Vol. 3 No. 2 - June 2017

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A Robust Method for Estimating B0 Inhomogeneity Field in the Liver by Mitigating Fat Signals and Phase-Wrapping Antonis Matakos 1 , James M. Balter 1,2 , and Yue Cao 1,2,3 1 Departments of Radiation Oncology; 2 Biomedical Engineering; and 3 Radiology, University of Michigan, Ann Arbor, Michigan Corresponding Author: Yue Cao, PhD Departments of Radiation Oncology, Biomedical Engineering, and Radiology, University of Michigan, Ann Arbor, MI 48109, USA; E-mail: Key Words: fat/water chemical-shift, B0 inhomogeneity field map, multiecho GRE Abbreviations: Magnetic resonance imaging (MRI), radiation treatment (RT), computed tomography (CT), magnetic resonance (MR), gradient-echo (GRE), poor signal-to-noise ratio (SNR), repetition time (TR), echo time (TE), 1-dimensional (1D), 3-dimensional (3D), volume interpolated breath-hold examination sequence (VIBE) We developed an optimized and robust method to estimate liver B0 field inhomogeneity for monitoring and correcting susceptibility-induced geometric distortion in magnetic resonance images for precision therapy. A triple-gradient-echo acquisition was optimized for the whole liver B0 field estimation within a single-exhale breath-hold scan on a 3 T scanner. To eliminate chemical-shift artifacts, fat signals were chosen in-phase be- tween 2 echoes with an echo time difference (DTE) of 2.3 milliseconds. To avoid phase-wrapping, other 2 echoes provided a large field dynamic range (1/DTE) to cover the B0 field inhomogeneity. In addition, using high parallel imaging factor of 4 and a readout-bandwidth of 1955 Hz/pixel, an ;18-second acquisition time for breath-held scans was achieved. A 2-step, 1-dimensional regularized method for the DB0 field map estimation was developed, tested and validated in phantom and patient studies. Our method was validated on a water phantom with fat components and air pockets; it yielded DB0-field maps that had no chemical- shift and phase-wrapping artifacts, and it had a ,0.5 mm of geometric distortion near the air pockets. The DB0-field maps of the patients' abdominal regions were also free from phase-wrapping and chemical-shift artifacts. The maximum field inhomogeneity was found near the lung–liver interface, up to ;300 Hz, result- ing in ;2 mm of distortions in anatomical images with a readout-bandwidth of 440 Hz/pixel. The field map- ping method in the abdominal region is robust; it can be easily integrated in clinical workflow for patient- based quality control of magnetic resonance imaging geometric integrity. INTRODUCTION Magnetic resonance imaging (MRI) is commonly used for plan- ning radiation treatment (RT) for patients with liver cancers because of its superior soft tissue contrast compared with com- puted tomography (CT). Recent investigations (1-3) show the potential to generate radiation attenuation maps based on mag- netic resonance (MR) images, thus eliminating the need for CT scans for RT planning, as well as the associated costs and burdens for the patients. However, ensuring geometric accuracy of MR images is crucial for its use as the sole modality for RT planning. Geometric distortion of clinical MR images needs to be assessed and corrected if necessary. System-level distortions are sufficiently corrected in state-of-the-art scanners, but the subject-induced distortions due to susceptibility and chemical- shift effects still need to be accounted for and corrected (4-9). A common approach for quantifying and correcting sub- ject-induced geometric distortions is to use the acquired B0- inhomogeneity field map from the phase difference of 2 gradient echo (GRE) images (5). Estimation of the DB0-field map from GRE images in the liver faces several challenges, some of which are common to all body sites such as poor signal-to-noise ratio (SNR) and phase-wrapping at locations with large susceptibility effects, and others that are more relevant for the liver such as respiratory motion, chemical-shift, acquisition time, and even geometric accuracy of the image data itself. The phase-wrapping artifact in the estimated field map is a serious challenge. Meth- ods have been proposed to unwrap the field map (10-13). Phase- unwrapping, when applied to the liver, significantly increases the computational load for image processing, and it may not guarantee that it will work in all cases. An alternative approach is to increase the available dynamic range of the field by short- ening the time difference between the 2 echoes (DTE), with a trade-off of reduced SNR. Clinical MRI scanner hardware limits the minimum separation of echo times (TEs) within a single repetition time (TR), usually to ;1 milliseconds or slightly less, corresponding to a field dynamic range of 6500 Hz. Further- more, if the TEs are not tuned for fat and water to be in-phase, the field map can be corrupted by chemical-shift artifacts, par- RESEARCH ARTICLE ABSTRACT © 2017 The Authors. Published by Grapho Publications, LLC This is an open access article under the CC BY-NC-ND license ( ISSN 2379-1381 TOMOGRAPHY.ORG | VOLUME 3 NUMBER 2 | JUNE 2017 79

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