Simultaneous acquisition of phase-contrast angiograms and stationary-tissue images with Hadamard encoding of flow-induced phase shifts

Enhancement of intra-cardiac flow-field data using adaptive Kernel filtering

A method of determining the optimal kernel size for filtering noise in vortex dominated flow-fields, as found in the cardiac chambers is presented in this paper. Using synthetic flow fields generated using harmonic functions and perturbed using Gaussian noises of different amplitudes and spreads, the effect of kernel size on noise removal using the Median filter is tested systematically. It is shown that there exists an optimal kernel size at which the Median filter works best. The size of the optimal kernel is shown to be related to the vortex size. When applied to MRI generated cardiac flow-fields, the approach is seen to reveal underlying vortex patterns thereby aiding as an effective tool in the diagnosis and prognosis of cardiac diseases based on vortices as clinical biomarkers. The behavior of the restored cardiac flow fields which are filtered with the optimal kernel size and also with some values preceding and succeeding it are similar to that observed in studies with synthetic flow fields. This confirms that the optimal size of the kernel is related to the cardiac vortex size as is observed in the case of synthetic flow fields.

Four-Dimensional Flow MR Imaging: Technique and Advances

4D Flow MRI is an advanced imaging technique for comprehensive non-invasive assessment of the cardiovascular system. The capture of the blood velocity vector field throughout the cardiac cycle enables measures of flow, pulse wave velocity, kinetic energy, wall shear stress, and more. Advances in hardware, MRI data acquisition and reconstruction methodology allow for clinically feasible scan times. The availability of 4D Flow analysis packages allows for more widespread use in research and the clinic and will facilitate much needed multi-center, multi-vendor studies in order to establish consistency across scanner platforms and to enable larger scale studies to demonstrate clinical value.

Accelerated 4D-flow MRI with 3-point encoding enabled by machine learning

PURPOSE

To investigate the acceleration of 4D-flow MRI using a convolutional neural network (CNN) that produces three directional velocities from three flow encodings, without requiring a fourth reference scan measuring background phase.

METHODS

A fully 3D CNN using a U-net architecture was trained in a block-wise fashion to take complex images from three flow encodings and to produce three real-valued images for each velocity component. Using neurovascular 4D-flow scans (n = 144), the CNN was trained to predict velocities computed from four flow encodings by standard reconstruction including correction for residual background phase offsets. Methods to optimize loss functions were investigated, including magnitude, complex difference, and uniform velocity weightings. Subsequently, 3-point encoding was evaluated using cross validation of pixelwise correlation, flow measurements in major arteries, and in experiments with data at differing acceleration rates than the training data.

RESULTS

The CNN-produced 3-point velocities showed excellent agreements with the 4-point velocities, both qualitatively in velocity images, in flow rate measures, and quantitatively in regression analysis (slope = 0.96, R²  = 0.992). Optimizing the training to focus on vessel velocities rather than the global velocity error and improved the correlation of velocity within vessels themselves. The lowest error was observed when the loss function used uniform velocity weighting, in which the magnitude-weighted inverse of the velocity frequency uniformly distributed weighting across all velocity ranges. When applied to highly accelerated data, the 3-point network maintained a high correlation with ground truth data and demonstrated a denoising effect.

CONCLUSION

The 4D-flow MRI can be accelerated using machine learning requiring only three flow encodings to produce three-directional velocity maps with small errors.

FlowRAU-Net: Accelerated 4D Flow MRI of Aortic Valvular Flows With a Deep 2D Residual Attention Network

In this work, we propose a novel deep learning reconstruction framework for rapid and accurate reconstruction of 4D flow MRI data. Reconstruction is performed on a slice-by-slice basis by reducing artifacts in zero-filled reconstructed complex images obtained from undersampled k-space. A deep residual attention network FlowRAU-Net is proposed, trained separately for each encoding direction with 2D complex image slices extracted from complex 4D images at each temporal frame and slice position. The network was trained and tested on 4D flow MRI data of aortic valvular flow in 18 human subjects. Performance of the reconstructions was measured in terms of image quality, 3-D velocity vector accuracy, and accuracy in hemodynamic parameters. Reconstruction performance was measured for three different k-space undersamplings and compared with one state of the art compressed sensing reconstruction method and three deep learning-based reconstruction methods. The proposed method outperforms state of the art methods in all performance measures for all three different k-space undersamplings. Hemodynamic parameters such as blood flow rate and peak velocity from the proposed technique show good agreement with reference flow parameters. Visualization of the reconstructed image and velocity magnitude also shows excellent agreement with the fully sampled reference dataset. Moreover, the proposed method is computationally fast. Total 4D flow data (including all slices in space and time) for a subject can be reconstructed in 69 seconds on a single GPU. Although the proposed method has been applied to 4D flow MRI of aortic valvular flows, given a sufficient number of training samples, it should be applicable to other arterial flows.

Multiparametric MRI identifies subtle adaptations for demarcation of disease transition in murine aortic valve stenosis

Aortic valve stenosis (AS) is the most frequent valve disease with relevant prognostic impact. Experimental model systems for AS are scarce and comprehensive imaging techniques to simultaneously quantify function and morphology in disease progression are lacking. Therefore, we refined an acute murine AS model to closely mimic human disease characteristics and developed a high-resolution magnetic resonance imaging (MRI) approach for simultaneous in-depth analysis of valvular, myocardial as well as aortic morphology/pathophysiology to identify early changes in tissue texture and critical transition points in the adaptive process to AS. AS was induced by wire injury of the aortic valve. Four weeks after surgery, cine loops, velocity, and relaxometry maps were acquired at 9.4 T to monitor structural/functional alterations in valve, aorta, and left ventricle (LV). In vivo MRI data were subsequently validated by histology and compared to echocardiography. AS mice exhibited impaired valve opening accompanied by significant valve thickening due to fibrotic remodelling. While control mice showed bell-shaped flow profiles, AS resulted not only in higher peak flow velocities, but also in fragmented turbulent flow patterns associated with enhanced circumferential strain and an increase in wall thickness of the aortic root. AS mice presented with a mild hypertrophy but unaffected global LV function. Cardiac MR relaxometry revealed reduced values for both T1 and T2 in AS reflecting subtle myocardial tissue remodelling with early alterations in mitochondrial function in response to the enhanced afterload. Concomitantly, incipient impairments of coronary flow reserve and myocardial tissue integrity get apparent accompanied by early troponin release. With this, we identified a premature transition point with still compensated cardiac function but beginning textural changes. This will allow interventional studies to explore early disease pathophysiology and novel therapeutic targets.

