Dynamically phase-cycled radial balanced SSFP imaging for efficient banding removal

Rapid quantitative magnetization transfer imaging: Utilizing the hybrid state and the generalized Bloch model

PURPOSE

To explore efficient encoding schemes for quantitative magnetization transfer (qMT) imaging with few constraints on model parameters.

THEORY AND METHODS

We combine two recently proposed models in a Bloch-McConnell equation: the dynamics of the free spin pool are confined to the hybrid state, and the dynamics of the semi-solid spin pool are described by the generalized Bloch model. We numerically optimize the flip angles and durations of a train of radio frequency pulses to enhance the encoding of three qMT parameters while accounting for all eight parameters of the two-pool model. We sparsely sample each time frame along this spin dynamics with a three-dimensional radial koosh-ball trajectory, reconstruct the data with subspace modeling, and fit the qMT model with a neural network for computational efficiency.

RESULTS

We extracted qMT parameter maps of the whole brain with an effective resolution of 1.24 mm from a 12.6-min scan. In lesions of multiple sclerosis subjects, we observe a decreased size of the semi-solid spin pool and longer relaxation times, consistent with previous reports.

CONCLUSION

The encoding power of the hybrid state, combined with regularized image reconstruction, and the accuracy of the generalized Bloch model provide an excellent basis for efficient quantitative magnetization transfer imaging with few constraints on model parameters.

Fast T2 mapping of short-T2 tissues in knee using 3D radial dual-echo balanced steady-state free precession

BACKGROUND

T2 mapping of short-T2 tissues in the knee (meniscus, tendon, and ligament) is needed to aid the clinical MRI knee diagnosis, which is hard to realize using traditional clinical methods.

PURPOSE

To accelerate the acquisition of T2 values for short-T2 tissues in the knee by analyzing the signal equation of balanced steady-state free precession (bSSFP) sequence in MRI.

METHODS

Effect of half-radial acquisition on pixel bandwidth was analyzed mathematically. A modified 3D radial dual-echo bSSFP sequence was proposed for 0.53 mm isotropic resolution knee imaging with 2 different TEs at 3 T, which alleviated the problem of off-resonance artifacts caused by traditional half-radial acquisition scheme. A novel pixel-based optimization method was proposed for efficient T2 mapping of short-T2 tissues in the knee given off-resonance values. Simulation was conducted to evaluate the sensitivity of the proposed method to other parameters. Phantom results were compared with 2D spin-echo (SE), and in vivo results were compared with SE and previously studies.

RESULTS

Simulation showed that the proposed method is insensitive to T1 and B1 variations (estimation error < 1% for T1/B1 error of ±90%), avoiding the need for separated T1 and B1 scans. High isotropic resolution knee imaging was achieved using the modified dual-echo bSSFP. The total scan time was within 3.5 min, including a separate off-resonance scan for T2 measurement. Measured mean T2 values for phantoms correlated well with SE (R2 = 0.99), and no significant difference was observed (P = 0.45). In vivo meniscus T2 measurements and ligament T2 measurements agreed with the literature, while tendon T2 measurements were much lower (31.7% lower for patellar tendon, and 13.5% lower for quadriceps tendon), which might result in its bi-component property.

CONCLUSIONS

The proposed method provides an efficient way for fast, robust, high-resolution imaging and T2 mapping of short-T2 tissues in the knee.

Patterns of Signal Intensity in CISS MRI of the Inner Ear and Eye

BACKGROUND

Constructive interference in steady state (CISS) is a gradient echo magnetic resonance imaging (MRI) pulse sequence that provides excellent contrast between cerebrospinal fluid and adjacent structures but is prone to banding artifacts due to magnetic field inhomogeneities. We aimed to characterize artifacts in the inner ear and eye.

METHODS

In 30 patients (60 ears/eyes) undergoing CISS sequence MRI, nine low-signal intensity regions were identified in the inner ear and compared to temporal bone histopathology. The number and angle of bands across the eye were examined.

