Have you watched the beginning of video 8? I try to re-explain the compressed sensing problem there and I think I do a clearer job than in video 7.
A: is the projection matrix, based on the location and angles where the x-ray passes (for each of the k x-rays taken). These are unraveled to make a k by (l x l) matrix.
This is always the same and doesn’t depend on what is being x-rayed.
Each box below is flattened to be one row of the projection matrix:
x: what we’re solving for, the thing being x-rayed
b: intensities measured by the x-ray passing through what is being x-rayed