Enhancement of cerebrovascular 4D flow MRI velocity fields using machine learning and computational fluid dynamics simulation data

Four-dimensional (4D) flow magnetic resonance imaging (MRI) often relies on the injection of gadolinium- or iron-oxide-based contrast agents to improve vessel delineation. In this work, a novel technique is developed to acquire and reconstruct 4D flow data with excellent dynamic visualization of blood vessels but without the need for contrast injection. Synchronization of Neighboring Acquisitions by Physiological Signals (SyNAPS) uses pilot tone (PT) navigation to retrospectively synchronize the reconstruction of two free-running three-dimensional radial acquisitions, to create co-registered anatomy and flow images. Thirteen volunteers and two Marfan syndrome patients were scanned without contrast agent using one free-running fast interrupted steady-state (FISS) sequence and one free-running phase-contrast MRI (PC-MRI) sequence. PT signals spanning the two sequences were recorded for retrospective respiratory motion correction and cardiac binning. The magnitude and phase images reconstructed, respectively, from FISS and PC-MRI, were synchronized to create SyNAPS 4D flow datasets. Conventional two-dimensional (2D) flow data were acquired for reference in ascending (AAo) and descending aorta (DAo). The blood-to-myocardium contrast ratio, dynamic vessel area, net volume, and peak flow were used to compare SyNAPS 4D flow with Native 4D flow (without FISS information) and 2D flow. A score of 0-4 was given to each dataset by two blinded experts regarding the feasibility of performing vessel delineation. Blood-to-myocardium contrast ratio for SyNAPS 4D flow magnitude images (1.5±0.3) was significantly higher than for Native 4D flow (0.7±0.1, p<0.01) and was comparable to 2D flow (2.3±0.9, p=0.02). Image quality scores of SyNAPS 4D flow from the experts (M.P.: 1.9±0.3, E.T.: 2.5±0.5) were overall significantly higher than the scores from Native 4D flow (M.P.: 1.6±0.6, p=0.03, E.T.: 0.8±0.4, p<0.01) but still significantly lower than the scores from the reference 2D flow datasets (M.P.: 2.8±0.4, p<0.01, E.T.: 3.5±0.7, p<0.01). The Pearson correlation coefficient between the dynamic vessel area measured on SyNAPS 4D flow and that from 2D flow was 0.69±0.24 for the AAo and 0.83±0.10 for the DAo, whereas the Pearson correlation between Native 4D flow and 2D flow measurements was 0.12±0.48 for the AAo and 0.08±0.39 for the DAo. Linear correlations between SyNAPS 4D flow and 2D flow measurements of net volume (r2 =0.83) and peak flow (r2 =0.87) were larger than the correlations between Native 4D flow and 2D flow measurements of net volume (r2 =0.79) and peak flow (r2 =0.76). The feasibility and utility of SyNAPS were demonstrated for joint whole-heart anatomical and flow MRI without requiring electrocardiography gating, respiratory navigators, or contrast agents. Using SyNAPS, a high-contrast anatomical imaging sequence can be used to improve 4D flow measurements that often suffer from poor delineation of vessel boundaries in the absence of contrast agents.

Hemodynamic assessment of left atrial (LA) flow using phase contrast MRI provides insight into thromboembolic risk in atrial fibrillation (AF). However, conventional flow imaging techniques are averaged over many heartbeats. To evaluate beat-to-beat variability and LA hemodynamics in patients with AF using real time phase contrast (RTPC) MRI. Prospective. Thirty-five patients with history of AF (68 ± 10 years, nine female), 10 healthy controls (57 ± 19 years, four female). 5T, 2D RTPC with through-plane velocity-encoded gradient echo sequence and 4D flow MRI with three-directional velocity-encoded gradient echo sequence. RTPC was continuously acquired for a mid-LA slice in all subjects. 4D flow data were interpolated at the RTPC location and normally projected for comparison with RTPC. RR intervals extracted from RTPC were used to calculate heart rate variability (HRV=interquartile range over median × 100%). Patients were classified into low (<9.7%) and high (>9.7%) HRV groups. LA peak/mean velocity and stasis (%velocities < 5.8cm/sec) were calculated from segmented 2D images. Variability in RTPC flow metrics was quantified by coefficient of variation (CV) over all cycles. Pearson's correlation coefficient (r), Bland-Altman analysis, Kruskal-Wallis test. A P value < 0.05 was considered statistically significant. RTPC and 4D flow measurements were strongly/significantly correlated for all hemodynamic parameters (R2 =0.75-0.83) in controls. Twenty-four patients had low HRV (mean=4 ± 2%) and 11 patients had high HRV (27 ± 9%). In patients, increased HRV was significantly correlated with CV of peak velocity (r=0.67), mean velocity (r=0.51), and stasis (r=0.41). A stepwise decrease in peak/mean velocity and increase in stasis was observed when comparing controls vs. low HRV vs. high HRV groups. Mean velocity and stasis differences were significant for control vs. high HRV groups. RTPC may be suitable for assessing the impact of HRV on hemodynamics and provide insight for AF management in highly arrhythmic patients. 1 TECHNICAL EFFICACY: Stage 2.

