Visualizing and quantifying flow stasis in abdominal aortic aneurysms in men using 4D flow MRI

Subjects with carotid webs demonstrate pro-thrombotic hemodynamics compared to subjects with carotid atherosclerosis

Carotid artery webs (CaW) are non-atherosclerotic projections into the vascular lumen and have been linked to up to one-third of cryptogenic strokes in younger patients. Determining how CaW affects local hemodynamics is essential for understanding clot formation and stroke risk. Computational fluid dynamics simulations were used to investigate patient-specific hemodynamics in carotid artery bifurcations with CaW, bifurcations with atherosclerotic lesions having a similar degree of lumen narrowing, and with healthy carotid bifurcations. Simulations were conducted using segmented computed tomography angiography geometries with inlet boundary conditions extracted from 2D phase contrast MRI scans. The study included carotid bifurcations with CaW (n = 13), mild atherosclerosis (n = 7), and healthy bifurcation geometries (n = 6). Hemodynamic parameters associated with vascular dysfunction and clot formation, including shear rate, oscillatory shear index (OSI), low velocity, and flow stasis were calculated and compared between the subject groups. Patients with CaW had significantly larger regions containing low shear rate, high OSI, low velocity, and flow stasis in comparison to subjects with mild atherosclerosis or normal bifurcations. These abnormal hemodynamic metrics in patients with CaW are associated with clot formation and vascular dysfunction and suggest that hemodynamic assessment may be a tool to assess stroke risk in these patients.

Regional and Global Aortic Pulse Wave Velocity in Patients with Abdominal Aortic Aneurysm

OBJECTIVE

Abdominal aortic aneurysm (AAA) is commonly defined as localised aortic dilatation with a diameter > 30 mm. The pathophysiology of AAA includes chronic inflammation and enzymatic degradation of elastin, possibly increasing aortic wall stiffness and pulse wave velocity (PWV). Whether aortic stiffness is more prominent in the abdominal aorta at the aneurysm site is not elucidated. The aim of this study was to evaluate global and regional aortic PWV in patients with AAA.

METHODS

Experimental study of local PWV in the thoracic descending and abdominal aorta in patients with AAA and matched controls. The study cohort comprised 25 patients with an AAA > 30 mm (range 36 - 70 mm, all male, age range 65 - 76 years) and 27 age and sex matched controls free of AAA. PWV was measured with applanation tonometry (carotid-femoral PWV, cfPWV) as well as a 4D flow MRI technique, assessing regional aortic PWV. Blood pressure and anthropometrics were measured.

RESULTS

Global aortic PWV was greater in men with an AAA than controls, both by MRI (AAA 8.9 ± 2.4 m/s vs. controls 7.1 ± 1.5 m/s; p = .007) and cfPWV (AAA 11.0 ± 2.1 m/s vs. controls 9.3 ± 2.3 m/s; p = .007). Regionally, PWV was greater in the abdominal aorta in the AAA group (AAA 7.0 ± 1.8 m/s vs. controls 5.8 ± 1.0 m/s; p = .022), but similar in the thoracic descending aorta (AAA 8.7 ± 3.2 m/s vs. controls 8.2 ± 2.4 m/s; p = .59). Furthermore, PWV was positively associated with indices of central adiposity both in men with AAA and controls.

CONCLUSION

PWV is higher in men with AAA compared with matched controls in the abdominal but not the thoracic descending aorta. Furthermore, aortic stiffness was linked with central fat deposition. It remains to be seen whether there is a causal link between AAA and increased regional aortic stiffness.

False lumen hemodynamics and partial thrombosis in chronic aortic dissection of the descending aorta

OBJECTIVES

Partial thrombosis of the false lumen (FL) in patients with chronic aortic dissection (AD) of the descending aorta has been associated with poor outcomes. Meanwhile, the fluid dynamic and biomechanical characteristics associated with partial thrombosis remain to be elucidated. This retrospective, single-center study tested the association between FL fluid dynamics and biomechanics and the presence and extent of FL thrombus.

METHODS

Patients with chronic non-thrombosed or partially thrombosed FLs in the descending aorta after an aortic dissection underwent computed tomography angiography, cardiovascular magnetic resonance (CMR) angiography, and a 4D flow CMR study. A comprehensive quantitative analysis was performed to test the association between FL thrombus presence and extent (percentage of FL with thrombus) and FL anatomy (diameter, entry tear location and size), fluid dynamics (inflow, rotational flow, wall shear stress, kinetic energy, and flow acceleration and stasis), and biomechanics (pulse wave velocity).

