Fat signal suppression for coronary MRA at 3T using a water-selective adiabatic T2-preparation technique

Fat-free noncontrast whole-heart cardiovascular magnetic resonance imaging with fast and power-optimized off-resonant water-excitation pulses

BACKGROUND

Cardiovascular magnetic resonance imaging (CMR) faces challenges due to the interference of bright fat signals in visualizing structures, such as coronary arteries. Effective fat suppression is crucial, especially when using whole-heart CMR techniques. Conventional methods often fall short due to rapid fat signal recovery, leading to residual fat content hindering visualization. Water-selective off-resonant radiofrequency (RF) pulses have been proposed but come with tradeoffs between pulse duration, which increases scan time, and increased RF energy deposit, which limits their applicability due to specific absorption rate (SAR) constraints. The study introduces a lipid-insensitive binomial off-resonant (LIBOR) RF pulse, which addresses concerns about SAR and scan time, and aims to provide a comprehensive quantitative comparison with published off-resonant RF pulses for CMR at 3T.

METHODS

A short (1 ms) LIBOR pulse, with reduced RF power requirements, was developed and implemented in a free-breathing respiratory-self-navigated three-dimensional radial whole-heart CMR sequence at 3T. A binomial off-resonant rectangular (BORR) pulse with matched duration, as well as previously published lipid-insensitive binomial off-resonant excitation (LIBRE) pulses (1 and 2.2 ms), were implemented and optimized for fat suppression in numerical simulations and validated in volunteers (n = 3). Whole-heart CMR was performed in volunteers (n = 10) with all four pulses. The signal-to-noise ratio (SNR) of ventricular blood, skeletal muscle, myocardium, and subcutaneous fat and the coronary vessel detection rates and sharpness were compared.

RESULTS

Experimental results validated numerical findings and near-homogeneous fat suppression was achieved with all four pulses. Comparing the short RF pulses (1 ms), LIBOR reduced the RF power nearly two-fold compared with LIBRE, and three-fold compared with BORR, and LIBOR significantly decreased overall fat SNR from cardiac scans, compared to LIBRE and BORR. The reduction in RF pulse duration (from 2.2 to 1 ms) shortened the whole-heart acquisition from 8.5 to 7 min. No significant differences in coronary arteries detection and sharpness were found when comparing all four pulses.

CONCLUSION

LIBOR pulses enabled whole-heart CMR under 7 min at 3T, with large volume fat signal suppression, while reducing RF power compared with LIBRE and BORR pulses. LIBOR is an excellent candidate to address SAR problems encountered in CMR sequences where fat suppression remains challenging and short RF pulses are required.

Turbo spin-echo-based enhanced acceleration-selective arterial spin labeling without electrocardiography or peripheral pulse unit triggering and contrast enhancement for lower extremity MRA

PURPOSE

Lower extremity magnetic resonance angiography (MRA) without electrocardiography (ECG) or peripheral pulse unit (PPU) triggering and contrast enhancement is beneficial for diagnosing peripheral arterial disease (PAD) while avoiding synchronization failure and nephrogenic systemic fibrosis. This study aimed to compare the diagnostic performance of turbo spin-echo-based enhanced acceleration-selective arterial spin labeling (eAccASL) (TSE-Acc) of the lower extremities with that of turbo field-echo-based eAccASL (TFE-Acc) and triggered angiography non-contrast enhanced (TRANCE).

METHODS

Nine healthy volunteers and a patient with PAD were examined on a 3.0 Tesla magnetic resonance imaging (MRI) system. The artery-to-muscle signal intensity ratio (SIR) and contrast-to-noise ratio (CNR) were calculated. The arterial visibility (1: poor, 4: excellent) and artifact contamination (1: severe, 4: no) were independently assessed by two radiologists. Phase-contrast MRI and digital subtraction angiography were referenced in a patient with PAD. Friedman's test and a post-hoc test according to the Bonferroni-adjusted Wilcoxon signed-rank test were used for the SIR, CNR, and visual assessment. p < 0.05 was considered statistically significant.

RESULTS

No significant differences in nearly all the SIRs were observed among the three MRA methods. Higher CNRs were observed with TSE-Acc than those with TFE-Acc (anterior tibial artery, p = 0.014; peroneal artery, p = 0.029; and posterior tibial artery, p = 0.014) in distal arterial segments; however, no significant differences were observed upon comparison with TRANCE (all p > 0.05). The arterial visibility scores exhibited similar trends as the CNRs. The artifact contamination scores with TSE-Acc were significantly lower (but within an acceptable level) compared to those with TFE-Acc. In the patient with PAD, the sluggish peripheral arteries were better visualized using TSE-Acc than those using TFE-Acc, and the collateral and stenosis arteries were better visualized using TSE-Acc than those using TRANCE.

