Abstract¶
Michael Loecher 1| Patrick Magrath 2| Eric Aliotta 3| Daniel B. Ennis 1,2,3
1 Department of Radiological Sciences, University of California, Los Angeles, California
2 Department of Bioengineering, University of California, Los Angeles, California
3Department of Biomedical Physics, University of California, Los Angeles, California
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, M0, M1, 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.
Conclusions: 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.
KEYWORDS
4D flow, convex optimization, eddy currents, peripheral nerve stimulation, phase contrast, pulse
sequence design
Corespondence: Daniel Ennis, Radiological Sciences Lab, 1201 Welch Road, Stanford, CA 94305. Email: dbe@stanford.edu