Beyond Vessel Diameters: Non-invasive Monitoring of Flow Patterns and Immune Cell Recruitment in Murine Abdominal Aortic Disorders by Multiparametric MRI

The pathophysiology of the initiation and progression of abdominal aortic aneurysms (AAAs) and aortic dissections (AADs) is still unclear. However, there is strong evidence that monocytes and macrophages are of crucial importance in these processes. Here, we utilized a molecular imaging approach based on background-free ¹⁹F MRI and employed perfluorocarbon nanoemulsions (PFCs) for in situ ¹⁹F labeling of monocytes/macrophages to monitor vascular inflammation and AAA/AAD formation in angiotensin II (angII)-treated apolipoproteinE-deficient (apoE^-/-) mice. In parallel, we used conventional ¹H MRI for the characterization of aortic flow patterns and morphology. AngII (1 μg/kg/min) was infused into apoE^-/- mice via osmotic minipumps for 10 days and mice were monitored by multiparametric ¹H/¹⁹F MRI. PFCs were intravenously injected directly after pump implantation followed by additional applications on day 2 and 4 to allow an efficient ¹⁹F loading of circulating monocytes. The combination of angiographic, hemodynamic, and anatomical measurements allowed an unequivocal classification of mice in groups with developing AAAs, AADs or without any obvious aortic vessel alterations despite the exposure to angII. Maximal luminal and external diameters of the aorta were enlarged in AAAs, whereas AADs showed either a slight decrease of the luminal diameter or no alteration. ¹H/¹⁹F MRI after intravenous PFC application demonstrated significantly higher ¹⁹F signals in aortae of mice that developed AAAs or AADs as compared to mice in which no aortic disorders were detected. High resolution ¹H/¹⁹F MRI of excised aortae revealed a patchy pattern of the ¹⁹F signals predominantly in the adventitia of the aorta. Histological analysis confirmed the presence of macrophages in this area and flow cytometry revealed higher numbers of immune cells in aortae of mice that have developed AAA/AAD. Importantly, there was a linear correlation of the ¹⁹F signal with the total number of infiltrated macrophages. In conclusion, our approach enables a precise differentiation between AAA and AAD as well as visualization and quantitative assessment of inflammatory active vascular lesions, and therefore may help to unravel the complex interplay between macrophage accumulation, vascular inflammation, and the development and progression of AAAs and AADs.

Human upper-airway respiratory airflow: In vivo comparison of computational fluid dynamics simulations and hyperpolarized 129Xe phase contrast MRI velocimetry

Computational fluid dynamics (CFD) simulations of respiratory airflow have the potential to change the clinical assessment of regional airway function in health and disease, in pulmonary medicine and otolaryngology. For example, in diseases where multiple sites of airway obstruction occur, such as obstructive sleep apnea (OSA), CFD simulations can identify which sites of obstruction contribute most to airway resistance and may therefore be candidate sites for airway surgery. The main barrier to clinical uptake of respiratory CFD to date has been the difficulty in validating CFD results against a clinical gold standard. Invasive instrumentation of the upper airway to measure respiratory airflow velocity or pressure can disrupt the airflow and alter the subject's natural breathing patterns. Therefore, in this study, we instead propose phase contrast (PC) velocimetry magnetic resonance imaging (MRI) of inhaled hyperpolarized 129Xe gas as a non-invasive reference to which airflow velocities calculated via CFD can be compared. To that end, we performed subject-specific CFD simulations in airway models derived from 1H MRI, and using respiratory flowrate measurements acquired synchronously with MRI. Airflow velocity vectors calculated by CFD simulations were then qualitatively and quantitatively compared to velocity maps derived from PC velocimetry MRI of inhaled hyperpolarized 129Xe gas. The results show both techniques produce similar spatial distributions of high velocity regions in the anterior-posterior and foot-head directions, indicating good qualitative agreement. Statistically significant correlations and low Bland-Altman bias between the local velocity values produced by the two techniques indicates quantitative agreement. This preliminary in vivo comparison of respiratory airway CFD and PC MRI of hyperpolarized 129Xe gas demonstrates the feasibility of PC MRI as a technique to validate respiratory CFD and forms the basis for further comprehensive validation studies. This study is therefore a first step in the pathway towards clinical adoption of respiratory CFD.

