Physics-informed graph neural networks for flow field estimation in carotid arteries.

The article "Assessment of superior mesenteric vascular flow quantitation in children using four-dimensional flow magnetic resonance imaging" suggests to use of four-dimensional (4D) flow magnetic resonance imaging (MRI) which is also to measure the blood flow in the superior mesenteric vein (SMV) in pediatric patients over the traditional method. The study focuses on assessing the potential of SMV and superior mesenteric artery (SMA) flow quantification in children utilizing 4D flow MRI. It included 9 pediatric patients aged 18 years and below where 5 were male and 4 were female patients, on whom magnetic resonance enterorrhaphy (MRE) with 4D flow MRI protocol was used. Statistical analysis was performed using MedCalc. Measurements of SMV and SMA between two readers were calculated using Bland-Altman analysis. The results stated that six patients showed no MRE evidence of active inflammatory bowel disease, two patients showed unmarkable bowel appearance on MRI and one patient showed normal MRE without endoscopy performed at the same timeframe. The study utilized available 4D flow MRI sequences in this study aiming to show the feasibility of 4D flow quantitation of SMA and SMV flow in pediatric patients. The study also discovered good agreement for both peak velocity and peak speed measurements of SMA and SMV.

ABSTRACTPurposeIdentifying factors associated with left ventricular (LV) remodeling is important for risk stratification of patients with moderate aortic stenosis (AS). The aim of this preliminary study was to explore aortic hemodynamics in these patients and assess their relationships with LV remodeling using four‐dimensional (4D) flow magnetic resonance imaging (MRI).MethodData from 17 patients with moderate AS involving the tricuspid aortic valves and normal LV ejection fraction (EF > 55%) were analyzed. All patients underwent 4D flow MRI of the ascending aorta and cardiac magnetic resonance (CMR) on the same day within 1 month of being diagnosed with moderate AS by transthoracic Doppler echocardiography. 4D flow MRI aortic parameters, including turbulent kinetic energy (TKE), both total and peak; peak wall shear stress (WSS); and the vortex and helix flow severity, were assessed. CMR measures comprised LV structure and function, late gadolinium enhancement (LGE), myocardial native T1 mapping, and extracellular volume fraction.ResultsThe minimum and maximum values of total TKE, peak TKE, and peak WSS were 39.4/160.3 mJ, 876.6/2059.6 J/m3, and 2.0/4.9 Pa, respectively. More than 50% of the patients had vortex and helix flow of Grade 2. Peak TKE significantly correlated with LV end‐diastolic volume index (r = 0.675, p = 0.003), LV end‐systolic volume index (r = 0.666, p = 0.003), and LV mass index (r = 0.653, p = 0.004). LGE was not observed, and peak TKE also showed an inverse correlation with native T1 value (r = −0.571, p = 0.017).ConclusionsAmong patients with moderate AS, 4D flow MRI revealed heterogeneous aortic hemodynamic profiles and peak TKE was a significant hemodynamic parameter related to early LV remodeling. However, these findings need to be confirmed in further studies with larger sample sizes and a long‐term serial follow‐up data.

The effectiveness of four-dimensional (4D) flow magnetic resonance imaging (MRI) for assessing hemodynamic changes before and after balloon-occluded retrograde transvenous obliteration (BRTO) remains unclear. To evaluate the feasibility of 4D flow MRI for assessing hemodynamic changes in the portal venous system before and after BRTO. We included 10 patients (7 men, 3 women; mean age = 67 years) with liver cirrhosis who had a high risk of gastric variceal bleeding or hepatic encephalopathy. Non-contrast 4D flow MRI of the upper abdomen was performed before and after BRTO. In addition, we compared the blood flow rates in the portal vein (PV), superior mesenteric vein (SMV), splenic vein (SV), left renal vein, and inferior vena cava before and after BRTO. Moreover, the flow directions of the SMV and SV before and after BRTO were assessed using both portography and 4D flow MRI. There was a significant post-BRTO increase in the blood flow rate in the PV and SV (P < 0.05). There was no significant post-BRTO change in the blood flow rates in the SMV, inferior vena cava, and left renal vein. In four patients, portography confirmed that hepatofugal flow in the SV and SMV changed to hepatopetal flow after BRTO. Moreover, 4D flow MRI correctly assessed the flow directions in the SMV and SV in 70%-100% of the patients. 4D flow MRI can be used to detect hemodynamic changes in the portal venous system before and after BRTO.

