Chance of infection is very, very meagre..
At the outset, I advice you not to worry.
Chance of HIV
through the described set of events is very, very, very meagre. I would rate your chances as almost nil. I have not heard or read about HIV transmissions indirectly from such means.
To validate further, presuming the blood was from the infected lady:
1. Viral particles on a small stain of blood is usually not very high to cause infection.
2. HIV virus is a highly fragile virus. It dies in few minutes of contact with external environment. I am pretty sure it wouldn't have survived radiations within the MRI
room though the radiations are not very strong.
3. Even if it had survived during the whole course of scan up to your scratching event, chances of it causing infection is said to be less than 1%
4. The current DNA PCR tests are developed from recent state of art and they are extremely sensitive. It hardly gives false negative result. And so there is no scope to doubt the negative result on 17th day.
I hope these facts help you realize that you will not be infected. So in my opinion, you should not be worried anymore.
Let me know if you need clarifications.