To assess the ability of four-dimensional (4D) flow MRI to measure hepatic arterial hemodynamics by determining the effects of spatial resolution and respiratory motion suppression in vitro and its applicability in vivo with comparison to two-dimensional (2D) phase-contrast MRI. A dynamic hepatic artery phantom and 20 consecutive volunteers were scanned. The accuracies of Cartesian 4D flow sequences with k-space reordering and navigator gating at four spatial resolutions (0.5- to 1-mm isotropic) and navigator acceptance windows (± 8 to ± 2 mm) and one 2D phase-contrast sequence (0.5-mm in -plane) were assessed in vitro at 3 T. Two sequences centered on gastroduodenal and hepatic artery branches were assessedin vivo for intra - and interobserver agreement and compared to 2D phase-contrast. In vitro, higher spatial resolution led to a greater decrease in error than narrower navigator window (30.5 to -4.67% vs -6.64 to -4.67% for flow). In vivo, hepatic and gastroduodenal arteries were more often visualized with the higher resolution sequence (90 vs 71%). Despite similar interobserver agreement (κ = 0.660 and 0.704), the higher resolution sequence had lower variability for area (CV = 20.04 vs 30.67%), flow (CV = 34.92 vs 51.99%), and average velocity (CV = 26.47 vs 44.76%). 4D flow had lower differences between inflow and outflow at the hepatic artery bifurcation (11.03 ± 5.05% and 15.69 ± 6.14%) than 2D phase-contrast (28.77 ± 21.01%). High-resolution 4D flow can assess hepatic artery anatomy and hemodynamics with improved accuracy, greater vessel visibility, better interobserver reliability, and internal consistency. • Motion-suppressed Cartesian four-dimensional (4D) flow MRI with higher spatial resolution provides more accurate measurements even when accepted respiratory motion exceeds voxel size. • 4D flow MRI with higher spatial resolution provides substantial interobserver agreement for visualization of hepatic artery branches. • Lower peak and average velocities and a trend toward better internal consistency were observed with 4D flow MRI as compared to 2D phase-contrast.

BackgroundDiseases related to cerebrospinal fluid flow, such as hydrocephalus, syringomyelia, and Chiari malformation, are often found in small dogs. Although studies in human medicine have revealed a correlation with cerebrospinal fluid flow in these diseases by magnetic resonance imaging, there is little information and no standard data for normal dogs.ObjectivesThe purpose of this study was to obtain cerebrospinal fluid flow velocity data from the cerebral aqueduct and subarachnoid space at the foramen magnum in healthy beagle dogs.MethodsSix healthy beagle dogs were used in this experimental study. The dogs underwent phase-contrast and time-spatial labeling inversion pulse magnetic resonance imaging. Flow rate variations in the cerebrospinal fluid were observed using sagittal time-spatial labeling inversion pulse images. The pattern and velocity of cerebrospinal fluid flow were assessed using phase-contrast magnetic resonance imaging within the subarachnoid space at the foramen magnum level and the cerebral aqueduct.ResultsIn the ventral aspect of the subarachnoid space and cerebral aqueduct, the cerebrospinal fluid was characterized by a bidirectional flow throughout the cardiac cycle. The mean ± SD peak velocities through the ventral and dorsal aspects of the subarachnoid space and the cerebral aqueduct were 1.39 ± 0.13, 0.32 ± 0.12, and 0.76 ± 0.43 cm/s, respectively.ConclusionsNoninvasive visualization of cerebrospinal fluid flow movement with magnetic resonance imaging was feasible, and a reference dataset of cerebrospinal fluid flow peak velocities was obtained through the cervical subarachnoid space and cerebral aqueduct in healthy dogs.