CONCLUSION

Peripheral arterial visualization was better with TSE-Acc than that with TFE-Acc in lower extremity MRA without ECG or PPU triggering and contrast enhancement, which was comparable with TRANCE as the reference standard. Furthermore, TSE-Acc may propose satisfactory diagnostic performance for diagnosing PAD in patients with arrhythmia and chronic kidney disease.

Fully automated pixel-wise quantitative CMR-myocardial perfusion with CMR-coronary angiography to detect hemodynamically significant coronary artery disease

OBJECTIVES

We applied a fully automated pixel-wise post-processing framework to evaluate fully quantitative cardiovascular magnetic resonance myocardial perfusion imaging (CMR-MPI). In addition, we aimed to evaluate the additive value of coronary magnetic resonance angiography (CMRA) to the diagnostic performance of fully automated pixel-wise quantitative CMR-MPI for detecting hemodynamically significant coronary artery disease (CAD).

METHODS

A total of 109 patients with suspected CAD were prospectively enrolled and underwent stress and rest CMR-MPI, CMRA, invasive coronary angiography (ICA), and fractional flow reserve (FFR). CMRA was acquired between stress and rest CMR-MPI acquisition, without any additional contrast agent. Finally, CMR-MPI quantification was analyzed by a fully automated pixel-wise post-processing framework.

RESULTS

Of the 109 patients, 42 patients had hemodynamically significant CAD (FFR ≤ 0.80 or luminal stenosis ≥ 90% on ICA) and 67 patients had hemodynamically non-significant CAD (FFR ˃ 0.80 or luminal stenosis < 30% on ICA) were enrolled. On the per-territory analysis, patients with hemodynamically significant CAD had higher myocardial blood flow (MBF) at rest, lower MBF under stress, and lower myocardial perfusion reserve (MPR) than patients with hemodynamically non-significant CAD (p < 0.001). The area under the receiver operating characteristic curve of MPR (0.93) was significantly larger than those of stress and rest MBF, visual assessment of CMR-MPI, and CMRA (p < 0.05), but similar to that of the integration of CMR-MPI with CMRA (0.90).

CONCLUSIONS

Fully automated pixel-wise quantitative CMR-MPI can accurately detect hemodynamically significant CAD, but the integration of CMRA obtained between stress and rest CMR-MPI acquisition did not provide significantly additive value.

KEY POINTS

• Full quantification of stress and rest cardiovascular magnetic resonance myocardial perfusion imaging can be postprocessed fully automatically, generating pixel-wise myocardial blood flow (MBF) and myocardial perfusion reserve (MPR) maps. • Fully quantitative MPR provided higher diagnostic performance for detecting hemodynamically significant coronary artery disease, compared with stress and rest MBF, qualitative assessment, and coronary magnetic resonance angiography (CMRA). • The integration of CMRA and MPR did not significantly improve the diagnostic performance of MPR alone.

The Road Toward Reproducibility of Parametric Mapping of the Heart: A Technical Review

Parametric mapping of the heart has become an essential part of many cardiovascular magnetic resonance imaging exams, and is used for tissue characterization and diagnosis in a broad range of cardiovascular diseases. These pulse sequences are used to quantify the myocardial T₁, T₂, T2*, and T_1ρ relaxation times, which are unique surrogate indices of fibrosis, edema and iron deposition that can be used to monitor a disease over time or to compare patients to one another. Parametric mapping is now well-accepted in the clinical setting, but its wider dissemination is hindered by limited inter-center reproducibility and relatively long acquisition times. Recently, several new parametric mapping techniques have appeared that address both of these problems, but substantial hurdles remain for widespread clinical adoption. This review serves both as a primer for newcomers to the field of parametric mapping and as a technical update for those already well at home in it. It aims to establish what is currently needed to improve the reproducibility of parametric mapping of the heart. To this end, we first give an overview of the metrics by which a mapping technique can be assessed, such as bias and variability, as well as the basic physics behind the relaxation times themselves and what their relevance is in the prospect of myocardial tissue characterization. This is followed by a summary of routine mapping techniques and their variations. The problems in reproducibility and the sources of bias and variability of these techniques are reviewed. Subsequently, novel fast, whole-heart, and multi-parametric techniques and their merits are treated in the light of their reproducibility. This includes state of the art segmentation techniques applied to parametric maps, and how artificial intelligence is being harnessed to solve this long-standing conundrum. We finish up by sketching an outlook on the road toward inter-center reproducibility, and what to expect in the future.

