Carotid blood flow measurement accelerated by compressed sensing: validation in healthy volunteers

A characterization of cardiac-induced noise in R2 * maps of the brain

PURPOSE

Cardiac pulsation increases the noise level in brain maps of the transverse relaxation rate R2 *. Cardiac-induced noise is challenging to mitigate during the acquisition of R2 * mapping data because its characteristics are unknown. In this work, we aim to characterize cardiac-induced noise in brain maps of the MRI parameter R2 *.

METHODS

We designed a sampling strategy to acquire multi-echo 3D data in 12 intervals of the cardiac cycle, monitored with a fingertip pulse-oximeter. We measured the amplitude of cardiac-induced noise in this data and assessed the effect of cardiac pulsation on R2 * maps computed across echoes. The area of k-space that contains most of the cardiac-induced noise in R2 * maps was then identified. Based on these characteristics, we introduced a tentative sampling strategy that aims to mitigate cardiac-induced noise in R2 * maps of the brain.

RESULTS

In inferior brain regions, cardiac pulsation accounts for R2 * variations of up to 3 s-1 across the cardiac cycle (i.e., ∼35% of the overall variability). Cardiac-induced fluctuations occur throughout the cardiac cycle, with a reduced intensity during the first quarter of the cycle. A total of 50% to 60% of the overall cardiac-induced noise is localized near the k-space center (k < 0.074 mm-1 ). The tentative cardiac noise mitigation strategy reduced the variability of R2 * maps across repetitions by 11% in the brainstem and 6% across the whole brain.

CONCLUSION

We provide a characterization of cardiac-induced noise in brain R2 * maps that can be used as a basis for the design of mitigation strategies during data acquisition.

Quality Control for 4D Flow MR Imaging

In recent years, 4D flow MRI has become increasingly important in clinical applications for the blood vessels in the whole body, heart, and cerebrospinal fluid. 4D flow MRI has advantages over 2D cine phase-contrast (PC) MRI in that any targeted area of interest can be analyzed post-hoc, but there are some factors to be considered, such as ensuring measurement accuracy, a long imaging time and post-processing complexity, and interobserver variability.Due to the partial volume phenomenon caused by low spatial and temporal resolutions, the accuracy of flow measurement in 4D flow MRI is reduced. For spatial resolution, it is recommended to include at least four voxels in the vessel of interest, and if possible, six voxels. In large vessels such as the aorta, large voxels can be secured and SNR can be maintained, but in small cerebral vessels, SNR is reduced, resulting in reduced accuracy. A temporal resolution of less than 40 ms is recommended. The velocity-to-noise ratio (VNR) of low-velocity blood flow is low, resulting in poor measurement accuracy. The use of dual velocity encoding (VENC) or multi-VENC is recommended to avoid velocity wrap around and to increase VNR. In order to maintain sufficient spatio-temporal resolution, a longer imaging time is required, leading to potential patient movement during examination and a corresponding decrease in measurement accuracy.For the clinical application of new technologies, including various acceleration techniques, in vitro and in vivo accuracy verification based on existing accuracy-validated 2D cine PC MRI and 4D flow MRI, as well as accuracy verification on the conservation of mass' principle, should be performed, and intraobserver repeatability, interobserver reproducibility, and test-retest reproducibility should be checked.

Quantitative assessment of velocity and flow using compressed SENSE in children and young adults with adequate acquired temporal resolution

BACKGROUND

Phase contrast (PC) cardiovascular magnetic resonance (CMR) imaging with parallel imaging acceleration is established and validated for measuring velocity and flow. However, additional acceleration to further shorten acquisition times would be beneficial in patients with complex vasculature who need multiple PC-CMR measurements, especially pediatric patients with higher heart rates.

METHODS

PC-CMR images acquired with compressed sensitivity encoding (C-SENSE) factors of 3 to 6 and standard of care PC-CMR with sensitivity encoding (SENSE) factor of 2 (S2) acquired as part of clinical CMR examinations performed between November 2020 and January 2021 were analyzed retrospectively. The velocity and flow through the ascending aorta (AAo), descending aorta (DAo), and superior vena cava (SVC) in a transverse plane at the level of pulmonary artery bifurcation were compared. Additionally, frequency power distribution and dynamic time warp distance were calculated for these acquisitions. To further validate the adequate temporal resolution requirement, patients with S2 PC-CMR in the same acquisition plane were added in frequency power distribution analysis.

