My neuro ordered my thymus scan with contrast, but the radiologist insisted on doing it without. He (the radiologist) said that contrast wouldn't help at all. So I don't know who's right.

Iodinated contrast dyes are on the list of "drugs to be avoided or used with caution" by myasthenics:
http://www.myasthenia.org/docs/MGFA_MedicalInfoCard.pdf

Abby