Laminar fMRI using T2-prepared multi-echo FLASH

Functional magnetic resonance imaging (fMRI) using blood oxygenation level dependent (BOLD) contrast at a sub-millimeter scale is a promising technique to probe neural activity at the level of cortical layers. While gradient echo (GRE) BOLD sequences exhibit the highest sensitivity, their signal is confounded by unspecific extravascular (EV) and intravascular (IV) effects of large intracortical ascending veins and pial veins leading to a downstream blurring effect of local signal changes. In contrast, spin echo (SE) fMRI promises higher specificity towards signal changes near the microvascular compartment. However, the T₂-weighted signal is typically sampled with a gradient echo readout imposing additional T₂'-weighting. In this work, we used a T₂-prepared (T₂-prep) sequence with short GRE readouts to investigate its capability to acquire laminar fMRI data during a visual task in humans at 7 T. By varying the T₂-prep echo time (TE_prep) and acquiring multiple gradient echoes (TE_GRE) per excitation, we studied the specificity of the sequence and the influence of possible confounding contributions to the shape of laminar fMRI profiles. By fitting and extrapolating the multi-echo GRE data to a TE_GRE = 0 ms condition, we show for the first time laminar profiles free of T₂'-pollution, confined to gray matter. This finding is independent of TE_prep, except for the shortest one (31 ms) where hints of a remaining intravascular component can be seen. For TE_GRE > 0 ms a prominent peak at the pial surface is observed that increases with longer TE_GRE and dominates the shape of the profiles independent of the amount of T₂-weighting. Simulations show that the peak at the pial surface is a result of static EV dephasing around pial vessels in CSF visible in GM due to partial voluming. Additionally, another, weaker, static dephasing effect is observed throughout all layers of the cortex, which is particularly obvious in the data with shortest T₂-prep echo time. Our simulations show that this cannot be explained by intravascular dephasing but that it is likely caused by extravascular effects of the intracortical and pial veins. We conclude that even for TE_GRE as short as 2.3 ms, the T₂'-weighting added to the T₂-weighting is enough to dramatically affect the laminar specificity of the BOLD signal change. However, the bulk of this corruption stems from CSF partial volume effects which can in principle be addressed by increasing the spatial resolution of the acquisition.

A robust broadband fat-suppressing phaser T2 -preparation module for cardiac magnetic resonance imaging at 3T

PURPOSE

Designing a new T₂ -preparation (T₂ -Prep) module to simultaneously provide robust fat suppression and efficient T₂ preparation without requiring an additional fat-suppression module for T₂ -weighted imaging at 3T.

METHODS

The tip-down radiofrequency (RF) pulse of an adiabatic T₂ -Prep module was replaced by a custom-designed RF-excitation pulse that induces a phase difference between water and fat, resulting in a simultaneous T₂ preparation of water signals and the suppression of fat signals at the end of the module (a phaser adiabatic T₂ -Prep). Numerical simulations and in vitro and in vivo electrocardiogram (ECG)-triggered navigator-gated acquisitions of the human heart were performed. Blood, myocardium, and fat signal-to-noise ratios and right coronary artery vessel sharpness were compared against previously published adiabatic T₂ -Prep approaches.

RESULTS

Numerical simulations predicted an increased fat-suppression bandwidth and decreased sensitivity to transmit magnetic field inhomogeneities using the proposed approach while preserving the water T₂ -Prep capabilities. This was confirmed by the tissue signals acquired in the phantom and the in vivo images, which show similar blood and myocardium signal-to-noise ratio, contrast-to-noise ratio, and significantly reduced fat signal-to-noise ratio compared with the other methods. As a result, the right coronary artery conspicuity was significantly increased.

CONCLUSION

A novel fat-suppressing T₂ -Prep method was developed and implemented that showed robust fat suppression and increased vessel sharpness compared with conventional techniques while preserving its T₂ -Prep capabilities.

Combined T2 -preparation and multidimensional outer volume suppression for coronary artery imaging with 3D cones trajectories

PURPOSE

To develop a modular magnetization preparation sequence for combined T2 -preparation and multidimensional outer volume suppression (OVS) for coronary artery imaging.

