Larmor - josalggui/MaRGE GitHub Wiki

Larmor Sequence Documentation

The Larmor sequence is designed to acquire data using a spin echo to determine the Larmor frequency. This sequence generates a time-domain signal and frequency spectrum and updates the Larmor frequency based on the signal’s central frequency. The acquired data can be visualized in both domains.

Parameters

The following parameters can be adjusted for the Larmor sequence:

Sequence Parameters:

  • Sequence Name (seqName): Identifier for the sequence.

  • Number of scans (nScans): Number of repetitions of the full scan.

RF Parameters:

  • Frequency (larmorFreq): The excitation and demodulation frequency in MHz.

  • Excitation flip angle (rfExFA): The flip angle for the excitation RF pulse in degrees.

  • Refocusing flip angle (rfReFA): The flip angle for the refocusing RF pulse in degrees.

  • Excitation time (rfExTime): Duration of the excitation RF pulse in microseconds (us).

  • Refocusing time (rfReTime): Duration of the refocusing RF pulse in microseconds (us).

Sequence Timing Parameters:

  • Repetition time (repetitionTime): The time between successive excitation pulses in milliseconds (ms).

  • Bandwidth (bw): The bandwidth in kilohertz (kHz).

  • Frequency resolution (dF): The frequency resolution in hertz (Hz).

Other Parameters:

  • Shimming (shimming): Shimming parameter to compensate for B0 linear inhomogeneity.

Sequence Workflow

  1. Initialization: The sequence begins by setting up the excitation and refocusing pulse durations and amplitudes, as well as calculating the acquisition time and echo time.
  2. RF Pulses: The RF excitation and refocusing pulses are applied according to the specified flip angles and pulse durations.
  3. Data Acquisition: The sequence acquires data using the defined bandwidth and frequency resolution.
  4. Larmor Frequency Determination: The frequency spectrum is calculated, and the central frequency is used to update the Larmor frequency.
  5. Results Processing: The time-domain signal and frequency spectrum are saved for visualization and analysis.
  6. Visualization: The time-domain signal and spectrum are plotted, and results are saved for later analysis.

Output Data

  • Signal vs. Time: The signal is plotted in the time domain, showing both the real and imaginary parts, as well as the absolute value.

  • Frequency Spectrum: The spectrum is plotted in the frequency domain, showing the amplitude of the signal in a frequency range defined by the bandwidth.

Notes

  • The Larmor frequency is recalculated from the signal’s central frequency, and this value is updated in the sequence parameters.
  • The sequence supports both standalone mode and iterative mode for repeated scans.
  • In standalone mode, the results are automatically plotted.
  • The frequency resolution (dF) and bandwidth (bw) are critical for the quality of the frequency spectrum analysis.
  • It is assumed that B1 efficiency has been previously calibrated. If RabiFlops was not launched, the efficiency is obtained from the hw_config.py file.