An important source of MRI acoustic noise-magnet cryostat warm-bore vibrations caused by eddy-current-induced forces-can be mitigated by a passive metal shield mounted on the outside of a vibration-isolated, vacuum-enclosed shielded gradient set. Finite-element (FE) calculations for a z-gradient indicate that a 2-mm-thick Cu layer wrapped on the gradient assembly can decrease mechanical power deposition in the warm bore and reduce warm-bore acoustic noise production by about 25 dB. Eliminating the conducting warm bore and other magnet parts as significant acoustic noise sources could lead to the development of truly quiet, fully functioning MRI systems with noise levels below 70 dB. Magn Reson Med 53: 1013-1017, 2005.
A method for finding closed-form solutions for the normal mode frequencies of systems with circulant (2 pi/n) symmetry was investigated. This method is particularly useful for questions of degeneracy that arise when one considers parallel imaging techniques like SENSE and SMASH in MRI. It is applicable to systems that include birdcage coils as well as planar coils with the appropriate rotational symmetry. A proof is given that complete degeneracy of all normal mode frequencies is impossible when all mutual inductive couplings are included. We tested the method against measurements made on a planar coil array and on an 8-element birdcage coil. The inclusion of the co-rotating end-ring mode changes the fundamental symmetry of the system from circulant to 'bordered circulant.' Closed-form solutions for the normal mode frequencies of a bordered circulant system are also given.
An improved functional approach for designing MRI (magnetic resonance imaging) main magnets with active shielding is presented. By nulling one or two external moments as well as a certain series of internal moments of the magnetic field, new designs with improved shielding in combination with or without shorter magnet lengths are obtained. The improved method can be employed to design short and practical superconducting magnets at any given field strength. The resulting designs yield the desired field homogeneity inside the region of interest without using superconducting shim coils. This approach requires only a modest amount of computing power. One of the design steps, a contour plot of the continuous current solutions, can be utilized to study stretch goals for favorable design parameters.
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