Since its introduction in the 1970's, magnetic resonance imaging (MRI) has become a standard imaging modality. With its broad and standardized application, it is firmly established in the clinical routine and an essential element in cardiovascular and abdominal imaging. In addition to sonography and computer tomography, MRI is a valuable tool for diagnosing cardiovascular and abdominal diseases, for determining disease severity, and for assessing therapeutic success. MRI techniques have improved over the last few decades, revealing not just morphologic information, but functional information about perfusion, diffusion and hemodynamics as well. Four-dimensional (4D) flow MRI, a time-resolved phase contrast-MRI with three-dimensional (3D) anatomic coverage and velocity encoding along all three flow directions has been used to comprehensively assess complex cardiovascular hemodynamics in multiple regions of the body. The technique enables visualization of 3D blood flow patterns and retrospective quantification of blood flow parameters in a region of interest. Over the last few years, 4D flow MRI has been increasingly performed in the abdominal region. By applying different acceleration techniques, taking 4D flow MRI measurements has dropped to a reasonable scanning time of 8 to 12 min. These new developments have encouraged a growing number of patient studies in the literature validating the technique's potential for enhanced evaluation of blood flow parameters within the liver's complex vascular system. The purpose of this review article is to broaden our understanding of 4D flow MRI for the assessment of liver hemodynamics by providing insights into acquisition, data analysis, visualization and quantification. Furthermore, in this article we highlight its development, focussing on the clinical application of the technique.

BackgroundFour dimensional (4D) flow magnetic resonance imaging (MRI) enables both visualisation and quantification of hemodynamics in abdominal aortic aneurysms (AAAs). However, its clinical implementation remains limited due to time-consuming and cumbersome post-processing, as well as a lack of standardisation. While previous work has proposed automated solutions for thoracic and intracranial cases, applications in the abdominal region, particularly for AAAs, remain underexplored. The aim of this study was to develop an automated post-processing pipeline for 4D flow MRI in AAA patients, incorporating aneurysm identification, via automated segmentation, blood flow visualisation and hemodynamic parameter quantification.Methods4D flow MRI and three dimensional (3D) cine balanced steady state free precession (bSSFP) scans were acquired from 16 patients with an AAA in the same field of view and with an isotropic resolution of 1 mm. Using 5-fold cross-validation, an nnU-Net was trained to segment the aorta based on 3D cine bSSFP MRI. The automated post-processing pipeline was built using MATLAB. Wall shear stress (WSS) values were assessed within the extracted aneurysm. nnU-Net performance was assessed with the Dice-similarity coefficient (DSC), 95% percentile Hausdorff distance (HD95) and by calculating the Pearson correlation coefficients between WSS values obtained in the aneurysm based on segmentation with both methods.ResultsThe resulting post-processing pipeline demonstrated robustness to anatomical variability, including tortuosity and intraluminal thrombus (ILT), both characteristic of AAA, and consistently produced output without user-interference. The nnU-Net performance was excellent for lumen segmentation and good for ILT segmentation, with DSCs of 0.93 (0.05) and 0.80 (0.26) respectively. The HD95 was 4.87 (8.59) mm and 6.04 (3.71) mm for respectively lumen and thrombus segmentation. Significant correlations (ρ =0.92 or higher) were found between WSS values derived using the manual and nnU-Net segmentations.ConclusionsAn automated post-processing pipeline was developed specifically for 4D flow MRI in patients with AAA. The pipeline is robust for varying anatomies and may facilitate 4D flow MRI implementation in the clinical workflow.

