Gradientenspulen

Gradientenspulen are electromagnets used in magnetic resonance imaging (MRI) and other NMR-based systems to generate controlled spatial variations of the main magnetic field. They produce three orthogonal gradient fields, commonly denoted Gx, Gy, and Gz, which are superimposed on the uniform B0 field to encode spatial information.

During an imaging sequence, the gradient coils are rapidly switched on and off. The resulting spatially varying field causes spins at different locations to experience different frequencies or phases, enabling the reconstruction of spatially resolved images. The gradient waveforms determine the trajectory in k-space and thus influence resolution and scan geometry.

Gradient coils are mounted around the bore and consist of windings on non-magnetic supports. The x-, y-, and z-gradient assemblies are often designed as saddle or Maxwell-type configurations to improve linearity of the resulting field over the region of interest. Active shielding is commonly used to confine the gradient field and reduce eddy currents in surrounding structures.

Performance is described by gradient strength and slew rate. Clinical MRI systems typically offer gradient amplitudes on the order of tens of millitesla per meter, with modern installations reaching higher values for research applications. Slew rates, which reflect how quickly the gradient can change, are a key factor for fast imaging techniques. Gradient operation also involves cooling to manage resistive heating and can generate acoustic noise and, in some cases, peripheral nerve stimulation risks.

Applications include spatial encoding for conventional MRI, diffusion-weighted imaging, and functional MRI. Gradientenspulen remain a central component of MRI hardware, balancing linearity, speed, patient safety, and comfort.