PGSE

PGSE, or Pulsed Gradient Spin Echo, is a diffusion-weighted imaging sequence used in nuclear magnetic resonance and magnetic resonance imaging to measure molecular diffusion. It encodes the displacements of spins by applying a pair of brief, equal-strength diffusion-sensitizing gradient pulses separated by a diffusion time. Variations in spin position during the interval lead to incomplete rephasing and reduced echo signal, with the attenuation reflecting the underlying diffusion process.

In MRI, the signal S from a voxel follows S = S0 exp(-b D), where D is the apparent diffusion coefficient (ADC) and b is a parameter determined by the gradient waveform. For two rectangular gradient pulses of duration delta separated by time Delta, b = gamma^2 G^2 delta^2 (Delta - delta/3). Here gamma is the gyromagnetic ratio of the nucleus (for protons, about 2.675×10^8 rad s^-1 T^-1), G is the gradient amplitude, delta is the gradient duration, and Delta is the separation between pulses. The diffusion-weighted signal decays with larger b or larger D, reflecting restricted molecular motion.

Applications include diffusion-weighted imaging to detect acute ischemia, characterize tumors, and quantify tissue microstructure. In diffusion tensor imaging (DTI), measurements are taken along multiple gradient directions to estimate a diffusion tensor, from which metrics such as fractional anisotropy are derived.

Limitations include sensitivity to subject motion, bulk flow, T2 weighting, and gradient nonlinearity. The PGSE framework originated with Stejskal and Tanner in 1965, describing the diffusion-encoded spin-echo approach that underpins much of diffusion MRI and NMR diffusion measurements.