Balanced SSFP Prephaser Error

If RF pulse is set very short, then the refocusing pulses are correspondingly short.

In the bSSFP sequence code, the SS refocus pulse duration is used for the readout prephaser, however this can lead to a case where the duration is too short for the respective k-space traverse.

A temporary fix would be to not pass the prephaser duration to the readout gradient calculation and allow it to generate the fastest possible gradient.

For reference:

readout definition in seqdefs.ssfp.balanced_ssfp, line 48.

The associated error comes from seqdefs.readout.gre_cartesian_line, line 135