Full-Field Comparison of MRV and CFD of Gas Flow through Regular Catalytic Monolithic Structures

Understanding the influence of gas flow maldistribution in honeycombs can be beneficial for the process design in various technical applications. Although recent studies have investigated the effect of maldistribution by comparing the results of numerical simulations with experimental measurements, an exhaustive 3D full-field comparison is still lacking. Such full-field comparisons are required to identify and eliminate possible limitations of numerical and experimental tools. For that purpose, spatially resolved flow patterns were simulated by computational fluid dynamics (CFD) and measured experimentally by non-invasive NMR velocimetry (MRV). While the latter might suffer from a misinterpretation of artefacts, the reliability of CFD is linked to correctly chosen boundary conditions. Here, a full-field numerical and experimental analysis of the gas flow within catalytic honeycombs is presented. The velocity field of thermally polarized methane gas was measured in a regular 3D-printed honeycomb and a commercial monolith using an optimized MRV pulse sequence to enhance the obtained signal-to-noise ratio. A second pulse sequence was used to show local flow propagators along the axial and radial direction of the honeycomb to quantify the contribution of diffusion to mass transport. A quantitative comparison of the axially averaged convective flow as determined by MRV and CFD shows a very good matching with an agreement of ±5% and 10% for printed and commercial samples, respectively. The impact of maldistribution on the gas flow pattern can be observed in both simulation and experiments, confirming the existence of an entrance effect. Gas displacement measurements, however, revealed that diffusive interchannel transport can also contribute to maldistribution, as was shown for the commercial sample. The good agreement between the simulation and experiments underpins the reliability of both methods for studying gas hydrodynamics within opaque monolith structures.

From 2D to 4D Phase-Contrast MRI in the Neurovascular System: Will It Be a Quantum Jump or a Fancy Decoration?

Considering the crosstalk between the flow and vessel wall, hemodynamic assessment of the neurovascular system may offer a well-integrated solution for both diagnosis and management by adding prognostic significance to the standard CT/MR angiography. 4D flow MRI or time-resolved 3D velocity-encoded phase-contrast MRI has long been promising for the hemodynamic evaluation of the great vessels, but challenged in clinical studies for assessing intracranial vessels with small diameter due to long scan times and low spatiotemporal resolution. Current accelerated MRI techniques, including parallel imaging with compressed sensing and radial k-space undersampling acquisitions, have decreased scan times dramatically while preserving spatial resolution. 4D flow MRI visualized and measured 3D complex flow of neurovascular diseases such as aneurysm, arteriovenous shunts, and atherosclerotic stenosis using parameters including flow volume, velocity vector, pressure gradients, and wall shear stress. In addition to the noninvasiveness of the phase contrast technique and retrospective flow measurement through the wanted windows of the analysis plane, 4D flow MRI has shown several advantages over Doppler ultrasound or computational fluid dynamics. The evaluation of the flow status and vessel wall can be performed simultaneously in the same imaging modality. This article is an overview of the recent advances in neurovascular 4D flow MRI techniques and their potential clinical applications in neurovascular disease. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 3.

Limits to flow detection in phase contrast MRI

Pulsed gradient spin echo (PGSE) complex signal behavior becomes dominated by attenuation rather than oscillation when displacements due to flow are similar or less than diffusive displacements. In this "slow-flow" regime, the optimal displacement encoding parameter q for phase contrast velocimetry depends on the diffusive length scale q s l o w = 1 / l D = 1 / 2 D Δ rather than the velocity encoding parameter v _enc = π/(qΔ). The minimum detectable mean velocity using the difference between the phase at +q _slow and -q _slow is 〈 v m i n 〉 = 1 / SNR D / Δ . These theories are then validated and applied to MRI by performing PGSE echo planar imaging experiments on water flowing through a column with a bulk region and a beadpack region at controlled flow rates. Velocities as slow as 6 μm/s are detected with velocimetry. Theories, MRI experimental protocols, and validation on a controlled phantom help to bridge the gap between porous media NMR and pre-clinical phase contrast and diffusion MRI.

Analysis and improvement of motion encoding in magnetic resonance elastography

Magnetic resonance elastography (MRE) utilizes phase contrast magnetic resonance imaging (MRI), which is phase locked to externally generated mechanical vibrations, to measure the three-dimensional wave displacement field. At least four measurements with linear-independent encoding directions are necessary to correct for spurious phase contributions if effects from imaging gradients are non-negligible. In MRE, three encoding schemes have been used: unbalanced four- and six-point and balanced four-point ('tetrahedral') encoding. The first two sensitize to motion with orthogonal gradients, with the four-point method acquiring a single reference scan without motion sensitization, whereas three additional scans with inverted gradients are used with six-point encoding, leading to two-fold higher displacement-to-noise ratio (DNR) and 50% longer scan duration. Balanced four-point (tetrahedral) encoding encodes along the four diagonals of a cube, with one direction serving as a reference for the other three encoding directions, similar to four-point encoding. The objective of this work is to introduce a theoretical framework to compare different motion sensitization strategies with respect to their motion encoding efficiency in two fundamental encoding limits, the gradient strength limit and the dynamic range limit, which are both placed in relation to conventional gradient recalled echo (GRE)- and spin echo (SE)-based MRE sequences. We apply the framework to the three aforementioned schemes and show that the motion encoding efficiency of unbalanced four- and six-point encoding schemes in the gradient-limited regime can be increased by a factor of 1.5 when using all physical gradient channels concurrently. Furthermore, it is demonstrated that reversing the direction of the reference in balanced four-point (tetrahedral) encoding results in the Hadamard encoding scheme, which leads to increased DNR by 2 compared with balanced four-point encoding and 2.8 compared with unbalanced four-point encoding. As an example, we show that optimal encoding can be utilized to reduce the acquisition time of standard liver MRE in vivo from four to two breath holds.

Cardiac 4D phase-contrast CMR at 9.4 T using self-gated ultra-short echo time (UTE) imaging

BACKGROUND

Time resolved 4D phase contrast (PC) cardiovascular magnetic resonance (CMR) in mice is challenging due to long scan times, small animal ECG-gating and the rapid blood flow and cardiac motion of small rodents. To overcome several of these technical challenges we implemented a retrospectively self-gated 4D PC radial ultra-short echo-time (UTE) acquisition scheme and assessed its performance in healthy mice by comparing the results with those obtained with an ECG-triggered 4D PC fast low angle shot (FLASH) sequence.