RESULTS

In the cochlea, all ears had regions of low signal corresponding to anatomy (modiolus (all), spiral lamina (n = 59, 98.3%), and interscalar septa (n = 50, 83.3%)). In the labyrinth, the lateral semicircular canal crista (n = 42, 70%) and utricular macula (n = 47, 78.3%) were seen. Areas of low signal in the vestibule seen in all ears may represent the walls of the membranous utricle. Zero to three banding artifacts were seen in both eyes (right: 96.7%, mean 1.5; left: 93.3%, mean 1.3).

CONCLUSION

Low signal regions in the inner ear on CISS sequences are common and have consistent patterns; most in the inner ear represent anatomy, appearing blurred due to partial volume averaging. Banding artifacts in the eye are more variable.

A dual-stage partially interpretable neural network for joint suppression of bSSFP banding and flow artifacts in non-phase-cycled cine imaging

PURPOSE

To develop a partially interpretable neural network for joint suppression of banding and flow artifacts in non-phase-cycled bSSFP cine imaging.

METHODS

A dual-stage neural network consisting of a voxel-identification (VI) sub-network and artifact-suppression (AS) sub-network is proposed. The VI sub-network provides identification of artifacts, which guides artifact suppression and improves interpretability. The AS sub-network reduces banding and flow artifacts. Short-axis cine images of 12 frequency offsets from 28 healthy subjects were used to train and test the dual-stage network. An additional 77 patients were retrospectively enrolled to evaluate its clinical generalizability. For healthy subjects, artifact suppression performance was analyzed by comparison with traditional phase cycling. The partial interpretability provided by the VI sub-network was analyzed via correlation analysis. Generalizability was evaluated for cine obtained with different sequence parameters and scanners. For patients, artifact suppression performance and partial interpretability of the network were qualitatively evaluated by 3 clinicians. Cardiac function before and after artifact suppression was assessed via left ventricular ejection fraction (LVEF).

RESULTS

For the healthy subjects, visual inspection and quantitative analysis found a considerable reduction of banding and flow artifacts by the proposed network. Compared with traditional phase cycling, the proposed network improved flow artifact scores (4.57 ± 0.23 vs 3.40 ± 0.38, P = 0.002) and overall image quality (4.33 ± 0.22 vs 3.60 ± 0.38, P = 0.002). The VI sub-network well identified the location of banding and flow artifacts in the original movie and significantly correlated with the change of signal intensities in these regions. Changes of imaging parameters or the scanner did not cause a significant change of overall image quality relative to the baseline dataset, suggesting a good generalizability. For the patients, qualitative analysis showed a significant improvement of banding artifacts (4.01 ± 0.50 vs 2.77 ± 0.40, P < 0.001), flow artifacts (4.22 ± 0.38 vs 2.97 ± 0.57, P < 0.001), and image quality (3.91 ± 0.45 vs 2.60 ± 0.43, P < 0.001) relative to the original cine. The artifact suppression slightly reduced the LVEF (mean bias = -1.25%, P = 0.01).

CONCLUSIONS

The dual-stage network simultaneously reduces banding and flow artifacts in bSSFP cine imaging with a partial interpretability, sparing the need for sequence modification. The method can be easily deployed in a clinical setting to identify artifacts and improve cine image quality.

Respiratory-Triggered Flow-Independent Noncontrast Non-ECG-Gated MRV (REACT) Versus CE-MRV for Central Venous Evaluation in Children and Young Adults: A Six-Reader Study

