Infographics / MRI Physics

Slice-select, phase & frequency encoding gradients

A pulse-timing diagram with five lanes. RF shows a 90-degree then a 180-degree pulse. Gs slice-select on the z-axis is switched on during each RF pulse and has a rephasing lobe of opposite polarity. Gp phase-encode on the y-axis is a stepped blip applied between the 180-degree pulse and readout, one k-space line per repetition. Gr frequency-encode or readout on the x-axis is switched on during signal acquisition, centred on TE. The signal lane shows the echo forming at TE. Dashed drop-lines mark that the RF pulses coincide with the slice-select lobes; the time axis marks the 90-degree pulse, the 180-degree pulse at TE over two, and the echo at TE.

Timing of the three orthogonal spatial-encoding gradients in a spin-echo sequence A pulse-timing diagram with five lanes. RF shows a 90-degree then a 180-degree pulse. Gs slice-select on the z-axis is switched on during each RF pulse and has a rephasing lobe of opposite polarity. Gp phase-encode on the y-axis is a stepped blip applied between the 180-degree pulse and readout, one k-space line per repetition. Gr frequency-encode or readout on the x-axis is switched on during signal acquisition, centred on TE. The signal lane shows the echo forming at TE. Dashed drop-lines mark that the RF pulses coincide with the slice-select lobes; the time axis marks the 90-degree pulse, the 180-degree pulse at TE over two, and the echo at TE. Timing of the three spatial-encoding gradients in a spin echo RF Gs (z) Gp (y) Gr (x) Signal 90° 180° slice-select on during 90° and 180° + rephasing lobe (opposite polarity) stepped: one k​y line per TR on during signal acquisition (readout) echo forms at TE time 90° 180° TE TE (90° to echo); 180° at TE/2 Gs slice-select (z) on during RF; Gp phase-encode (y) stepped, one line per TR; Gr frequency-encode (x) reads out the echo at TE. Together they localise signal in 3D and fill k-space.
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