METHODS

Cardiac 4D PC CMR images were acquired at 9.4 T in healthy mice using the proposed self-gated radial center-out UTE acquisition scheme (TE/TR of 0.5 ms/3.1 ms) and a standard Cartesian 4D PC imaging sequence (TE/TR of 2.1 ms/5.0 ms) with a four-point Hadamard flow encoding scheme. To validate the proposed UTE flow imaging technique, experiments on a flow phantom with variable pump rates were performed.

RESULTS

The anatomical images and flow velocity maps of the proposed 4D PC UTE technique showed reduced artifacts and an improved SNR (left ventricular cavity (LV): 8.9 ± 2.5, myocardium (MC): 15.7 ± 1.9) compared to those obtained using a typical Cartesian FLASH sequence (LV: 5.6 ± 1.2, MC: 10.1 ± 1.4) that was used as a reference. With both sequences comparable flow velocities were obtained in the flow phantom as well as in the ascending aorta (UTE: 132.8 ± 18.3 cm/s, FLASH: 134.7 ± 13.4 cm/s) and pulmonary artery (UTE: 78.5 ± 15.4 cm/s, FLASH: 86.6 ± 6.2 cm/s) of the animals. Self-gated navigator signals derived from information of the oversampled k-space center were successfully extracted for all animals with a higher gating efficiency of time spent on acquiring gated data versus total measurement time (UTE: 61.8 ± 11.5%, FLASH: 48.5 ± 4.9%).

CONCLUSIONS

The proposed self-gated 4D PC UTE sequence enables robust and accurate flow velocity mapping of the mouse heart in vivo at high magnetic fields. At the same time SNR, gating efficiency, flow artifacts and image quality all improved compared to the images obtained using the well-established, ECG-triggered, 4D PC FLASH sequence.

Adapted MR velocimetry of slow liquid flow in porous media

MR velocimetry of liquid flow in opaque porous filters may play an important role in better understanding the mechanisms of deep bed filtration. With this knowledge, the efficiency of separating the suspended solid particles from the vertically flowing liquid can be improved, and thus a wide range of industrial applications such as wastewater treatment and desalination can be optimized. However, MR velocimetry is challenging for such studies due to the low velocities, the severe B₀ inhomogeneity in porous structures, and the demand for high spatial resolution and an appropriate total measurement time during which the particle deposition will change velocities only marginally. In this work, a modified RARE-based MR velocimetry method is proposed to address these issues for velocity mapping on a deep bed filtration cell. A dedicated RF coil with a high filling factor is constructed considering the limited space available for the vertical cell in a horizontal MR magnet. Several means are applied to optimize the phase contrast RARE MRI pulse sequence for accurately measuring the phase contrast in a long echo train, even in the case of a low B₁ homogeneity. Two means are of particular importance. One uses data acquired with zero flow to correct the phase contrast offsets from gradient imperfections, and the other combines the phase contrast from signals of both odd and even echoes. Results obtained on a 7T preclinical MR scanner indicate that the low velocities in the heterogeneous system can be correctly quantified with high spatial resolution and an adequate total measurement time, enabling future studies on flow during the filtration process.

Selective arterial spin labeling in conjunction with phase-contrast acquisition for the simultaneous visualization of morphology, flow direction, and velocity of individual arteries in the cerebrovascular system

PURPOSE

In various cerebrovascular diseases the visualization of individual arteries and knowledge about their hemodynamic properties, like flow velocity and direction, can become important for an accurate diagnosis. Magnetic resonance angiography methods are intended to acquire this information, but often a single acquisition is not sufficient to retrieve all of this desired information.

METHODS

Using selective arterial spin labeling (ASL) methods, a single artery of interest can be tagged and visualized, whereas quantitative information about hemodynamics can be retrieved using phase-contrast techniques that are often limited regarding their selectivity. In this study, a method that allows for velocity mapping of individual arteries by incorporating phase-contrast preparation into selective ASL angiography measurements is presented. Several postprocessing steps are required to generate velocity and directional-encoded maps of selected arteries from the data acquired in a single scan.

RESULTS

The method was successfully evaluated in healthy volunteers, and a first application in two selected patients is presented. In one patient, an aneurysm of the middle cerebral artery is investigated, and in the second patient it is used to visualize an arterio-venous malformation.

CONCLUSION

Selective ASL imaging in conjunction with phase-contrast acquisition allows for investigating hemodynamic properties of individual arteries. Magn Reson Med 78:1469-1475, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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.

4D spiral imaging of flows in stenotic phantoms and subjects with aortic stenosis

PURPOSE

The utility of four-dimensional (4D) spiral flow in imaging of stenotic flows in both phantoms and human subjects with aortic stenosis is investigated.

METHODS

The method performs 4D flow acquisitions through a stack of interleaved spiral k-space readouts. Relative to conventional 4D flow, which performs Cartesian readout, the method has reduced echo time. Thus, reduced flow artifacts are observed when imaging high-speed stenotic flows. Four-dimensional spiral flow also provides significant savings in scan times relative to conventional 4D flow.

