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Enhancing the quantification of tissue sodium content by MRI: time‐efficient sodium B 1 mapping at clinical field strengths
Author(s) -
Lommen Jonathan,
Konstandin Simon,
Krämer Philipp,
Schad Lothar R.
Publication year - 2016
Publication title -
nmr in biomedicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.278
H-Index - 114
eISSN - 1099-1492
pISSN - 0952-3480
DOI - 10.1002/nbm.3292
Subject(s) - flip angle , phase (matter) , amplitude , signal (programming language) , nuclear magnetic resonance , field (mathematics) , materials science , chemistry , optics , analytical chemistry (journal) , biomedical engineering , physics , magnetic resonance imaging , computer science , mathematics , chromatography , medicine , pure mathematics , radiology , organic chemistry , programming language
Tissue sodium content (TSC) is a sensitive measure of pathological changes and can be detected non‐invasively by MRI. For the absolute quantification of TSC, B 1 inhomogeneities must be corrected, which is not well established beyond research applications. An in‐depth analysis of B 1 mapping methods which are suitable for application in TSC quantification is presented. On the basis of these results, a method for simultaneous B 1 mapping and imaging is proposed in order to enhance accuracy and to reduce measurement time at clinical field strengths. The B 1 mapping techniques used were phase‐sensitive (PS), Bloch‐Siegert shift (BSS), double‐angle (DAM) and actual flip–angle imaging (AFI) methods. Experimental and theoretical comparisons demonstrated that the PS technique yields the most accurate field profiles and exhibits the highest signal‐to‐noise ratio (SNR). Simultaneous B 1 mapping and imaging was performed for the PS method, employing both degrees of freedom of the MR signal: the B 1 field is encoded into signal phase and the amplitude provides the concentration information. In comparison with the more established DAM, a 13% higher SNR was obtained and field effects could be corrected more accurately without the need for additional measurement time. The protocol developed was applied to measure TSC in the healthy human head at an isotropic resolution of 4 mm. TSC was determined to be 35 ± 1 mM in white matter and 134 ± 3 mM in vitreous humor. By employing the proposed simultaneous characterization of the B 1 field and acquisition of the spin density‐weighted sodium signal, the accuracy of the non‐invasive measurement of TSC is enhanced and the measurement time is reduced. This should allow 23 Na MRI to be better incorporated into clinical studies and routine. Copyright © 2015 John Wiley & Sons, Ltd.