ISMRM 24th Annual Meeting & Exhibition • 07-13 May 2016 • Singapore

Scientific Session: Artefacts: System Imperfections & Implants

Monday, May 9, 2016
Room 334-336
10:45 - 12:45
Moderators: Pablo Irarrazaval, Dinghui Wang

Evolution-time encoded single-scan cross spatiotemporal encoding imaging near metal implants
Zhiyong Zhang1,2, Amir Seginer1, and Lucio Frydman1
1Chemical Physics, Weizmann Institute of Science, Rehovot, Israel, 2Electronic Science, Xiamen University, Xiamen, China, People's Republic of
Magnetic resonance imaging (MRI) near metallic implants remains an unmet need because of severe artifacts, which mainly stem from large metal-induced field inhomogeneities. The single-scan cross spatiotemporal encoding (xSPEN) technique delivers in-plane distortion-free 2D images under such large field inhomogeneity condition, while the slice-plane displacement, “signal voids” and “pile-up” effects are proposed to be solved by applying t1-evolution-time encoding on the multi-slicing 2D xSPEN technique. Compared to the popular “SEMAC” and “MAVIC” techniques, the remarkable time efficiency of this t1-encoding xSPEN thus enable many advanced MRI applications near metal implants with another additional dimension, such as diffusing MRI, function MRI.

Fast Fourier transform-based susceptibility-to-B0 calculation without aliasing artifacts
Lee Seungkyun1,2
1Center for Neuroscience Imaging Research (CNIR), Institute for Basic Science (IBS), Suwon, Korea, Republic of, 2Department of Biomedical Engineering, Sungkyunkwan University (SKKU), Suwon, Korea, Republic of
In the Fourier transform-based susceptibility-to-B0 calculation, the dipolar field kernel (1/3-kz2/k2) is discretely sampled in the k-space, which leads to aliasing artifacts in the spatial domain. We show that calculating and discretizing the dipolar field kernel in the spatial domain, before the Fourier transform, can effectively reduce the aliasing effect without resorting to large zero-filled buffers. In particular, aliasing is eliminated if the spatial-domain grid size is larger than the combined dimensions of the susceptibility source and the B0 target regions. The new method can accelerate repeated calculations of susceptibility-induced B0 fields.

Concomitant gradient effects on chemical shift encoded imaging
Timothy J Colgan1,2, Diego Hernando1, Samir D Sharma1, Ann Shimakawa3, and Scott B Reeder1,2,4,5,6
1Radiology, University of Wisconsin, Madison, WI, United States, 2Medical Physics, University of Wisconsin, Madison, WI, United States, 3Global Applied Science Lab, GE Healthcare, Menlo Park, CA, United States, 4Biomedical Engineering, University of Wisconsin, Madison, WI, United States, 5Medicine, University of Wisconsin, Madison, WI, United States, 6Emergency Medicine, University of Wisconsin, Madison, WI, United States
Quantitative chemical shift-encoded (CSE) MRI techniques acquire complex-valued (magnitude and phase) images at multiple echo times (TE), enabling simultaneous mapping of fat-fraction, R2* (=1/T2*) and B0field. Applications of CSE-MRI include tissue fat quantification, iron quantification and quantitative susceptibility mapping (QSM). Recently, phase shifts due to concomitant gradients (CG) have been identified as a source of error for quantitative CSE techniques, so their effects on fat-fraction, R2* and B0 maps are characterized in this study.   CG correction of experimental data demonstrates that the detrimental effects of CG phase shifts can be removed before reconstruction to produce more accurate estimates of the fat-fraction, R2*, and field map measurements.

Real-Time Field Control Using Full 3rd-Order Matrix Pre-Emphasis
Yolanda Duerst1, Bertram J. Wilm1, Benjamin E. Dietrich1, Simon Gross1, Thomas Schmid1, David O. Brunner1, and Klaas P. Pruessmann1
1ETH Zurich, Zurich, Switzerland
Update steps of real-time field control suffer from imperfect shim responses which degrade control quality. By including full 3rd-order matrix pre-emphasis as an additional filter in the control loop, all self-term responses are shaped to be equal and all cross-term responses are directly suppressed. This leads to disturbances being rejected faster and less noise amplification. Thus enables better field control in demanding situations such as caused by disturbance of high spatial and temporal variability.

Reducing Brain MRI Artifacts Caused by Ferromagnetic Orthodontic Appliances Using Permanent Magnets
Zhiyue J Wang1,2, Yong Jong Park1,2, Youngseob Seo1,2, Michael C Morriss1,2, and Nancy K Rollins1,2
1UT Southwestern Medical Center, Dallas, TX, United States, 2Children's Medical Center, Dallas, TX, United States
Stainless steel orthodontic appliances are commonly found in adolescents undergoing clinical brain MRI examinations. They cause severe magnetic susceptibility artifacts and failure to obtain diagnostic information from many MR techniques. The B0 shimming capability present on clinical MR scanners cannot remove these artifacts. We have constructed devices for the correction of these artifacts at 1.5 T using small pieces of permanent magnets mounted on intra-oral mouth guards or an extra-oral mouth-band. The magnetic field from the permanent magnets cancels the B0 inhomogeneity induced by ferromagnetic orthodontic appliances, resulting in drastic improvement of MR image quality.