RESULTS

In vitro experiments were performed under both steady and pulsatile flows in a phantom model of severe stenosis (one inch diameter at the inlet, with 87% area reduction at the throat of the stenosis) while imaging a 6-cm axial extent of the phantom, which included the Gaussian-shaped stenotic narrowing. In all cases, gradient strength and slew rate for standard clinical acquisitions, and identical field of view and resolution were used. For low steady flow rates, quantitative and qualitative results showed a similar level of accuracy between 4D spiral flow (echo time [TE] = 2 ms, scan time = 40 s) and conventional 4D flow (TE = 3.6 ms, scan time = 1:01 min). However, in the case of high steady flow rates, 4D spiral flow (TE = 1.57 ms, scan time = 38 s) showed better visualization and accuracy as compared to conventional 4D flow (TE = 3.2 ms, scan time = 51 s). At low pulsatile flow rates, a good agreement was observed between 4D spiral flow (TE = 2 ms, scan time = 10:26 min) and conventional 4D flow (TE = 3.6 ms, scan time = 14:20 min). However, in the case of high flow-rate pulsatile flows, 4D spiral flow (TE = 1.57 ms, scan time = 10:26 min) demonstrated better visualization as compared to conventional 4D flow (TE = 3.2 ms, scan time = 14:20 min). The feasibility of 4D spiral flow was also investigated in five normal volunteers and four subjects with mild-to-moderate aortic stenosis. The approach achieved TE = 1.68 ms and scan time = 3:44 min. The conventional sequence achieved TE = 2.9 ms and scan time = 5:23 min. In subjects with aortic stenosis, we also compared both MRI methods with Doppler ultrasound (US) in the measurement of peak velocity, time to peak systolic velocity, and eject time. Bland-Altman analysis revealed that, when comparing peak velocities, the discrepancy between Doppler US and 4D spiral flow was significantly less than the discrepancy between Doppler and 4D Cartesian flow (2.75 cm/s vs. 10.25 cm/s), whereas the two MR methods were comparable (-5.75 s vs. -6 s) for time to peak. However, for the estimation of eject time, relative to Doppler US, the discrepancy for 4D conventional flow was smaller than that of 4D spiral flow (-16.25 s vs. -20 s).

CONCLUSION

Relative to conventional 4D flow, 4D spiral flow achieves substantial reductions in both the TE and scan times; therefore, utility for it should be sought in a variety of in vivo and complex flow imaging applications.

Velocity mapping of the aortic flow at 9.4 T in healthy mice and mice with induced heart failure using time-resolved three-dimensional phase-contrast MRI (4D PC MRI)

Objectives

In this study, we established and validated a time-resolved three-dimensional phase-contrast magnetic resonance imaging method (4D PC MRI) on a 9.4 T small-animal MRI system. Herein we present the feasibility of 4D PC MRI in terms of qualitative and quantitative flow pattern analysis in mice with transverse aortic constriction (TAC).

Materials and methods

4D PC FLASH images of a flow phantom and mouse heart were acquired at 9.4 T using a four-point phase-encoding scheme. The method was compared with slice-selective PC FLASH and ultrasound using Bland–Altman analysis. Advanced 3D streamlines were visualized utilizing Voreen volume-rendering software.

Results

In vitro, 4D PC MRI flow profiles showed the transition between laminar and turbulent flow with increasing velocities. In vivo, 4D PC MRI data of the ascending aorta and the pulmonary artery were confirmed by ultrasound, resulting in linear regressions of R² > 0.93. Magnitude- and direction-encoded streamlines differed substantially pre- and post-TAC surgery.

Conclusions

4D PC MRI is a feasible tool for in vivo velocity measurements on high-field small-animal scanners. Similar to clinical measurement, this method provides a complete spatially and temporally resolved dataset of the murine cardiovascular blood flow and allows for three-dimensional flow pattern analysis.

Electronic supplementary material

The online version of this article (doi:10.1007/s10334-014-0466-z) contains supplementary material, which is available to authorized users.

4D UTE flow: a phase-contrast MRI technique for assessment and visualization of stenotic flows

PURPOSE

Inaccuracy of conventional four-dimensional (4D) flow MR imaging in the presence of random unsteady and turbulent blood flow distal to a narrowing has been an important challenge. Previous investigations have revealed that shorter echo times (TE) decrease the errors, leading to more accurate flow assessments.

METHODS

In this study, as part of a 4D flow acquisition, an Ultra-Short TE (UTE) method was adopted. UTE works based on a center-out radial k-space trajectory that inherently has a short TE. By employing free induction decay sampling starting from read-out gradient ramp-up, and by combining the refocusing lobe of the slice select gradient with the bipolar flow encoding gradient, TEs of ≈1 msec may be achieved.

RESULTS

Both steady and pulsatile flow regimes, and in each case a range of Reynolds numbers, were studied in an in-vitro model. Flow assessment at low and medium flow rates demonstrated a good agreement between 4D UTE and conventional 4D flow techniques. However, 4D UTE flow significantly outperformed conventional 4D flow, at high flow rates for both steady and pulsatile flow regimes. Feasibility of the method in one patient with Aortic Stenosis was also demonstrated.

CONCLUSION

For both steady and pulsatile high flow rates, the measured flow distal to the stenotic narrowing using conventional 4D flow revealed more than 20% error compared to the ground-truth flow. This error was reduced to less than 5% using the 4D UTE flow technique.

Shared velocity encoding: a method to improve the temporal resolution of phase-contrast velocity measurements

Phase-contrast magnetic resonance imaging (PC-MRI) is used routinely to measure fluid and tissue velocity with a variety of clinical applications. Phase-contrast magnetic resonance imaging methods require acquisition of additional data to enable phase difference reconstruction, making real-time imaging problematic. Shared Velocity Encoding (SVE), a method devised to improve the effective temporal resolution of phase-contrast magnetic resonance imaging, was implemented in a real-time pulse sequence with segmented echo planar readout. The effect of SVE on peak velocity measurement was investigated in computer simulation, and peak velocities and total flow were measured in a flow phantom and in volunteers and compared with a conventional ECG-triggered, segmented k-space phase-contrast sequence as a reference standard. Computer simulation showed a 36% reduction in peak velocity error from 8.8 to 5.6% with SVE. A similar reduction of 40% in peak velocity error was shown in a pulsatile flow phantom. In the phantom and volunteers, volume flow did not differ significantly when measured with or without SVE. Peak velocity measurements made in the volunteers using SVE showed a higher concordance correlation (0.96) with the reference standard than non-SVE (0.87). The improvement in effective temporal resolution with SVE reconstruction has a positive impact on the precision and accuracy of real-time phase-contrast magnetic resonance imaging peak velocity measurements.

