Yuri Lvovsky

Superconducting systems for MRI-present solutions and new trends

Over the past two decades, magnetic resonance imaging (MRI) has developed into a mature technology, and is the leading commercial large-scale application of superconductivity. This is still a rapidly evolving field, characterized by constantly emerging configurations requiring innovative technical solutions, with trends toward increasing field strength B/sub 0/ and more advanced magnetic, cryogenic and integrated system design. This paper describes the main technical approaches and challenges in MRI superconducting electromagnetic design, trade-offs in the magnet parametric design space and their effect on the superconducting design. Recent trends, advanced novel configurations and different applications are discussed. The trend toward higher fields manifests itself in the ongoing shift of clinical applications to 3 T, as well as in recent developments of whole-body MRI with field strength up to 9.4 T. Illustrations are presented from commercial and developing MRI systems.

Index Loss Effect in n-Value Measurements of Commercial Superconductors

n-value of the superconductor is key parameter in the persistent magnet design, affecting cost and performance. Analysis is presented that shows that a few mK overheat from the nonlinear index loss during standard Ic measurement of NbTi and Nb3Sn conductors in the helium bath can significantly affect the measurements and produce artificially high n-values. The error increases in conductors with higher n. Effect of low heat transfer, Eref criterion and index stability crisis are discussed. Analytical correlation is presented between the apparent and actual n-values.

Parametric Relations in Multicoil Cylindrical MRI Magnet Design

In MRI magnet design, optimized configurations are generated by different manufacturers, which are driven by the pursuit of competitive cost and performance. It is the magnet envelope and the field constraints such as homogeneity and stray field that largely define the coil topology. Understanding the scaling laws and relations that govern sensitivities in the design space gives a designer a tool for scoping and tradeoff analyses, guiding his selection of design options. A study is presented that produces scaling relations and characterizes the amount of conductor, the number and location of coils as a function of the magnet envelope, and field requirements. Described is the birth of coils and their movement with an expanding field of view, as well as impacts of shielding and of shorter envelopes. Derived relations are given in parametric form, which is described by dimensionless complexes.