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Magnetic resonance imaging (MRI) has become one of the routine modalities for clinical imaging. It allows for excellent image contrast, especially in soft tissue, without applying any radiation to the patient.
MRI scanner An MRI scanner comprises essentially four parts: the main magnet, providing a stable homogeneous magnetic field Bz to align the proton spins; at least one RF coil sending and receiving at the Larmor frequency ω = γBz, γ being the gyromagnetic ratio, to excite spins into their higher energy state and detect the emitted radiation from relaxation into the lower state; some hardware to collect these signals and transform them into images; and a set of three gradient coils to superimpose magnetic fields to vary Bz in three orthogonal directions, that is, creating magnetic fields with a given equation. These superimposed fields encode the spatial information into the signal, ω = ω(w,y,z).
Gradient coils Design of gradient coils is an inverse problem: We know the effect, a magnetic field with a given gradient of its component collinear to a and want to find the cause, a current distribution in the gradient coil. Several constraints need to be satisfied, for example maximum current density, maximum force and torque, size restrictions, and shielding. Shielding means that the stray field of the gradient coil has to be kept away from the superconducting main magnet to avoid image distortions caused by eddy currents in the conductive cryovessel. Another important objective is minimizing the coil inductance, since fast imaging methods require rapid switching of gradient fields.
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