Minimum TR radiofrequency‐pulse design for rapid gradient echo sequences

Abstract

A framework to design radiofrequency (RF) pulses specifically to minimize the TR of gradient echo sequences is presented, subject to hardware and physiological constraints. Methods: Single-band and multiband (MB) RF pulses can be reduced in duration using variable-rate selective excitation (VERSE) VERSE for a range of flip angles; however, minimum-duration pulses do not guarantee minimum TR because these can lead to a high specific absorption rate (SAR). The optimal RF pulse is found by meeting spatial encoding, peripheral nerve stimulation (PNS) and SAR constraints. A TR reduction for a range of designs is achieved and an application of this in an MB cardiac balanced steady-state free-precession (bSSFP) experiment is presented. Gradient imperfections and their imaging effects are also considered. Results: Sequence TR with low-time bandwidth product (TBP) pulses, as used in bSSFP, was reduced up to 14%, and the TR when using high TBP pulses, as used in slab-selective imaging, was reduced by up to 72%. A breath-hold cardiac exam was reduced by 46% using both MB and the TR-optimal framework. The importance of RF-based correction of gradient imperfections is demonstrated. PNS was not a practical limitation. Conclusion: The TR-optimal framework designs RF pulses for a range of pulse parameters, specifically to minimize sequence TR. K E Y W O R D S cardiac, radiofrequency pulse design, rapid imaging, simultaneous multislice (SMS) 1 | INTRODUCTION Rapid gradient echo sequences, including spoiled gradient echo (SPGR) and steady-state free precession (SSFP) are used across MRI for a wide range of clinical and research applications. These sequences typically acquire one line of k-space per radiofrequency (RF) excitation; therefore, minimizing TR is key to fast image acquisition. This is particularly relevant for cine cardiac imaging 1-3 in which the TR determines the breath-hold duration. Furthermore, reducing TR for bSSFP also limits the impact of banding artifacts, which can become problematic for challenging B 0-shimming scenarios such as cardiac applications, especially at 3T and above. The minimum TR achievable is determined by multiple factors including the durations of image encoding gradients, peripheral nerve stimulation (PNS) predictions, 4 and specific absorption rate (SAR). The latter is particularly limiting for applications

Open‐source myocardial T1 mapping with simultaneous multi‐slice acceleration: Combining an auto‐calibrated blipped‐bSSFP readout with VERSE‐MB pulses

“…16 We integrate variable-rate selective excitation (VERSE) MB pulses, where a time variable slice gradient is optimized in combination with the MB pulse. 18,19 The goal is to increase flexibility when designing the RF pulses, enabling to find a better tradeoff between minimum RF pulse duration and hence the achievable TR, 20 the resulting specific absorption rate (SAR) level and the production of undesirable side lobes (SLs). 5,19 VERSE can be expected to become particularly relevant when pushing for very high SMS acceleration factors at high fields or if very sharp slice profiles are required.…”