I paid for an MRI for my reassurance when I was convinced I had a brain tumour. Regardless what people around me had to say on that it was so worth it, I was so deep in my brain tumour fear I honestly feel like nothing else would be able to get me out if it, so if someone reaches that point and feels like that is the only way for them - I guess it can help.
That being said, it definitely doesn't make your HA go away, a week after my MRI I started obsessing over MS and got anxiety symptoms consisted with that. I got MRI without contrast, which as far as I know doesn't always show MS. I did ask for a contrast one at the time as I was worried this would happen, but the radiologist doing it thought it was not necessary (he was probably right, but tell that to my anxiety). So if you get it for reassurance fair enough, but it definitely isn't a magical cure, sadly anxiety is way trickier than that.