gradientpulsen

Gradientpulsen, in the context of magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR), are brief, time-limited variations of a magnetic field gradient produced by dedicated gradient coils. When a gradient is applied, the magnetic field varies linearly with position along the gradient axis, causing a position-dependent Larmor frequency. This allows spatial encoding of the signal or selective manipulation of spins.

In MRI, gradient pulses are essential for encoding spatial information. Three independent gradient axes (x, y, z) enable three-dimensional encoding. A gradient applied during radiofrequency excitation selects a slice, while gradients used during data acquisition encode phase (phase-encoding) and frequency (readout) information. The phase accumulated by spins is proportional to the integral of the gradient over time and the spin position, φ = γ ∫ G(t) · r dt, where γ is the gyromagnetic ratio. The design and timing of gradient waveforms affect image resolution, distortion, and artifact levels. Gradient amplitude, duration, and slew rate (how quickly the gradient can be switched) are key specifications, with typical clinical scanners delivering tens of mT/m and slew rates enabling rapid switching.

Gradient pulses are also used to sensitize MRI signals to diffusion. Diffusion-weighted sequences apply a pair of gradient lobes (often bipolar) to dephase and rephase spins; the net signal loss depends on molecular diffusion and is summarized by the diffusion weighting parameter b. The resulting images highlight tissue microstructure and are widely used in neuroradiology and other areas.

Outside MRI, gradient pulses are employed in NMR spectroscopy for coherence selection and phase cycling. Safety and hardware constraints, including patient comfort, peripheral nerve stimulation, and coil heating, govern practical gradient design and operation.