View ordering for magnetization prepared steady state free precession acquisition: Application in contrast-enhanced MR angiography

mcLARO: Multi-contrast learned acquisition and reconstruction optimization for simultaneous quantitative multi-parametric mapping

PURPOSE

To develop a method for rapid sub-millimeter T1 , T2 ,T 2 * $$ {\mathrm{T}}_2^{\ast } $$ , and QSM mapping in a single scan using multi-contrast learned acquisition and reconstruction optimization (mcLARO).

METHODS

A pulse sequence was developed by interleaving inversion recovery and T2 magnetization preparations and single-echo and multi-echo gradient echo acquisitions, which sensitized k-space data to T1 , T2 ,T 2 * $$ {\mathrm{T}}_2^{\ast } $$ , and magnetic susceptibility. The proposed mcLARO optimized both the multi-contrast k-space under-sampling pattern and image reconstruction based on image feature fusion using a deep learning framework. The proposed mcLARO method withR = 8 $$ R=8 $$ under-sampling was validated in a retrospective ablation study and compared with other deep learning reconstruction methods, including MoDL and Wave-MoDL, using fully sampled data as reference. Various under-sampling ratios in mcLARO were investigated. mcLARO was also evaluated in a prospective study using separately acquired conventionally sampled quantitative maps as reference standard.

RESULTS

The retrospective ablation study showed improved image sharpness of mcLARO compared to the baseline network without the multi-contrast sampling pattern optimization or image feature fusion module. The under-sampling ratio experiment showed that higher under-sampling ratios resulted in blurrier images and lower precision of quantitative values. The prospective study showed that small or negligible bias and narrow 95% limits of agreement on regional T1 , T2 ,T 2 * $$ {\mathrm{T}}_2^{\ast } $$ , and QSM values by mcLARO (5:39 mins) compared to reference scans (40:03 mins in total). mcLARO outperformed MoDL and Wave-MoDL in terms of image sharpness, noise suppression, and artifact removal.

CONCLUSION

mcLARO enabled fast sub-millimeter T1 , T2 ,T 2 * $$ {\mathrm{T}}_2^{\ast } $$ , and QSM mapping in a single scan.

LARO: Learned acquisition and reconstruction optimization to accelerate quantitative susceptibility mapping

Quantitative susceptibility mapping (QSM) involves acquisition and reconstruction of a series of images at multi-echo time points to estimate tissue field, which prolongs scan time and requires specific reconstruction technique. In this paper, we present our new framework, called Learned Acquisition and Reconstruction Optimization (LARO), which aims to accelerate the multi-echo gradient echo (mGRE) pulse sequence for QSM. Our approach involves optimizing a Cartesian multi-echo k-space sampling pattern with a deep reconstruction network. Next, this optimized sampling pattern was implemented in an mGRE sequence using Cartesian fan-beam k-space segmenting and ordering for prospective scans. Furthermore, we propose to insert a recurrent temporal feature fusion module into the reconstruction network to capture signal redundancies along echo time. Our ablation studies show that both the optimized sampling pattern and proposed reconstruction strategy help improve the quality of the multi-echo image reconstructions. Generalization experiments show that LARO is robust on the test data with new pathologies and different sequence parameters. Our code is available at https://github.com/Jinwei1209/LARO-QSM.git.

Single-shot pseudo-centric EPI for magnetization-prepared imaging

PURPOSE

To implement a single-shot centric-reordered EPI (1sh-CenEPI), which reduces TE significantly, thus enabling to improve SNR for magnetization-prepared imaging.

METHODS

We proposed a 1sh-CenEPI in which grouped oscillating readout gradients, phase-encoding blips within each group, and big phase-encoding jumps between two consecutive groups are incorporated to encode whole k-space from the center to the edges in a single shot. The concept was tested on phantoms and human brains at 3 T. In addition, the proposed reordering scheme was applied to pseudo-continuous arterial spin labeling for evaluating the efficiency of the centric reordering in magnetization-prepared imaging.