RESULTS

Sixty-eight patients were included. In multivariate logistic regression FL kinetic energy (p = 0.038) discriminated the 33 patients with partial FL thrombosis from the 35 patients with no thrombosis. Similarly, in separated multivariate linear correlations kinetic energy (p = 0.006) and FL inflow (p = 0.002) were independently related to the extent of the thrombus. FL vortexes, flow acceleration and stasis, wall shear stress, and pulse wave velocity showed limited associations with thrombus presence and extent.

CONCLUSION

In patients with chronic descending aorta dissection, false lumen kinetic energy is related to the presence and extent of false lumen thrombus.

CLINICAL RELEVANCE STATEMENT

In patients with chronic aortic dissection of the descending aorta, false lumen hemodynamic parameters are closely linked with the presence and extent of false lumen thrombosis, and these non-invasive measures might be important in patient management.

KEY POINTS

• Partial false lumen thrombosis has been associated with aortic growth in patients with chronic descending aortic dissection; therefore, the identification of prothrombotic flow conditions is desirable. • The presence of partial false lumen thrombosis as well as its extent was related with false lumen kinetic energy. • The assessment of false lumen hemodynamics may be important in the management of patients with chronic aortic dissection of the descending aorta.

Current Imaging Strategies in Patients with Abdominal Aortic Aneurysms

BACKGROUND

 An abdominal aortic aneurysm (AAA) is defined as a localized dilatation of the abdominal aorta of ≥ 3 cm. With a prevalence of 4-8 %, AAA is one of the most common vascular diseases in Western society. Radiological imaging is an elementary component in the diagnosis, monitoring, and treatment planning of AAA patients.

METHOD

 This is a narrative review article on preoperative imaging strategies of AAA, incorporating expert opinions based on the current literature and standard-of-care practices from our own center. Examples are provided to illustrate clinical cases from our institution.

RESULTS AND CONCLUSION

 Radiological imaging plays a pivotal role in the initial diagnosis and monitoring of patients with AAA. Ultrasound is the mainstay imaging modality for AAA screening and surveillance. Contrast-enhanced CT angiography is currently considered the gold standard for preoperative imaging and image-based treatment planning in AAA repair. New non-contrast MR angiography techniques are robustly applicable and allow precise determination of aortic diameters, which is of critical importance, particularly with regard to current diameter-based surgical treatment guidelines. 3D imaging with multiplanar reformation and automatic centerline positioning enables more accurate assessment of the maximum aortic diameter. Modern imaging techniques such as 4D flow MRI have the potential to further improve individualized risk stratification in patients with AAA.

KEY POINTS

  · Ultrasound is the mainstay imaging modality for AAA screening and monitoring. · Contrast-enhanced CT angiography is the gold standard for preoperative imaging in AAA repair. · Non-contrast MR angiography allows for accurate monitoring of aortic diameters in AAA patients. · Measurement of aortic diameters is more accurate with 3D-CT/MRI compared to ultrasound. · Research seeks new quantitative imaging biomarkers for AAA risk stratification, e. g., using 4D flow MRI.

Four-Dimensional Flow MRI-Derived Hemodynamics in Abdominal Aortic Aneurysms: Reproducibility and Associations With Diameter, Intraluminal Thrombus Volume, and Vorticity

BACKGROUND

Maximum diameter measurements are used to assess the rupture risk of abdominal aortic aneurysms (AAAs); however, these are not precise enough to predict all ruptures. Four-dimensional (4D) flow MRI-derived parameters provide additional information by visualizing hemodynamics in AAAs but merit further investigation before they are clinically applicable.

PURPOSE

To assess the reproducibility of 4D flow MRI-derived hemodynamics, to investigate possible correlations with lumen and maximum diameter, and to explore potential relationships with vorticity and aneurysm growth.

STUDY TYPE

Prospective single-arm study.

POPULATION

A total of 22 (71.5 ± 6.1 years, 20 male) asymptomatic AAA patients with a maximum diameter of at least 30 mm.

FIELD STRENGTH/SEQUENCE

A 3.0 T/Free-breathing 4D flow MRI phase-contrast acquisition with retrospective ECG-gating.