BackgroundChanges in blood flow after brain aneurysm treatment are linked to treatment success. While 4D flow magnetic resonance imaging (MRI) can measure these changes, it is sensitive to metal artifacts from implants. Black-blood MRI, less affected by artifacts, may complement 4D flow MRI. We investigated whether changes in black-blood signal reflect reductions in blood flow and could indicate the success of aneurysm treatment.MethodsWe performed 22 flow experiments using 3D printed models of two patient-derived brain aneurysms and two straight vessels. Flow-modulating devices, including flow-diverter stents and intrasaccular devices, were inserted into 15 aneurysm models, while untreated models served as controls. All models were imaged with 4D flow and black-blood MRI on a 3 T clinical system. Numerical flow simulations were also performed. Blood flow velocity and black-blood signal were compared between treated and untreated models using non-parametric statistical tests, and their relationship was evaluated with correlation analysis.ResultsHere we show that the black-blood signal in straight vessels decreases with increasing velocity (rho = −0.92, p-value = 3.29E-04). Implanted devices reduce blood flow within aneurysms while leaving flow in the parent vessels essentially unchanged. Treated aneurysms exhibit a significant increase in black-blood signal, which correlates inversely with measured velocity (rho = −0.82, p-value = 6.24E-04). The experimentally observed flow reductions match the numerical simulations. Metal artifacts are more pronounced on 4D flow compared to black-blood MRI.ConclusionsBlack-blood MRI may serve as a surrogate marker of blood flow reduction after aneurysm treatment, particularly in cases where metal artifacts limit conventional imaging.

Our goal was to study both right and left ventricular blood flow in patients with precapillary pulmonary hypertension (pre-PH) with 4-dimensional (4D) flow magnetic resonance imaging (MRI) and to analyze their correlation with cardiac functional metrics on cardiovascular magnetic resonance (CMR) and hemodynamics from right heart catheterization (RHC). 129 patients (64 females, mean age 47 ± 13 y) including 105 patients with pre-PH (54 females, mean age 49 ± 13 y) and 24 patients without PH (10 females, mean age 40 ± 12 y) were retrospectively included. All patients underwent CMR and RHC within 48 hours. 4D flow MRI was acquired using a 3-dimensional retrospectively electrocardiograph-triggered, navigator-gated phase contrast sequence. Right and left ventricular flow components including the percentages of direct flow (PDF), retained inflow (PRI), delayed ejection flow (PDE), and residual volume (PRVo) were respectively quantified. The ventricular flow components between patients with pre-PH and non-PH were compared and correlations of flow components with CMR functional metrics and hemodynamics measured with RHC were analyzed. Biventricular flow components were compared between survivors and deceased patients during the perioperative period. Right ventricular (RV) PDF and PDE significantly correlated with RVEDV and RV ejection fraction. RV PDF negatively correlated with pulmonary arterial pressure (PAP) and pulmonary vascular resistance. When the RV PDF was <11%, the sensitivity and specificity of RV PDF for predicting mean PAP ≥25 mm Hg were 88.6% and 98.7%, respectively, with an area under the curve value of 0.95 ± 0.02. When RV PRVo was more than 42%, the sensitivity and specificity of RV PRVo for predicting mean PAP ≥25 mm Hg were 85.7% and 98.5%, respectively, with an area under the curve value of 0.95 ± 0.01. Nine patients died during the perioperative period. Biventricular PDF, RV PDE, and PRI of survivors were higher than nonsurvivors whereas RV PRVo increased in deceased patients. Biventricular flow analysis with 4D flow MRI provides comprehensive information about the severity and cardiac remodeling of PH and may be a predictor of perioperative death of patients with pre-PH.