HighlightsWhat are the main findings?Cine phase contrast MRI for time-resolved assessment of leg muscle function during voluntary and electrically evoked motion was feasible in typically developing children.The success rates of cine phase contrast MRI during both voluntary and electrically evoked leg motion were reduced in pediatric patients with cerebral palsy.What is the implication of the main finding?Further optimization of the electrical muscle stimulation protocol, the voluntary motion task, and the MRI acquisition—particularly with regard to scan acceleration—is required.These improvements would enable cine phase-contrast MRI to serve as a feasible method for studying muscle dynamics and assessing treatment effects in pediatric patients with neuromuscular diseases.Background/Objectives: Magnetic resonance imaging (MRI) can be used to assess muscle function while performing a motion task within the scanner. Quantitative measures such as contraction velocity and strain can be derived from the images. Cine phase contrast (PC) MRI for time-resolved imaging of muscle function relies on the consistently repeated execution of the motion task for several minutes until data acquisition is complete. This may be difficult for patients with neuromuscular dysfunctions. To date, this approach has been applied only in adults, but not pediatric populations. The aim of this pilot study was to investigate the feasibility of PC MRI for assessing calf muscle function during electrically evoked and voluntary motion in children with cerebral palsy (CP) using open-source hardware and software. Methods: Cine PC MRI was performed at 3T in ambulatory pediatric patients with CP and typically developing children under electrical muscle stimulation (EMS) (n = 14/13) and during voluntary plantarflexion (n = 4/4) using a home-built pedal with a force sensor. A visual feedback software was developed to enable synchronized imaging of voluntary muscle contractions. Muscle contraction velocity and strain were calculated from the MRI data. Data quality was rated by two readers. Results: During EMS, the velocity data quality was rated as sufficient in 21% of scans in patients compared with 82% of scans in controls. During the voluntary task, all patients demonstrated increased compliance and greater generated force output than during EMS. Voluntary motion imaging was successful in all controls but none of the patients, as motion periodicity in patients was worse during voluntary than during stimulated contraction. Conclusions: Cine phase-contrast MRI combined with EMS or voluntary motion proved challenging in pediatric patients with CP, particularly in those with more severe baseline muscle dysfunction or reduced tolerance to stimulation. In contrast, the approach was successfully implemented in typically developing children. Although the scope of the patient-based findings is limited by data heterogeneity, the method demonstrates considerable potential as a tool for monitoring treatment-related changes in muscle function, particularly in less severely affected patients. Further refinement of the EMS and voluntary motion protocols, together with a reduction in MRI acquisition time, is required to improve motion periodicity, tolerability, and consequently the overall success rate in the intended pediatric patient cohort.

Fully automated on-scanner aortic four-dimensional flow magnetic resonance imaging processing and hemodynamic analysis

Rapid slice-to-volume four-dimensional flow in pediatric congenital heart disease: a feasibility study

A segmented k-space acquisition technique using noninterleaved velocity encodings is presented to reduce spatial and temporal blur in phase-contrast cardiovascular magnetic resonance imaging. A translating phantom with pulsatile flow was used to simulate imaging of coronary arteries on a 1.5-T GE Echospeed scanner, using both interleaved and noninterleaved velocity encodings. The results demonstrate that the use of noninterleaved velocity encodings reduces spatial and temporal blur by improving the temporal resolution.

A method to implement the reservoir-wave hypothesis using phase-contrast magnetic resonance imaging

Diastolic dysfunction, a leading cause of heart failure in the US, is a complex pathology which manifests morphological and hemodynamic changes in the heart and circulatory system. Recent advances in time-resolved phase-contrast cardiac magnetic resonance imaging (4D Flow) have allowed for characterization of blood flow in the right ventricle (RV) and right atrium (RA), including calculation of vorticity and qualitative visual assessment of coherent flow patterns. We hypothesize that right ventricular diastolic dysfunction (RVDD) is associated with changes in vorticity and right heart blood flow. This paper presents background on RVDD, and 4D Flow tools and techniques used for quantitative and qualitative analysis of cardiac flows in the normal and disease states. In this study, 20 patients with RVDD and 14 controls underwent cardiac 4D Flow and echocardiography. A method for determining the time-step for peak early diastole using 4D Flow data is described. Spatially integrated early diastolic vorticity was extracted from the RV, RA, and combined RV/RA regions of each subject using a range of vorticity thresholding and scaling methods. Statistically significant differences in vorticity were found in the RA and combined RA/RV in RVDD subjects compared to controls when vorticity vectors were both thresholded and scaled by cardiac index.

