Vol. 3 No. 2 - June 2017

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Dynamic Susceptibility Contrast MRI at 7 T: Tail-Scaling Analysis and Inferences About Field Strength Dependence Linda Knutsson 1,2 , Xiang Xu 3,4 , Freddy Ståhlberg 1,5,6 , Peter B. Barker 3,4 , Emelie Lind 1 , Pia C. Sundgren 5 , Peter C. M. van Zijl 3,4 , and Ronnie Wirestam 1 1 Department of Medical Radiation Physics, Lund University, Lund, Sweden; 2 Department of Radiology (Adjunct), Johns Hopkins School of Medicine, Baltimore, Maryland; 3 Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland; 4 F. M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland; 5 Department of Diagnostic Radiology, Lund University, Lund, Sweden; and 6 Lund University Bioimaging Center, Lund University, Lund, Sweden Corresponding Author: Linda Knutsson, PhD Department of Medical Radiation Physics, Lund University, Skane University Hospital, SE-221 85 Lund, Sweden; E-mail: Key Words: DSC-MRI, perfusion, high field, tail scaling, 7 T Abbreviations: Dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI), contrast agent (CA), cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), echo time (TE), arterial input function (AIF), partial volume effect (PVE), venous output function (VOF), gray matter (GM), regions of interest (ROIs), white matter (WM) Dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) following bolus injection of gadolin- ium contrast agent (CA) is widely used for the estimation of brain perfusion parameters such as cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) for both clinical and research purposes. Although it is predicted that DSC-MRI will have superior performance at high magnetic field strengths, to the best of our knowledge, there are no reports of 7 T DSC-MRI in the literature. It is plausible that the transfer of DSC-MRI to 7 T may be accompanied by increased R 2 * relaxivity in tissue and a larger difference in DR 2 *-versus-concentration relationships between tissue and large vessels. If not accounted for, this will subsequently result in apparent CBV and CBF estimates that are higher than those reported previ- ously at lower field strengths. The aims of this study were therefore to assess the feasibility of 7 T DSC-MRI and to investigate the apparent field-strength dependence of CBV and CBF estimates. In total, 8 healthy vol- unteers were examined using DSC-MRI at 7 T. A reduced CA dose of 0.05 mmol/kg was administered to decrease susceptibility artifacts. CBV, CBF, and MTT maps were calculated using standard DSC-MRI tracer- kinetic theory. Subject-specific arterial partial volume correction factors were obtained using a tail-scaling approach. Compared with literature values obtained using the tail-scaling approach at 1.5 T and 3 T, the CBV and CBF values of the present study were found to be further overestimated. This observation is poten- tially related to an inferred field-strength dependence of transverse relaxivities, although issues related to the CA dose must also be considered. INTRODUCTION Dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) is used for the estimation of cerebral perfusion pa- rameters such as cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) using the theory of intravascular tracers (1). Considering the ability of MRI to com- bine morphological and functional information during a single imaging session, this method can be used in both clinical and research environments. In the clinic, DSC-MRI is widely used for visualizing perfusion deficits in patients with vascular disease, particularly acute stroke, and for evaluating angiogenesis and blood volume in brain tumors. High magnetic field strength (B 0 ) has a number of theoret- ical advantages for DSC-MRI; the larger magnetization can be used either to create images with higher signal-to-noise ratio or higher spatial resolution, while the increased susceptibility ef- fect (which scales linearly with field strength) increases contrast. The increased susceptibility effect can be used to reduce contrast agent (CA) dose or adjust imaging parameters (eg, shorter echo time [TE]) while maintaining similar contrast. However, when performing DSC-MRI at higher field strengths, susceptibility and chemical shift artifacts, B 1 inhomogeneity, and concerns about specific absorption rate are amplified. However, with appropri- ate sequence design, these problems can be overcome. The purpose of this study was to design a DSC-MRI protocol for 7 T that provides images with a visual appearance similar to those seen at lower field strengths. An additional aspect of this study was to investigate whether CBV and CBF estimates, in absolute terms, show any apparent field strength dependence when ap- plying a conventional DSC-MRI quantification approach, that 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 74 TOMOGRAPHY.ORG | VOLUME 3 NUMBER 2 | JUNE 2017

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