The readout MR signal is a mix of RF waves with different amplitudes, frequencies and phases, containing spatial information. This signal is digitized and raw data are written into a data matrix called K-space. K-space data are equivalent to a Fourier plane. To go from a k-space data to an image requires using a 2D inverse Fourier Transform.
This chapter introduces the most complex concepts of MRI image formation for many students, so take to your time to follow this step.
Understanding k-space and Fourier transform will help you understand the trade offs that are made for fast imaging and sequences optimisation, as well as the source of various artifacts.