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Time‐interleaved acquisition of modes: An analysis of SAR and image contrast implications
Author(s) -
Orzada S.,
Maderwald S.,
Poser B. A.,
Johst S.,
Kannengiesser S.,
Ladd M. E.,
Bitz A. K.
Publication year - 2012
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.23081
Subject(s) - specific absorption rate , homogeneity (statistics) , computer science , data acquisition , homogeneous , image contrast , physics , nuclear magnetic resonance , artificial intelligence , telecommunications , machine learning , antenna (radio) , thermodynamics , operating system
As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole‐body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time‐interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high‐field MRI. The basic premise is to excite two (or more) different B 1 +modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time‐interleaved acquisition of mode on image contrast as well as on time‐averaged specific absorption rate is addressed in detail. Time‐interleaved acquisition of mode is superior in B 1 +homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time‐interleaved acquisition of modes can enable almost homogeneous high‐field imaging throughout the entire field of view in PD, T 2 , and T 2 *‐weighted imaging and, if a specified homogeneity criterion is met, in T 1 ‐weighted imaging as well. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.