Conventional ultrasound scanners are already cheap. Why can't a big ultrasound scanner be cheap too?

What makes MRI machines expensive is that they are big helium-cooled superconducting magnets that have to be continuously kept at a few Kelvin.

As others are saying in these comments, MRI machines themselves aren't particularly expensive machines on a per-scan basis, to the extent the machines themselves are often left underutilised.

But even if you disregard that, there's this:

  It starts by stepping into a shallow pool of golden light. You then begin to descend into the water. Your body passes through a ring of underwater sensors, each acting like a dolphin, using its echolocation. The sensors send ultrasonic sound waves through your body from every angle. With enough waves, and enough angles, we form an image of what's happening inside your body.

  The goal is for this process to take no more than 60 seconds.

  You go into the water, you come out of the water, and you're done.

Note this does not mean I think the hardware proposed here is totally impossible*. Sure you could make an ultrasound scanner. Why not? But then, hyperloop was always physically possible, just never turned out to be a good idea to actually build**.

* That said, I am suspicious about the claim in the video "Each sensor resolves motions smaller than the width of an atom - not micrometers or nanometers but picometers!", which does sound impossible to me given the movement of atoms is the sense field itself, albeit I'm not an expert in this domain and may just be wrong like how there's weird tricks for photolithography smaller than the wavelength of light used.

** Back when hyperloop was taken seriously and I was still looking for genius behind things Musk said, I thought hyperloop was an excuse to develop here on Earth a transport system that for a Mars colony made more sense than cars and roads (and indeed I still think that, just there's no evidence Musk ever did).