BACKGROUND. Contrast-enhanced MRI is commonly used to evaluate thoracic central venous patency in children and young adults. A flow-independent noncontrast non-ECG-gated 3D MRA-MR venography (MRV) technique described in 2019 as "relaxation-enhanced angiography without contrast and triggering (REACT)" may facilitate such evaluation. OBJECTIVE. The purpose of our study was to compare image quality, diagnostic confidence, and interreader agreement between respiratory-triggered REACT and 3D Dixon-based contrast-enhanced MRV (CE-MRV) for evaluating thoracic central venous patency in children and young adults. METHODS. This retrospective study included 42 consecutive children and young adults who underwent MRI of the neck and chest to evaluate central venous patency between August 2019 and January 2021 (median age, 5.2 years; IQR, 1.4-15.1 years; 22 female patients and 20 male patients). Examinations included respiratory-triggered REACT and navigator-gated CE-MRV sequences based on the institution's standard-of-care protocol. Six pediatric radiologists from four different institutions independently reviewed REACT and CE-MRV sequences; they assessed overall image quality (scale, 1-5; 5 = excellent), diagnostic confidence (scale, 1-5; 5 = extremely confident), and presence of clinically relevant artifact(s). Readers classified seven major central vessels as normal or abnormal (e.g., narrowing, thrombosis, or occlusion). Analysis used Wilcoxon signed rank and McNemar tests and Fleiss kappa coefficients. RESULTS. The distribution of overall image quality scores was higher (p = .02) for REACT than for CE-MRV for one reader (both sequences: median score, 5). Image quality scores were not significantly different between the sequences for the remaining five readers (all p > .05). Diagnostic confidence scores and frequency of clinically relevant artifact(s) were not significantly different between sequences for any reader (all p > .05). Interreader agreement for vessel classification as normal or abnormal was similar between sequences for all seven vessels (REACT: κ = 0.37-0.81; CE-MRV: κ = 0.34-0.81). Pooling readers and vessels, 65.4% of vessels were normal by both sequences; 18.7%, abnormal by both sequences; 9.8%, abnormal by REACT only; and 6.1%, abnormal by CE-MRV only. CONCLUSION. Respiratory-triggered REACT, in comparison with CE-MRV, showed no significant difference in image quality (aside from for one of six readers), diagnostic confidence, or frequency of artifact(s), with similar interreader agreement for vessel classification as normal or abnormal. CLINICAL IMPACT. High-resolution 3D MRV performed without IV contrast material can be used to assess central venous patency in children and young adults.

Phase-cycled balanced SSFP imaging for non-contrast-enhanced functional lung imaging

PURPOSE

To introduce phase-cycled balanced SSFP (bSSFP) acquisition as an alternative in Fourier decomposition MRI for improved robustness against field inhomogeneities.

METHODS

Series 2D dynamic lung images were acquired in 5 healthy volunteers at 1.5 T and 3 T using bSSFP sequence with multiple RF phase increments and compared with conventional single RF phase increment acquisitions. The approach was evaluated based on functional map homogeneity analysis, while ensuring image and functional map quality by means of SNR and contrast-to-noise ratio analyses.

RESULTS

At both field strengths, functional maps obtained with phase-cycled acquisitions displayed improved robustness against local signal losses compared with single-phase acquisitions. The coefficient of variation (mean ± SD, across volunteers) measured in the ventilation maps resulted in 29.7 ± 2.6 at 1.5 T and 37.5 ± 3.1 at 3 T for phase-cycled acquisitions, compared with 39.9 ± 5.2 at 1.5 T and 49.5 ± 3.7 at 3 T for single-phase acquisitions, indicating a significant improvement ( p < 0.05 $$ p<0.05 $$ ) in ventilation map homogeneity.

CONCLUSIONS

Phase-cycled bSSFP acquisitions improve robustness against field inhomogeneity artifacts and significantly improve ventilation map homogeneity at both field strengths. As such, phase-cycled bSSFP may serve as a robust alternative in lung function assessments.

Constrained Ellipse Fitting for Efficient Parameter Mapping With Phase-Cycled bSSFP MRI

