Time-resolved Three-dimensional Phase-contrast MRI Research Articles

Pseudo-spiral sampling and compressed sensing reconstruction provides flexibility of temporal resolution in accelerated aortic 4D flow MRI: A comparison with k-t principal component analysis.

IntroductionTime‐resolved three‐dimensional phase contrast MRI (4D flow) of aortic blood flow requires acceleration to reduce scan time. Two established techniques for highly accelerated 4D flow MRI are k‐t principal component analysis (k‐t PCA) and compressed sensing (CS), which employ either regular or random k‐space undersampling. The goal of this study was to gain insights into the quantitative differences between k‐t PCA‐ and CS‐derived aortic blood flow, especially for high temporal resolution CS 4D flow MRI.MethodsThe scan protocol consisted of both k‐t PCA and CS accelerated 4D flow MRI, as well as a 2D flow reference scan through the ascending aorta acquired in 15 subjects. 4D flow scans were accelerated with factor R = 8. For CS accelerated scans, we used a pseudo‐spiral Cartesian sampling scheme, which could additionally be reconstructed at higher temporal resolution, resulting in R = 13. 4D flow data were compared with the 2D flow scan in terms of flow, peak flow and stroke volume. A 3D peak systolic voxel‐wise velocity and wall shear stress (WSS) comparison between k‐t PCA and CS 4D flow was also performed.ResultsThe mean difference in flow/peak flow/stroke volume between the 2D flow scan and the 4D flow CS with R = 8 and R = 13 was 4.2%/9.1%/3.0% and 5.3%/7.1%/1.9%, respectively, whereas for k‐t PCA with R = 8 the difference was 9.7%/25.8%/10.4%. In the voxel‐by‐voxel 4D flow comparison we found 13.6% and 3.5% lower velocity and WSS values of k‐t PCA compared with CS with R = 8, and 15.9% and 5.5% lower velocity and WSS values of k‐t PCA compared with CS with R = 13.ConclusionPseudo‐spiral accelerated 4D flow acquisitions in combination with CS reconstruction provides a flexible choice of temporal resolution. We showed that our proposed strategy achieves better agreement in flow values with 2D reference scans compared with using k‐t PCA accelerated acquisitions.

Physiological significance of helical flow in the arterial system and its potential clinical applications.

Helical flow in the human aorta is possibly a typical example of 'form follows function' in the vascular system. The helical blood flow may provide guaranties for the inner surface of the ascending aortic wall to get smooth and even washing by the blood so that atherosclerotic plaques can hardly form in the area of the ascending aorta. It has been documented that the phenomenon of helical flow of blood is not just localized in the ascending aorta, it also exists in several large arteries and veins as well. Preliminary studies demonstrated the widely existing helical flow might play positive physiological roles in facilitating blood flow transport, suppressing disturbed blood flow, preventing the accumulation of atherogenic low density lipoproteins on the luminal surfaces of arteries, enhancing oxygen transport from the blood to the arterial wall and reducing the adhesion of blood cells on the arterial surface. These roles of helical blood flow may lessen the burden of arteries and protect the arteries from the pathology of atherosclerosis, thrombosis, and intimal hyperplasia. The great development of time-resolved three-dimensional phase contrast MRI (flow-sensitive 4D-MRI) and the advent of dimensionless indices such as helical flow index proposed to characterize helical flow make clinic quantification of the helical flow in the human large arteries possible. Moreover, researchers probed into the possibility to apply the mechanism of helical flow to the design of vascular interventions to reduce thrombus formation and intimal hyperplasia caused by abnormal flow conditions.

Incorporating time-resolved three-dimensional phase contrast (4D flow) MRI in clinical workflow: initial experiences at a large tertiary care medical center

Background Time-resolved three-dimensional phase contrast (4D flow) MRI allows for visualization of three-dimensional cardiovascular anatomy and pulsatile flow with full volumetric coverage in a single, easy to prescribe 3D acquisition. The technique provides comprehensive flow visualization and permits retrospective flow quantification at any user-defined region of interest. [1] To our knowledge, no center has incorporated 4D flow MRI as a part of standard clinical cardiovascular MRI (CMR). The goals of this study include: 1) reporting on the incorporation of 4D flow MRI acquisition and processing as part of clinical CMR workflow and 2) better understanding the clinical impact of 3D flow visualization and retrospective flow quantification derived from 4D flow MRI in CMR.