Accelerated Imaging of Metallic Implants Using Model-Based Nonlinear Reconstruction
Xinwei Shi1,2, Evan G Levine1,2, and Brian A Hargreaves1,2
1Radiology, Stanford University, Stanford, CA, United States, 2Electrical Engineering, Stanford University, Stanford, CA, United States
3D Multi-Spectral Imaging (MSI) methods, including SEMAC, MAVRIC, and MAVRIC-SL, enable MRI near metallic implants by correcting for the metal-induced off-resonance artifacts, but their widespread application is limited by prolonged scan time. In this work, we introduce a novel model-based reconstruction method to accelerate 3D MSI. We demonstrate in phantom and in vivo experiments that the proposed method can accelerate MAVRIC-SL acquisitions by a factor of 4 when used alone, and 13-17 when combined with parallel imaging and half-Fourier acquisition. The images reconstructed by the proposed method showed sharper details and lower level of noise, compared with model-free L1-ESPIRiT. 

Bayesian correction of bias field and Venetian blind for high resolution ex vivo MRI with clinical scanners
Juan Eugenio Iglesias1, Pedro Manuel Paz-Alonso1, Garikoitz Lerma-Usabiaga1, Ricardo Insausti2, Karla Miller3, and César Caballero-Gaudes1
1Basque Center on Cognition, Brain and Language (BCBL), Donostia - San Sebastián, Spain, 2Human Neuroanatomy Laboratory, University of Castilla-La Mancha, Albacete, Spain, 3Centre for Functional MRI of the Brain, University of Oxford, Oxford, United Kingdom
Multi-slab MRI enables the acquisition of ultra-high resolution ex vivo MRI of the whole human brain with clinical scanners, by overcoming their hardware limitations (e.g., memory size). However, multi-slab MRI produces slab boundary artifacts (SBA) that degrade the image quality and bias subsequent image analyses. Here we propose a Bayesian method that corrects for SBA and intensity inhomogeneities / bias field (BF) simultaneously. The method, which combines a probabilistic brain atlas and the Expectation Maximization algorithm, takes advantage of the interplay between the two artifacts to outperform state-of-the-art SBA and BF correction algorithms (even when used in combination).

Breathing-induced B0 field fluctuations in the cervical spinal cord at 7T
Signe Johanna Vannesjo1, Falk Eippert1, Yazhuo Kong1, Stuart Clare1, Karla L Miller1, and Irene Tracey1
1FMRIB centre, NDCN, University of Oxford, Oxford, United Kingdom
Spinal cord MRI at ultra-high field poses considerable technical challenges, especially related to static and dynamic B0 field variations. We here investigated the magnitude and spatial profile of breathing-induced B0 field fluctuations in the cervical spinal cord at 7T, by comparing field maps acquired during breath-holds in an expired vs. inspired breathing state. Breathing-related field fluctuations of up to 140Hz at the level of C7 were observed. We further implemented a proof-of-principle shim correction, demonstrating the feasibility of using the shim system to compensate for the breathing-induced fields.

Robust Nyquist Ghost Correction by Incorporating Phase Errors Correction in SENSE
Victor B. Xie1,2, Mengye Lyu1,2, Yilong Liu1,2, Yangqiu Feng1,2, and Ed X. Wu1,2
1Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong SAR, China, People's Republic of, 2Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong SAR, China, People's Republic of
In this abstract, we proposed a novel method that can fully and robustly correct EPI Nyquist ghost by incorporating high-order phase error correction into SENSE reconstruction. More importantly, this method does not induce SNR loss, greatly benefiting the final reconstructed images. Phantom and in vivo imaging results clearly demonstrated the efficacy of this method in ghost correct as well as its superior SNR performance, particularly in accelerated data set that can suffer from amplified noise problems. This novel method has great potentials to be applied in all kinds of EPI-based MRI studies, such as fMRI and DTI.

B0 Eddy Current Correction for Spiral MRI
Ryan K Robison1, Dinghui Wang1, Zhiqiang Li1, and James G Pipe1
1Imaging Research, Barrow Neurological Institute, Phoenix, AZ, United States
Eddy currents are a common source of artifacts in Spiral MRI. Eddy currents that effect the k-space trajectory are often the focus of eddy current correction. However, the spatially uniform but time-varying B0 eddy currents can also be a subtle but important source of artifacts in spiral images. This work demonstrates the improvement in image quality that can result from measuring and correcting the phase produced by B0 eddy currents in spiral MRI.

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