RESULTS

Twenty-eight patients (25 males; 15.9 ± 1.9 years; body surface area (BSA) 1.7 ± 0.2 m2; heart rate 81 ± 16 bpm) underwent all five PC-CMR acquisitions during the study period. An additional 22 patients (16 males; 17.5 ± 7.7 years; BSA 1.6 ± 0.5 m2; heart rate 91 ± 16 bpm) were included for frequency power spectrum analysis. As expected, scan time decreased with increasing C-SENSE acceleration factor = 3 (37.5 ± 6.5 s, 26.4 ± 7.6%), 4 (28.1 ± 4.9 s, 44.7 ± 5.6%), 5 (21.6 ± 3.6 s, 57.6 ± 4.4%), and 6 (19.1 ± 3.2 s, 62.3 ± 4.2%) relative to SENSE = 2 (51.3 ± 10.1 s) PC-CMR acquisition. Mean peak velocity, net flow, and cardiac output were comparable (p > 0.87) between the five PC-CMR acquisitions with mean differences less than < 4%, < 2%, and < 3% respectively. All individual blood vessels showed a non-significant dependence of difference in fmax99 (< 4 Hz, p > 0.2), and dynamic time warp distance (p > 0.3) on the C-SENSE acceleration factor used. There was a strongly correlated (r = 0.74) increase in fmax99 (10.5 ± 2.2, range: 7.1-16.4 Hz) with increasing heart rate. The computed minimum required cardiac phase number was 15 ± 2.0 (range: 11-20) over the heart rate of 86 ± 15 bpm (range: 58-113 bpm).

CONCLUSIONS

Stroke volume, cardiac output, and mean peak velocity measurements using PC-CMR with C-SENSE of up to 6 agree with measurements by standard of care PC-CMR with SENSE = 2 and resulted in up to a 65% reduction in acquisition time. Adequate temporal sampling can be ensured by acquiring 20 cardiac phases throughout the entire cardiac cycle over a wide range of pediatric and young adult heart rates.

Comparison of data-driven and general temporal constraints on compressed sensing for breast DCE MRI

PURPOSE

Current breast DCE-MRI strategies provide high sensitivity for cancer detection but are known to be insufficient in fully capturing rapidly changing contrast kinetics at high spatial resolution across both breasts. Advanced acquisition and reconstruction strategies aim to improve spatial and temporal resolution and increase specificity for disease characterization. In this work, we evaluate the spatial and temporal fidelity of a modified data-driven low-rank-based model (known as MOCCO, model consistency condition) compressed-sensing (CS) reconstruction compared to CS with temporal total variation with radial acquisition for high spatial-temporal breast DCE MRI.

METHODS

Reconstruction performance was characterized using numerical simulations of a golden-angle stack-of-stars breast DCE-MRI acquisition at 5-second temporal resolution. Specifically, MOCCO was compared with CS total variation and conventional SENSE reconstructions. The temporal model for MOCCO was prelearned over the source data, whereas CS total variation was performed using a first-order temporal gradient sparsity transform.

RESULTS

The MOCCO reconstruction was able to capture rapid lesion kinetics while providing high image quality across a range of optimal regularization values. It also recovered kinetics in small lesions (1.5 mm) in line-profile analysis and error images, whereas g-factor maps showed relatively low and constant values with no significant artifacts. The CS-TV method demonstrated either recovery of high spatial resolution with reduced temporal accuracy using large regularization values, or recovery of rapid lesion kinetics with reduced image quality using low regularization values.

CONCLUSION

Simulations demonstrated that MOCCO with radial acquisition provides a robust imaging technique for improving temporal fidelity, while maintaining high spatial resolution and image quality in the setting of bilateral breast DCE MRI.

