Time‐optimized 4D phase contrast MRI with real‐time convex optimization of gradient waveforms and fast excitation methods

摘要

Purpose To shorten 4D flow acquisitions by shortening TRs with fast RF pulses and gradient waveforms. Real‐time convex optimization is used to generate these gradients waveforms on the scanner. Theory and Methods RF and slab‐select waveforms were shortened with a minimum phase SLR excitation and the time‐optimal variable‐rate selective excitation method. Real‐time convex optimization was used to shorten bipolar and spoiler gradients by finding the shortest gradient waveforms that satisfied constraints on scan parameters, gradient hardware, M 0 , M 1 , and peripheral nerve stimulation. Waveforms were calculated and TE and/or TR values were compared for a range of scan parameters and compared to a conventional 4D flow sequence. The method was tested in flow phantoms, and in the aorta and neurovasculature of volunteers ( N = 10). Additionally, eddy current error was measured in a large phantom. Results TEs and TRs were shortened by 21–32% and 20–34%, respectively, compared to the conventional sequence over a range of scan parameters. Bland‐Altman analysis of 2 flow phantom configurations showed flow rate bias of 0.3 mL/s and limits of agreement (LOA) of [?6.9, 7.5] mL/s for a cardiac phantom and a bias of ?0.1 mL/s with LOA = [?0.4, 0.2] mL/s for a neuro phantom. Similar agreement was also seen for flow measurements in volunteers (bias = ?1.0 and ?0.1 mL/s, LOA = [?34.9, 33.0] and [?0.7, 0.6] mL/s). Measured eddy currents were 39% larger with the CVX + mpVERSE method. Conclusion The real‐time optimized 4D flow gradients and fast slab‐selection excitation methods produced up to 34% faster TRs with excellent flow measurement agreement compared to a conventional 4D flow sequence.