Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields. Issue 4 (21st June 2013)

• Record Type: Journal Article
• Authors: Hsu, Yi‐Cheng; Chern, I‐Liang; Zhao, Wei; Gagoski, Borjan; Witzel, Thomas; Lin, Fa‐Hsuan
• Is Part Of: Magnetic resonance in medicine. Volume 71:Issue 4(2014:Apr.)
• Page Start: 1458
• Page End: 1469
• Publication Date: 2013-06-21
• Subjects: Nuclear magnetic resonance -- Periodicals; Electron paramagnetic resonance -- Periodicals; 616.07548
• DOI: 10.1002/mrm.24801
• Languages: English
• ISSNs: 0740-3194

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <sec id="mrm24801-sec-0001" sec-type="section"> <title>Purpose</title> <p>High‐field magnetic resonance imaging (MRI) has the challenge of inhomogeneous <italic>B</italic><sub>1</sub><sup>+</sup>, and consequently inhomogeneous flip angle distribution, which causes spatially dependent contrast and makes clinical diagnosis difficult.</p> </sec> <sec id="mrm24801-sec-0002" sec-type="section"> <title>Method</title> <p>We propose a two‐step pulse design procedure in which (1) a combination of linear and nonlinear spatial encoding magnetic fields (SEMs) is used to remap the <italic>B</italic><sub>1</sub><sup>+</sup> map in order to reduce the dimensionality of the problem, (2) the locations, amplitudes, and phases of spoke pulses are estimated in one dimension. The advantage of this <italic>B</italic><sub>1</sub><sup>+</sup> remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of <italic>B</italic><sub>1</sub><sup>+</sup>, a simple pulse sequence design using time‐varying SEMs can achieve a homogenous flip‐angle distribution efficiently.</p> </sec> <sec id="mrm24801-sec-0003" sec-type="section"> <title>Results</title> <p>We demonstrate that spatially selective radiofrequency (RF) excitation with generalized SEMs (SAGS) using both linear and quadratic SEMs in a multi‐spoke <italic>k</italic>‐space trajectory can mitigate the <italic>B</italic><sub>1</sub><sup>+</sup> inhomogeneity at 7T efficiently. Numerical simulations based on experimental data suggest that, compared with other methods, SAGS provide a formulation allowing multiple‐pulse design, a similar average flip‐angle distribution with less RF power, and/or a more homogeneous flip‐angle distribution.</p> </sec> <sec id="mrm24801-sec-0004" sec-type="section"> <title>Conclusion</title> <p>Without using multiple RF coils for parallel transmission, SAGS can be used to mitigate the <italic>B</italic><sub>1</sub><sup>+</sup> inhomogeneity in high‐field MRI experiments. <bold>Magn Reson Med 71:1458–1469, 2014. © 2013 Wiley Periodicals, Inc.</bold></p> </sec> </abstract>