Improved encoding strategy for CPMG-based Bloch-Siegert B 1+ mapping

• Published in: Magnetic resonance in medicine 2012-08, Vol.68 (2), p.507-515
• Main Authors: Sturm, V. J. F., Basse-Lüsebrink, T. C., Kampf, T., Stoll, G., Jakob, P. M.
• Format: Article
• Language: English
• Subjects: B1 mapping; Bloch-Siegert shift; CPMG; multi-spin echo; turbo-spin echo
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.23232

Bloch‐Siegert (BS) based B 1+ mapping methods use off‐resonant pulses to encode quantitative B 1+ information into the signal phase. It was recently shown that the principle behind BS‐based B 1+ mapping can be expanded from spin echo (BS‐SE) and gradient‐echo (BS‐FLASH) based BS B 1+ mapping to methods such as Carr, Purcell, Meiboom, Gill (CPMG)‐based turbo‐spin echo (BS‐CPMG‐TSE) and multi‐spin echo (BS‐CPMG‐MSE) imaging. If CPMG conditions are preserved, BS‐CPMG‐TSE allows fast acquisition of the B 1+ information and BS‐CPMG‐MSE enables simultaneous mapping of B 1+, M0, and T2. To date, however, two separate MRI experiments must be performed to enable the calculation of B 1+ maps. This study investigated a modified encoding strategy for CPMG BS‐based methods to overcome this limitation. By applying a “bipolar” off‐resonant BS pulse before the refocusing pulse train, the needed phase information was able to be encoded into different echo images of one echo train. Thus, this technique allowed simultaneous B 1+ and T2 mapping in a single BS‐CPMG‐MSE experiment. To allow single‐shot B 1+ mapping, this method was also applied to turbo‐spin echo imaging. Furthermore, the presented modification intrinsically minimizes phase‐based image artifacts in BS‐CPMG‐TSE experiments. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.