup coil

Steps:

  1. Tune and match the solenoid RF coil to 50 Ohms at the target frequency, ensuring it is inside the scanner.
  2. Connect:
    • The solenoid coil to Port 1 of the VNA.
    • The pick-up loop to Port 2 of the VNA.
  3. Measure the $S_{21}$ parameter using the VNA.

setup

The RF solenoid efficiency $\varepsilon$ in $T/\sqrt{W}$ can be calculated as:

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

Where:

  • $R_L$: Load resistance (50 Ohms).
  • $n$: Number of turns in the pick-up loop.
  • $S$: Surface area of the pick-up loop.
  • $S_{21}$: Transmission coefficient (linear scale).
  • $f$: Working frequency.

Example

In this example, the efficiency was 134 $\mu T/\sqrt{W}$.

Gross Efficiency Estimation

Using the RFPA gain and the Red Pitaya's maximum output voltage amplitude (225 mV for a 50 Ohms load):

  • RFPA: Barthel model RFPA-1/4-250 with 55 dB gain.
  • At maximum output, power is 158 W, with a $\pi$-flip time of 14 $\mu \text{s}$.
  • Red Pitaya amplitude is controlled by a unitless parameter (0 to 1). The gross efficiency estimate for the RF coil is $\pi/14$.

2. Fine-Tuning Efficiency Using the RabiFlops Sequence

Once a rough estimate is available, the RabiFlops sequence can refine the efficiency value.

Steps:

  1. Select RabiFlops in the custom tab.
  2. Configure the inputs. Example:
    • Amplitude: 0.3, 30% of maximum output.
    • Sweep: 5 $\mu \text{s}$ to 100 $\mu \text{s}$.

Rabi example

In the example, the flip angle of $\pi$ was achieved in 55 $\mu \text{s}$ for an amplitude of 0.3. The refined efficiency is:

$$ \varepsilon = \frac{\pi}{0.3 \cdot 55} $$

3. Adding the Antenna to the Configuration

Once the efficiency is determined, update the hw_config.py file:

b1Efficiency = np.pi / (0.3 * 55)
antenna_dict = {"RF01": np.pi / (0.3 * 55)}
  • b1Efficiency: Used for standalone sequences.
  • antenna_dict: Used by MaRGE after RF coil selection in the session window.

With this setup, your new RF coil is ready for use in MaRGE!