The aim of this study was to evaluate the application of phase-contrast cine magnetic resonance imaging (MRI) in endoscopic aqueductoplasty (EA) for patients with obstructive hydrocephalus. The clinical diagnosis of hydrocephalus caused by aqueduct obstruction in 23 patients was confirmed by phase-contrast cine MRI examination. The patients were treated with EA and MRI was repeated during the follow-up. The cerebrospinal fluid (CSF) flow velocity in the aqueduct was measured to determine whether the aqueduct was obstructed. The results of phase-contrast cine MRI examinations indicated that there was no CSF flow in the aqueduct for all patients prior to surgery. Aqueductoplasty was successfully performed in all patients. The results of phase-contrast cine MRI examinations performed a week after surgery demonstrated an average CSF flow velocity of 4.74±1.77 cm/sec. During the follow-up, intracranial hypertension recurred in two patients in whom CSF flow was not observed in the aqueduct by the phase-contrast cine MRI scan. Aqueduct re-occlusion was revealed by an endoscopic exploration. By measuring the CSF flow velocity, phase-contrast cine MRI accurately identifies aqueduct obstruction. Cine MRI is a nontraumatic, simple and reliable method for determining whether the aqueduct is successfully opened following aqueductoplasty.

Background Phase-contrast magnetic resonance imaging (PC-MRI) provides a powerful method for the quantification of blood velocity. Accuracy of flow measurement with PCMRI has been validated with several techniques such as Doppler ultrasound and electromagnetic flowmeters. However, these methods suffer from low accuracy, especially in pulsating flows where short response times are required. Methods Herein, a series of detailed experiments are reported for validation of MR measurements of steady and pulsatile flows with stereoscopic particle image velocimetry (SPIV) on three different stenotic models with 50%, 74%, and 87% area occlusions. Mean inlet Reynolds number was 190 for both steady and pulsatile cases, mimicking the flow of the human common iliac artery. Axial PC-MRI images were acquired at three sites: inlet (two diameters proximal to the stenosis), throat, and outlet (two diameters distal to the stenosis) using a 3T TX Achieva Philips MRI scanner with slice thickness = 4 mm, resolution = 1 × 1 mm, TE/TR = 3.0/4.0 ms, field of view =6 4 ×6 4 mm, and velocity encoding (Venc) = 30-200 cm/s depending on the imaging section. For SPIV purposes, a laser light sheet was passed perpendicular to the axis of the phantom to illuminate the flowing fluorescent particles (Fig 1). A set of image pairs were captured using two cameras looking at the phantom at different angles and the fluid velocity was extracted using a cross-correlation scheme, yielding a nominal spatial resolution of 0.2 mm for the velocity data. The temporal resolution of pulsatile flow measurements was 25 ms, corresponding to 40 measurements per second. Results Agreement between PC-MRI and SPIV was demonstrated for both steady and pulsatile flow measurements at the inlet by evaluating the linear regression between the two methods, which showed a correlation coefficient of >0.99 and >0.96 for steady and pulsatile flows, respectively. The difference between SPIV and PC-MRI measurements for steady and pulsatile mean flows was less than 5% for both inlet and throat and showed good agreement in all cases (Fig 2). The agreement, however, was weaker at the outlet especially for the 87% stenosis. The flow rate error distal to the stenosis was shown to be a function of narrowing severity. Conclusions Our experiments revealed that the most accurate measures of flow by PC-MRI are found at the throat of the stenosis. This study also illustrates that SPIV provides an excellent approach to in-vitro validation of new or existing PC-MRI flow measurement techniques. Funding

Staged palliation with the creation of a Fontan circulation is the standard surgical approach in patients with a single ventricle. The Fontan circulation is a complex circuit that is associated with various complications that may present early or later in life and can limit life quality and expectancy. In this context, a good understanding of the Fontan physiology is important to improve outcomes for single-ventricle patients. Cardiovascular magnetic resonance (CMR) is recommended for the long-term follow-up of Fontan patients, as it provides functional and haemodynamic information. Four-dimensional (4D) Flow MRI is a time-resolved, three-dimensional, velocity-encoded cardiovascular magnetic resonance technique that is increasingly used in Fontan patients because it not only enables measuring blood flow within a three-dimensional (3D) volume, but also allows for assessing more advanced haemodynamic parameters that may help in understanding the Fontan physiology and pathophysiology. Furthermore, 4D Flow is used for image-based simulations using computational fluid dynamics. In this review, we provide an overview of the use of cardiovascular magnetic resonance flow assessment, with a focus on four-dimensional flow ('4D Flow').