ASSESSMENT

Patients underwent two consecutive 4D flow MRI scans 1-week apart. Aortic volumes were segmented from time-averaged phase contrast magnetic resonance angiographies. Reproducibility was assessed by voxelwise analysis after registration. Mean flow velocity, mean wall shear stress (WSS), mean lumen diameter, and qualitative vorticity scores were assessed. In addition, Dixon MRI and retrospective surveillance data were used to study maximum diameter (including thrombus), intraluminal thrombus volume (ILT), and growth rate.

STATISTICAL TESTS

For reproducibility assessment, Bland-Altman analyses, Pearson correlation, Spearman's correlation, and orthogonal regression were conducted. Potential correlations between hemodynamics and vorticity scores were assessed using linear regression. P < 0.05 was considered statistically significant.

RESULTS

Test-retest median Pearson correlation coefficients for flow velocity and WSS were 0.85 (IQR = 0.08) m/sec and 0.82 (IQR = 0.10) Pa, respectively. Mean WSS significantly correlated with mean flow velocity (R = 0.75) and inversely correlated with mean lumen diameter (R = -0.73). No significant associations were found between 4D flow MRI-derived hemodynamic parameters and maximum diameter (flow velocity: P = 0.98, WSS: P = 0.22).

DATA CONCLUSION

A 4D flow MRI is robust for assessing the hemodynamics within AAAs. No correlations were found between hemodynamic parameters and maximum diameter, ILT volume and growth rate.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 2.

Quantitative Magnetic Resonance Imaging to Assess Progression and Rupture Risk of Aortic Aneurysms: A Scoping Review

PURPOSE

In current practice, the diameter of an aortic aneurysm is utilized to estimate the rupture risk and decide upon timing of elective repair, although it is known to be imprecise and not patient-specific. Quantitative magnetic resonance imaging (MRI) enables the visualization of several biomarkers that provide information about processes within the aneurysm and may therefore facilitate patient-specific risk stratification. We performed a scoping review of the literature on quantitative MRI techniques to assess aortic aneurysm progression and rupture risk, summarized these findings, and identified knowledge gaps.

METHODS

Literature concerning primary research was of interest and the medical databases PubMed, Scopus, Embase, and Cochrane were systematically searched. This study used the PRISMA protocol extension for scoping reviews. Articles published between January 2010 and February 2023 involving animals and/or humans were included. Data were extracted by 2 authors using a predefined charting method.

RESULTS

A total of 1641 articles were identified, of which 21 were included in the scoping review. Quantitative MRI-derived biomarkers were categorized into hemodynamic (8 studies), wall (5 studies) and molecular biomarkers (8 studies). Fifteen studies included patients and/or healthy human subjects. Animal models were investigated in the other 6 studies. A cross-sectional study design was the most common, whereas 5 animal studies had a longitudinal component and 2 studies including patients had a prospective design. A promising hemodynamic biomarker is wall shear stress (WSS), which is estimated based on 4D-flow MRI. Molecular biomarkers enable the assessment of inflammatory and wall deterioration processes. The ADAMTS4-specific molecular magnetic resonance (MR) probe showed potential to predict abdominal aortic aneurysm (AAA) formation and rupture in a murine model. Wall biomarkers assessed using dynamic contrast-enhanced (DCE) MRI showed great potential for assessing AAA progression independent of the maximum diameter.

CONCLUSION

This scoping review provides an overview of quantitative MRI techniques studied and the biomarkers derived from them to assess aortic aneurysm progression and rupture risk. Longitudinal studies are needed to validate the causal relationships between the identified biomarkers and aneurysm growth, rupture, or repair. In the future, quantitative MRI could play an important role in the personalized risk assessment of aortic aneurysm rupture.

CLINICAL IMPACT

The currently used maximum aneurysm diameter fails to accurately assess the multifactorial pathology of an aortic aneurysm and precisely predicts rupture in a patient-specific manner. Quantitative magnetic resonance imaging (MRI) enables the detection of various quantitative parameters involved in aneurysm progression and subsequent rupture. This scoping review provides an overview of the studied quantitative MRI techniques, the biomarkers derived from them, and recommendations for future research needed for the implementation of these biomarkers. Ultimately, quantitative MRI could facilitate personalized risk assessment for patients with aortic aneurysms, thereby reducing untimely repairs and improving rupture prevention.