Compressed-sensing accelerated 4D flow MRI of cerebrospinal fluid dynamics

BACKGROUND

4D flow magnetic resonance imaging (MRI) of CSF can make an important contribution to the understanding of hydrodynamic changes in various neurological diseases but remains limited in clinical application due to long acquisition times. The aim of this study was to evaluate the accuracy of compressed SENSE accelerated MRI measurements of the spinal CSF flow.

METHODS

In 20 healthy subjects 4D flow MRI of the CSF in the cervical spine was acquired using compressed sensitivity encoding [CSE, a combination of compressed sensing and parallel imaging (SENSE) provided by the manufacturer] with acceleration factors between 4 and 10. A conventional scan using SENSE was used as reference. Extracted parameters were peak velocity, absolute net flow, forward flow and backward flow. Bland-Altman analysis was performed to determine the scan-rescan reproducibility and the agreement between SENSE and compressed SENSE. Additionally, a time accumulated flow error was calculated. In one additional subject flow of the spinal canal at the level of the entire spinal cord was assessed.

RESULTS

Averaged acquisition times were 10:21 min (SENSE), 9:31 min (CSE4), 6:25 min (CSE6), 4:53 min (CSE8) and 3:51 min (CSE10). Acquisition of the CSF flow surrounding the entire spinal cord took 14:40 min. Bland-Altman analysis showed good agreement for peak velocity, but slight overestimations for absolute net flow, forward flow and backward flow (< 1 ml/min) in CSE4-8. Results of the accumulated flow error were similar for CSE4 to CSE8.

CONCLUSION

A quantitative analysis of acceleration factors CSE4-10 showed that CSE with an acceleration factor up to 6 is feasible. This allows a scan time reduction of 40% and enables the acquisition and analysis of the CSF flow dynamics surrounding the entire spinal cord within a clinically acceptable scan time.

Sparse Analyzer Tool for Biomedical Signals

The virtual (software) instrument with a statistical analyzer for testing algorithms for biomedical signals' recovery in compressive sensing (CS) scenario is presented. Various CS reconstruction algorithms are implemented with the aim to be applicable for different types of biomedical signals and different applications with under-sampled data. Incomplete sampling/sensing can be considered as a sort of signal damage, where missing data can occur as a result of noise or the incomplete signal acquisition procedure. Many approaches for recovering the missing signal parts have been developed, depending on the signal nature. Here, several approaches and their applications are presented for medical signals and images. The possibility to analyze results using different statistical parameters is provided, with the aim to choose the most suitable approach for a specific application. The instrument provides manifold possibilities such as fitting different parameters for the considered signal and testing the efficiency under different percentages of missing data. The reconstruction accuracy is measured by the mean square error (MSE) between original and reconstructed signal. Computational time is important from the aspect of power requirements, thus enabling the selection of a suitable algorithm. The instrument contains its own signal database, but there is also the possibility to load any external data for analysis.

Improved acceleration of phase-contrast flow imaging with magnitude difference regularization

PURPOSE

To develop a regularized image reconstruction algorithm for improved scan acceleration of phase-contrast (PC) flow MRI.

METHODS

Based on the magnitude similarity between bipolar-encoded k-space data, magnitude-difference regularization was incorporated into the conventional compressed sensing (CS) reconstruction. The gradient of the magnitude regularization was derived so the reconstruction problem can be solved using non-linear conjugate gradient with backtracking line search. Phase contrast flow data obtained in the peripheral arteries of healthy and patient subjects were retrospectively undersampled for testing the proposed reconstruction method. Three-dimensional velocity-encoded PC flow MRI was performed with prospective 4-fold undersampling for measuring arotic flow velocity in a healthy volunteer.

RESULTS

In the femoral arteries of healthy volunteers, the root-mean-square (RMS) errors of mean velocities were 0.56 ± 0.09 cm/s with CS-only reconstruction and 0.46 ± 0.08 cm/s with addition of magnitude regularization for three-fold acceleration; 1.34 ± 0.17 cm/s (CS only) and 1.08 ± 0.15 cm/s (magnitude regularized) for four-fold acceleration. In the iliac arteries of the patient, the RMS errors of mean velocities were 0.72 ± 0.12 cm/s and 0.56 ± 0.10 for three-fold acceleration, and 1.75 ± 0.21 and 1.24 ± 0.19 cm/s for four-fold acceleration (in the order of CS-only and magnitude regularized reconstructions). In the popliteal arteries, the RMS errors were 0.61 ± 0.10 cm/s and 0.42 ± 0.11 for three-fold acceleration, and 1.41 ± 0.19 and 1.12 ± 0.17 cm/s for four-fold acceleration. The maximum through-plane mean flow velocities were measured as 63.2 cm/s and 84.5 cm/s in ascending and descending aortas, respectively.

