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Quantitative pulsed CEST‐MRI using Ω ‐plots
Author(s) -
Meissner JanEric,
Goerke Steffen,
Rerich Eugenia,
Klika Karel D.,
Radbruch Alexander,
Ladd Mark E.,
Bachert Peter,
Zaiss Moritz
Publication year - 2015
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.3362
Subject(s) - specific absorption rate , saturation (graph theory) , magnetization transfer , nuclear magnetic resonance , chemistry , computational physics , flip angle , radio frequency , analytical chemistry (journal) , materials science , magnetic resonance imaging , physics , medicine , telecommunications , mathematics , chromatography , combinatorics , computer science , antenna (radio) , radiology
Chemical exchange saturation transfer (CEST) allows the indirect detection of dilute metabolites in living tissue via MRI of the tissue water signal. Selective radio frequency (RF) with amplitude B 1 is used to saturate the magnetization of protons of exchanging groups, which transfer the saturation to the abundant water pool. In a clinical setup, the saturation scheme is limited to a series of short pulses to follow regulation of the specific absorption rate (SAR). Pulsed saturation is difficult to describe theoretically, thus rendering quantitative CEST a challenging task. In this study, we propose a new analytical treatment of pulsed CEST by extending a former interleaved saturation–relaxation approach. Analytical integration of the continuous wave (cw) eigenvalue as a function of the RF pulse shape leads to a formula for pulsed CEST that has the same structure as that for cw CEST, but incorporates two form factors that are determined by the pulse shape. This enables analytical Z ‐spectrum calculations and permits deeper insight into pulsed CEST. Furthermore, it extends Dixon's Ω ‐plot method to the case of pulsed saturation, yielding separately, and independently, the exchange rate and the relative proton concentration. Consequently, knowledge of the form factors allows a direct comparison of the effect of the strength and B 1 dispersion of pulsed CEST experiments with the ideal case of cw saturation. The extended pulsed CEST quantification approach was verified using creatine phantoms measured on a 7 T whole‐body MR tomograph, and its range of validity was assessed by simulations. Copyright © 2015 John Wiley & Sons, Ltd.