Particle Tracing Based on 4D Flow Magnetic Resonance Imaging: A Systematic Review into Methods, Applications, and Current Developments

BACKGROUND

Particle tracing based on 4D Flow MRI has been applied as a quantitative and qualitative postprocessing technique to study temporally evolving blood flow patterns.

PURPOSE

To systematically review the various methods to perform 4D Flow MRI-based particle tracing, as well as the clinical value, clinical applications, and current developments of the technique.

STUDY TYPE

The study type is systematic review.

SUBJECTS

Patients with cardiovascular disease (such as Marfan, Fontan, Tetralogy of Fallot), healthy controls, and cardiovascular phantoms that received 4D Flow MRI with particle tracing.

FIELD STRENGTH/SEQUENCE

Three-dimensional three-directional cine phase-contrast MRI, at 1.5 T and 3 T.

ASSESSMENT

Two systematic searches were performed on the PubMed database using Boolean operators and the relevant key terms covering 4D Flow MRI and particle tracing. One systematic search was focused on particle tracing methods, whereas the other on applications. Additional articles from other sources were sought out and included after a similar inspection. Particle tracing methods, clinical applications, clinical value, and current developments were extracted.

STATISTICAL TESTS

The main results of the included studies are summarized, without additional statistical analysis.

RESULTS

Of 127 unique articles retrieved from the initial search, 56 were included (28 for methods and 54 for applications). Most articles that described particle tracing methods used an adaptive timestep, a fourth order Runge-Kutta integration method, and linear interpolation in the time dimension. Particle tracing was applied in heart chambers, aorta, venae cavae, Fontan circulation, pulmonary arteries, abdominal vasculature, peripheral arteries, carotid arteries, and cerebral vasculature. Applications were grouped as intravascular, intracardiac, flow stasis, and research.

DATA CONCLUSIONS

Particle tracing based on 4D Flow MRI gives unique insight into blood flow in several cardiovascular diseases, but the quality depends heavily on the MRI data quality. Further studies are required to evaluate the clinical value of the technique for different cardiovascular diseases.

EVIDENCE LEVEL

5.

TECHNICAL EFFICACY

Stage 1.

Abdominal applications of quantitative 4D flow MRI

4D flow MRI is a quantitative MRI technique that allows the comprehensive assessment of time-resolved hemodynamics and vascular anatomy over a 3-dimensional imaging volume. It effectively combines several advantages of invasive and non-invasive imaging modalities like ultrasound, angiography, and computed tomography in a single MRI acquisition and provides an unprecedented characterization of velocity fields acquired non-invasively in vivo. Functional and morphological imaging of the abdominal vasculature is especially challenging due to its complex and variable anatomy with a wide range of vessel calibers and flow velocities and the need for large volumetric coverage. Despite these challenges, 4D flow MRI is a promising diagnostic and prognostic tool as many pathologies in the abdomen are associated with changes of either hemodynamics or morphology of arteries, veins, or the portal venous system. In this review article, we will discuss technical aspects of the implementation of abdominal 4D flow MRI ranging from patient preparation and acquisition protocol over post-processing and quality control to final data analysis. In recent years, the range of applications for 4D flow in the abdomen has increased profoundly. Therefore, we will review potential clinical applications and address their clinical importance, relevant quantitative and qualitative parameters, and unmet challenges.

Association between blood flow pattern and rupture risk of abdominal aortic aneurysm based on computational fluid dynamics

OBJECTIVES

This study aimed to derive a novel classification of blood flow pattern in AAA based on computational fluid dynamics, and determine the predicting value of flow pattern in abdominal aortic aneurysm (AAA) rupture.

DESIGN

Age, gender matched case-control study MATERIALS: Case patients were identified as those who underwent emergent endovascular or open repair due to ruptured or impending rupture AAA. Control patients were those age and gender matched AAA patients who were asymptomatic and confirmed unruptured from CTA images from the same period.

METHODS

Classification of blood flow pattern (Type I: non-helical main flow channel with multiple vortices; Type II: non-helical main flow channel with single vortices; Type III, helical main flow channel with helical vortices) and hemodynamic parameters [areas of low wall shear stress (A _{low WSS}), aneurysm pressure drop (Δ pressure), etc.] were derived from computational fluid dynamic (CFD) analyses. Multivariate regression was used to determine independent risk factors of AAA rupture. The incremental discriminant and reclassification abilities for AAA rupture were compared among different models.