METHODS

A combined T2 -prepared 1D OVS sequence with fat saturation was defined to contain a 90°-60 180°60 composite nonselective tip-down pulse, two 180°Y hard pulses for refocusing, and a -90° spectral-spatial sinc tip-up pulse. For 2D OVS, 2 modules were concatenated, selective in X and then Y. Bloch simulations predicted robustness of the sequence to B0 and B1 inhomogeneities. The proposed sequence was compared with a T2 -prepared 2D OVS sequence proposed by Luo et al, which uses a spatially selective 2D spiral tip-up. The 2 sequences were compared in phantom studies and in vivo coronary artery imaging studies with a 3D cones trajectory.

RESULTS

Phantom results demonstrated superior OVS for the proposed sequence compared with the Luo sequence. In studies on 15 healthy volunteers, the proposed sequence had superior image edge profile acutance values compared with the Luo sequence for the right (P < .05) and left (P < .05) coronary arteries, suggesting superior vessel sharpness. The proposed sequence also had superior signal-to-noise ratio (P < .05) and passband-to-stopband ratio (P < .05). Reader scores and reader preference indicated superior coronary image quality of the proposed sequence for both the right (P < .05) and left (P < .05) coronary arteries.

CONCLUSION

The proposed sequence with concatenated 1D spatially selective tip-ups and integrated fat saturation has superior image quality and suppression compared with the Luo sequence with 2D spatially selective tip-up.

Improved respiratory self-navigation for 3D radial acquisitions through the use of a pencil-beam 2D-T2 -prep for free-breathing, whole-heart coronary MRA

PURPOSE

In respiratory self-navigation (SN), signal from static structures, such as the chest wall, may complicate motion detection or introduce post-correction artefacts. Suppressing signal from superfluous tissues may therefore improve image quality. We thus test the hypothesis that SN whole-heart coronary magnetic resonance angiography (MRA) will benefit from an outer-volume suppressing 2D-T2 -Prep and present both phantom and in vivo results.

METHODS

A 2D-T2 -Prep and a conventional T2 -Prep were used prior to a free-breathing 3D-radial SN sequence. Both techniques were compared by imaging a home-built moving cardiac phantom and by performing coronary MRA in nine healthy volunteers. Reconstructions were performed using both a reference-based and a reference-independent approach to motion tracking, along with several coil combinations. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were compared, along with vessel sharpness (VS).

RESULTS

In phantoms, using the 2D-T2 -Prep increased SNR by 16% to 53% and mean VS by 8%; improved motion tracking precision was also achieved. In volunteers, SNR increased by an average of 29% to 33% in the blood pool and by 15% to 25% in the myocardium, depending on the choice of reconstruction coils and algorithm, and VS increased by 34%.

CONCLUSION

A 2D-T2 -Prep significantly improves image quality in both phantoms and volunteers when performing SN coronary MRA. Magn Reson Med 79:1293-1303, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A Cylindrical, Inner Volume Selecting 2D-T2-Prep Improves GRAPPA-Accelerated Image Quality in MRA of the Right Coronary Artery

BACKGROUND

Two-dimensional (2D) spatially selective radiofrequency (RF) pulses may be used to excite restricted volumes. By incorporating a "pencil beam" 2D pulse into a T2-Prep, one may create a "2D-T2-Prep" that combines T2-weighting with an intrinsic outer volume suppression. This may particularly benefit parallel imaging techniques, where artefacts typically originate from residual foldover signal. By suppressing foldover signal with a 2D-T2-Prep, image quality may therefore improve. We present numerical simulations, phantom and in vivo validations to address this hypothesis.

METHODS

A 2D-T2-Prep and a conventional T2-Prep were used with GRAPPA-accelerated MRI (R = 1.6). The techniques were first compared in numerical phantoms, where per pixel maps of SNR (SNRmulti), noise, and g-factor were predicted for idealized sequences. Physical phantoms, with compartments doped to mimic blood, myocardium, fat, and coronary vasculature, were scanned with both T2-Preparation techniques to determine the actual SNRmulti and vessel sharpness. For in vivo experiments, the right coronary artery (RCA) was imaged in 10 healthy adults, using accelerations of R = 1,3, and 6, and vessel sharpness was measured for each.

RESULTS

In both simulations and phantom experiments, the 2D-T2-Prep improved SNR relative to the conventional T2-Prep, by an amount that depended on both the acceleration factor and the degree of outer volume suppression. For in vivo images of the RCA, vessel sharpness improved most at higher acceleration factors, demonstrating that the 2D-T2-Prep especially benefits accelerated coronary MRA.

CONCLUSION

Suppressing outer volume signal with a 2D-T2-Prep improves image quality particularly well in GRAPPA-accelerated acquisitions in simulations, phantoms, and volunteers, demonstrating that it should be considered when performing accelerated coronary MRA.