PC HYPR flow: a technique for rapid imaging of contrast dynamics

PURPOSE

To improve spatial and temporal resolution and signal-to-noise ratio (SNR) in three-dimensional (3D) radial contrast-enhanced (CE) time-resolved MR angiography by means of a novel hybrid phase contrast (PC) and CE MRA acquisition and HYPR reconstruction (PC HYPR Flow).

MATERIALS AND METHODS

PC HYPR Flow consists of a CE exam immediately followed by a PC scan used to constrain the HYPR reconstruction of the time series. Temporal resolution of the new method was studied in computer simulations. The feasibility of the new technique was studied in healthy subjects and patients with brain arteriovenous malformations and in a canine model of aneurysms.

RESULTS

Simulations demonstrated preservation of contrast agent dynamics in proximal vessels, showing better performance than peer methods for acceleration up to 20 in 2D. In vivo, PC HYPR Flow yielded 3D time series with frame rate of 0.5 s and significantly outperformed two peer methods by means of a major increase in spatial resolution (0.8 x 0.8 x 0.8 mm(3)) and arterial/venous ratio, while maintaining necessary temporal waveform fidelity and high SNR.

CONCLUSION

This initial study indicates that PC HYPR Flow simultaneously provides 3D isotropic sub-millimeter spatial resolution, sub-second temporal reconstruction windows and high SNR level, which may benefit a wide range of CE MRA applications.

Improved SNR in phase contrast velocimetry with five-point balanced flow encoding

Phase contrast velocimetry can be utilized to measure complex flow for both quantitative and qualitative assessment of vascular hemodynamics. However, phase contrast requires that a maximum measurable velocity be set that balances noise and phase aliasing. To efficiently reduce noise in phase contrast images, several investigators have proposed extended velocity encoding schemes that use extra encodings to unwrap phase aliasing; however, existing techniques can lead to significant increases in echo and scan time, limiting their clinical benefits. In this work, we have developed a novel five-point velocity encoding scheme that efficiently reduces noise with minimal increases in scan and echo time. Investigations were performed in phantoms, demonstrating a 63% increase in velocity-to-noise ratio compared to standard four-point encoding schemes. Aortic velocity measurements were performed in healthy volunteers, showing similar velocity-to-noise ratio improvements. In those volunteers, it was also demonstrated that, without sacrificing accuracy, low-resolution images can be used for the fifth encoding point, reducing the scan time penalty from 25% down to less than 1%.

Velocity-selective RF pulses in MRI

A family of velocity-selective pulses consisting of a series of RF hard pulses followed by bipolar gradients was designed. The succession of required pulses was deduced using a k-space approach within a small tip-angle approximation. Fourier transform of the desired velocity excitation determined the flip-angle series, and the corresponding position in the generalized k-space identified the bipolar-gradient first moments. Spins from any velocity class can be selected. To illustrate this approach we designed and experimentally tested a velocity-slice selection that is analogous to standard spatial-slice selection but involves excitation of spins moving at a chosen velocity (velocity-slice center) and within a given interval (velocity-slice thickness). The assumed approximation does not limit the design to small angles, because velocity selection still holds for angles up to 90 degrees. Velocity slices were experimentally selected, centered on velocities ranging from -1 m s(-1) to 1 m s(-1) with a velocity-slice thickness of 0.4 m s(-1). The experimental velocity-slice profile was assessed and the flow was quantified.

Quantitative analysis of PC MRI velocity maps: pulsatile flow in cylindrical vessels

The accuracy of MR phase contrast (PC) velocity mapping, and the subsequent derivation of wall shear stress (WSS) values, has been quantitatively assessed. Using a retrospectively gated PC gradient-echo technique, the temporal-spatial velocity fields were measured for pulsatile flow in a rigid cylindrical vessel. The experimental data were compared with values derived from the Womersley solution of the Navier-Stokes equations. For a sinusoidal waveform, the overall root-mean-square (rms) difference between the measured and analytical velocities corresponded to 13% of the peak fluid velocity. The WSS derived from the data displayed a 14% rms difference with the analytical model. As an example of a more complicated flow, a triangular saw-tooth waveform was deconstructed into its Fourier components. Velocity maps and the WSS were calculated by the superposition of the individual solutions, weighted by the Fourier series coefficient, for each harmonic. The velocity and experimentally derived WSS agreed with the analytical results (4% and 12% rms difference, respectively). Evaluation of the analytical models allowed an estimate of the inherent accuracy in the measurement of velocity maps and WSS values.

Complex-valued stiffness reconstruction for magnetic resonance elastography by algebraic inversion of the differential equation

Noninvasive quantitation of the mechanical properties of tissue could improve early detection of pathology. Previously a method for detecting displacement from propagating shear waves using a phase-contrast MRI technique was developed. In this work it is demonstrated how a collection of data representing the full vector displacement field could be used to potentially estimate the full complex stiffness tensor. An algebraic inversion approach useful for piece-wise homogeneous materials is described in detail for the general isotropic case, which is then specialized to incompressible materials as a model for tissue. Results of the inversion approach are presented for simulated and experimental phantom data that show the technique can be used to obtain shear wave-speed and attenuation in regions where there is sufficient signal-to-noise ratio in the displacement and its second spatial derivatives. The sensitivity to noise is higher in the attenuation estimates than the shear wave-speed estimates. Magn Reson Med 45:299-310, 2001.

MR-guided percutaneous angioplasty: assessment of tracking safety, catheter handling and functionality

PURPOSE

Magnetic resonance (MR)-guided percutaneous vascular interventions have evolved to a practical possibility with the advent of open-configuration MR systems and real-time tracking techniques. The purpose of this study was to assess an MR-tracking percutaneous transluminal angioplasty (PTA) catheter with regard to its safety profile and functionality.

