[Freesurfer] improvement of segmentation qualitiy by setting control points

Dear freesurfer community,

we are conducting cortical-thickness analysis and we have encountered some problems with setting control points to improve segmentation quality. Mainly, white matter was classified as grey matter after completion of the first run that incorporated control points. Now we have several questions, we would be very thankful for your comments:

1. According to your expereince, how many control points for the overall brain are usually necessary to obtain good segmentation results?

2. Are control points necessary at all? Is there a way to control the segmentation quality?

3. Is there any visual example on how to best set control points?

Thank you for your help,

Annemarie