Synthesis methods of multiple phase-cycled SSFP images to reduce the band artifact and noise more reliably

Varying undersampling directions for accelerating multiple acquisition magnetic resonance imaging

In this study, we propose a new sampling strategy for efficiently accelerating multiple acquisition MRI. The new sampling strategy is to obtain data along different phase-encoding directions across multiple acquisitions. The proposed sampling strategy was evaluated in multicontrast MR imaging (T1, T2, proton density) and multiple phase-cycled (PC) balanced steady-state free precession (bSSFP) imaging by using convolutional neural networks with central and random sampling patterns. In vivo MRI acquisitions as well as a public database were used to test the concept. Based on both visual inspection and quantitative analysis, the proposed sampling strategy showed better performance than sampling along the same phase-encoding direction in both multicontrast MR imaging and multiple PC-bSSFP imaging, regardless of sampling pattern (central, random) or datasets (public, retrospective and prospective in vivo). For the prospective in vivo applications, acceleration was performed by sampling along different phase-encoding directions at the time of acquisition with a conventional rectangular field of view, which demonstrated the advantage of the proposed sampling strategy in the real environment. Preliminary trials on compressed sensing (CS) also demonstrated improvement of CS with the proposed idea. Sampling along different phase-encoding directions across multiple acquisitions is advantageous for accelerating multiacquisition MRI, irrespective of sampling pattern or datasets, with further improvement through transfer learning.

Factorized sensitivity estimation for artifact suppression in phase-cycled bSSFP MRI

OBJECTIVE

Balanced steady-state free precession (bSSFP) imaging suffers from banding artifacts in the presence of magnetic field inhomogeneity. The purpose of this study is to identify an efficient strategy to reconstruct banding-free bSSFP images from multi-coil multi-acquisition datasets.

METHOD

Previous techniques either assume that a naïve coil-combination is performed a priori resulting in suboptimal artifact suppression, or that artifact suppression is performed for each coil separately at the expense of significant computational burden. Here we propose a tailored method that factorizes the estimation of coil and bSSFP sensitivity profiles for improved accuracy and/or speed.

RESULTS

In vivo experiments show that the proposed method outperforms naïve coil-combination and coil-by-coil processing in terms of both reconstruction quality and time.

CONCLUSION

The proposed method enables computationally efficient artifact suppression for phase-cycled bSSFP imaging with modern coil arrays. Rapid imaging applications can efficiently benefit from the improved robustness of bSSFP imaging against field inhomogeneity.

Frequency-modulated SSFP with radial sampling and subspace reconstruction: A time-efficient alternative to phase-cycled bSSFP

PURPOSE

A novel subspace-based reconstruction method for frequency-modulated balanced steady-state free precession (fmSSFP) MRI is presented. In this work, suitable data acquisition schemes, subspace sizes, and efficiencies for banding removal are investigated.

THEORY AND METHODS

By combining a fmSSFP MRI sequence with a 3D stack-of-stars trajectory, scan efficiency is maximized as spectral information is obtained without intermediate preparation phases. A memory-efficient reconstruction routine is implemented by introducing the low-frequency Fourier transform as a subspace which allows for the formulation of a convex reconstruction problem. The removal of banding artifacts is investigated by comparing the proposed acquisition and reconstruction technique to phase-cycled bSSFP MRI. Aliasing properties of different undersampling schemes are analyzed and water/fat separation is demonstrated by reweighting the reconstructed subspace coefficients to generate virtual spectral responses in a post-processing step.

RESULTS

A simple root-of-sum-of-squares combination of the reconstructed subspace coefficients yields high-SNR images with the characteristic bSSFP contrast but without banding artifacts. Compared to Golden-Angle trajectories, turn-based sampling schemes were superior in minimizing aliasing across reconstructed subspace coefficients. Water/fat separated images of the human knee were obtained by reweighting subspace coefficients.

CONCLUSIONS

The novel subspace-based fmSSFP MRI technique emerges as a time-efficient alternative to phase-cycled bSFFP. The method does not need intermediate preparation phases, offers high SNR and avoids banding artifacts. Reweighting of the reconstructed subspace coefficients allows for generating virtual spectral responses with applications to water/fat separation.

Mitigation of near-band balanced steady-state free precession through-plane flow artifacts using partial dephasing

PURPOSE

To mitigate artifacts from through-plane flow at the locations of steady-state stopbands in balanced steady-state free precession (SSFP) using partial dephasing.

METHODS

A 60° range in the phase accrual during a TR was created over the voxel by slightly unbalancing the slice-select dephaser. The spectral profiles of SSFP with partial dephasing for various constant flow rates and during pulsatile flow were simulated to determine if partial dephasing decreases through-plane flow artifacts originating near SSFP dark bands while maintaining on-resonant signal. Simulations were then validated in a flow phantom. Lastly, phase-cycled SSFP cardiac cine images were acquired with and without partial dephasing in six subjects.

