The raw scan is saved as a DPX or EXR sequence. These are uncompressed (or losslessly compressed) log files. Even with modern compression, a feature film fits on a hard drive the size of a pizza box. But that drive weighs a lot.
These scanners use a pin-registered gate. Unlike cheap "sprocket" transports, pin registration pushes precision pins into the perforations of the film to lock the frame perfectly flat. For IMAX, even a micron of wobble translates to visible blur when projected on a 100-foot screen.
Christopher Nolan is the foremost proponent of this process. For the Dark Knight Blu-ray releases, the IMAX sequences were scanned separately from the 35mm sequences. imax film scan
There is a growing community of "cine-archivists" trying to DIY an IMAX film scan. With the bankruptcy of Kodak’s motion picture division (unless revived), some collectors own actual IMAX prints.
The DIY method:
Result: You will get optical resolution of about 2K due to lens softness. You will also scratch the film. It is not recommended unless you are treating the film as "expendable."
When you scan 35mm, you see grain. When you scan IMAX at 11K, you see the structure of the grain. You see the texture of the emulsion. You see dust that is smaller than a human blood cell. The raw scan is saved as a DPX or EXR sequence
Here is the hardest part: Focus.
Because an IMAX lens captures so much depth, scanning focus is a nightmare. A human operator zooms into 2000% on a specific speck of dust on the edge of the frame. They adjust the scanner’s lens by micrometers. Why? Because if the sprocket hole is sharp but the center of the frame is soft, the entire three-second shot is ruined. These scanners use a pin-registered gate