High power systems for dynamic field control and shielding in the MR environment

This thesis addresses several aspects of gradient and shim coil design and fabrication. New design techniques are coupled with experimental construction methods to expand small animal insert gradient and shim technology. The design techniques are also applied to other areas of magnetic resonance hardware. A custom 2-axis gradient insert coil is designed and fabricated for the purpose of eddy current characterization. The construction tolerances were examined via bench top inductance measurements and eddy currents measurement inside a 7.0 T head-only MR system. A great deal of freedom is available when positioning shielding coils with respect to their corresponding primary coils in small animal inserts before eddy currents become prohibitive for imaging. A new method for actively shielding electromagnets is presented. The minimum energy method for designing shielding coils of any geometry is developed and validated against historical methods. Several shielded gradient insert coils are designed, including a cylindrical gradient set with rectangular shields, which demonstrates the versatility of this new method. The performance of the shielded insert coils is reported. A high power custom shim insert coil is designed and optimized for dynamic shimming applications. This 10-axis shim insert coil is designed to operate at currents higher than any previously existing shim sets. Several experimental fabrication methods are tested during the construction of the insert coil. Inductance, resistance and cooling measurements are conducted and compared to design specifications. Field measurements are taken using a 3-axis field transducer and the shim efficiencies are calculated. Finally mutual inductance measurements are taken between strongly coupled axes to verify active shielding performance. Lastly, the minimum energy method for active shielding is applied to several MR fringe field type problems. Shields are designed to conform to rooms within an imaging facility for the purpose of controlling the magnetic footprint of an MR system. The MR room itself it designed to house an active shield, along with rooms adjacent to the MR room and a small equipment cabinet located inside the MR room is also fitted with a shield. The performance of the shields is calculated, and the feasibility of such shields is discussed

Shielded resistive electromagnets of arbitrary surface geometry using the boundary element method and a minimum energy constraint.

Eddy currents are generated in MR by the use of rapidly switched electromagnets, resulting in time varying and spatially varying magnetic fields that must be either minimized or corrected. This problem is further complicated when non-cylindrical insert magnets are used for specialized applications. Interruption of the coupling between an insert coil and the MR system is typically accomplished using active magnetic shielding. A new method of actively shielding insert gradient and shim coils of any surface geometry by use of the boundary element method for coil design with a minimum energy constraint is presented. This method was applied to shield x- and z-gradient coils for two separate cases: a traditional cylindrical primary gradient with cylindrical shield and, to demonstrate its versatility in surface geometry, the same cylindrical primary gradients with a rectangular box-shaped shield. For the cylindrical case this method produced shields that agreed with analytic solutions. For the second case, the rectangular box-shaped shields demonstrated very good shielding characteristics despite having a different geometry than the primary coils

Dense sampling of bird diversity increases power of comparative genomics

Whole-genome sequencing projects are increasingly populating the tree of life and characterizing biodiversity(1-4). Sparse taxon sampling has previously been proposed to confound phylogenetic inference(5), and captures only a fraction of the genomic diversity. Here we report a substantial step towards the dense representation of avian phylogenetic and molecular diversity, by analysing 363 genomes from 92.4% of bird families-including 267 newly sequenced genomes produced for phase II of the Bird 10,000 Genomes (B10K) Project. We use this comparative genome dataset in combination with a pipeline that leverages a reference-free whole-genome alignment to identify orthologous regions in greater numbers than has previously been possible and to recognize genomic novelties in particular bird lineages. The densely sampled alignment provides a single-base-pair map of selection, has more than doubled the fraction of bases that are confidently predicted to be under conservation and reveals extensive patterns of weak selection in predominantly non-coding DNA. Our results demonstrate that increasing the diversity of genomes used in comparative studies can reveal more shared and lineage-specific variation, and improve the investigation of genomic characteristics. We anticipate that this genomic resource will offer new perspectives on evolutionary processes in cross-species comparative analyses and assist in efforts to conserve species. A dataset of the genomes of 363 species from the Bird 10,000 Genomes Project shows increased power to detect shared and lineage-specific variation, demonstrating the importance of phylogenetically diverse taxon sampling in whole-genome sequencing.Peer reviewe