Back pain for three months i just went to the doctor last week and he said i needed a mri but i don't know if i can get one. should i just do it soon?

Health comes first!: Yes, back pain that has lasted 12 weeks in an 18 year old male is something that deserves a thorough evaluation. Hopefully, you have already had basic blood tests and plain X-rays, along with a comprehensive physical exam and history. The next study would be an MRI. I'm also going to assume you've had a testicular exam (it's very relevant) . Trauma/sports injuries have been considered.