RESULTS

The proposed 1sh-CenEPI reduced TE from 50 ms to 1.4 ms for gradient-echo EPI, and from 100 ms to 7 ms for spin-echo EPI, while the elongation of readout duration was below 10% of the whole readout duration in most cases. The 1sh-CenEPI images exhibited no distinct geometric distortion both in phantom and human brain, comparable to the conventional two-shot center-out EPI. In pseudo-continuous arterial spin labeling results, 3-fold temporal SNR increase and 2-fold spatial SNR increase in the perfusion-weighted images were achieved with 1sh-CenEPI compared with the conventional linear ordering, whereas the cerebral blood flow values were consistent with previous studies.

CONCLUSION

The proposed 1sh-CenEPI significantly reduced TE while maintaining similar readout window and providing images comparable to the conventional linear and multishot center-out EPI images. It can be a qualified candidate as a new readout for various magnetization-prepared imaging techniques.

Prospective GIRF-based RF phase cycling to reduce eddy current-induced steady-state disruption in bSSFP imaging

Purpose

To propose an explicit Balanced steady‐state free precession (bSSFP) signal model that predicts eddy current‐induced steady‐state disruptions and to provide a prospective, practical, and general eddy current compensation method.

Theory and Methods

Gradient impulse response functions (GIRF) were used to simulate trajectory‐specific eddy current‐induced phase errors at the end of a repetition block. These phase errors were included in bloch simulations to establish a bSSFP signal model to predict steady‐state disruptions and their corresponding image artifacts. The signal model was embedded in the MR system and used to compensate the phase errors by prospectively modifying the phase cycling scheme of the RF pulse. The signal model and eddy current compensation method were validated in phantom and in vivo experiments. In addition, the signal model was used to analyze pre‐existing eddy current mitigation methods, such as 2D tiny golden angle radial and 3D paired phase encoded Cartesian acquisitions.

Results

The signal model predicted eddy current‐induced image artifacts, with the zeroth‐order GIRF being the primary factor to predict the steady‐state disruption. Prospective RF phase cycling schemes were automatically computed online and considerably reduced eddy current‐induced image artifacts. The signal model provides a direct relationship for the smoothness of k‐space trajectories, which explains the effectiveness of phase encode pairing and tiny golden angle trajectory.

Conclusions

The proposed signal model can accurately predict eddy current‐induced steady‐state disruptions for bSSFP imaging. The signal model can be used to derive the eddy current‐induced phase errors required for trajectory‐specific RF phase cycling schemes, which considerably reduce eddy current‐induced image artifacts.

A novel profile/view ordering with a non-convex star shutter for high-resolution 3D volumetric T1 mapping under multiple breath-holds

PURPOSE

To examine a novel non-convex star ordering/shutter for reducing the number of breath-holds in cardiac three-dimensional (3D) T₁ Mapping MRI with multiple breath-holds.

METHODS

A novel ordering, Non-Convex Star (NCS) was designed to acquire 3D volumes in a modified look-locker inversion recovery (MOLLI) T₁ mapping sequence to provide more spatial resolution and coverage in fewer breath-holds. The proposed 3D-MOLLI approach using NCS was first validated in two phantoms using artifact power (AP) measurement against the fully sampled phantom. This was followed by an in vivo study in seven swine, in which the T₁ values of the left ventricular (LV) myocardium divided into the American Heart Association (AHA) 16-segment model was compared against the reference multislice two-dimensional (2D) clinical reference and 3D volume without NCS breath-hold reduction.

RESULTS

NCS breath-hold reduction yielded less AP compared with the matched SENSE accelerated phantom volume (P < 0.0005), and was shown to be optimal at 25% fewer breath-holds. Calculated T₁ values from 3D in vivo volumes with/without NCS were comparable in all AHA segments (P = NS), whereas 3D-NCS yielded significantly higher T₁ values than 2D at midslice of the LV myocardium in each AHA segment (P < 0.05).

