Abstract
PurposeQuantitative MRI applications, such as mapping the T1 time of tissue, puts high demands on the accuracy and precision of transmit field (B1+) estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch‐Siegert frequency shift (BSS) of 2 acquisitions with opposite off‐resonance frequency radiofrequency pulses. Interleaving these radiofrequency pulses ensures robustness to motion and scanner drifts; however, here we demonstrate that doing so also introduces a bias in the B1+ estimates.Theory and MethodsIt is shown here by means of simulation and experiments that the amplitude of the error depends on MR pulse sequence parameters, such as repetition time and radiofrequency spoiling increment, but more problematically, on the intrinsic properties, T1 and T2, of the investigated tissue. To solve these problems, a new approach to BSS‐based B1+ estimation that uses a multi‐echo acquisition and a general linear model to estimate the correct BSS‐induced phase is presented.ResultsIn line with simulations, phantom and in vivo experiments confirmed that the general linear model‐based method removed the dependency on tissue properties and pulse sequence settings.ConclusionThe general linear model‐based method is recommended as a more accurate approach to BSS‐based B1+ mapping.
Highlights
Knowledge of the spatial distribution of the radiofrequency transmit field (B1+) is crucial to many MRI applications
Effect of acquisition order and RF spoiling For sequential acquisition ordering, once steady-state was reached for each off-resonance frequency blocks (Fig.[2] a), the phase difference between these was zero before the BS pulse (Fig.[2] c)
Interleaved acquisitions are recommended for Bloch-Siegert based B1+ mapping to increase robustness to motion and scanner drift
Summary
Knowledge of the spatial distribution of the radiofrequency transmit field (B1+) is crucial to many MRI applications. In the Bloch-Siegert (BS)[5,6] approach, an off-resonance RF pulse leads to the Bloch-Siegert frequency shift (BSS), and an associated phase accumulation, which is proportional to the square of the pulse amplitude thereby encoding the B1+ field. This technique performed favourably in a recent review of the accuracy, precision and practicality of a range of prominent B1+ mapping techniques[7], and has been shown to be less sensitive to B0 inhomogeneities and chemical shifts[8] than other phase-based methods
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