CONCLUSION

The addition of magnitude-difference regularization into conventional CS reconstruction improves the accuracy of image reconstruction using highly undersampled phase-contrast flow MR data.

Accelerated aortic 4D flow cardiovascular magnetic resonance using compressed sensing: applicability, validation and clinical integration

BACKGROUND

Three-dimensional time-resolved phase-contrast cardiovascular magnetic resonance (4D flow CMR) enables the quantification and visualisation of blood flow, but its clinical applicability remains hampered by its long scan time. The aim of this study was to evaluate the use of compressed sensing (CS) with on-line reconstruction to accelerate the acquisition and reconstruction of 4D flow CMR of the thoracic aorta.

METHODS

4D flow CMR of the thoracic aorta was acquired in 20 healthy subjects using CS with acceleration factors ranging from 4 to 10. As a reference, conventional parallel imaging (SENSE) with acceleration factor 2 was used. Flow curves, net flows, peak flows and peak velocities were extracted from six contours along the aorta. To measure internal data consistency, a quantitative particle trace analysis was performed. Additionally, scan-rescan, inter- and intraobserver reproducibility were assessed. Subsequently, 4D flow CMR with CS factor 6 was acquired in 3 patients with differing aortopathies. The flow patterns resulting from particle trace visualisation were qualitatively analysed.

RESULTS

All collected data were successfully acquired and reconstructed on-line. The average acquisition time including respiratory navigator efficiency with CS factor 6 was 5:02 ± 2:23 min while reconstruction took approximately 9 min. For CS factors of 8 or less, mean differences in net flow, peak flow and peak velocity as compared to SENSE were below 2.2 ± 7.8 ml/cycle, 4.6 ± 25.2 ml/s and - 7.9 ± 13.0 cm/s, respectively. For a CS factor of 10 differences reached 5.4 ± 8.0 ml/cycle, 14.4 ± 28.3 ml/s and - 4.0 ± 12.2 cm/s. Scan-rescan analysis yielded mean differences in net flow of - 0.7 ± 4.9 ml/cycle for SENSE and - 0.2 ± 8.5 ml/cycle for CS factor of 6.

CONCLUSIONS

A six- to eightfold acceleration of 4D flow CMR using CS is feasible. Up to a CS acceleration rate of 6, no statistically significant differences in measured flow parameters could be observed with respect to the reference technique. Acquisitions in patients with aortopathies confirm the potential to integrate the proposed method in a clinical routine setting, whereby its main benefits are scan-time savings and direct on-line reconstruction.

Local Pulse Wave Velocity: Theory, Methods, Advancements, and Clinical Applications

Local pulse wave velocity (PWV) is evolving as one of the important determinants of arterial hemodynamics, localized vessel stiffening associated with several pathologies, and a host of other cardiovascular events. Although PWV was introduced over a century ago, only in recent decades, due to various technological advancements, has emphasis been directed toward its measurement from a single arterial section or from piecewise segments of a target arterial section. This emerging worldwide trend in the exploration of instrumental solutions for local PWV measurement has produced several invasive and noninvasive methods. As of yet, however, a univocal opinion on the ideal measurement method has not emerged. Neither have there been extensive comparative studies on the accuracy of the available methods. Recognizing this reality, makes apparent the need to establish guideline-recommended standards for the measurement methods and reference values, without which clinical application cannot be pursued. This paper enumerates all major local PWV measurement methods while pinpointing their salient methodological considerations and emphasizing the necessity of global standardization. Further, a summary of the advancements in measuring modalities and clinical applications is provided. Additionally, a detailed discussion on the minimally explored concept of incremental local PWV is presented along with suggestions of future research questions.