Aortic coarctation augments changes in thoracic aortic hemodynamics in pediatric and young adult patients with bicuspid aortic valve

Background Bicuspid aortic valve (BAV) is a congenital abnormality that is often associated with aortic coarctation. Evidence suggests this combination may increase the risk of secondary aortic complications compared to patients with BAV alone.[1] Altered aortic hemodynamics likely induce vascular remodeling[2] and may contribute to these complications. No study has investigated the impact of aortic coarctation on aortic hemodynamics in pediatric and young adult patients with BAV using time-resolved three-dimensional phase contrast (4D flow) MRI. The aim of this study is to utilize 4D flow MRI to compare thoracic aorta flow patterns in pediatric and young adult patients with BAV with and without aortic coarctation or coarctation repair. Methods In accordance with an IRB-approved protocol, 17 patients with BAV (age = 11.9 +/- 5.3 years) who underwent cardiovascular MRI including 4D flow as part of follow-up care were included. Seven patients had aortic coarctation including 5 patients with coarctation repairs. Images were processed using in-house software for noise reduction, anti-aliasing, and eddy current correction. Flow visualization and quantification were performed in EnSight (CEI, Apex, NC). Aortic root z-scores were calculated from MRI measurements and height and weight at time of scan in EchoIMS (Merge, Chicago, IL). Flow jet pattern and helicity were assessed in the ascending aorta, aortic arch, and descending aorta. Flow jet pattern was determined by the region of peak velocity in systole corresponding to anatomically designated quadrants of

Time-resolved three-dimensional phase contrast MRI evaluation of bicuspid aortic valve and coarctation of the aorta

A 41-year-old man with a bicuspid aortic valve (BAV) and coarctation of the aorta, as visualized by 2D phase contrast cardiovascular magnetic resonance imaging (CMR) ( Figure 1D , Supplementary data online, Videos S1 ) and contrast enhanced MR angiography ( Figure 1A ), was referred for evaluation by time-resolved 3D phase contrast (4D flow) CMR to assess the flow velocity through the coarctation and provide aortic flow characterization. Findings of the comprehensive examination demonstrate the complex haemodynamic patterns that emerge throughout the aorta secondary …

K-t BLAST and SENSE accelerated time-resolved three-dimensional phase contrast MRI in an intracranial aneurysm

The objective of this study was to investigate the performance of k-t BLAST (Broad-use Linear Acquisition Speed-up Technique) accelerated time-resolved 3D PC-MRI compared to SENSE (SENSitivity Encoding) acceleration in an in vitro and in vivo intracranial aneurysm. Non-accelerated, SENSE and k-t BLAST accelerated time-resolved 3D PC-MRI measurements were performed in vivo and in vitro. We analysed the consequences of various temporal resolutions in vitro. Both in vitro and in vivo measurements showed that the main effect of k-t BLAST was underestimation of velocity during systole. In the phantom, temporal blurring decreased with increasing temporal resolution. Quantification of the differences between the non-accelerated and accelerated measurements confirmed that in systole SENSE performed better than k-t BLAST in terms of mean velocity magnitude. In both in vitro and in vivo measurements, k-t BLAST had higher SNR compared to SENSE. Qualitative comparison between measurements showed good similarity. Comparison with SENSE revealed temporal blurring effects in k-t BLAST accelerated measurements.

Quantification and visualization of flow in the Circle of Willis: Time‐resolved three‐dimensional phase contrast MRI at 7 T compared with 3 T

The assessment of both geometry and hemodynamics of the intracranial arteries has important diagnostic value in internal carotid occlusion, sickle cell disease, and aneurysm development. Provided that signal to noise ratio (SNR) and resolution are high, these factors can be measured with time-resolved three-dimensional phase contrast MRI. However, within a given scan time duration, an increase in resolution causes a decrease in SNR and vice versa, hampering flow quantification and visualization. To study the benefits of higher SNR at 7 T, three-dimensional phase contrast MRI in the Circle of Willis was performed at 3 T and 7 T in five volunteers. Results showed that the SNR at 7 T was roughly 2.6 times higher than at 3 T. Therefore, segmentation of small vessels such as the anterior and posterior communicating arteries succeeded more frequently at 7 T. Direction of flow and smoothness of streamlines in the anterior and posterior communicating arteries were more pronounced at 7 T. Mean velocity magnitude values in the vessels of the Circle of Willis were higher at 3 T due to noise compared to 7 T. Likewise, areas of the vessels were lower at 3 T. In conclusion, the gain in SNR at 7 T compared to 3 T allows for improved flow visualization and quantification in intracranial arteries.