Balanced steady-state free precession (bSSFP) imaging enables high scan efficiency in MRI, but differs from conventional sequences in terms of elevated sensitivity to main field inhomogeneity and nonstandard [Formula: see text]-weighted tissue contrast. To address these limitations, multiple bSSFP images of the same anatomy are commonly acquired with a set of different RF phase-cycling increments. Joint processing of phase-cycled acquisitions serves to mitigate sensitivity to field inhomogeneity. Recently phase-cycled bSSFP acquisitions were also leveraged to estimate relaxation parameters based on explicit signal models. While effective, these model-based methods often involve a large number of acquisitions (N ≈ 10-16), degrading scan efficiency. Here, we propose a new constrained ellipse fitting method (CELF) for parameter estimation with improved efficiency and accuracy in phase-cycled bSSFP MRI. CELF is based on the elliptical signal model framework for complex bSSFP signals; and it introduces geometrical constraints on ellipse properties to improve estimation efficiency, and dictionary-based identification to improve estimation accuracy. CELF generates maps of [Formula: see text], [Formula: see text], off-resonance and on-resonant bSSFP signal by employing a separate [Formula: see text] map to mitigate sensitivity to flip angle variations. Our results indicate that CELF can produce accurate off-resonance and banding-free bSSFP maps with as few as N = 4 acquisitions, while estimation accuracy for relaxation parameters is notably limited by biases from microstructural sensitivity of bSSFP imaging.

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.

Cardiac balanced steady-state free precession MRI at 0.35 T: a comparison study with 1.5 T

Background

While low-field MRI is disadvantaged by a reduced signal-to-noise ratio (SNR) compared to higher fields, it has a number of useful features such as decreased SAR and shorter T1, and has shown promise for diagnostic imaging. This study demonstrates the feasibility of cardiac balanced steady-state free precession (bSSFP) MRI at 0.35 T and compares cardiac bSSFP MRI images at 0.35 T with those at 1.5 T.

Methods

Cardiac images were acquired in 7 healthy volunteers using an ECG-gated bSSFP cine sequence on a 0.35 T superconducting MR system as well as a clinical 1.5 T system. Blood and myocardium SNR and contrast-to-noise ratio (CNR) were computed. Subjective image scoring was used to compare the image quality between 0.35 and 1.5 T.

Results

Cardiac images at 0.35 T were successfully acquired in all volunteers. While the 0.35 T images were noisier than those at 1.5 T, blood, myocardium and papillary muscles could be clearly delineated. At 0.35 T, bSSFP images were acquired at flip angles as high as 150°. Maximum CNR was achieved at 130°. Image quality scoring showed that while at lower flip angles, the 0.35 T images had poorer quality than the 1.5 T, but with flip angles of 110 and 130, the image quality at 0.35 T had scores similar to those at 1.5 T.

Conclusions

This study demonstrates that cardiac bSSFP imaging is highly feasible at 0.35 T.

Frequency-modulated bSSFP for phase-sensitive separation of water and fat

Our study proposes the use of a frequency-modulated acquisition which suppresses banding artefacts in combination with a phase-sensitive water-fat separation algorithm. The performance of the phase-sensitive separation for standard bSSFP, complex sum combination thereof, and frequency-modulated bSSFP were compared in in vivo measurements of the upper and lower legs at 1.5 and 3 T. It is shown, that the standard acquisition suffered from banding artefacts and major swaps between tissues. The dual-acquisition bSSFP could alleviate banding artefacts and only minor swaps occurred, but it comes at the expense of a doubled acquisition. In the frequency-modulated acquisitions all banding artefacts and the associated phase jumps were eliminated and no swaps between tissues occurred. It therefore provides a means to robustly separate water and fat, in one single radial bSSFP scan, using the phase-sensitive approach, even in the presence of high field inhomogeneities.

PLANET: An ellipse fitting approach for simultaneous T1 and T2 mapping using phase-cycled balanced steady-state free precession

Purpose

To demonstrate the feasibility of a novel, ellipse fitting approach, named PLANET, for simultaneous estimation of relaxation times T₁ and T₂ from a single 3D phase‐cycled balanced steady‐state free precession (bSSFP) sequence.

Methods

A method is presented in which the elliptical signal model is used to describe the phase‐cycled bSSFP steady‐state signal. The fitting of the model to the acquired data is reformulated into a linear convex problem, which is solved directly by a linear least squares method, specific to ellipses. Subsequently, the relaxation times T₁ and T₂, the banding free magnitude, and the off‐resonance are calculated from the fitting results.

Results

Maps of T₁ and T₂, as well as an off‐resonance and a banding free magnitude can be simultaneously, quickly, and robustly estimated from a single 3D phase‐cycled bSSFP sequence. The feasibility of the method was demonstrated in a phantom and in the brain of healthy volunteers on a clinical MR scanner. The results were in good agreement for the phantom, but a systematic underestimation of T₁ was observed in the brain.

