Hey there,
How is the RF amplitude defined within the jupyter notebooks? It appears to be in arbitrary units, for example, in the RARE sequence’s 180° block pulse, the amplitude is set to “1”, which I back-calculated to correspond to ~587.17 µT based on the duration. Is the pulse calibration tool the only way to determine the true amplitude?
Thanks,
Christina
Hi Christina,
Yes, the units for the amplitude setting are arbitrary. Amplitude is defined as a ratio of the maximum available amplitude, so a value of 1.0 is the maximum amplitude.
Most of our dashboards work with precalibrated pulses, just loading the amplitude and width from the appropriate file, but you may calculate the B1 at maximum amplitude from the pulse calibration and then use that to calculate the amplitude ratio.
e.g. if B1_max=600uT and you want B1_desired=100uT then you should set the pulse_update amplitude value to (B1_desired/B1_max). It may be easier to take a_180 and t_180 as parameters into your pulse sequence and calculate internally the B1_max:
B1_max = (1/(2*a_180*t_180))*(1/42.58e6)
Here (2*a_180*t_180) is calculating the duration of a maximum power 360 degree flip angle pulse, so 1/(2*a_180*t_180) is the precession frequency at maximum power, and dividing by the gyromagnetic ratio (gamma-bar in Hz/T) gives the field strength.
Then if you define your pulse amplitude parameter in units of Tesla you can simply divide by B1_max inside the pulse sequence to calculate the amplitude ratio.
Amplitude may also be set negative (valid range is -1.0 to 1.0) for convenience. A (phase=0, amp=-1.0) pulse is identical to a (phase=180, amp=1.0) pulse.
The B1 amplitude depends on the probe size and tuning/matching, so it needs calibration.
- Cameron Dykstra
Hi Cameron,
Following up on this from Christina, thanks so much for clarifying! I’m working on some custom pulse sequences which use frequency modulation, so I might have some questions about that to follow!
Best,
Alex