Magnetic Resonance Imaging of the Brain Using Compressed Sensing - Quality Assessment in Daily Clinical Routine

PURPOSE

To assess the effect of compressed sensing (CS) on image quality and acquisition speed in routine brain magnetic resonance imaging (MRI).

METHODS

During a 2-month implementation period of CS, two senior neuroradiologists, one MRI physicist and one application specialist optimized the CS acceleration factor to reduce scan time and improve spatial resolution, while maintaining image quality. Afterwards, two neuroradiologists independently scored image quality on a 5-point Likert scale in 3‑dimensional (3D) fluid attenuation inversion recovery (FLAIR), 3D double inversion recovery (DIR), 3D T2, 3D T1, 3D T1 + gadoteric acid, axial T2, axial FLAIR, axial T2*, and 3D arterial time-of-flight MR angiography (art. TOF) sequences acquired during 1 week before (CS-) and after (CS+) the implementation of CS. Time of acquisition was recorded for all sequences.

RESULTS

A total of 51 CS- and 48 CS+ patients were included. The median scan time reduction was 29.3% (range 0.0-58.4%), median voxel size reduction was 10.5% (0.0-33.3%). The CS+ image quality was rated superior for 3D FLAIR (p < 0.001), 3D T2 (p = 0.001), and axial T2* sequences (p = 0.024). For all other sequences, no statistical difference in image quality was observed. Interreader agreement regarding image quality was good for all sequences (weighted Cohen's κ > 0.5).

CONCLUSION

The use of CS saves considerable imaging time while allowing to increase spatial resolution in routine clinical brain MRI without loss in image quality.

Carotid artery flow as determined by real-time phase-contrast flow MRI and neurovascular ultrasound: A comparative study of healthy subjects

BACKGROUND

The assessment of carotid artery flow by neurovascular ultrasound (nvUS) can be complemented by real-time phase-contrast (RT-PC) flow MRI which apart from quantitative flow parameters offers velocity distributions across the entire vessel lumen.

MATERIALS AND METHODS

The feasibility and diagnostic potential of RT-PC flow MRI was evaluated in 20 healthy volunteers in comparison to conventional nvUS. RT-PC flow MRI at 40 ms temporal resolution and 0.8 mm in-plane resolution resulted in velocity maps with low phase noise and high spatiotemporal accuracy by exploiting respective advances of a recent nonlinear inverse model-based reconstruction. Peak-systolic velocities (PSV), end-diastolic velocities (EDV), flow volumes and comprehensive velocity profiles were determined in the common, internal and external carotid artery on both sides.

RESULTS

Flow characteristics such as pulsatility and individual abnormalities shown on nvUS could be reproduced and visualized in detail by RT-PC flow MRI. PSV to EDV differences revealed good agreement between both techniques, mean PSV and EDV were significantly lower and flow volumes were higher for MRI.

CONCLUSION

Our findings suggest that RT-PC flow MRI adds to clinical diagnostics, e.g. by alterations of dynamic velocity distributions in patients with carotid stenosis. Lower PSV and EDV values than for nvUS mainly reflect the longer MRI acquisition time which attenuates short peak velocities, while higher flow volumes benefit from a proper assessment of the true vessel lumen.

Highly accelerated intracranial 4D flow MRI: evaluation of healthy volunteers and patients with intracranial aneurysms

OBJECTIVES

To evaluate an accelerated 4D flow MRI method that provides high temporal resolution in a clinically feasible acquisition time for intracranial velocity imaging.

MATERIALS AND METHODS

Accelerated 4D flow MRI was developed by using a pseudo-random variable-density Cartesian undersampling strategy (CIRCUS) with the combination of k-t, parallel imaging and compressed sensing image reconstruction techniques (k-t SPARSE-SENSE). Four-dimensional flow data were acquired on five healthy volunteers and eight patients with intracranial aneurysms using CIRCUS (acceleration factor of R = 4, termed CIRCUS4) and GRAPPA (R = 2, termed GRAPPA2) as the reference method. Images with three times higher temporal resolution (R = 12, CIRCUS12) were also reconstructed from the same acquisition as CIRCUS4. Qualitative and quantitative image assessment was performed on the images acquired with different methods, and complex flow patterns in the aneurysms were identified and compared.