RESULTS

This study included 53 ruptured and 53 intact AAA patients. Ruptured AAA showed higher prevalence of type III flow pattern (60.38% vs. 15.09%, P<.001) compared to intact AAA. Type III flow pattern was associated with a significantly increased risk of aneurysm rupture (OR 10.22, 95%CI 3.43-30.49). Among all predicting models, combination of AAA diameter, hemodynamic parameters (A _{low WSS} or Δ pressure) and flow pattern showed highest discriminant abilities in both overall population (concordance statistic [c-index] .862) and subgroup patients with AAAs <55mm (c-index .972). Compared to AAA diameter, adding flow pattern could significantly improve the reclassification abilities in both overall population (net reclassification index [NRI] .321; p<.001) and subgroup of AAAs < 55mm (NRI .732, P<.001).

CONCLUSION

Type III flow pattern was associated with a significantly increased risk of AAA rupture. Integration of blood flow pattern may improve the identification of high-risk aneurysms in both overall population and AAAs smaller than 55mm.

False lumen rotational flow and aortic stiffness are associated with aortic growth rate in patients with chronic aortic dissection of the descending aorta: a 4D flow cardiovascular magnetic resonance study

BACKGROUND

Patency of the false lumen in chronic aortic dissection (AD) is associated with aortic dilation and long-term aortic events. However, predictors of adverse outcomes in this population are limited. The aim of this study was to evaluate the relationship between aortic growth rate and false lumen flow dynamics and biomechanics in patients with chronic, patent AD.

METHODS

Patients with a chronic AD with patent false lumen in the descending aorta and no genetic connective tissue disorder underwent an imaging follow-up including a contrast-enhanced 4D flow cardiovascular magnetic resonance (CMR) protocol and two consecutive computed tomography angiograms (CTA) acquired at least 1 year apart. A comprehensive analysis of anatomical features (including thrombus quantification), and false lumen flow dynamics and biomechanics (pulse wave velocity) was performed.

RESULTS

Fifty-four consecutive patients with a chronic, patent false lumen in the descending aorta were included (35 surgically-treated type A AD with residual tear and 19 medically-treated type B AD). Median follow-up was 40 months. The in-plane rotational flow, pulse wave velocity and the percentage of thrombus in the false lumen were positively related to aortic growth rate (p = 0.006, 0.017, and 0.037, respectively), whereas wall shear stress showed a trend for a positive association (p = 0.060). These results were found irrespectively of the type of AD.

CONCLUSIONS

In patients with chronic AD and patent false lumen of the descending aorta, rotational flow, pulse wave velocity and wall shear stress are positively related to aortic growth rate, and should be implemented in the follow-up algorithm of these patients. Further prospective studies are needed to confirm if the assessment of these parameters helps to identify patients at higher risk of adverse clinical events.

Wall shear stress and relative residence time as potential risk factors for abdominal aortic aneurysms in males: a 4D flow cardiovascular magnetic resonance case-control study

BACKGROUND

Abdominal aortic aneurysms (AAA) can lead to catastrophic events such as dissection or rupture, and are an expression of general aortic disease. Low wall shear stress (WSS), high oscillatory shear index (OSI), and high relative residence time (RRT) have been correlated against increased uptake of inflammatory markers in the vessel wall and may improve risk stratification of AAA. We sought to obtain a comprehensive view of WSS, OSI, and RRT in the whole aorta for patients with AAA and age-matched elderly controls and young normal controls.

METHODS

4D Flow cardiovascular magnetic resonance images of the whole aorta were acquired in 18 AAA patients (70.8 ± 3.4 years), 22 age-matched controls (71.4 ± 3.4 years), and 23 young subjects (23.3 ± 3.1 years), all males. Three-dimensional segmentations of the whole aorta were created for all timeframes using a semi-automatic approach. The aorta was divided into five segments: ascending aorta, arch, descending aorta, suprarenal and infrarenal abdominal aorta. For each segment, average values of peak WSS, OSI, and RRT were computed. Student's t-tests were used to compare values between the three cohorts (AAA patients vs elderly controls, and elderly controls vs young controls) where the data were normally distributed, and the non-parametric Wilcoxon rank sum tests were used otherwise.