CONCLUSION

We successfully demonstrate the feasibility of the NCS approach for a 3D T₁ mapping acquisition requiring fewer breath-holds. Magn Reson Med 77:2215-2224, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Three-dimensional magnetization-prepared imaging using a concentric cylinders trajectory

PURPOSE

To develop new magnetization-prepared imaging schemes based on a three-dimensional (3D) concentric cylinders trajectory.

METHODS

The 3D concentric cylinders trajectory, which is robust to off-resonance effects and timing delays while requiring fewer excitations than a comparable 3D Cartesian (3DFT) sequence, is used as the readout for magnetization-prepared sequences exploiting its inherently centric-ordered structure. Two applications: (i) T1 -weighted brain imaging with an inversion-recovery-prepared radiofrequency-spoiled gradient-echo (IR-SPGR) sequence, (ii) non-contrast-enhanced (NCE) peripheral angiography with a magnetization-prepared balanced steady-state free precession (bSSFP) sequence are presented to demonstrate the effectiveness of the proposed method. For peripheral angiography, the scan efficiency is further improved by interleaving different preparations at different rates and by carefully designing the sampling geometry for an efficient parallel imaging method.

RESULTS

In vivo brain scans with an IR-SPGR sequence and lower extremity scans with a magnetization-prepared bSSFP sequence for NCE peripheral angiography both demonstrate that the proposed sequences with concentric cylinders effectively capture the transient magnetization-prepared contrast with faster scan times than a corresponding 3DFT sequence. The application of peripheral angiography also shows the feasibility of the proposed interleaving schemes and parallel imaging method.

CONCLUSION

The 3D concentric cylinders trajectory is a robust and efficient readout that is well-suited for magnetization-prepared imaging.

Monitoring and compensating phase imperfections in cine balanced steady-state free precession

PURPOSE

To analyze and correct for eddy current-induced phase imperfections in cardiac cine balanced steady-state free precession (bSSFP) imaging.

METHODS

Eddy current-induced phase offsets were measured for different phase-encoding schemes using a higher order dynamic field camera. Based on these measurements, offset phases were corrected for in postprocessing and by run-time phase compensation applying radiofrequency phase increments and additional compensatory gradient areas. The findings were validated using numerical simulations, phantom experiments, and in vivo cardiac scans.

RESULTS

Depending on the phase-encoding scheme, significant eddy current-induced phase offsets were detected. Time-varying phase offsets were observed at subsequent excitations leading to steady-state distortions and hence to profile-dependent amplitude modulations in k-space. Taking into account measured k-space trajectories algebraic image reconstruction allowed compensating imperfect spatial encoding. Correction of amplitude modulations was successfully accomplished by run-time phase compensation.

CONCLUSION

Using magnetic field monitoring, artifacts in cine balanced steady-state free precession caused by uncompensated eddy current fields can be significantly reduced.

Magnetization-prepared IDEAL bSSFP: a flow-independent technique for noncontrast-enhanced peripheral angiography

PURPOSE

To propose a new noncontrast-enhanced flow-independent angiography sequence based on balanced steady-state free precession (bSSFP) that produces reliable vessel contrast despite the reduced blood flow in the extremities.

MATERIALS AND METHODS

The proposed technique addresses a variety of factors that can compromise the exam success including insufficient background suppression, field inhomogeneity, and large volumetric coverage requirements. A bSSFP sequence yields reduced signal from venous blood when long repetition times are used. Complex-sum bSSFP acquisitions decrease the sensitivity to field inhomogeneity but retain phase information, so that data can be processed with the Iterative Decomposition of Water and Fat with Echo Asymmetry and Least-Squares Estimation (IDEAL) method for robust fat suppression. Meanwhile, frequent magnetization preparation coupled with parallel imaging reduces the muscle and long-T(1) fluid signals without compromising scan efficiency.

