A pulse sequence is the pattern of RF pulses and gradient waveforms used to acquire MRI data. Different sequences exploit T1, T2, and proton density contrast to highlight different tissue properties.

Spin Echo (SE)

A 90° excitation pulse followed by a 180° refocusing pulse after time TE/2 creates an echo at time TE. The 180° pulse reverses static field inhomogeneity dephasing but not true T2 decay, making SE sensitive to tissue T2. Repetition time TR controls T1 weighting. SE is robust to B0 inhomogeneity.

Gradient Echo (GRE)

Uses a flip angle < 90° and a gradient reversal (instead of 180° RF pulse) to form the echo. Faster than SE but sensitive to B0 inhomogeneity (T2* rather than T2). Used for fast imaging, functional MRI (BOLD), and angiography.

Inversion Recovery (IR)

A 180° inversion pulse followed by a delay TI before a spin-echo acquisition. By choosing TI = T1·ln(2) for a tissue, its signal is nulled. FLAIR (Fluid Attenuated Inversion Recovery) nulls CSF to improve lesion conspicuity. STIR nulls fat.

k-Space

MRI data is acquired in the spatial frequency domain (k-space). Each data point corresponds to a different spatial frequency component of the image. The image is reconstructed by a 2D Fourier transform. Low spatial frequencies (near k-space centre) determine image contrast; high frequencies (periphery) determine edge sharpness. Trajectories through k-space define the acquisition strategy: Cartesian, radial, spiral, EPI, etc.