MRI DBS: Parameters Modification on GE Scanners

1) Starting the exam:
Gradient slew rate(dB/dt): Use Normal mode
Specific Absorption Rate(SAR): Use Normal mode

2) The estimated SAR and B1+rms are displayed in scan planning page:
Specific Absorption Rate(SAR) is the RF power in watts(W) divided by the patient weight(kg) and therefore has units of W/kg.
B1+ measures the magnitude of the excitation RF field. B1+rms is the time-weighted average measured in micro-tesla (uT). It is independent of the patient characteristics, but depends upon flip angle, pulse type, number of echoes, slices, and TR.
For MRI Conditional DBS system, the typical conditions require B1+rms below 2uT.

3) Sequence Selection
In General, SE and FSE sequences will deposit more heat compared to GE/GRE sequences because they use a 90/180 excitation/refocusing pulse while GE/GRE use a single, low angle excitation pulse. SSFSE sequences are unlikely to meet B1+rms requirements.

4) General FSE sequences:
In order of effectiveness:
1) Decrease re-focusing flip angle, this can be brough down to 110 without too large of an effect on image quality
2) Increase TR - guidelines for T2 and T1 weighted sequences below
3) Decrease ETL
4) Remove Fat Saturation and Saturation bands if not necessary

5) T2 FSE TR Guidelines:
-Increase TR time, this will not affect T2 Contrast

6) T1 FSE TR Guidelines sequences:
-Increase TR time, but understand lower TR means better T1 contrast. TR>800 for FSE and TR>2400 for T1Flair will result in poor inherent T1 contrast


0:17 Whole body SAR
0:28 Head SAR
0:36 B1+RMS
2:45 T1 Volumetric
3:32 DWI
3:49 Axial T2
7:57 T2 FLAIR
9:35 Axial T1 FLAIR