Optimized amplitude modulated multiband RF pulse design

Section: Discussionmentioning

confidence: 99%

“…41 To reduce this RF distortion, we design real valued RF pulses that are less prone to RF distortions. 42 Additionally, we used a time grid of 10 ls, 400 times the minimal RF duration of 25 ns. Therefore, we do not expect significant alterations of the RF chain on the optimized RF shapes.…”

Section: Discussionmentioning

confidence: 99%

Simultaneous multislice refocusing via time optimal control

Rund

Aigner

Kunisch

et al. 2018

“…For this work, the ripple relations were set to design a single refocusing pulse instead of a matched‐excitation as originally proposed. Moreover, the Monte Carlo search for optimal root‐patterns was replaced by a genetic algorithm as implemented in MATLAB 2015b, which we previously found to give slightly improved results …”

Section: Methodsmentioning

confidence: 99%

“…Optimal phase‐offsets were chosen to produce real‐valued RF pulses (i.e. not complex‐valued), which could be described using purely signed AM . This was done to circumvent an additional known hardware issue with faithfully reproducing rapidly varying FM waveforms.…”

Section: Methodsmentioning

confidence: 99%

“…where

γ

is the gyromagnetic ratio,

G

is the amplitude of the constant selection gradient,

t

is a time‐variable,

x_{n}

is the spatial location of the n th excited slice and

ϕ_{n}

is the phase‐offset of this slice, numerically optimized as in Wong and Abo Seada et al After application of VERSE the gradient waveform is time‐variable, and this must be accounted for in the modulation function

f_{N}^{v} (t)

f_{N}^{v} (t) = \sum_{n = 1}^{N} e^{i (k ()(, t) x_{n} + ϕ_{n})}

…”

Section: Theorymentioning

confidence: 99%

See 1 more Smart Citation

Multiband RF pulse design for realistic gradient performance

Seada

Price

Schneider

et al. 2018

Self Cite

PurposeSimultaneous multi‐slice techniques are reliant on multiband RF pulses, for which conventional design strategies result in long pulse durations, lengthening echo‐times so lowering SNR for spin‐echo imaging, and lengthening repetition times for gradient echo sequences. Pulse durations can be reduced with advanced RF pulse design methods that use time‐variable selection gradients. However, the ability of gradient systems to reproduce fast switching pulses is often limited and can lead to image artifacts when ignored. We propose a time‐efficient pulse design method that inherently produces gradient waveforms with lower temporal bandwidth.MethodsEfficient multiband RF pulses with time‐variable gradients were designed using time‐optimal VERSE. Using VERSE directly on multiband pulses leads to gradient waveforms with high temporal bandwidth, whereas VERSE applied first to singleband RF pulses and then modulated to make them multiband, significantly reduces this. The relative performance of these approaches was compared using simulation and experimental measurements.ResultsApplying VERSE before multiband modulation was successful at removing out‐of‐band slice distortion. This effectively removes the need for high frequency modulation in the gradient waveform while preserving the benefit of time‐efficiency inherited from VERSE.ConclusionWe propose a time‐efficient RF pulse design that produces gradient pulses with lower temporal bandwidth, reducing image artifacts associated with finite temporal bandwidth of gradient systems.

Simultaneous multi‐slice cardiac cine with Fourier‐encoded self‐calibration at 7 Tesla

Rapacchi

Troalen

Bentatou

et al. 2018

Purpose To accelerate cardiac cine at 7 tesla using simultaneous multi‐slice (SMS) acquisition with self‐calibration to resolve misalignment between calibration and imaging data due to breathing motion. Methods A spoiled‐gradient echo cine sequence was modified with radiofrequency phase‐cycled SMS excitations. A Fourier encoding strategy was applied along the cardiac phase dimension to allow for slice untangling and split‐slice GRAPPA calibration. Split‐slice GRAPPA was coupled with regular GRAPPA (SMS‐GRAPPA) and L1‐SPIRiT (SMS‐L1SPIRiT) for image reconstruction. 3‐slice SMS cine MRI was evaluated in ten subjects against single‐slice cine MRI in terms of SNR and contrast‐to‐noise ratio and slice leakage. Results SNR decreased significantly from 10.1 ± 7.1 for single‐slice cine to 7.4 ± 2.8 for SMS‐GRAPPA (P = 0.02) and was recovered to 9.0 ± 4.5 with SMS‐L1SPIRiT (P = 0.02). Contrast to noise ratio decreased significantly from 14.5 ± 8.1 for single‐slice cine to 5.6 ± 3.6 for SMS‐GRAPPA (P < 0.0001) and increased slightly but significantly back to 6.7 ± 4.4 for SMS‐L1SPIRiT (P = 0.03). Specific absorption rate restrictions imposed a reduced nominal flip angle (−37 ± 7%, P = 0.02) for 3‐slice SMS excitations compared to single‐slice acquisitions. SMS slice leakage increased significantly from apex (8.6 ± 6.5 %) to base (13.1 ± 4.1 %, P = 0.03) in the left ventricle. Conclusion Three‐fold acceleration of cine at 7T was achieved using the proposed SMS technique. Fourier encoding self‐calibration and regularized image reconstruction enabled simultaneous acquisition of three slices without significant SNR decrease but significant CNR decrease linked to the reduced nominal excitation flip angle.