Quantification and visualization of flow in small vessels of the Circle of Willis: time-resolved three-dimensional phase contrast MRI at 7T compared with 3T

Background A promising technique to measure blood flow is timeresolved three-dimensional phase contrast MRI (PCMRI). In small structures as the Circle of Willis (CoW), the resolution of the measurement needs to be high (< 1 mm). Image quality may be compromised when SNR decreases with increasing resolution which leads to blood flow direction uncertainty and flow quantification inaccuracies. To increase SNR, PC-MRI can be conducted at higher field strengths. In this study timeresolved 3D PC-MRI is performed in the CoW of five volunteers at 3T and 7T to investigate the advantages of increased SNR.

Four‐dimensional flow MRI using spiral acquisition

Time-resolved three-dimensional phase-contrast MRI is an important tool for physiological as well as clinical studies of blood flow in the heart and vessels. The application of the technique is, however, limited by the long scan times required. In this work, we investigate the feasibility of using spiral readouts to reduce the scan time of four-dimensional flow MRI without sacrificing quality. Three spiral approaches are presented and evaluated in vivo and in vitro against a conventional Cartesian acquisition. In vivo, the performance of each method was assessed in the thoracic aorta in 10 volunteers using pathline-based analysis and cardiac output analysis. Signal-to-noise ratio and background phase errors were investigated in vitro. Using spiral readouts, the scan times of a four-dimensional flow acquisition of the thoracic aorta could be reduced 2-3-fold, with no statistically significant difference in pathline validity or cardiac output. The shortened scan time improves the applicability of four-dimensional flow MRI, which may allow the technique to become a part of a clinical workflow for cardiovascular functional imaging.

Aortic flow patterns in patients with Marfan syndrome assessed by flow‐sensitive four‐dimensional MRI

To apply time-resolved three-dimensional (3D) phase contrast MRI with three-directional velocity encoding (flow-sensitive 4D MRI) for the characterization of flow pattern changes in patients with Marfan syndrome (MFS) compared with normal controls. Flow-sensitive 4D MRI of the thoracic aorta (temporal resolution ∼45 ms, spatial resolution ∼2.4 × 2.1 × 2.8 mm(3) ) was performed in 24 MFS patients and 10 volunteers. Aortic flow patterns were visualized by 3D particle traces and streamlines. Global (affecting the complete lumen) and local (parts of the vessel lumen) helix and vortex flow in the ascending aorta (AAo), aortic arch, and descending aorta (DAo) were graded in 3 categories (blinded reading, two observers): none = 0, moderate = 1, pronounced = 2. Flow grading revealed similar global helix and vortex flow in the AAo and arch for MFS patients and controls. Local helix flow in the AAo was significantly (P = 0.011) increased in patients and was associated with aortic sinus dilatation. The incidence of global helix and vortex flow in the DAo was increased in patients (77% and 50% of subjects) compared with controls (none and 10%). The 4D flow analysis revealed marked differences of the aortic flow patterns between Marfan patients and controls: Local helix flow in the patients' AAo may be associated with the increased incidence of aortic root dilatation. The flow alterations in the proximal DAo could explain the occurrence of Type-B dissection originating from this site.

In vivo hemodynamic analysis of intracranial aneurysms obtained by magnetic resonance fluid dynamics (MRFD) based on time-resolved three-dimensional phase-contrast MRI

Hemodynamics is thought to play a very important role in the initiation, growth, and rupture of intracranial aneurysms. The purpose of our study was to perform in vivo hemodynamic analysis of unruptured intracranial aneurysms of magnetic resonance fluid dynamics using time-resolved three-dimensional phase-contrast MRI (4D-Flow) at 1.5T and to analyze relationships between hemodynamics and wall shear stress (WSS) and oscillatory shear index (OSI). This study included nine subjects with 14 unruptured aneurysms. 4D-Flow was performed by a 1.5-T magnetic resonance scanner with a head coil. We calculated in vivo streamlines, WSS, and OSI of intracranial aneurysms based on 4D-Flow with our software. We evaluated the number of spiral flows in the aneurysms and compared the differences in WSS or OSI between the vessel and aneurysm and between whole aneurysm and the apex of the spiral flow. 3D streamlines, WSS, and OSI distribution maps in arbitrary direction during the cardiac phase were obtained for all intracranial aneurysms. Twelve aneurysms had one spiral flow each, and two aneurysms had two spiral flows each. The WSS was lower and the OSI was higher in the aneurysm compared to the vessel. The apex of the spiral flow had a lower WSS and higher OSI relative to the whole aneurysm. Each intracranial aneurysm in this study had at least one spiral flow. The WSS was lower and OSI was higher at the apex of the spiral flow than the whole aneurysmal wall.