Interval cross-sectional imaging plays an important role in aortopathy surveillance. Often, cardiac magnetic resonance imaging (MRI) is used over computed tomography (CT) due to the lack of radiation in repeated surveillance and the option of additional hemodynamic assessment. Primarily, assessment includes the orthogonal measurement of aortic dimensions in a three-dimensional (3D) structure. Lately, four-dimensional (4D) flow MRI is becoming more widespread as it can be acquired within 1–5 minutes using advanced techniques. In addition to standard flow quantification, 4D flow MRI offers advanced hemodynamic quantification. This review discusses important advanced imaging biomarkers, including helical flow pattern, wall shear stress (WSS), flow displacement and systolic flow reversal ratio (sFRR). It focuses on those parameters that can be analyzed using commercially available post-processing platforms and are accessible for clinical centers without the need for research setup and collaboration. WSS plays a role in the assessment of bicuspid aortic valve disease. Here it is elevated even without the presence of stenosis. Flow displacement is also of value in bicuspid aortic valve disease and is abnormal in heart failure with preserved ejection fraction (HFpEF) as well as chronic aortic dissection. 4D flow MRI is also useful in understanding and assessing flow changes in aortic valve replacement.

Hemodynamic evaluation using four-dimensional flow magnetic resonance imaging for a patient with multichanneled aortic dissection

In Vitro Modeling of Human Brain Arteriovenous Malformation for Endovascular Simulation and Flow Analysis

We investigated the reduction in regional brain volume and cerebral blood flow (CBF) with aging and explored potential sex differences in healthy brains. Three-dimensional (3D) T1-weighted magnetic resonance imaging (MRI), time-of-flight magnetic resonance angiography, and four-dimensional (4D) flow MRI were performed on 129 healthy volunteers aged 22-92 years. The brains of healthy volunteers were segmented into 21 subregions using 3D T1-weighted MRI and CBFs in 16 major intracranial arteries were measured using 4D flow MRI. The cortical gray matter volume decreased linearly with aging, whereas the cerebral white matter volume increased until the 40s and then decreased, and the subcortical gray matter volume changed little with aging. The cortical gray matter volume was significantly associated with the total CBF of the major intracranial arteries distal to the circle of Willis; however, the cerebral white matter and subcortical gray matter volumes were not. Generally, women have higher total CBF than men, particularly in their 40s and younger, despite the smaller intracranial volume and smaller diameters of intracranial arteries than men. This may contribute to the higher incidence of subarachnoid hemorrhage due to cerebral aneurysms and migraine in women.

To evaluate the inflow pattern and flow quantification in patients with functional univentricular heart after Fontan's operation using 4D flow magnetic resonance imaging (MRI) with streamline visualization when compared with the conventional 2D flow approach. Seven patients with functional univentricular heart after Fontan's operation and twenty-three healthy controls underwent 4D flow MRI. In two orthogonal two-chamber planes, streamline visualization was applied, and inflow angles with peak inflow velocity (PIV) were measured. Transatrioventricular flow quantification was assessed using conventional 2D multiplanar reformation (MPR) and 4D MPR tracking the annulus and perpendicular to the streamline inflow at PIV, and they were validated with net forward aortic flow. Inflow angles at PIV in the patient group demonstrated wide variation of angles and directions when compared with the control group (P < .01). The use of 4D flow MRI with streamlines visualization in quantification of the transatrioventricular flow had smaller limits of agreement (2.2 ± 4.1 mL; 95% limit of agreement -5.9-10.3 mL) when compared with the static plane assessment from 2DFlow MRI (-2.2 ± 18.5 mL; 95% limit of agreement agreement -38.5-34.1 mL). Stronger correlation was present in the 4D flow between the aortic and trans-atrioventricular flow (R2 correlation in 4D flow: 0.893; in 2D flow: 0.786). Streamline visualization in 4D flow MRI confirmed variable atrioventricular inflow directions in patients with functional univentricular heart with previous Fontan's procedure. 4D flow aided generation of measurement planes according to the blood flood dynamics and has proven to be more accurate than the fixed plane 2D flow measurements when calculating flow quantifications.

