Paul Chang, Sahar Nassirpour, Ali Aghaeifar, Klaus Scheffler, Anke Henning

In this work, we evaluated the performance of two B₀ shimming approaches (i.e. a very high order spherical harmonic shim system versus a multi-coil shim setup), for dynamic shimming in the context of high resolution and multi-slice metabolite mapping of the human brain at 9.4T. 

Ryan Topfer, Alexandru Foias, Nibardo Lopez Rios, Angel Chauffray, Grégoire Germain, Nick Arango, Lawrence Wald, Jason Stockmann, Julien Cohen-Adad

Spinal cord imaging is hampered by static and respiration-induced dynamic variations of B0, causing serious issues for EPI-based protocols and spectroscopy. Integrated ΔB0/Rx arrays could provide an effective means to further compensate small-scale variations, complementary to the existing spherical harmonic coils. Here, we designed, built and tested in a human subject an 8-channel integrated ΔB0/Rx coil for the cervical spinal cord. Results showed improvements for correcting static field variations as well as respiratory-induced variations. Future studies will investigate dynamic and real time shimming applications for the spinal cord.

Yuhang Shi, S. Johanna Vannesjo, Stuart Clare

The implementation of dynamic shimming relies on determining robust and accurate shim currents. This work presents a novel, analytical, and fully automated regularized shim determination technique to solve ill-conditioned least-square problems and regularize current constraint challenges for dynamic shimming applications. The method is based on the Tikhonov regularization whereby the L-curve method is utilized to search for an optimal shim solution. The method improves shim current use efficiency and conditioning of the shim determination problem, outperforming the truncated singular value decomposition regularization based least-square method. 

Michael Schwerter, Chan Hong Moon, Hoby Hetherington, Jullie Pan, Lutz Tellmann, Jörg Felder, N. Jon Shah

Dynamic shim updating using spherical harmonics is an effective B₀ shim technique, but known to induce eddy currents which degrades the achievable shim quality. Current DSU implementations therefore use pre-emphasis which requires additional hardware and time-consuming system calibrations. To reduce eddy current generation, we have implemented an optimization algorithm which limits the maximum inter-slice shim current change. It is based on the assumption that a smooth variation of shim currents with small current steps will substantially reduce eddy currents. Simulations and initial experiments have shown that eddy currents can be drastically reduced without significant impact on the achievable shim quality.

Jiazheng Zhou, Jason Stockmann, Nicolas Arango, Klaus Scheffler, Lawrence Wald, Fa-Hsuan Lin

We purpose an integrated RF-shim coil array, where the shimming current path and the RF receiving coil are arranged on two orthogonal planes to minimize the mutual coupling. This design was implemented as an array consisting of seven shim coils and a 32-channel RF-shim array. The 39-channel RF-shim coil array has marginal SNR loss and improved the global shimming by 9%, compared to the 32-channel RF-shim array. 

Devin Willey, Dean Darnell, Allen Song, Trong-Kha Truong

Integrated parallel reception, excitation, and shimming (iPRES) coil arrays enable simultaneous image acquisition and localized B₀ shimming with a single coil array. We propose to apply this novel technology to recover signal loss in fMRI by using a new shim optimization algorithm. Simulations and in vivo experiments performed with a 32-channel iPRES head coil array demonstrate that the proposed method can reduce both the B₀ inhomogeneity and the signal loss in gradient-echo EPI images, particularly in the inferior frontal brain region, resulting in more activation in that region in a breath-holding fMRI experiment.

Thomas Foo, Dominic Graziani, Yihe Hua, Ye Bai, Justin Ricci, Christina Vasil, Naveenan Thiagarajan, Eric Fiveland, Alex Kagan, Joseph Piel, Ek Tan, Mark Vermilyea, Charles Helms, Franklyn Snell, Derek Seeber, William Einziger, David Lee, Peter Roemer, Jean-Baptiste Mathieu, Maureen Hood, Heechin Chae, Vincent Ho

An ultra-high gain asymmetric gradient coil design for imaging the brain microstructure is described. This design has greater than 4 times the gain of existing whole-body gradient coils and 3x that of the Compact 3T gradient coil with similar inner diameter. With a 1 MVA driver, this gradient coil is designed to deliver 200 mT/m at a maximum slew rate of 500 T/m/s.

Chan Hong Moon, Michael Schwerter, Jullie Pan, N Jon Shah, Hoby Hetherington

Dynamic shimming compared to global shimming improves B₀ homogeneity over whole brain. However, typical shim optimization could result in high currents, particularly for small volume, e.g. small slab or slice. To address this we investigated the extent to which large reductions in aggregate shim current could be achieved through constrained optimization at 7T. We also investigated the extent to which alternate imaging orientation could improve homogeneity. The results show constrained optimization can provide robust shimming at substantially reduced aggregate current values, and angulated imaging improved shimming condition. The proposed methods enhanced multi-slice imaging by reducing both susceptibility-induced signal losses and distortions.

Gary Liney, Bin Dong, Kevin Zhang, Urszula Jelen, Jarrad Begg, Amy Walker, Robba Rai, Ewald Weber, Ricky O'Brien, Michael Barton, Stuart Crozier, Paul Keall

This study describes the progress that has been made using a dedicated split bore 1.0 Tesla magnet to provide an MRI-guided radiotherapy system. This magnet design has a number of advantages including patient entry in two orientations to investigate competing magnetic field-beam configurations. A number of beam and imaging experiments, including first in vivo results, are described which show the efficacy of the system as an integrated inline MRI-Linac.

Yixin Ma, Dean Darnell, Huimin Zhang, Fraser Robb, Allen Song, Trong-Kha Truong

In breast imaging, the image quality is severely degraded by susceptibility-induced B₀ inhomogeneities. Here, we apply the novel integrated parallel reception, excitation, and shimming (iPRES) technology to enable simultaneous image acquisition and localized B₀ shimming of the breasts with the same coil array. Proof-of-concept in vivo experiments with only four iPRES(1) breast coil elements already show up to 43.3% reduction in B₀ root-mean-square error with no SNR compromise. Simulations show that the shimming can be further improved by implementing more complex iPRES(N) geometries with N smaller shim loops per RF coil element.