Evaluation of intracranial stenoses and aneurysms with accelerated 4D flow

The aim of this study was to evaluate intracranial arterial stenoses and aneurysms with accelerated time-resolved three-dimensional (3D) phase-contrast MRI or 4D flow. The 4D flow technique was utilized to image four normal volunteers, two patients with intracranial stenoses and two patients with intracranial aneurysms. In order to reduce scan time, parallel imaging was combined with an acquisition strategy that eliminates the corners of k-space. In the two patients with intracranial stenoses, 4D flow velocity measurements showed that one patient had normal velocity profiles in agreement with a previous magnetic resonance angiogram (MRA), while the second showed increased velocities that indicated a less significant narrowing than suspected on a previous MRA, as confirmed by catheter angiography. This result may have prevented an invasive angiogram. In the two patients with 4-mm intracranial aneurysm, one had a stable helical flow pattern with a large jet, while the other had a temporally unstable flow pattern with a more focal jet possibly indicating that the second aneurysm may have a higher likelihood of rupture. Accelerated 4D flow provides time-resolved 3D velocity data in an 8- to 10-min scan. In the stenosis patients, the addition of 4D flow to a traditional MRA adds the velocity data provided from transcranial Doppler ultrasound (TCD) possibly allowing for more accurate grading of stenoses. In the aneurysm patients, visualization of flow patterns may help to provide prognostic information about future risk of rupture.

Initial experience characterizing a type I endoleak from velocity profiles using time-resolved three-dimensional phase-contrast MRI

We report the first utilization of time-resolved three-dimensional phase contrast magnetic resonance imaging, termed 4D flow, to image a type I endoleak after endovascular aneurysm repair. The combination of 4D flow and a traditional magnetic resonance angiogram can aid in the accurate detection and characterization of endoleaks by combining the three-dimensional resolution of cross-sectional imaging with the temporally resolved velocity data of Doppler ultrasound.

Kinematics of the heart: strain-rate imaging from time-resolved three-dimensional phase contrast MRI.

A four-dimensional mapping (three spatial dimensions + time) of myocardial strain-rate would help to describe the mechanical properties of the myocardium, which affect important physiological factors such as the pumping performance of the ventricles. Strain-rate represents the local instantaneous deformation of the myocardium and can be calculated from the spatial gradients of the velocity field. Strain-rate has previously been calculated using one-dimensional (ultrasound) or two-dimensional (2-D) magnetic resonance imaging) techniques. However, this assumes that myocardial motion only occurs in one direction or in one plane, respectively. This paper presents a method for calculation of the time-resolved three-dimensional (3-D) strain-rate tensor using velocity vector information in a 3-D spatial grid during the whole cardiac cycle. The strain-rate tensor provides full information of both magnitude and direction of the instantaneous deformation of the myocardium. A method for visualization of the full 3-D tensor is also suggested. The tensors are visualized using ellipsoids, which display the principal directions of strain-rate and the ratio between strain-rate magnitude in each direction. The presented method reveals the principal strain-rate directions without a priori knowledge of myocardial motion directions.

Noninvasive measurement of time-varying three-dimensional relative pressure fields within the human heart.

Understanding cardiac blood flow patterns is important in the assessment of cardiovascular function. Three-dimensional flow and relative pressure fields within the human left ventricle are demonstrated by combining velocity measurements with computational fluid mechanics methods. The velocity field throughout the left atrium and ventricle of a normal human heart is measured using time-resolved three-dimensional phase-contrast MRI. Subsequently, the time-resolved three-dimensional relative pressure is calculated from this velocity field using the pressure Poisson equation. Noninvasive simultaneous assessment of cardiac pressure and flow phenomena is an important new tool for studying cardiac fluid dynamics.