Cardiac Phase-Resolved B1 Mapping at 3T
Sebastian Weingartner1,2,3, Greg Metzger2, Pierre-Francois Van de Moortele2, and Mehmet Akcakaya1,2

1Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, United States, 2Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, 3Computer Assisted Clinical Medicine, University Medical Center Mannheim, Heidelberg University, Mannheim, Germany

Synopsis

For various cardiac MRI appications, having B1 maps that are cardiac phase-resolved may be beneficial at high fields. In this work, we developa retrospectively ECG-gated sequence based on the actual flip angle imaging technique for 2D cardiac phase-resolved B1 mapping at 3T.

Introduction

A spatial mapping of the transmitted radiofrequency (RF) field is necessary in a number of scenarios, especially at high field strengths, where the nominal flip-angle (FA) may be significantly different than the applied angle. For cardiac MRI, having B1 maps that are cardiac phase-resolved may be beneficial in various applications, such as parallel transmission [1], characterizing effects of implantable objects [2] and myocardial quantification at various cardiac phases [3, 4]. Actual flip angle imaging (AFI) has been proposed [5] as an efficient 3D B1 mapping technique, which utilizes two interleaved acquisitions with the same nominal FA but different repetition times. Its 2D extensions have also been studied [6]. In this work, we propose a retrospectively ECG-gated sequence based on the AFI technique for 2D cardiac phase-resolved B1 mapping.

Methods

Sequence: An AFI sequence consisting of two interleaved FLASH acquisitions with different repetition times (TR1, TR2=nTR1), was implemented as a cardiac cine sequence, enabling image acquisition during steady-state. Retrospective ECG gating was used to calculate images for various cardiac phases. To achieve the rapid TRs required for cardiac imaging and to have sufficient spoiling for AFI quantification, FLASH dephasing gradients were applied at 80 mT/m maximum amplitude and the maximum slew rate. For the kth cardiac phase, the ratio between the two interleave images (I1k, I2k), rk = I1k/I2k is used to calculate the FA for that phase using the formula [5], αk = arccos[(rk·n - 1)/(n - rk)].

Imaging: All imaging was performed on a Siemens 3T Prisma scanner. Typical sequence parameters were 340×220mm2 matrix=144/108, TE/TR1/TR2 =1.5ms/3.9ms/19.9ms, slice thickness= 14mm, duration = 18-20 heart-beats.

Phantom Imaging: A homogenous water/NaCl phantom was imaged with the proposed sequence using a symmetric sinc pulse with bandwidth-time-product (BWTP)=2, and nominal FA between 8 and 80 degrees. Since 2D AFI estimates the total FA experienced by the slice, the slice-profile was simulated offline to calculate the slice-corrected expected FA as reference value. The effect of improved slice-profiles on the effective FA was studied using symmetric sinc pulses with BWTP between 1 and 16.

In-vivo Imaging: Imaging was performed on 2 healthy adult subjects, using a symmetric sinc pulse with BWTP=4. A mid-ventricular short-axis slice was acquired in a breath-hold using the proposed sequence. B1 homogeneity within the myocardium was studied throughout the cardiac cycle. Furthermore, in-vivo B1 maps were acquired with varying nominal FA, and compared to the expected FA from the specific slice-profile simulations. To avoid partial voluming effects from the blood pool, myocardium was manually segmented using the high acquisition resolution. Subsequently, the myocardium signal was smoothed using a 3×3 moving average filter prior to fitting.

Results

The effect of the slice-profiles on the nominal FA is depicted in Fig 1. The imperfect slice-profiles lead to a lower effective FA. These are well estimated by the proposed sequence, with a maximum deviation <13% with the simulations. Fig. 2 shows the effect of different RF pulses, where the pulses with increased BWTP lead to visually-improved slice-profiles, which in turn increases the effective FA for the slice. An in-vivo example is depicted in Fig 3 for the proposed acquisition with nominal FA=35°, and slice-corrected expected FA=22°. The estimated average B1 values were 18°, with standard deviation 4.5° throughout the myocardium. 10% (1.9°) B1 variation was observed across the cardiac cycle. Fig. 4 shows another in-vivo experiment for different nominal FA, showing example B1 maps from the end-diastolic phase. There is good agreement between the estimated FA using the proposed sequence and the slice-corrected expected FA.

