A theoretical and experimental comparison of different techniques for B₁ mapping at very high fields

Abstract

With the increasing use of ultrahigh-field MR with multiple transmit channels, mapping of the B₁(+) field has become a critical factor in many studies, leading to the publication of a large number of sequences for the measurement of the flip angle in recent years. In this article, the accuracy, precision and practicability of some of the most prominent of these techniques are investigated both theoretically, using error propagation computations and Monte-Carlo simulations, and experimentally for different settings. For an exemplary experiment, which is typical for high-field applications, the flip angle uncertainty is calculated and measured for two- and three-dimensional acquisitions for techniques based on both magnitude and phase data. Simulated and measured results show good agreement. An experimental assessment of T₁ and B₀ dependence yields weak variations with these parameters for only a few of the sequences. Measurements on human scanners show crucial influences of specific absorption rate limitations, especially at ultrahigh field.

Analysis of the Precision of Variable Flip Angle T1 Mapping with Emphasis on the Noise Propagated from RF Transmit Field Maps

“…Although bias resulting from the spatial non-uniformity of the transmit RF transmit field is also well known and is commonly corrected by incorporating a $B_1^{+}$ map into the calculations (Helms et al, 2008; Yarnykh, 2010; Lutti et al, 2013; Stikov et al, 2015), the precision with which the $B_1^{+}$ map is obtained is typically ignored. Pohmann and Scheffler compared the precision of several $B_1^{+}$ mapping methods and found widely varying results depending on the method used and the nominal flip angle to be measured (Pohmann and Scheffler, 2013). Our results further demonstrate the necessity to consider the precision of $B_1^{+}$ mapping when using these to correct bias in the T 1 relaxation time maps.…”

“…In this study we used the AFI method for $B_1^{+}$ mapping, which was proposed and optimized by Yarnykh (Yarnykh, 2007, 2010) and has been shown to perform comparatively well (Pohmann and Scheffler, 2013). From Experiments 1–2 , we showed that increasing spoiler gradients makes the estimation of T 1 not only more accurate as previously reported (Yarnykh, 2010) but also more precise due to the complete spoiling of the transverse magnetization.…”

“…However, the precision of the $B_1^{+}$ map and how this diminishes the precision of the estimated T 1 values has not been thoroughly addressed, especially not in vivo . Recently, a systematic comparison of the precision of different $B_1^{+}$ mapping methods was performed by Pohmann and Scheffler (2013). They reported the uncertainty in the measurements of the $B_1^{+}$ maps and found that the error could be up to approximately 30% for 3D variants.…”

Tissue Electrical Property Mapping From Zero Echo-Time Magnetic Resonance Imaging

“…Phase acquisition in both the conventional and the new methods can be quick; for example, a UTE [15] and a steady-state precession [37] sequence have been used for rapid conductivity mapping using phase acquisition alone. However, in order to map permittivity, or to improve the accuracy in conductivity, conventional methods require an additional B 1 + mapping scan.…”

Direct imaging of radio-frequency modes via traveling wave magnetic resonance imaging

“…In general, the relationship between the MR image intensity and magnetic field map is non-trivial and specially designed MRI sequences exist to provide magnitude and phase magnetic field maps 23,24 . Such sequences usually target spatial profiles of H + 1 and H − 1 separately, which are the rightand left-hand polarized components of H 1 that correspond to transmit and receive sensitivity profiles of the rf coil respectively.…”