RESULTS

Four-dimensional flow MRI with CIRCUS was achieved in 5 min and allowed further improved temporal resolution of <30 ms. Volunteer studies showed similar qualitative and quantitative evaluation obtained with the proposed approach compared to the reference (overall image scores: GRAPPA2 3.2 ± 0.6; CIRCUS4 3.1 ± 0.7; CIRCUS12 3.3 ± 0.4; difference of the peak velocities: -3.83 ± 7.72 cm/s between CIRCUS4 and GRAPPA2, -1.72 ± 8.41 cm/s between CIRCUS12 and GRAPPA2). In patients with intracranial aneurysms, the higher temporal resolution improved capturing of the flow features in intracranial aneurysms (pathline visualization scores: GRAPPA2 2.2 ± 0.2; CIRCUS4 2.5 ± 0.5; CIRCUS12 2.7 ± 0.6).

CONCLUSION

The proposed rapid 4D flow MRI with a high temporal resolution is a promising tool for evaluating intracranial aneurysms in a clinically feasible acquisition time.

Phase-contrast MRI with hybrid one and two-sided flow-encoding and velocity spectrum separation

PURPOSE

To develop and evaluate a phase-contrast MRI (PC-MRI) technique with hybrid one and two-sided flow-encoding and velocity spectrum separation (HOTSPA) for accelerated blood flow and velocity measurement.

METHODS

In the HOTSPA technique, the two-sided flow encoding (FE) is used for two FE directions and one-sided is used for the remaining FE direction. Such a temporal modulation of the FE strategy allows for separations of the Fourier velocity spectrum into components for the flow-compensated and the three-directional velocity waveforms, accelerating PC-MRI by encoding three-directional velocities using only two repetition times (TRs) instead of four TRs as in standard PC-MRI. The HOTSPA was evaluated and compared with standard PC-MRI in the common carotid arteries of six healthy volunteers.

RESULTS

Total volumetric flow and peak velocity measurements based on HOTSPA and the conventional PC-MRI were in good agreement with a bias of -0.005 mL (-0.1% relative bias error) for total volumetric flow and 1.21 cm/s (1.1% relative bias error) for peak velocity, although the total acquisition time was 50% of the conventional PC-MRI.

CONCLUSION

The proposed HOTSPA technique achieved nearly two-fold acceleration of PC-MRI while maintaining accuracy for total volumetric flow and peak velocity quantification by separating the paired acquisitions in the Fourier velocity spectrum domain. Magn Reson Med 78:182-192, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Four-dimensional MRI flow examinations in cerebral and extracerebral vessels - ready for clinical routine?

PURPOSE OF REVIEW

To evaluate the feasibility of 4-dimensional (4D) flow MRI for the clinical assessment of cerebral and extracerebral vascular hemodynamics in patients with neurovascular disease.

RECENT FINDINGS

4D flow MRI has been applied in multiple studies to qualitatively and quantitatively study intracranial aneurysm blood flow for potential risk stratification and to assess treatment efficacy of various neurovascular lesions, including intraaneurysmal and parent artery blood flow after flow diverter stent placement and staged embolizations of arteriovenous malformations and vein of Galen aneurysmal malformations. Recently, the technique has been utilized to characterize age-related changes of normal cerebral hemodynamics in healthy individuals over a broad age range.

SUMMARY

4D flow MRI is a useful tool for the noninvasive, volumetric and quantitative hemodynamic assessment of neurovascular disease without the need for gadolinium contrast agents. Further improvements are warranted to overcome technical limitations before broader clinical implementation. Current developments, such as advanced acceleration techniques (parallel imaging and compressed sensing) for faster data acquisition, dual or multiple velocity encoding strategies for more accurate arterial and venous flow quantification, ultrahigh-field strengths to achieve higher spatial resolution and streamlined postprocessing workflow for more efficient and standardized flow analysis, are promising advancements in 4D flow MRI.