RESULTS

Partial dephasing decreased the strength and non-linearity of the dependence of the signal at the stopbands on the through-plane flow rate. It thus mitigated hyper-enhancement from out-of-slice signal contributions and transient-related artifacts caused by variable flow both in the phantom and in vivo. In six volunteers, partial dephasing noticeably decreased artifacts in all of the phase-cycled cardiac cine datasets.

CONCLUSION

Partial dephasing can mitigate the flow artifacts seen at the stopbands in balanced SSFP while maintaining the sequence's desired signal. By mitigating hyper-enhancement and transient-related artifacts originating from the stopbands, partial dephasing facilitates robust multiple-acquisition phase-cycled SSFP in the heart. Magn Reson Med 79:2944-2953, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Analytical corrections of banding artifacts in driven equilibrium single pulse observation of T2 (DESPOT2)

PURPOSE

DESPOT2 is a single-component T₂ mapping technique based on bSSFP imaging. It has seen limited application because of banding artifacts and magnetization transfer (MT) effects. In this work, acquisitions are optimized to minimize MT effects, while exact and approximate analytical equations enable automatic correction of banding artifacts within the T₂ maps in mere seconds.

THEORY AND METHODS

The technique was verified on an agar phantom at 3 tesla. The T₂ resulting from four different data combination techniques was compared with the T₂ from CPMG. Two comparable DESPOT2 scan protocols (short vs. long TR/T_RF ) designed to minimize MT effects, were tested both in the phantom and in vivo. A third protocol was tested in the brain of 8 volunteers and analytical correction schemes were compared with DESPOT2-FM.

RESULTS

The T₂ measurements in agar agree with CPMG within ∼7% and in vivo results agree with values reported in the literature. The approximate analytical solutions provide increased robustness to hardware imperfections and higher T₂ -to-noise ratio than the exact solutions.

CONCLUSION

New analytical solutions enable fast and accurate whole-brain T₂ mapping from previously measured T₁ and B₁ maps, and bSSFP images with at least two phase offsets and two flip angles (=4 datasets, 8 min scan). Magn Reson Med 76:1790-1804, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Layer-specific functional and anatomical MRI of the retina with passband balanced SSFP

The retina consists of multiple cellular and synaptic layers and is nourished by two distinct (retinal and choroidal) circulations bounding the retina, separated by an avascular layer. High spatiotemporal resolution, layer-specific MRI of the retina remains challenging due to magnetic inhomogeneity-induced artifacts. This study reports passband balanced steady-state free-precession (bSSFP) MRI at 45×45×500 μm and 1.6 s temporal resolution to image the mouse retina, overcoming geometric distortion and signal dropout while maintaining rapid acquisition and high signal-to-noise ratio. bSSFP images revealed multiple alternating dark-bright-dark-bright retinal layers. Hypoxic (10% O(2) ) inhalation decreased bSSFP signals in the two layers bounding the retina, corresponding to the retinal and choroidal vasculatures. The layer in between showed no substantial response and was assigned the avascular photoreceptor layers. Choroidal responses (-25.9 ± 6.4%, mean ± SD, n=6) were significantly (P<0.05) larger than retinal vascular responses (-11.6±2.4%). bSSFP offers very high spatiotemporal resolution and could have important applications in imaging layer-specific changes in retinal diseases.

Brain MR perfusion-weighted imaging with alternate ascending/descending directional navigation

In this study, a new arterial spin labeling technique that requires no separate spin preparation pulse was developed. Sequential two-dimensional slices were acquired in ascending and descending orders by turns using balanced steady state free precession for pair-wise subtraction. Simulation studies showed this new technique, alternate ascending/descending directional navigation (ALADDIN), has high sensitivity to both slow- (1-10 cm/sec) and fast-moving (>10 cm/sec) blood because of the presence of multiple labeling planes proximal to imaging planes and sensitivity of balanced steady state free precession to initial magnetization differences. ALADDIN provided high-resolution multislice perfusion-weighted images in ∼ 3 min. About 80-90% of signals in a slice were ascribed to spins saturated in the four prior slices. Three to five edge slices on each side of imaging group were affected by transient magnetization transfer effects and incomplete T(1) recovery between successive acquisitions. ALADDIN signals were dependent on many imaging parameters, implying room for improvement. Sagittal and coronal ALADDIN images demonstrated perfusion direction in gray matter regions was mostly from center to lateral, anterior, or posterior, whereas that in some white matter regions was reversed. ALADDIN is likely useful for many studies requiring perfusion-weighted imaging with short scan time, insensitiveness to arterial transit time, directional information, high resolution, and/or wide coverage.