Fusion produces much, much less helium than that. D-T fusion that produces 100kg of He-4 would produce something like 25PJ of net primary energy. If you managed to turn 20% of that into electricity, you'd have 5PJ. This is within an order of magnitude of the yearly electricity consumption of the entire world.
I do actually have receipts. For example numbers I picked Washington DC, which has 700k people that each use an average of 600 kWh per month. That's 5 TWh / year, which I rounded up to 10 TWh. This yields 475 m^3 of helium, which is 85 kg. I rounded up to 100. DC is a modest city and all of humanity uses 15000 TWh / year (of heat). This "100 kg / city / year" estimate is even on the lower end since I started with electric consumption and never put in the factor of 3 to convert to thermal.
Your mass-energy conversion number is within error of mine, but your estimate of how much power we use is much lower.
A single MRI needs around 1000 liters, 1L is roughly 130g. For those who wonder prices vary between $15-35 per liter. Recent MRIs have recycling systems that reduce refill needs by 90% when they work properly.
