Design of parallel transmission radiofrequency pulses robust against respiration in cardiac MRI at 7 Tesla

Abstract

Purpose Two-spoke parallel transmission (pTX) RF pulses have been demonstrated in cardiac MRI at 7T. However, current pulse designs rely on a single set of B1+/B0 maps that may not be valid for subsequent scans acquired at another phase of the respiration cycle, because of organs displacement. Such mismatches may yield severe excitation profile degradation. Methods B1+/B0 maps were obtained, using 16 transmit channels at 7T, at three breath-hold positions: exhale, half-inhale and inhale. Standard and robust RF pulses were designed using maps obtained at exhale only, and at multiple respiratory positions, respectively. Excitation patterns were analyzed for all positions using Bloch simulations. Flip-angle homogeneity was compared in-vivo in cardiac CINE acquisitions. Results Standard 1- and 2-spoke pTX RF pulses are sensitive to breath-hold position, primarily due to B1+ alterations, with high dependency on excitation trajectory for 2-spokes. In-vivo excitation inhomogeneity varied from nRMSE=8.2% (exhale) up to 32.5% (inhale) with the standard design; much more stable results were obtained with the robust design with nRMSE=9.1% (exhale) and 10.6% (inhale). Conclusion A new pTX RF pulse design robust against respiration induced variations of B1+/B0 maps is demonstrated and is expected to positively impact cardiac MRI in breath-hold, free-breathing and real-time acquisitions.

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

“…As robust mapping is a prerequisite for these shimming and tailored RF excitation methods, it constitutes a crucial need for overall image quality at ultrahigh fields. Additionally, the field around the heart at 7T has been shown to be dependent on the breathing state, affecting shimming and calibration . An extension of the proposed motion‐robust cardiac mapping method to ultrahigh field applications potentially allows for robust mapping at various positions in the respiratory cycle.…”

B1 artifact reduction in abdominal DCE‐MRI using kT‐points: First clinical assessment of dynamic RF shimming at 3T

“…13 rCE is a visual parameter that illustrates the potential gain of contrast obtained with dynamic RF shimming. This can potentially be attributed to subject-movement-induced Df 0 fluctuations between calibration and acquisitions, as k Tpoints are inherently less broadband than hard pulses.…”

“…It consists of transmitting the same RF shape on multiple radiating elements with channel-dependent amplitude and phase coefficients adjusted to create a more homogeneous B 1 1 field. Among the methods proposed, the "spokes" technique [10][11][12] has stood out for slice-selective excitation in the brain, heart, 13 and liver, 14 while k T -points 15,16 have shown good results for nonselective pulses. With the advent of two-transmit-channel clinical scanners and automated B 1 1 -mapping calibration, patient-tailored static RF shimming has been recognized as a more efficient approach.…”

Myocardial T2* Mapping with Ultrahigh Field Magnetic Resonance: Physics and Frontier Applications

“…This would also help to further investigate the effect of through-plane motion. Acquisition approaches like simultaneous multi-slice excitation can be used to reduce scan times while multichannel transmit systems can be employed to balance excitation field homogeneity and RF power deposition constraints [114,115].…”