Introduction: Postoperative anticoagulation therapy after atrial fibrillation (AF) surgery remains a controversial topic because left atrial (LA) blood flow velocity and pattern remain unclear. This study utilizes four-dimensional (4D) flow magnetic resonance imaging (MRI) to analyze postoperative LA hemodynamics following various atrial fibrillation surgeries. Methods: This retrospective study included nine patients (mean age, 73 ± 8 years; range, 56-83 years) who underwent AF surgeries from August 2021 to June 2022. Maze surgery was performed in 3, pulmonary vein isolation (PVI) and left atrial appendage closure (LAAC) in 3, and the LAAC alone in 3 patients. Postoperative 4D flow MRI scans of the left atrium were performed, and image analysis was conducted. Blood flow velocity in the left atrium was measured, with velocities below 10 cm/s considered flow stasis. The percentage of flow stasis within one systolic-diastolic cycle was evaluated as % stasis. Furthermore, the presence or absence of a physiological vortex flow was assessed from the flow images. Results: Sinus rhythm was postoperatively maintained in 6 patients, and 3 patients were analyzed in AF. The left atrial % stasis was significantly low in the sinus rhythm group (46.0 ± 22.7 in the sinus rhythm group vs. 100 in the AF group, p=0.027). The absence of vortex flow was observed during sinus rhythm in one patient after Maze surgery. Conversely, vortex flow was identified during AF in the patient after LAAC, being accompanied by moderate mitral regurgitation. Conclusions: Any type of AF surgery does not always provide physiological LA blood flow. Further, 4D flow MRI analysis contributes to our understanding of the discrepancy between ideals and reality which may or may not require anticoagulant therapy regardless of postoperative rhythm.

Atrial fibrillation (AF) is characterized by rapid and irregular contraction of the left atrium (LA). Impacting LA haemodynamics, this increases the risk of thrombi development and stroke. Flow conditions preceding stroke in these patients are not well defined, partly due the limited resolution of 4D flow magnetic resonance imaging (MRI). In this study, we combine a high-resolution computed tomography (CT) LA reconstruction with motion and pulmonary inflows from 4D flow MRI to create a novel multimodal computational fluid dynamics (CFD) model, applying it to five AF patients imaged in sinus rhythm (24 ± 39 days between acquisitions). The dynamic model was compared with a rigid wall equivalent and the main flow structures were validated with 4D flow MRI. Point-by-point absolute differences between the velocity fields showed moderate differences given the sensitivity to registration. The rigid wall model significantly underestimated LA time-averaged wall shear stress (TAWSS) (p = 0.02) and oscillatory shear index (OSI) (p = 0.02) compared to the morphing model. Similarly, in the left atrial appendage (LAA), TAWSS (p = 0.003) and OSI (p < 0.001) were further underestimated. The morphing model yielded a more accurate mitral valve waveform and showed low TAWSS and high OSI in the LAA, both associated with thrombus formation. We also observed a positive correlation between indexed LA volume and endothelial cell activation potential (ECAP) (R2 = 0.83), as well as LAA volume and LAA OSI (R2 = 0.70). This work demonstrates the importance of LA motion in modelling LAA flow. Assessed in larger cohorts, LAA haemodynamic analysis may be beneficial to refine stroke risk assessment for AF.

Traditionally, magnetic resonance imaging (MRI) of flow using phase contrast (PC) methods is accomplished using methods that resolve single-directional flow in two spatial dimensions (2D) of an individual slice. More recently, three-dimensional (3D) spatial encoding combined with three-directional velocity-encoded phase contrast MRI (here termed 4D flow MRI) has drawn increased attention. 4D flow MRI offers the ability to measure and to visualize the temporal evolution of complex blood flow patterns within an acquired 3D volume. Various methodological improvements permit the acquisition of 4D flow MRI data encompassing individual vascular structures and entire vascular territories such as the heart, the adjacent aorta, the carotid arteries, abdominal, or peripheral vessels within reasonable scan times. To subsequently analyze the flow data by quantitative means and visualization of complex, three-directional blood flow patterns, various tools have been proposed. This review intends to introduce currently used 4D flow MRI methods, including Cartesian and radial data acquisition, approaches for accelerated data acquisition, cardiac gating, and respiration control. Based on these developments, an overview is provided over the potential this new imaging technique has in different parts of the body from the head to the peripheral arteries.