Sparse Reconstruction Techniques in Magnetic Resonance Imaging: Methods, Applications, and Challenges to Clinical Adoption

The family of sparse reconstruction techniques, including the recently introduced compressed sensing framework, has been extensively explored to reduce scan times in magnetic resonance imaging (MRI). While there are many different methods that fall under the general umbrella of sparse reconstructions, they all rely on the idea that a priori information about the sparsity of MR images can be used to reconstruct full images from undersampled data. This review describes the basic ideas behind sparse reconstruction techniques, how they could be applied to improve MRI, and the open challenges to their general adoption in a clinical setting. The fundamental principles underlying different classes of sparse reconstructions techniques are examined, and the requirements that each make on the undersampled data outlined. Applications that could potentially benefit from the accelerations that sparse reconstructions could provide are described, and clinical studies using sparse reconstructions reviewed. Lastly, technical and clinical challenges to widespread implementation of sparse reconstruction techniques, including optimization, reconstruction times, artifact appearance, and comparison with current gold standards, are discussed.

Advanced flow MRI: emerging techniques and applications

Magnetic resonance imaging (MRI) techniques provide non-invasive and non-ionising methods for the highly accurate anatomical depiction of the heart and vessels throughout the cardiac cycle. In addition, the intrinsic sensitivity of MRI to motion offers the unique ability to acquire spatially registered blood flow simultaneously with the morphological data, within a single measurement. In clinical routine, flow MRI is typically accomplished using methods that resolve two spatial dimensions in individual planes and encode the time-resolved velocity in one principal direction, typically oriented perpendicular to the two-dimensional (2D) section. This review describes recently developed advanced MRI flow techniques, which allow for more comprehensive evaluation of blood flow characteristics, such as real-time flow imaging, 2D multiple-venc phase contrast MRI, four-dimensional (4D) flow MRI, quantification of complex haemodynamic properties, and highly accelerated flow imaging. Emerging techniques and novel applications are explored. In addition, applications of these new techniques for the improved evaluation of cardiovascular (aorta, pulmonary arteries, congenital heart disease, atrial fibrillation, coronary arteries) as well as cerebrovascular disease (intra-cranial arteries and veins) are presented.

Compressed sensing MRI: a review of the clinical literature

MRI is one of the most dynamic and safe imaging techniques available in the clinic today. However, MRI acquisitions tend to be slow, limiting patient throughput and limiting potential indications for use while driving up costs. Compressed sensing (CS) is a method for accelerating MRI acquisition by acquiring less data through undersampling of k-space. This has the potential to mitigate the time-intensiveness of MRI. The limited body of research evaluating the effects of CS on MR images has been mostly positive with regards to its potential as a clinical tool. Studies have successfully accelerated MRI with this technology, with varying degrees of success. However, more must be performed before its diagnostic efficacy and benefits are clear. Studies involving a greater number radiologists and images must be completed, rating CS based on its diagnostic efficacy. Also, standardized methods for determining optimal imaging parameters must be developed.

4D flow cardiovascular magnetic resonance consensus statement

Pulsatile blood flow through the cavities of the heart and great vessels is time-varying and multidirectional. Access to all regions, phases and directions of cardiovascular flows has formerly been limited. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has enabled more comprehensive access to such flows, with typical spatial resolution of 1.5×1.5×1.5 - 3×3×3 mm(3), typical temporal resolution of 30-40 ms, and acquisition times in the order of 5 to 25 min. This consensus paper is the work of physicists, physicians and biomedical engineers, active in the development and implementation of 4D Flow CMR, who have repeatedly met to share experience and ideas. The paper aims to assist understanding of acquisition and analysis methods, and their potential clinical applications with a focus on the heart and greater vessels. We describe that 4D Flow CMR can be clinically advantageous because placement of a single acquisition volume is straightforward and enables flow through any plane across it to be calculated retrospectively and with good accuracy. We also specify research and development goals that have yet to be satisfactorily achieved. Derived flow parameters, generally needing further development or validation for clinical use, include measurements of wall shear stress, pressure difference, turbulent kinetic energy, and intracardiac flow components. The dependence of measurement accuracy on acquisition parameters is considered, as are the uses of different visualization strategies for appropriate representation of time-varying multidirectional flow fields. Finally, we offer suggestions for more consistent, user-friendly implementation of 4D Flow CMR acquisition and data handling with a view to multicenter studies and more widespread adoption of the approach in routine clinical investigations.

