Universal nonselective excitation and refocusing pulses with improved robustness to off‐resonance for Magnetic Resonance Imaging at 7 Tesla with parallel transmission

• Published in: Magnetic resonance in medicine Vol. 85; no. 2; pp. 678 - 693
• Main Authors: Van Damme, L., Mauconduit, F., Chambrion, T., Boulant, N., Gras, V.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.02.2021; Wiley
• Subjects: Algorithms; Brain - diagnostic imaging; Calibration; Computational neuroscience; Design parameters; Excitation; GRAPE; Grapes; kT‐point; Magnetic Resonance Imaging; Mathematics; Medical imaging; Neuroimaging; Offsets; optimal control; Optimization and Control; parallel transmission; Parameterization; Phantoms, Imaging; Pulse duration; Radio Waves; RF pulse design; Robustness (mathematics); SPINS; ultra high field; Vibration; Waveforms; GRAPE; SPINS; optimal control; ultra high field; RF pulse design; parallel transmission; kT-point; k_T$-point
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.28441

Purpose In MRI at ultra‐high field, the kT‐point and spiral nonselective (SPINS) pulse design techniques can be advantageously combined with the parallel transmission (pTX) and universal pulse techniques to create uniform excitation in a calibration‐free manner. However, in these approaches, pulse duration is typically increased as compared to standard hard pulses, and excitation quality in regions exhibiting large resonance frequency offsets often suffer. This limitation is inherent to structure of kT‐point or SPINS pulse, and likely can be mitigated using parameterization‐free pulse design approaches. Methods The Gradient Ascent Pulse Engineering (GRAPE) algorithm was used to design parameterization‐free RF and magnetic field gradient (MFG) waveforms for creating 8∘ excitation, up to 105∘ scalable refocusing and inversion, nonselectively across the brain. Simulations were performed to provide flip angle normalized root‐mean‐squares error (FA‐NRMSE) estimations for the 8∘ and the 180∘kT‐point, SPINS, and GRAPE pulses. GRAPE pulses were tested experimentally with anatomical head scans at 7T. Results As compared to kT‐points and SPINS, GRAPE provided substantial improvement of excitation, refocusing, and inversion quality at off‐resonance while at least preserving the same global FA‐NRMSE performance. As compared to kT‐points, GRAPE allowed for a substantial reduction of the pulse duration for the 8∘ excitation and the 105∘ refocusing. Conclusions Parameterization‐free universal nonselective pTX‐pulses were successfully computed using GRAPE. Performance gains as compared to kT‐points were validated numerically and experimentally for three imaging protocols. In its current implementation, the computational burden of GRAPE limits its use to applications where pulse computations are not subject to time constraints.