METHODS

Real-time, biplanar tracking of the PTA catheter was made possible by incorporating a small radiofrequency (RF) coil in the catheter tip and connecting it to a coaxial cable embedded in the catheter wall. To evaluate potentially hazardous thermal effects due to the incorporation of the coil, temperature measurements were performed within and around the coil under various scanning and tracking conditions at 1.5 Tesla (T). Catheter force transmission and balloon-burst pressure of the MR-tracking PTA catheter were compared with those of a standard PTA catheter. The dilatative capability of the angioplasty balloon was assessed in vitro as well as in vivo, in an isolated femoral artery segment in a swine.

RESULTS

The degree of heating at the RF coil was directly proportional to the power of the RF pulses. Heating was negligible with MR tracking, conventional spin-echo and low-flip gradient-echo sequences. Sequences with higher duty cycles, such as fast spin echo, produced harmful heating effects. Force transmission of the MR-tracking PTA catheter was slightly inferior to that of the standard PTA catheter, while balloon-burst pressures were similar to those of conventional catheters. The MR-tracking PTA catheter functioned well both in vitro and in vivo.

CONCLUSION

The in vivo use of an MR-tracking PTA catheter is safe under most scanning conditions.

MR versus fluoroscopic guidance of a catheter/guidewire system: in vitro comparison of steerability

Our purpose in this study was to evaluate the steerability of a combined catheter guidewire system with MR tracking in an open-configuration .5T MR system and compare it with fluoroscopic guidance. Experiments were performed with an aorta-shaped glass phantom with different-size branches connected to a roller flow pump to simulate pulsatile flow. A .035" guidewire was used in conjunction with a 5F Cobra 2-shaped catheter. For active MR guidance, a small RF coil was incorporated into the tips of all devices. In addition to fluoroscopic guidance, we used MR tracking devices to selectively catheterize all branches in the .5T open magnet (Signa SP, GEMS). Time requirements for the latter were compared with those needed under conventional angiographic conditions using standard catheters and guidewires. Active MR tracking permitted the simultaneous real time (4 updates/sec) guidance of both guidewire and catheter. Under MR guidance, all branch vessels were successfully catheterized. We found no significant differences in time requirements between guidance with the MR tracking device and fluoroscopic guidance. However, cannulation under fluoroscopic guidance with standard angiography devices required significantly less time (P < .05). Selective catheterization of small branch vessels is possible with active MR tracking of a combined catheter/guidewire system. Limitations of MR tracking device material and design result in a considerable lengthening of the cannulation procedures.

Velocity encoding using both phase and magnitude

Imaging time constitutes a major limitation of phase-contrast (PC) angiography. It is possibly the main disadvantage of PC methods over the time-of-flight (TOF) methods that actually are used clinically. This relatively long imaging time comes from the fact that conventional PC methods require the acquisition of at least four images with different velocity sensitization to reconstruct a single angiogram (1, 2). However, more than one-half of the information gathered through the acquisition of these four images is either redundant or simply discarded. We propose a faster approach to making PC angiograms in which the quantity of data acquired is diminished by as much as a factor 2. This is made possible by encoding velocity information in both the phase and magnitude of the image. Due to the use of extra radiofrequency (RF) and gradient waveforms, decreases in data requirements do not translate in a direct manner into decreases in imaging time. Nevertheless, significant reductions in imaging time are achieved with the present approach.

MR-guided cholecystostomy: assessment of biplanar, real-time needle tracking in three pigs

PURPOSE

To demonstrate the feasibility of magnetic resonance (MR)-guided cholecystostomy using active, real-time, biplanar MR tracking in animal experiments.

METHODS

Experiments were performed on three fully anesthetized pigs in an interventional MR system (GE open). The gallbladder was displayed in two orthogonal planes using a heavily T2-weighted fast spin-echo sequence. These "cholangio roadmaps" were displayed on LCD monitors positioned in front of the interventionalist. A special coaxial MR-tracking needle, equipped with a small receive-only coil at its tip, was inserted percutaneously into the gallbladder under continuous, biplanar MR guidance. The MR-tracking sequence allowed sampling of the coil (needle tip) position every 120 msec. The position of the coil was projected onto the two orthogonal "cholangio roadmap" images.

RESULTS

Successful insertion of the needle was confirmed by aspiration of bile from the gallbladder. The process of aspiration and subsequent instillation of Gd-DTPA into the gallbladder was documented with fast gradient-recalled echo imaging.

CONCLUSION

Biplanar, active, real-time MR tracking in combination with "cholangio roadmaps" allows for cholecystostomies in an interventional MRI environment.

Two methods for peak RF power minimization of multiple inversion-band pulses

Two novel methods to minimize peak RF power for high order longitudinal Hadamard encoding are described and demonstrated experimentally. The first method uses the fact that the choice of a reference phase in an inversion process does not affect the final frequency response. In this method, the different single inversion-band pulses are added together, each with a different reference phase. For a proper phase choice, minimization of the peak RF power is obtained. Scaling laws are defined allowing the use of a given phase-set in multiple cases. In the second method, single inversion-band pulses are added together, each partially shifted in time. This results in a significant reduction in peak power with only a moderate increase in pulse length. Theoretical conditions outlining the optimal addition order are defined. Experimental results verify the theoretical conditions and demonstrate that the frequency response is not affected by the peak power minimization process. With the new low peak RF power, longitudinal Hadamard encoding of 8TH (or 16TH) order can be performed in any clinical setting.

Magnetic resonance angiography

Magnetic resonance angiography (MRA) permits the non-invasive visualization of blood flow through the effects of moving spins on the magnetic resonance signal. MRA techniques can be divided into two main classifications depending upon the primary effect responsible for contrast in the image. Angiograms can be produced using either the time-of-flight (TOF) or phase contrast (PC) methods. Each method has particular advantages and limitations as an angiographic imaging technique and these are reflected in their respective applications. This review article is intended to outline the scientific and technical development of MRA from its basis in the earliest in vitro nuclear magnetic resonance (NMR) experiments, through the implementation of in vivo angiographic techniques on whole body MRI systems, to the recent rapid expansion in MRA acquisition and processing techniques.

