Fast CPMG‐based Bloch‐Siegert B₁⁺ mapping

Abstract

A novel method for B þ 1 mapping based on the Bloch-Siegert (BS) shift was recently presented. This method applies offresonant pulses before signal acquisition to encode B 1 information into the signal phase. BS-based methods possess significant advantages in measurement time and accuracy compared to magnitude-based B þ 1 methods. This study extends the idea of BS B þ 1 mapping to Carr, Purcell, Meiboom, Gill (CPMG)-based multi-spin-echo (BS-CPMG-MSE) and turbo-spin-echo (BS-CPMG-TSE) imaging. Compared to BSbased spin echo imaging (BS-SE), faster acquisition of the B þ 1 information was possible using the BS-CPMG-TSE sequence. Furthermore, signal loss by T 2 * effects could be minimized using these spin echo-based techniques. These effects are critical for gradient echo-based BS methods at high field strengths. However, multi-spin-echo-based BS B 1 methods inherently possess high specific absorption rates. Thus, the relative specific absorption rate of BS-CPMG-TSE sequences was estimated and compared with the specific absorption rate produced by BS-SE sequences. A novel method for fast B þ 1 mapping was recently proposed (1). The authors demonstrated that a B 1 dependent phase shift is introduced by applying off-resonant pulses after signal excitation and before signal acquisition. This phase shift is named after Bloch-Siegert (BS), who gave the theoretical description of this phenomenon in 1940 (2). If certain requirements are met, the BS shift is directly proportional to (B 1 ) 2 , the duration of the off-resonant BS pulse, and inversely proportional to the off-resonance (v off ) of the BS pulse (1). The phase-based BS method is robust, contrary to magnitude-based B 1 methods, to biases caused by several parameters such as TR, T 1 relaxation, and magnetization transfer (1). Due to the robustness to TR and T 1 relaxation fast B 1 mapping is enabled. For B 1 mapping, a gradient echo-based BS (BS-FLASH) as well as a spin echo-based BS sequence (BS-SE) were introduced in (1). Several initial studies showed the wide applicability of these BS-based B 1 mapping methods (1,3-7).This work extends the idea of using the BS shift for B þ 1 mapping to multi/turbo-spin-echo-based (MSE/TSE) MRI. To avoid image artifacts inherent to MSE/TSE sequences, the same phase must be present before every refocusing pulse. This scenario is referred to as Carr, Purcell, Meiboom, Gill (CPMG)-based conditions (8-10). Alternatives for avoiding image artifacts introduced by uncorrelated echoes are either crushing the unwanted echo pathway (11) or separating the pathways by adding an unbalanced read gradient (12,13). When compared against CPMG MSE/TSE imaging, however, these solutions would lead to a signal loss.With the proposed sequences, simultaneous B 1 and T 2 mapping can be performed using BS-based MSE (BS-CPMG-MSE) sequences when the produced magnetization transfer is negligible. To demonstrate the applicability of the proposed BS-CPMG-MSE sequence, 2D phantom and in vivo T 2 and B 1 maps were simultaneously acquired. Using the BS CPMG...

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

“…This agrees with Ref. 4, which describes this issue in greater detail. Briefly, TSE images with a longer TR can provide higher SNR and thus better defined phases (12) in the presence of long T 2 values.…”

“…This agrees with the findings in Ref. 4. In contrast, almost no phase jumping could be observed for the BS-MSE-B experiments.…”

“…The line v off indicates the different off-resonance BS pulse signs used for the different sequences. a: BE-SE sequence (1,4). b: BS-TSE/MSE sequence (4).…”

“…BS-B þ 1 mapping previously required two independent BS experiments with inverted off-resonant (6v off ) BS pulse frequencies (1,4). This work proposes a modification of the BS encoding strategy for CPMG-based BS MSE/TSE imaging.…”

“…This modification enables preservation of CPMG conditions (5-7) while encoding both required phase information for B þ 1 mapping in a single echo train. In contrast to (4), all the BS pulses used in this method have the same parameters except for the v off sign. Thus, image artifacts caused by slightly differing BS phase values introduced, for example by amplifier scaling problems (4) are intrinsically avoided in BS-CPMG-TSE experiments.…”