RESULTS

In vivo flow-independent peripheral angiograms with reliable background suppression and high spatial resolution are produced. Comparisons with phase-sensitive bSSFP angiograms (that yield out-of-phase fat and water signals, and exploit this phase difference to suppress fat) demonstrate enhanced vessel depiction with the proposed technique due to reduced partial-volume effects and improved venous suppression.

CONCLUSION

Magnetization-prepared complex-sum bSSFP with IDEAL fat/water separation can create reliable flow-independent angiographic contrast in the lower extremities.

Profile order and time-dependent artifacts in contrast-enhanced coronary MR angiography at 3T: origin and prevention

To enhance the clinical value of coronary magnetic resonance angiography (MRA), high-relaxivity contrast agents have recently been used at 3T. Here we examine a uniform bilateral shadowing artifact observed along the coronary arteries in MRA images collected using such a contrast agent. Simulations were performed to characterize this artifact, including its origin, to determine how best to mitigate this effect, and to optimize a data acquisition/injection scheme. An intraluminal contrast agent concentration model was used to simulate various acquisition strategies with two profile orders for a slow-infusion of a high-relaxivity contrast agent. Filtering effects from temporally variable weighting in k-space are prominent when a centric, radial (CR) profile order is applied during contrast infusion, resulting in decreased signal enhancement and underestimation of vessel width, while both pre- and postinfusion steady-state acquisitions result in overestimation of the vessel width. Acquisition during the brief postinfusion steady-state produces the greatest signal enhancement and minimizes k-space filtering artifacts.

Reduction of flow- and eddy-currents-induced image artifacts in coronary magnetic resonance angiography using a linear centric-encoding SSFP sequence

Coronary magnetic resonance angiography (MRA) acquired using steady-state free precession (SSFP) sequences tends to suffer from image artifacts caused by local magnetic field inhomogeneities. Flow- and gradient-switching-induced eddy currents are important sources of such phase errors, especially under off-resonant conditions. In this study, we propose to reduce these image artifacts by using a linear centric-encoding (LCE) scheme in the phase-encoding (PE) direction. Abrupt change in gradients, including magnitude and polarity between consecutive radiofrequency cycles, is minimized using the LCE scheme. Results from numeric simulations and phantom studies demonstrated that signal oscillation can be markedly reduced using LCE as compared to conventional alternating centric-encoding (ACE) scheme. The image quality of coronary arteries was improved at both 1.5 and 3.0 T using LCE compared to those acquired using ACE PE scheme (1.5 T: ACE/LCE=2.2+/-0.8/3.0+/-0.6, P=.02; 3.0 T: ACE/LCE=2.1+/-1.1/3.0+/-0.8, P=.01). In conclusion, flow- and eddy-currents-induced imaging artifacts in coronary MRA using SSFP sequence can be markedly reduced with LCE acquisition of PE lines.

T2 measurement of the human myocardium using aT2-prepared transient-state trueFISP sequence

A fast and motion-insensitive technique suitable for myocardial BOLD contrast imaging is presented. The method, termed T2-TrueFISP, combines T2 magnetization preparation with steady-state free precession (SSFP) imaging for T2 relaxation mapping of the myocardium in healthy volunteers. The T2 contrast-to-noise ratio (CNR) was optimized with the use of transient-state TrueFISP readout and half-Fourier readout with linear phase encoding. Single-slice myocardial T2-weighted image was obtained within one heartbeat, and a single slice T2 map of the myocardium was obtained in under 5-7 s. A respiratory navigator-gating method was incorporated for serial measurements and signal averaging, with the subjects breathing freely. The mean myocardial T2 relaxation time measured in 12 healthy volunteers was 54 +/- 5.7 ms. Regional variations of T2 values across the myocardium were 7%. Temporal variations across serial T2 measurements in a transmural region covering approximately 0.5 cc of the left ventricular (LV) wall were 3.6% without signal averaging (number of excitations (NEX) = 1) and 1.7% with signal averaging (NEX = 10). According to our preliminary results, the T2-TrueFISP method is expected to provide a robust and sensitive tool for clinical application of myocardial BOLD contrast imaging.