Frequency response of multi-phase segmented k-space phase-contrast

A theoretical analysis of the temporal frequency response of multi-phase segmented k-space phase-contrast was developed. This includes the effects of both segment duration and the number of cardiac phases that are reconstructed. An increase in the number of views per segment and the corresponding increase in segment duration results in an increased smoothing or low-pass filtering of the time-resolved flow waveform. Reconstruction of all intermediate cardiac phases makes the Nyquist sampling frequency independent of the number of views per segment. This analysis was verified experimentally using a multi-phase phase-contrast segmented k-space MR pulse sequence. This sequence reconstructs all intermediate cardiac phases and uses fractional segments at the end of the cardiac cycle if an entire segment does not fit. The use of fractional segments increases the portion of the cardiac cycle over which data are acquired.

MR angiography of abdominal and peripheral arteries. Techniques and clinical applications

This review article deals with MR angiography (MRA) of abdominal and peripheral arteries. Pulsatile flow, respiratory motion and peristalsis impose difficulties in imaging the vascular structures in the abdomen and the lower extremities. Development of new techniques, such as segmentation of the data acquisition, using specific acquisition windows in relation to a cardiac trigger, magnetization preparation of the tissue and phase-encoding re-ordering or sorting, have reduced the artifacts associated with abdominal and peripheral MRA. Clinical MR investigations of the arteries branching from the abdominal aorta such as the renal and mesenteric arteries and arteries in the lower extremities have revealed that severe stenoses or occlusions can be diagnosed accurately while the grading of less severe stenosis is more difficult. The phase-contrast method has been used to quantify blood flow and study the hemodynamics in abdominal and peripheral vessels. Quantitative flow information can be used to diagnose vascular disease and provides important physiological information. More prospective clinical studies, in which recently developed MRA techniques are compared with conventional angiography, are necessary before conclusive decisions can be made as to whether MRA may replace these methods.

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.

Reduction of partial-volume artifacts with zero-filled interpolation in three-dimensional MR angiography

Partial-volume artifacts reduce vessel contrast and continuity (especially in small vessels) in magnetic resonance (MR) angiography. The authors applied zero-filled (band-limited) interpolation to three-dimensional (3D) MR angiograms to reduce partial-volume artifacts. They demonstrated that zero-filled interpolation can also be implemented by means of voxel shifting in real space. Voxel-shifted interpolation is much less computer memory intensive than conventional zero-filled interpolation. They numerically simulated the contrast loss due to partial-volume artifacts and contrast recovery obtained with zero-filled interpolation. Zero-filled interpolation in all three orthogonal directions was applied to 3D MR angiography data sets from 29 human studies. These studies were obtained with the three commonly used 3D MR angiography techniques: 3D time of flight, multislab 3D time of flight, and 3D phase contrast. A substantial improvement in vessel contrast and vessel continuity was observed in all cases.

Magnetic resonance angiography: today and tomorrow

Magnetic resonance angiography (MRA) has enjoyed enthusiastic success at many research institutions where it is now routinely used in place of invasive x-ray angiography (XRA) for a variety of applications. While the physical principles of MRA are well understood, there is still plenty of opportunity for growth in the coming years. Recent improvements in instrumentation have permitted more rapid acquisition and manipulation of larger data sets. Instruments in the future are sure to continue this trend as computer hardware becomes more capable and less expensive. New clinical applications will also expand the utility of MRA beyond its current use. MRA is already being used in peripheral vessels and it appears to have great potential in the abdomen. Research into MRA methods for coronary vessel imaging is also beginning to show intriguing results. In addition, preliminary research results suggest that interventional MRA may one day become a reality.

Correlation of cine MR velocity measurements in the internal carotid artery with collateral flow in the circle of Willis: preliminary study

The velocity-phase relationship intrinsic to phase-contrast magnetic resonance (MR) angiography permits the quantitative and qualitative assessment of blood flow. The ability to measure velocity and vessel cross-sectional area allows noninvasive assessment of volume flow rate (VFR) in the internal carotid artery (ICA). Phase-contrast techniques also demonstrate flow direction. Using two-dimensional cine phase-contrast angiography, the authors evaluated VFR in the ICA and collateral flow about the circle of Willis in 15 patients with ischemic neurologic symptoms. The VFR in each carotid artery was correlated with the degree of stenosis and presence or absence of abnormal circle of Willis collateral flow. There was a correlation between a decrease in VFR and abnormal circle of Willis collateral flow. In addition, a correlation between severe stenosis and a decrease in VFR was found. In patients with ischemic neurologic symptoms without severe stenosis (< 70% diameter stenosis), no decrease in VFR was seen. It is hoped that flow quantification and directional flow imaging with phase-contrast angiography will help further characterize carotid artery occlusive disease by enabling assessment of VFR changes associated with ischemic neurologic symptoms. This study also supports the hypothesis that two mechanisms--hemodynamic and embolic--play a role in ischemic neurologic symptoms.

Real-time position monitoring of invasive devices using magnetic resonance

Techniques which can be used to follow the position of invasive devices in real-time using magnetic resonance (MR) are described. Tracking of an invasive device is made possible by incorporating one or more small RF coils into the device. These coils detect MR signals from only those spins near the coil. Pulse sequences which employ nonselective RF pulses to excite all nuclear spins within the field-of-view are used. Readout magnetic field gradient pulses, typically applied along one of the primary axes of the imaging system, are then used to frequency encode the position of the receive coil(s). Data are Fourier transformed and one or more peaks located to determine the position of each receiver coil in the direction of the applied field gradient. Subsequent data collected on orthogonal axes permits the localization of the receiver coil in three dimensions. The process can be repeated rapidly and the position of each coil can be displayed in real-time.