“Medical grade” is less pure than what MRIs (research grade).
Medical grade or grade-4.0 is 99% pure and is meant for inhalation. “Research grade” or grade-6 is 99.9999% pure.
https://www.westairgases.com/blog/exploring-the-most-essential-and-underappreciated-uses-for-helium
Medical grade or grade-4.0 is less pure than research grade or grade-6. That’s 99% vs 99.9999%.
Medical grade might be used for inhalation by patients and traces of other air is perfectly acceptable. MRI cryocoolers need more purity to keep the magnet cold.
https://www.westairgases.com/blog/exploring-the-most-essential-and-underappreciated-uses-for-helium