Motion-robust cardiac B1+ mapping at 3T using interleaved bloch-siegert shifts

Abstract

Purpose To develop and evaluate a robust motion-insensitive Bloch-Siegert shift based B1+ mapping method in the heart. Methods Cardiac Bloch-Siegert B1+ mapping was performed with interleaved positive and negative off-resonance shifts and diastolic spoiled gradient echo imaging in twelve heartbeats. Numerical simulations were performed to study the impact of respiratory motion. The method was compared to 3D actual flip angle imaging (AFI) and 2D saturated double angle method (SDAM) in phantom scans. Cardiac B1+ maps were acquired in six healthy volunteers, using Bloch-Siegert and SDAM in different views (SHAX, 4CH, 2CH) during breath-hold and free-breathing. In vivo maps were evaluated for inter-view consistency using the correlation coefficients of the B1+ profiles along the lines of intersection between the views. Results For the Bloch-Siegert sequence numerical simulations indicate high similarity between breath-hold and free-breathing scans and phantom results show low deviation from the 3D AFI reference (normalized-root mean square error, NRMSE=2.0%). Increased deviation is observed with 2D SDAM (NRMSE=5.0%) due to underestimation caused by imperfect excitation slice-profiles. Breath-hold and free-breathing Bloch-Siegert in vivo B1+ maps are visually comparable with no significant difference in the inter-view consistency (p>0.36). SDAM shows strongly impaired B1+ map quality during free-breathing. Inter-view consistency is significantly lower than with the Bloch-Siegert method (breath-hold: p=0.014, free-breathing: p<0.0001). Conclusion The proposed interleaved Bloch-Siegert sequence enables cardiac B1+ mapping with improved inter-view consistency and high resilience to respiratory motion.

Robust 3D Bloch‐Siegert based mapping using multi‐echo general linear modeling

“…The greater robustness of the interleaved acquisition scheme has led to its adoption in more recent work using this technique. 11,26 In the interleaved case, however, our numerical simulations showed that the phase never reached steady-state but rather a pseudo-steady-state that alternated between two conditions depending on the frequency of the preceding offresonance BS pulse. As a result, the difference in phase between interleaves is not solely due to the BSS effect and, therefore, does not match the phase difference of the sequential ordering scheme (Figure 2).…”

“…The resulting maps had visible artefacts and large biases (see Figure , right column). The greater robustness of the interleaved acquisition scheme has led to its adoption in more recent work using this technique …”

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

“…The resulting B1 + maps had visible artefacts and large biases (Fig.8, right column). The greater robustness of the interleaved acquisition scheme has led to its adoption in more recent work using this technique 9,24 .…”

Demonstration and suppression of respiration‐related artifacts in Bloch–Siegert shift‐based B1+ maps of the human brain

“…For the second set of experiments, the manufacturer's GRE sequence was modified for BSS mapping by adding a Fermi‐shaped off‐resonance RF pulse (frequency shift: ±4 kHz; duration: 8 ms; nominal B 1 + ‐peak: 5.5 μT (the maximum allowed due to SAR limits)). Positive and negative images were acquired in an interleaved fashion 30,35–38 (Figure 1A). To reduce the effects of on‐resonance excitation, the off‐resonance RF pulse was surrounded by bipolar crusher gradient lobes in the slice‐selection direction that provides 4π phase difference within a voxel 33 (Figure 1B).…”

“…Additionally, BSS mapping works for a relatively large dynamic range of , and it is insensitive to T 1 relaxation and magnetization transfer effects 27 . With these promising features, BSS mapping has attracted considerable attention since its discovery supported by numerous studies 28–38 …”