Conclusion

The presented method allows for accurate mapping of relaxation times and off‐resonance, and for the reconstruction of banding free magnitude images at realistic signal‐to‐noise ratios. Magn Reson Med 79:711–722, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.

Whole-brain perfusion imaging with balanced steady-state free precession arterial spin labeling

Recently, balanced steady-state free precession (bSSFP) readout has been proposed for arterial spin labeling (ASL) perfusion imaging to reduce susceptibility artifacts at a relatively high spatial resolution and signal-to-noise ratio (SNR). However, the main limitation of bSSFP-ASL is the low spatial coverage. In this work, methods to increase the spatial coverage of bSSFP-ASL are proposed for distortion-free, high-resolution, whole-brain perfusion imaging. Three strategies of (i) segmentation, (ii) compressed sensing (CS) and (iii) a hybrid approach combining the two methods were tested to increase the spatial coverage of pseudo-continuous ASL (pCASL) with three-dimensional bSSFP readout. The spatial coverage was increased by factors of two, four and six using each of the three approaches, whilst maintaining the same total scan time (5.3 min). The number of segments and/or CS acceleration rate (R) correspondingly increased to maintain the same bSSFP readout time (1.2 s). The segmentation approach allowed whole-brain perfusion imaging for pCASL-bSSFP with no penalty in SNR and/or total scan time. The CS approach increased the spatial coverage of pCASL-bSSFP whilst maintaining the temporal resolution, with minimal impact on the image quality. The hybrid approach provided compromised effects between the two methods. Balanced SSFP-based ASL allows the acquisition of perfusion images with wide spatial coverage, high spatial resolution and SNR, and reduced susceptibility artifacts, and thus may become a good choice for clinical and neurological studies. Copyright © 2015 John Wiley & Sons, Ltd.

Phase-cycled simultaneous multislice balanced SSFP imaging with CAIPIRINHA for efficient banding reduction

PURPOSE

To present a time-efficient technique for banding reduction in balanced steady-state free precession (bSSFP) imaging using phase-cycled simultaneous multislice (SMS) acquisition with CAIPIRINHA (controlled aliasing in parallel imaging results in higher acceleration).

THEORY

The proposed technique exploits the inherent phase modulation of SMS imaging with CAIPIRINHA to acquire multiple phase-cycled images, which can be combined for efficient banding reduction within the same scan time of a single-band bSSFP scan.

METHODS

Bloch equation simulation, phantom and in vivo brain, abdominal and cardiac imaging experiments were performed on healthy volunteers at 3T using multi-channel head and body array coils with SMS acceleration factors of two to four. The performance of banding reduction was quantitatively evaluated based on the percent ripple of signal distribution and signal-to-noise ratio (SNR) efficiency in both phantom and human studies.

RESULTS

The banding artifact was successfully removed or suppressed using phase-cycled SMS bSSFP imaging across SMS factors of two to four. The performance of banding reduction improved with higher SMS factors along with increased SNR efficiency.

CONCLUSION

Phase-cycled SMS bSSFP with CAIPIRINHA is a promising technique for efficient band reduction in bSSFP without prolonged scan time. Further evaluation of this technique in clinical applications is warranted. Magn Reson Med 76:1764-1774, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Physiological and Functional Magnetic Resonance Imaging Using Balanced Steady-state Free Precession

Balanced steady-state free precession (bSSFP) is a highly efficient pulse sequence that is known to provide the highest signal-to-noise ratio per unit time. Recently, bSSFP is getting increasingly popular in both the research and clinical communities. This review will be focusing on the application of the bSSFP technique in the context of probing the physiological and functional information. In the first part of this review, the basic principles of bSSFP are briefly covered. Afterwards, recent developments related to the application of bSSFP, in terms of physiological and functional imaging, are introduced and reviewed. Despite its long development history, bSSFP is still a promising technique that has many potential benefits for obtaining high-resolution physiological and functional images.