Discussion

Unlike other B1 mapping methods, 2D AFI allows estimation of the actual gross total FA seen by the slice, including the slice-profile, rather than the B1 scaling factor. This allows the in-vivo evaluation of different pulses and resulting slice-profiles. However, for highly non-rectangular slice-profiles, signal averaging across the slice in AFI steady-state may deviate from that in the target application, potentially leading to a mismatch in FA estimation. Due to in-flow effects, blood signal can hardly be driven to steady state in 2D cardiac applications. However, assuming a sufficiently low amount of blood regurgitation and a sufficiently long nTR, the blood-signal can be assumed to be almost fully-relaxed, allowing for B1 quantification, although this was beyond the scope of this study. Instead to avoid the impact of in-flow on the myocardial signal, B1 maps were acquired with relatively high in-plane resolution and the myocardium was manually segmented before further processing.

Conclusion

The proposed sequence allows for cardiac phase-resolved B1 mapping, with applications in phase-specific shimming or quantification.

Acknowledgements

Authors acknowledge grant support from NIH R00HL111410, NIH P41EB015894.

References

[1] Schmitter S, Wu X, Ugurbil K, Van de Moortele PF, “Design of parallel transmission radiofrequency pulses robust against respiration in cardiac MRI at 7 Tesla,” Magn Reson Med, 74(5):1291-305, 2015.

[2] Vashaeea S, Gooraa F, Brittonb MM, Newlinga B, Balcom BJ, “Mapping B1-induced eddy current effects near metallic structures in MR images: A comparison of simulation and experiment,” Journal of Magnetic Resonance, 250:17-24, Jan. 2015.

[3] Clique H, Cheng HL, Marie PY, Felblinger J, Beaumont M, “3D myocardial T1 mapping at 3T using variable flip angle method: pilot study,” Magn Reson Med, 71(2):823-9, 2014.

[4] Hamilton JI, Griswold MA, Seiberlich N, “MR Fingerprinting with chemical exchange (MRF-X) to quantify subvoxel T1 and extracellular volume fraction,” Journal of Cardiovascular Magnetic Resonance, 17(Suppl 1):W35, 2015.

[5] Yarnykh VL, “Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field,” Magn Reson Med., 57(1):192-200, Jan. 2007.

[6] Wu X, Deelchand DK, Yarnkyh VL, Ugurbil K, Van de Moortele PF. Actual flip angle imaging: from 3D to 2D. Proceedings of the 17th Annual Meeting of ISMRM, p. 372, 2009.

Figures

Figure 1: The effect of the slice-profile on the nominal flip angle (FA). Simulation results show the imperfect slice profiles leads to a lower effective FA. FA estimated from phantom measurements are in agreement with the simulations.

Figure 2: Effect of different RF pulses on the estimated FA. The increased bandwidth time product leads to visually improved slice profiles, which in turn leads to an increased expected FA for that slice.

Figure 3: In-vivo results from a healthy subject using the proposed sequence, acquired at a nominal FA = 35°. The slice-profile corrected expected FA is 22°. The estimated B1 maps and the corresponding FA in the myocardium are depicted (top and bottom respectively). The B1 maps are homogenous in the myocardium, and the estimated FA using the proposed AFI approach have an average value of 18±4.5°. A 10% variation (1.9°) of the B1 values across the cardiac cycle was also observed.

Figure 4: In-vivo experiment showing the estimated FA vs. the nominal (and slice-corrected expected) FA, for different nominal FA. There is a good agreement between the estimated FA using the proposed sequence and the slice-corrected expected FA. Example B1 maps are shown for the end-diastolic phase.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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