Hello Kacee, Welcome!
Please excuse my tardy response. I've been off the boards due to an illness - but your question and concern is very important to us.
I understand your concern. I'd have concerns also. You are going about this decision making process correctly. Learn as much as you can about your DX (diagnosis) it's treatments and the pros and cons of each one. Educate yourself so your decision can be based on facts.
There are several types of "dyes" or contrasts used when having an MRI. which contrast will often depend on the "area" they are trying to view. Ask which contrast be used and do your reading. Understand that your relatives may not have had the same contrast - and as always each one of us can react differently to a substance or medication. The contrasts used today are considered (for the most part) to be safe. In the past ( I won't go into details or specific names of contrasts) that has not always been true. Science has learned a lot. I am not dismissing your concern. Again, I'd have the same. Research the specific contrast that will be utilized.
I have read that "Anaphylactoid reactions are rare, occurring in approx. 0.03–0.1%." Indeed I have never heard of one. If that's your concern know that you will be observed closely with medical personal very close by. They are trained in handling any reaction.
The fusion!! Wow! You are so young! That is a major step - and again concerning. It's factual that often the vertebrae above and below a fusion weaken. More surgery is often indicated - or required. Again I can only tell you to educate yourself. Remember we often have different experiences. While one may have little issues following a spinal fusion or repair - another may not be as fortunate. It can be a crap shoot. What I do know is the skill of your surgeon, including experience factors and outcomes often has a large bearing on surgical repair. You have a right to know the number of same surgeries the surgeon has preformed - along with his statistics. Ask for this information - and research his/her background on the various Physician Review and Reports. Select your surgeon wisely.
I hope something I have said has been of help. If you have more questions - or want more specifics please feel free to ask. We'll do our best to provide you with information. We are not physicians - nothing any of us have to say should be used for medical decisions - just food for thought.
I wish you the very best and hope to hear from you again, soon. Continue to be diligent - your approach and course is exactly what I and most of us would take.
Take Care,
~Tuck