Background Transjugular intrahepatic portosystemic shunt (TIPS) dysfunction in patients with liver cirrhosis and recurrent symptoms of portal hypertension is primarily assessed with US and confirmed with invasive catheter venography, which can be used to measure the portosystemic pressure gradient (PSPG) to identify TIPS-refractory portal hypertension. To avoid the risks and costs of invasive catheter venography, noninvasive PSPG evaluation strategies are needed. Purpose To demonstrate the feasibility of the combination of four-dimensional (4D) flow MRI with computational fluid dynamics (CFD) for noninvasive PSPG assessment in participants with cirrhosis and TIPS. Materials and Methods Abdominal 4D flow MRI was performed prospectively in participants with cirrhosis and TIPS between January 2019 and September 2020. Flow rates were measured within the TIPS and inferior vena cava (IVC). The portal vein (PV), TIPS, right hepatic vein, and IVC were segmented on MRI scans to create a CFD mesh. The PV and infrahepatic IVC were defined as inflows for 4D flow MRI-derived flow rates. The suprahepatic IVC was defined as the outflow. CFD simulations were used to noninvasively estimate PSPG as the difference between the simulated pressures in the PV and suprahepatic IVC. Invasive venographic measurements of the PSPG served as the reference standard, and Pearson correlation analysis was conducted to evaluate the relationship between noninvasive estimates and invasive measurements. Results In all 20 participants with cirrhosis (mean age, 58 years ± 9 [SD]; 11 men), 4D flow MRI-based CFD simulations enabled visualization of flow velocities and pressure distributions within the segmented vasculature and TIPS. Noninvasive estimates and invasive measures of PSPG were strongly correlated (r = 0.77; P < .001). The 4D flow MRI-based CFD simulations correctly classified the presence or absence of a post-TIPS PSPG greater than 12 mm Hg in 16 of 20 participants (80%). Conclusion The combination of 4D flow MRI and CFD was feasible for noninvasive PSPG assessment in participants with cirrhosis, portal hypertension, and TIPS. © RSNA, 2024 See also the editorial by Motosugi and Watanabe in this issue.

The hemodynamic changes that occur after high-flow (extracranial-intracranial) EC-IC bypass surgery with internal carotid artery (ICA) ligation are not well known. To assess blood flow changes after high-flow EC-IC bypass with ICA ligation by time-resolved 3-dimensional phase-contrast (4D Flow) magnetic resonance imaging (MRI). We enrolled 11 patients who underwent high-flow EC-IC bypass. 4D Flow MRI was performed before and after surgery to quantify the blood flow volume (BFV) of the ipsilateral ICA (BFViICA), bypass artery (BFVbypass), contralateral ICA (BFVcICA), and basilar artery (BFVBA). Subsequently, we calculated the total BFV (BFVtotal=BFViICA+BFVcICA+BFVBA [before surgery], BFVcICA+BFVBA+BFVbypass [after surgery]). The BFV changes after bypass was statistically analyzed. BFVbypass was slightly lower than BFViICA, but the difference was not statistically significant (3.84±0.94 vs 4.42±1.38 mL/s). The BFVcICA and BFVBA significantly increased after bypass surgery (BFVcICA 5.89±1.44 vs 7.22±1.37 mL/s [P=.0018], BFVBA 3.06±0.41 vs 4.12±0.38 mL/s [P<.001]). The BFVtotal significantly increased after surgery (13.37±2.58 vs 15.18±1.77 mL/s [P=.015]). There was no evidence of hyperperfusion syndrome in any cases. After high-flow EC-IC bypass with permanent ICA ligation, the bypass artery could partially compensate for the loss of BFV of the sacrificed ICA. The increased flow of the contralateral ICA and BA supply collateral blood flow. Clinically irrelevant hyperperfusion was observed.