Cheryl and Zuni gave good advice. Unfortunately, one of the side effects of Effexor is anxiety. Don't get me wrong--I'm not saying that's why you're having anxiety attacks, just it's one of side effects listed. Do you think you're having these anxiety attacks because you're nervous about the PET scan? If so, keeping busy should work. Or do they just come out of the blue? If that's the case, you might want to discuss this with your physician, or therapist, if you see one.
Hugs & prayers,