I would like to convert an MRI (a set of image slices) into an open USD file. This will allow visualization, eh?

Thoughts on approaches?

I am not sure what you are asking. An MRI image is 2D. You are looking to apply the image to a plane and add it to a usd ? Multiple mri images added to planes and turned into a collection of slices ?

Or are you asking how to get a 2D image converted into 3D ?

Or are you starting with a 3D data scan from an mri and trying to convert that into usd ? This is already possible. You just need to convert your 3D scan to a common CAD format and then from there convert it to usd.