Infographics / MRI Physics

k-space filling & trajectories (FSE, EPI)

A square k-space grid with kx and ky axes. The centre (kx and ky near zero) is highlighted as low spatial frequencies that set overall signal and image contrast; the periphery holds high spatial frequencies that give fine detail and sharp edges. Horizontal raster lines show Cartesian line-by-line filling where one line equals one phase-encoding step per TR. Four small panels compare sampling trajectories: Cartesian line-by-line, single-shot EPI zig-zag, radial spokes through the centre, and a centre-out spiral.

k-space: centre encodes contrast, periphery encodes detail, and common sampling trajectories A square k-space grid with kx and ky axes. The centre (kx and ky near zero) is highlighted as low spatial frequencies that set overall signal and image contrast; the periphery holds high spatial frequencies that give fine detail and sharp edges. Horizontal raster lines show Cartesian line-by-line filling where one line equals one phase-encoding step per TR. Four small panels compare sampling trajectories: Cartesian line-by-line, single-shot EPI zig-zag, radial spokes through the centre, and a centre-out spiral. k-space stores the image’s spatial frequencies centre encodes contrast and signal · periphery encodes fine detail and edges kx ky Periphery — high spatial frequencies fine detail and sharp edges (resolution) Centre (kx ≈ 0, ky ≈ 0) low spatial frequencies — signal and contrast one line = one phase-encoding step (per TR) filling order sets which echo fills the centre, so it fixes the effective TE and contrast How k-space is filled — sampling trajectories Cartesian line-by-line, one line per TR EPI single-shot zig-zag, alternating readout polarity Radial spokes through the centre Spiral centre-out
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