brain mri timing?

I'm not quite sure what to do and I welcome any input. My wife, Caryn, was diagnosed with mets to lungs and liver by PET and CAT in July. She just completed her 3rd week of gemzar/herceptin and I'm nervous about the prospect of brain mets. When I questioned him on brain scan he said that it was covered by the Pet. From all of the great input here I know that the PET is not nearly as sensitive as an MRI. As a result, I want her to get an MRI but I'm not sure the best timing for it. Because of the cost I'm sure they won't do these frequently so you almost have to get lucky to catch anything early enough. She's scheduled for follow up scans in September and I was thinking of pressing the issue then but I do get very nervous with every headache she has (most likely from medications). Plus doing it along with the other scans would'nt alert her to any fears that I'm having. Any suggestions are welcome.

Thank you, Eric