Multi-dimensional flow-preserving compressed sensing (MuFloCoS) for time-resolved velocity-encoded phase contrast MRI

4-D time-resolved velocity-encoded phase-contrast MRI (4-D PCI) is a fully non-invasive technique to assess hemodynamics in vivo with a broad range of potential applications in multiple cardiovascular diseases. It is capable of providing quantitative flow values and anatomical information simultaneously. The long acquisition time, however, still inhibits its wider clinical use. Acceleration is achieved at present using parallel MRI (pMRI) techniques which can lead to substantial loss of image quality for higher acceleration factors. Both the high-dimensionality and the significant degree of spatio-temporal correlation in 4-D PCI render it ideally suited for recently proposed compressed sensing (CS) techniques. We propose the Multi-Dimensional Flow-preserving Compressed Sensing (MuFloCoS) method to exploit these properties. A multi-dimensional iterative reconstruction is combined with an interleaved sampling pattern (I-VT), an adaptive masked and weighted temporal regularization (TMW) and fully automatically obtained vessel-masks. The performance of the novel method was analyzed concerning image quality, feasibility of acceleration factors up to 15, quantitative flow values and diagnostic accuracy in phantom experiments and an in vivo carotid study with 18 volunteers. Comparison with iterative state-of-the-art methods revealed significant improvements using the new method, the temporal normalized root mean square error of the peak velocity was reduced by 45.32% for the novel MuFloCoS method with acceleration factor 9. The method was furthermore applied to two patient cases with diagnosed high-grade stenosis of the ICA, which confirmed the performance of MuFloCoS to produce valuable results in the presence of pathological findings in 56 s instead of over 8 min (full sampling).

4D flow imaging: current status to future clinical applications

4D flow MRI permits a comprehensive in-vivo assessment of three-directional blood flow within 3-dimensional vascular structures throughout the cardiac cycle. Given the large coverage permitted from a 4D flow acquisition, the distribution of vessel wall and flow parameters along an entire vessel of interest can thus be derived from a single measurement without being dependent on multiple predefined 2D acquisitions. In addition to qualitative 3D visualizations of complex cardiac and vascular flow patterns, quantitative flow analysis can be performed and is complemented by the ability to compute sophisticated hemodynamic parameters, such as wall shear stress or 3D pressure difference maps. These metrics can provide information previously unavailable with conventional modalities regarding the impact of cardiovascular disease or therapy on global and regional changes in hemodynamics. This review provides an introduction to the methodological aspects of 4D flow MRI to assess vascular hemodynamics and describes its potential for the assessment and understanding of altered hemodynamics in the presence of cardiovascular disease.

Phase contrast MRI with flow compensation view sharing

PURPOSE

To develop and evaluate a technique for accelerating phase contrast MRI (PC-MRI) acquisitions without significant compromise in flow quantification accuracy.

METHODS

PC-MRI is commonly acquired using interleaved flow-compensated (FC) and flow-encoded (FE) echoes. We hypothesized that FC data, which represent background phase, do not change significantly over time. Therefore, we proposed to undersample the FC data and use an FC view sharing (FCVS) approach to synthesize a composite FC frame for each corresponding FE frame. FCVS was evaluated in a flow phantom and healthy volunteers and compared with a standard FC/FE PC-MRI.

RESULTS

The FCVS sequence resulted in an error of 0.0% for forward flow and 2.0% for reverse flow volume when compared with FC/FE PC-MRI in a flow phantom. Measurements in the common carotid arteries showed that the FCVS method had -1.16 cm/s bias for maximum peak velocity and -0.019 mL bias in total flow, when compared with FC/FE with the same temporal resolution, but double the total acquisition time. These results represent ≤1.3% bias error in velocity and volumetric flow quantification.

CONCLUSION

FCVS can accelerate PC-MRI acquisitions while maintaining flow and velocity measurement accuracy when there is limited temporal variation in the FC data.