Contrast-Enhanced Magnetic Resonance Angiography with Biodegradable (Gd-DTPA)-Cystamine Copolymers: Comparison with MS-325 in a Swine Model

The purpose of this study is to evaluate the use of (Gd-DTPA)-cystamine copolymers (GDCC), a novel biodegradable intravascular polydisulfide-based macromolecular gadolinium(III) contrast agent, for first-pass and steady-state contrast-enhanced magnetic resonance angiography (MRA) in a swine model. A breath-hold background-suppressed 3D MRA of the thorax was performed for first-pass imaging and repeated every 10 min after GDCC injection to monitor the tissue enhancement time course. A navigator-gated 3D MRA of the coronary arteries was performed during steady state following the first-pass imaging. Imaging with intravascular agent MS-325 approximately 1 h after GDCC injection was also included for comparison. Experimental results indicated that GDCC provided significant blood signal-to-noise ratio (SNR) improvement, approximately 1633% for first-pass and 33% for steady-state contrast-enhanced MRA. Compared to MS-325, GDCC provided similar blood enhancement for first-pass and steady-state imaging but with a different tissue enhancement time course. The blood SNR enhancement half-time was 10 +/- 6 min for GDCC and 46 +/- 33 min for MS-325. GDCC provided less enhancement in the liver, bone growth plates, and muscle than MS-325.

Fluoroscopic Contrast-Enhanced MR Angiography with a Magnetization-Prepared Steady-State Free Precession Technique in Peripheral Arterial Occlusive Disease

OBJECTIVE

The objective of our study was to evaluate the feasibility of intraarterial (IA) near-real-time contrast-enhanced MR angiography (CE-MRA) with a frame rate of 1.3 frames per second in seven patients with lower extremity peripheral arterial occlusive disease (PAOD). For optimized background suppression, a modified 2D steady-state free precession (SSFP) technique with magnetization preparation and mask subtraction was developed. The femoropopliteal and infrapopliteal arteries were covered in two separate steps. Acceptable contrast-to-noise ratios were obtained, and road maps were reconstructed from the same data set.

CONCLUSION

Mastering IA near-real-time CE-MRA, including road map reconstruction, with an SSFP technique in the lower extremity of patients with PAOD is an important building block toward successfully performing endovascular catheter MR-guided interventions.

Analysis and compensation of eddy currents in balanced SSFP

Balanced steady-state free precession (SSFP) completely compensates for all gradients within each repetition time (TR), and is thus very sensitive to any magnetic field imperfection that disturbs the perfectly balanced acquisition scheme. It is demonstrated that balanced SSFP is especially sensitive to changing eddy currents that are induced by stepwise changing phase-encoding (PE) gradients. In contrast to the linear k-space trajectory, which has small variations between consecutive encoding steps, other encoding schemes (e.g., centric, random, or segmented orderings) exhibit significant jumps in k-space between adjacent PE steps, and consequently induce rapidly changing eddy currents. The resulting disturbances induce significant image artifacts, such that compensation strategies are essential when nonlinear PE schemes are applied. Although direct annihilation of the induced eddy currents by additional, opposing magnetic fields has been investigated, it is limited by uncertainty regarding the time evolution of induced eddy currents. A generic (and thus system-unrelated) compensation strategy is proposed that consists of "pairing" of consecutive PE steps. Another approach is based on partial dephasing along the slice direction that annihilates eddy-current-induced signal oscillations. Both pairing of the PE steps and "through-slice equilibration" are easy to implement and allow the use of arbitrary k-space trajectories for balanced SSFP.