Figure 2.

The evolution of MRI sampling schemes. Illustrated using 2D k-space sampling patterns. (a) Shows a radial scheme, the one MRI actually started with. (b) Cartesian sampling which took over due to its simplicity and robustness against hardware imperfections. (c) Spiral k-space sampling which maximises the sampling efficiency. (d) Hybrid radial–Cartesian sampling which is motion robust (each blade allows low resolution motion navigation). (e,f) Sub-sampling schemes for the dominant Cartesian schemes: (e) uniform sub-sampling as used in parallel imaging applications like SENSE and (f) variable density sub-sampling. A partially uniform version of (f) is used in GRAPPA while a more random version is used for compressed sensing.