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Sodium imaging optimization under specific absorption rate constraint
Author(s) -
Stobbe Robert,
Beaulieu Christian
Publication year - 2008
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.21468
Subject(s) - flip angle , specific absorption rate , relaxation (psychology) , pulse (music) , nuclear magnetic resonance , absorption (acoustics) , signal to noise ratio (imaging) , physics , radio frequency , signal (programming language) , magnetic resonance imaging , optics , materials science , chemistry , computer science , telecommunications , medicine , programming language , detector , antenna (radio) , radiology
The concept of sodium imaging RF pulse parameter optimization for signal‐to‐noise ratio (SNR) under specific absorption rate (SAR) constraints is introduced. This optimization concept is unique to sodium imaging, as sodium exhibits ultrarapid T 2 relaxation in vivo, and involves minimizing echo time (TE). For 3D radial k ‐space acquisition, minimizing TE (and T 2 loss) requires minimizing the RF pulse length. SNR optimization also involves exploiting rapid T 1 relaxation with shortened repetition time (TR) values. However, especially at higher fields, both RF pulse length and TR are constrained by SAR, which is also dependent on the flip angle. Quantum mechanical simulations were performed for SAR equivalent sets of RF pulse length, TR, and flip angle. It was determined that an SNR advantage is associated with a spoiled steady‐state approach to sodium imaging with radial acquisition even though significantly longer RF pulses (and TE) are required to implement this approach under the SAR constraint at 4.7T. This advantage, compared to RF pulse sequences implementing ultrashort echo times, 90° flip angles, and longer repetition times, was confirmed in healthy volunteers (measured SNR increase of ≈38%) and used to produce excellent quality sodium images of the human brain. Magn Reson Med 59:345–355, 2008. © 2008 Wiley‐Liss, Inc.