Quantification of liver fat: A comprehensive review

Fat accumulation in the liver causes metabolic diseases such as obesity, hypertension, diabetes or dyslipidemia by affecting insulin resistance, and increasing the risk of cardiac complications and cardiovascular disease mortality. Fatty liver diseases are often reversible in their early stage; therefore, there is a recognized need to detect their presence and to assess its severity to recognize fat-related functional abnormalities in the liver. This is crucial in evaluating living liver donors prior to transplantation because fat content in the liver can change liver regeneration in the recipient and donor. There are several methods to diagnose fatty liver, measure the amount of fat, and to classify and stage liver diseases (e.g. hepatic steatosis, steatohepatitis, fibrosis and cirrhosis): biopsy (the gold-standard procedure), clinical (medical physics based) and image analysis (semi or fully automated approaches). Liver biopsy has many drawbacks: it is invasive, inappropriate for monitoring (i.e., repeated evaluation), and assessment of steatosis is somewhat subjective. Qualitative biomarkers are mostly insufficient for accurate detection since fat has to be quantified by a varying threshold to measure disease severity. Therefore, a quantitative biomarker is required for detection of steatosis, accurate measurement of severity of diseases, clinical decision-making, prognosis and longitudinal monitoring of therapy. This study presents a comprehensive review of both clinical and automated image analysis based approaches to quantify liver fat and evaluate fatty liver diseases from different medical imaging modalities.

On the accuracy and precision of cardiac magnetic resonance T2 mapping: A high-resolution radial study using adiabatic T2 preparation at 3 T

PURPOSE

The goal of this study was to characterize the accuracy and precision of cardiac T₂ mapping as a function of different factors including low signal-to-noise ratio (SNR), imaging in systole, and off-resonance frequencies.

METHODS

Bloch equation and Monte Carlo simulations were used to determine the influence of SNR and the choice of T₂ preparation echo time (TE_T2prep ) increments on the accuracy and precision of high-resolution radial cardiac T₂ mapping at 3.0 T. Healthy volunteers were scanned to establish the difference in precision and inter- and intraobserver variability between T₂ mapping in diastole and systole, as well as the effect of SNR and off-resonance frequencies on the accuracy of T₂ maps.

RESULTS

The simulations demonstrated that a TE_T2prep increment of ∼0.75 times the T₂ value of interest optimally increases the precision of the T₂ fit. Systolic T₂ maps were found to have a higher precision (P = 0.002), but similar inter- and intraobserver variability compared with diastolic T₂ maps, whereas off-resonance frequencies beyond ± 100 Hz cause a significant decrease in both accuracy and precision (P < 0.05).

CONCLUSION

This evaluation of the accuracy and precision of cardiac T₂ mapping characterizes the major vulnerabilities of the technique and will help guide protocol definition of studies that include T₂ mapping. Magn Reson Med 77:159-169, 2017. © 2016 Wiley Periodicals, Inc.

Combined outer volume suppression and T2 preparation sequence for coronary angiography

PURPOSE

To develop a magnetization preparation sequence for simultaneous outer volume suppression (OVS) and T2 weighting in whole-heart coronary magnetic resonance angiography.

METHODS

A combined OVS and T2 preparation sequence (OVS-T2 Prep) was designed with a nonselective adiabatic 90° tipdown pulse, two adiabatic 180° refocusing pulses, and a 2D spiral -90° tipup pulse. The OVS-T2 Prep preserves the magnetization inside an elliptic cylinder with T2 weighting, while saturating the magnetization outside the cylinder. Its performance was tested on phantoms and on 13 normal subjects with coronary magnetic resonance angiography using 3D cones trajectories.

RESULTS

Phantom studies showed expected T2 -dependent signal amplitude in the spatial passband and suppressed signal in the spatial stopband. In vivo studies with full-field-of-view cones yielded a passband-to-stopband signal ratio of 3.18 ± 0.77 and blood-myocardium contrast-to-noise ratio enhancement by a factor of 1.43 ± 0.20 (P < 0.001). In vivo studies with reduced-field-of-view cones showed that OVS-T2 Prep well suppressed the aliasing artifacts, as supported by significantly reduced signal in the regions with no tissues compared to the images acquired without preparation (P < 0.0001).

CONCLUSION

OVS-T2 Prep is a compact sequence that can accelerate coronary magnetic resonance angiography by suppressing signals from tissues surrounding the heart while simultaneously enhancing the blood-myocardium contrast.