If I had MRI in 2019 and I am still paying for it, does any of my payments qualify as medical expense?

Who are you on a payment plan with?  The medical facility?  If so, you can only use the amount you actually paid to them in 2019, not any payments that you will make in 2020 or later.

Medical payments are deductible on the date you pay the provider.  If you have been paying the provider, let's say $50 per month, then your 2019 deductible amount is the amount you paid to the provider.  If you paid the provider in full from a credit card, bank loan, or other payment arrangement, then the provider was paid in full and you can deduct t the entire amount, even though you are making payments to the lender or credit card.  (Your payments to the lender are never deductible medical expenses.)