Timothy Patrick Eagan

Active-passive gradient shielding for MRI acoustic noise reduction

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.

Application of the SUSHI method to the design of gradient coils

An approach to potential improvements in magnetic field shielding for a gradient coil system with cylindrical geometry is presented, utilizing “supershielding” conditions for the currents on both the primary and the secondary coils. It is demonstrated that the field can be strongly suppressed everywhere outside a cylindrical shield coil radius, even though the finite‐length active shield only partially surrounds a primary coil. The supershielding method, which is aimed at controlling eddy currents, still has sufficient freedom to maintain the desired magnetic field behavior inside the imaging volume. The trade‐off is an additional primary current oscillation and increased current peaks and field energy. This method has been applied to design short transverse and axial gradient coils, giving substantially improved shielding compared to an apodization method. Magn Reson Med 45:147–155, 2001.

Design of actively shielded main magnets: an improved functional method

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.

A comparison of two design methods for MRI magnets

Designs of magnetic resonance imaging (MRI) main magnets obtained from both a functional method and a genetic algorithm method have been compared. While most features in the two approaches are similar, there are several important differences. The functional method leads to fewer coil bundles and a reduced total current, i.e., total ampere turns, (e.g., 6-8 MA) that can be as much as 70% of the total current found with the genetic analysis. While the conclusion about stress is that it is a sensitive function of the choice of wire current density, the designs found with the functional method have a larger hoop stress than that of the genetic design, which may require new or refined manufacturing techniques. Furthermore, the functional approach requires much less computing power (i.e., a personal computer is quite sufficient) while the genetic algorithm method in general demands massively parallel computing techniques. However, in order to search for the optimal magnetic resonance design at a given field strength, it is likely that a combination of these two methods will lead to the best results.

Active-Passive Shielding for MRI Acoustic Noise Reduction: Network Analysis

We have modeled the effect of passive copper shielding applied to the outside of an actively shielded, axisymmetric z-gradient coil assembly, with the aim of substantially reducing induced eddy currents in the cryostat inner bore that create acoustic noise. For the purpose of calculation, the cylindrical cryostat inner bore and the passive copper shielding are coaxial and are imagined to be sliced into thin ring sections. Each ring section has a finite thickness and is further divided into several concentric layers. The thin cylindrical sections become elements in an electrical network that includes the actively shielded gradient coil. We calculate eddy currents both for single frequency excitations and for time-dependent excitations by modeling a series of trapezoidal pulses. Our results take into account time dependence, diffusion of eddy currents among cylindrical sections and skin depth effects. Two configurations are analyzed. The first is a thin copper layer wrapped around the outer diameter of the gradient assembly, and the second extends the copper over the ends of the gradient assembly. A 2-mm-thick copper layer around the gradient assembly reduces power deposited in the cryostat inner bore by 13.5 dB for a 1 kHz gradient excitation. Extending the passive shield, for the same 2 mm thickness, to cover the ends of the gradient reduces cryostat inner bore power deposition, by 26.7 dB for the same frequency

Toward shielding improvements in MRI gradients and other systems

Abstract A new method is described for reducing the shielding-error function in the ‘supershielding’ approach to designing MRI systems. The method is thus shown to lead to significantly better shielding and better control of eddy current effects associated with gradient coils. To illustrate this technique, a set of results for a z-gradient coil is presented. A generalization to non-standard geometries can be made in a straightforward manner with the new method. The usefulness of the relationship of all fringe-field quantities to the shielding-error function is emphasized. The formal limit of perfect shielding in a ‘least-squares’ sense is shown for a simple strip-shield model along with a numerical eigenvalue study for comparison with the theoretical limit.

An Antenna-Theory Method for Modeling High-Frequency RF Coils: A Segmented Birdcage Example

We suggest that center-fed dipole antenna analytics can be employed in the optimized design of high-frequency MRI RF coil applications. The method is illustrated in the design of a single-segmented birdcage model and a short multisegmented birdcage model. As a byproduct, it is shown that for a long single-segmented birdcage model, the RF field within it is essentially a TEM mode and has excellent planar uniformity. For a short shielded multisegmented birdcage model, the RF field is optimized with a target-field approach with an average SAR functional. The planar homogeneity of the optimized RF field is significantly improved compared with that of a single-segmented birdcage model with the same geometry. The accuracy of the antenna formulae is also verified with numerical simulations performed via commercial software. The model discussed herein provides evidence for the effectiveness of antenna methods in future RF coil analysis.