Vol. 2 No. 2 - Jun 2016

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Fast Padé Transform Accelerated CSI for Hyperpolarized MRS Esben Szocska Søvsø Hansen 1,2,3 , Sun Kim 4 , Jack J. Miller 2,5 , Marcus Geferath 6 , Glen Morrell 7,8 , and Christoffer Laustsen 1 1 MR Research Centre, Institute of Clinical Medicine, Aarhus University Hospital, Aarhus, Denmark; 2 Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford; 3 Danish Diabetes Academy, Odense, Denmark; 4 Department of Neurology & Neurological Sciences, Stanford Hospital & Clinics, Palo Alto, California; 5 Department of Physics, University of Oxford, Oxford; 6 School of Mathematical Sciences, University College Dublin, Belfield, Dublin, Ireland; 7 University of Utah School of Medicine, Salt Lake City, Utah; and 8 Utah Center for Advanced Imaging Research, Salt Lake City, Utah Corresponding Author: Christoffer Laustsen Aarhus University Hospital Skejby, Palle Juul-Jensens Boulevard 99, DK-8200 Aarhus N, M 145 24439141, Denmark; E-mail: Key Words: hyperpolarization, MRI, spectral reconstruction, Padé approximant Abbreviations: Fast Padé transform (FPT), chemical shift imaging (CSI), magnetic resonance (MR), free induction decay (FID), nuclear magnetic resonance (NMR), iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL), magnetic resonance spectroscopy (MRS), region of interest (ROI), signal-to-noise ratio (SNR) The fast Padé transform (FPT) is a method of spectral analysis that can be used to reconstruct nuclear mag- netic resonance spectra from truncated free induction decay signals with superior robustness and spectral resolution compared with conventional Fourier analysis. The aim of this study is to show the utility of FPT in reducing of the scan time required for hyperpolarized 13 C chemical shift imaging (CSI) without sacrificing the ability to resolve a full spectrum. Simulations, phantom, and in vivo hyperpolarized [1- 13 C] pyruvate CSI data were processed with FPT and compared with conventional analysis methods. FPT shows improved sta- bility and spectral resolution on truncated data compared with the fast Fourier transform and shows results that are comparable to those of the model-based fitting methods, enabling a reduction in the needed acquisi- tion time in 13 C CSI experiments. Using FPT can reduce the readout length in the spectral dimension by 2-6 times in 13 C CSI compared with conventional Fourier analysis without sacrificing the spectral resolution. This increased speed is crucial for 13 C CSI because T1 relaxation considerably limits the available scan time. In addition, FPT can also yield direct quantification of metabolite concentration without the additional peak analysis required in conventional Fourier analysis. INTRODUCTION Hyperpolarized Magnetic Resonance (MR) with Dissolution-Dy- namic Nuclear Polarization is a clinically emerging technique, and it shows great promise in investigating diseases of meta- bolic dysregulation such as cancer and heart diseases (1). Chem- ical shift imaging (CSI) is a well-known and powerful tool for the investigation of metabolic processes following the injection of hyperpolarized MR media. Because the T1 relaxation rate of hyperpolarized 13 C places an ultimate limit on the time of acquisition of CSI data (forming a window of 50-80 seconds in vivo) (2, 3), spectroscopic 13 C imaging is typically performed with a multipoint Dixon approach (4) to limit the number of time points required for spectral analysis. However, this approach requires a priori knowledge of the number and spectral location of spectral peaks expected to be present in the spectrum. If other signals are present at unexpected resonant frequencies (ie, un- expected reactions occur), these multipoint methods are prone to producing incorrect results. In contrast, full-spectrum CSI yields a complete MR spectrum for each image voxel, with zero to few prior assumptions. However, the acquisition time necessary to obtain full spectra with adequate resolution from spatially re- solved voxels represents a significant limitation in hyperpolar- ized 13 C imaging. Numerous novel methods have been proposed for gaining higher spatial and temporal resolution at the expense of spectral resolution (4-10). Undersampling the spectral dimension can considerably reduce the amount of data that is required to be acquired and hence the acquisition time; this is typically offset by the introduction of uncertainties in the spectral domain of the acquired data, which often require prior spectral knowledge to reconstruct, causing a potential for misinterpretation. Conse- quently, despite an increasing number of alternatives, the CSI sequence is still considered a gold standard sequence for hyper- polarized imaging experiments and a preferred starting point for testing experimental hyperpolarized setups (7, 11-18). The CSI sequence makes few prior assumptions about ob- tained spectral frequencies and is therefore both more robust experimentally and less prone to misinterpretation than faster alternatives. This flexibility in the acquisition and processing makes the CSI sequence a valuable tool for clinical and preclin- ical investigations. The per-shot acquisition time for optimal signal-to-noise ratio (SNR) is typically taken to be in the range RESEARCH ARTICLE ABSTRACT © 2016 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 2 NUMBER 2 | JUNE 2016 117

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