Improved encoding strategy for CPMG‐based Bloch‐Siegert B mapping

Abstract

Bloch‐Siegert (BS) based B mapping methods use off‐resonant pulses to encode quantitative B information into the signal phase. It was recently shown that the principle behind BS‐based B mapping can be expanded from spin echo (BS‐SE) and gradient‐echo (BS‐FLASH) based BS B 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 information and BS‐CPMG‐MSE enables simultaneous mapping of B, M0, and T2. To date, however, two separate MRI experiments must be performed to enable the calculation of B 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 and T2 mapping in a single BS‐CPMG‐MSE experiment. To allow single‐shot B 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.

Visualization of inflammation using 19F‐magnetic resonance imaging and perfluorocarbons

“…Different B 1 mapping methods have recently been developed that could be applied to 19 F‐MRI 52–55. For example, Bloch‐Siegert shift‐based B 1 mapping methods can provide density‐weighted magnitude images while the B 1 profile is being encoded in the image phase 53–55. Thus, in principle, no additional 19 F images must be acquired for quantification purposes.…”

Improved Bloch‐Siegert based B1 mapping by reducing off‐resonance shift

“…Based on this, the effect of off‐resonance can be determined by Equation or via pre‐computed lookup tables . Artifacts related to direct excitation effect can be suppressed by crusher gradients . Consequently, crushers in BS sequences also help reduce some of the non‐linear effects associated with low frequency offsets due to averaging across magnetization phases.…”

“…Unfortunately, speed and sensitivity of BS methods are often limited by the amount of RF power deposited in the tissue, which can exceed safety limits for allowable tissue heating, as indicated by the specific absorption rate (SAR) . For this reason, several BS variants have been proposed to reduce SAR by changing the imaging approach and image readout scheme, by optimizing BS irradiation pulse parameters, and by introducing gradient spoiling schemes to eliminate spurious signals excited by the BS pulse. These variants have improved mapping sensitivity, accuracy, and speed .…”

Robust 3D Bloch-Siegert based B1+ mapping using Multi-Echo General Linear Modelling

“…The BSS technique is flexible and can be integrated into a multitude of pulse sequences, such as 2D 5 or 3D gradient echo (GRE) [7][8][9] , Interleaved EPI, spiral GRE 10 and spin echo [11][12][13][14] acquisitions. The BSS technique, as typically implemented, requires two acquisitions with opposite off-resonance frequencies.…”