Tomography

Vol. 3 No. 2 - June 2017

Tomography is a scientific journal for publication of articles in imaging research

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Mask-Adapted Background Field Removal for Artifact Reduction in Quantitative Susceptibility Mapping of the Prostate Sina Straub 1 , Julian Emmerich 1 , Heinz-Peter Schlemmer 2 , Klaus H. Maier-Hein 3 , Mark E. Ladd 1 , Matthias C. Röthke 2 , David Bonekamp 2 , and Frederik B. Laun 1,4 1 Department of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; 2 Department of Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; 3 Junior Group Medical Image Computing, German Cancer Research Center (DKFZ), Heidelberg, Germany; and 4 Institute of Radiology, University Hospital Erlangen, Erlangen, Germany Corresponding Author: Sina Straub, PhD Department of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany; E-mail: sina.straub@dkfz.de Key Words: quantitative susceptibility mapping, prostate cancer, calcification, artifact reduction, background field removal Abbreviations: Magnetic resonance imaging (MRI), quantitative susceptibility mapping (QSM), gradient-echo (GRE), magnetic resonance (MR), volumes of interest (VOIs) We propose an alternative processing method for quantitative susceptibility mapping of the prostate that reduces artifacts and enables better visibility and quantification of calcifications and other lesions. Three-di- mensional gradient-echo magnetic resonance data were obtained from 26 patients at 3 T who previously received a planning computed tomography of the prostate. Phase images were unwrapped using Laplacian- based phase unwrapping. The background field was removed with the V-SHARP method using tissue masks for the entire abdomen (Method 1) and masks that excluded bone and the rectum (Method 2). Susceptibility maps were calculated with the iLSQR method. The quality of susceptibility maps was assessed by one radiol- ogist and two physicists who rated the data for visibility of lesions and data quality on a scale from 1 (poor) to 4 (good). The readers rated susceptibility maps computed with Method 2 to be, on average, better for visibility of lesions with a score of 2.9 6 1.1 and image quality with a score of 2.8 6 0.8 compared with maps computed with Method 1 (2.4 6 1.2/2.3 6 1.0). Regarding strong artifacts, these could be removed using adapted masks, and the susceptibility values seemed less biased by the artifacts. Thus, using an adapted mask for background field removal when calculating susceptibility maps of the prostate from phase data reduces artifacts and improves visibility of lesions. INTRODUCTION Prostate cancer is the most common cancer among males in the Western world (1). However, in magnetic resonance imaging (MRI), prostate cancer detection can be hampered by postbiopsy hemorrhage and/or calcifications (2); in particular, the detection of intraprostatic calcifications is difficult because of variations in signal intensity and the small size of the lesions (3). In addition, the treatment outcome may be influenced by the pres- ence of calcifications (4, 5). After quantitative susceptibility mapping (QSM) (6-8) had been successfully applied in several clinical studies of the brain (9-13), there have been in vivo applications of QSM in the liver (14, 15). Moreover, it has recently been shown that QSM can be a valuable tool to detect intra- and periprostatic calcifications (16), but in cases of intestinal movement or air in the rectum, severe artifacts can occur when inverting unreliable phase in- formation because of nonlocality of the inversion kernel (17-21) for QSM and can render the detection of calcifications difficult. Because streaking artifacts in susceptibility maps not only corrupt image quality and hamper detection of lesions (calcifi- cations or hemorrhage), but also bias the accurate quantification of susceptibility values, the reduction or elimination of artifacts is crucial. The purpose of this study is to propose a method to reduce or eliminate artifacts and enable better visibility of calcifications and other lesions, for example, hemorrhage, in the prostate. METHODOLOGY Patients The study was conducted in accordance with the Declaration of Helsinki. Institutional review board approval was obtained, and all subjects provided written informed consent. Patient data were collected over the period of 1.3 years. In this study, 26 patients (age range: 55 to 80 years; mean age: 67.9 6 6.8 years; RESEARCH ARTICLE ABSTRACT © 2017 The Authors. Published by Grapho Publications, LLC This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ISSN 2379-1381 http://dx.doi.org/10.18383/j.tom.2017.00005 96 TOMOGRAPHY.ORG | VOLUME 3 NUMBER 2 | JUNE 2017

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