RESULTS

AAA patients had lower peak WSS in the descending aorta as well as in the abdominal aorta compared to elderly controls (p ≤ 0.001), similar OSI, but higher RRT in the descending and abdominal aorta (p ≤ 0.001). Elderly controls had lower peak WSS compared to young controls throughout the aorta (p < 0.001), higher OSI in all segments except for the infrarenal aorta (p < 0.001), and higher RRT throughout the aorta, except the infrarenal aorta (p < 0.001).

CONCLUSIONS

This study provides novel insights into WSS, OSI, and RRT in patients with AAA in relation to normal ageing, highlighting how AAA patients have markedly abnormal hemodynamic stresses not only in the infrarenal, but in the entire aorta. Moreover, we identified RRT as a marker for abnormal AAA hemodynamics. Further investigations are needed to explore if RRT or other measures of hemodynamics stresses best predict AAA growth and/or rupture.

Efficient parallel simulation of hemodynamics in patient-specific abdominal aorta with aneurysm

Surgical planning for aortic aneurysm repair is a difficult task. In addition to the morphological features obtained from medical imaging, alternative features obtained with computational modeling may provide additional useful information. Though numerical studies are noninvasive, they are often time-consuming, especially when we need to study and compare multiple repair scenarios, because of the high computational complexity. In this paper, we present a highly parallel algorithm for the numerical simulation of unsteady blood flows in the patient-specific abdominal aorta before and after the aneurysmic repair. We model the blood flow with the unsteady incompressible Navier-Stokes equations with different outlet boundary conditions, and solve the discretized system with a highly scalable domain decomposition method. With this approach, a high resolution simulation of a full-size adult aorta can be obtained in less than an hour, instead of days with older methods and software. In addition, we show that the parallel efficiency of the proposed method is near 70% on a parallel computer with 2, 880 processor cores.

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

Blood flow metrics obtained with four-dimensional (4D) flow phase contrast (PC) magnetic resonance imaging (MRI) can be of great value in clinical and experimental cerebrovascular analysis. However, limitations in both quantitative and qualitative analyses can result from errors inherent to PC MRI. One method that excels in creating low-error, physics-based, velocity fields is computational fluid dynamics (CFD). Augmentation of cerebral 4D flow MRI data with CFD-informed neural networks may provide a method to produce highly accurate physiological flow fields. In this preliminary study, the potential utility of such a method was demonstrated by using high resolution patient-specific CFD data to train a convolutional neural network, and then using the trained network to enhance MRI-derived velocity fields in cerebral blood vessel data sets. Through testing on simulated images, phantom data, and cerebrovascular 4D flow data from 20 patients, the trained network successfully de-noised flow images, decreased velocity error, and enhanced near-vessel-wall velocity quantification and visualization. Such image enhancement can improve experimental and clinical qualitative and quantitative cerebrovascular PC MRI analysis.

Impact of prosthetic mitral valve orientation on the ventricular flow field: Comparison using patient-specific computational fluid dynamics

Significant mitral valve regurgitation creates progressive adverse remodeling of the left ventricle (LV). Replacement of the failing valve with a prosthesis generally improves patient outcomes but leaves the patient with non-physiological intracardiac flow patterns that might contribute to their future risk of thrombus formation and embolism. It has been suggested that the angular orientation of the implanted valve might modify the postoperative distortion of the intraventricular flow field. In this study, we investigated the effect of prosthetic valve orientation on LV flow patterns by using heart geometry from a patient with LV dysfunction and a competent native mitral valve to calculate intracardiac flow fields with computational fluid dynamics (CFD). Results were validated using in vivo 4D Flow MRI. The computed flow fields were compared to calculations following virtual implantation of a mechanical heart valve oriented in four different angles to assess the effect of leaflet position. Flow patterns were visualized in long- and short-axes and quantified with flow component analysis. In comparison to a native valve, valve implantation increased the proportion of the mitral inflow remaining in the basal region and further increased the residual volume in the apical area. Only slight changes due to valve orientation were observed. Using our numerical framework, we demonstrated quantitative changes in left ventricular blood flow due to prosthetic mitral replacement. This framework may be used to improve design of prosthetic heart valves and implantation procedures to minimize the potential for apical flow stasis, and potentially assist personalized treatment planning.

Abnormal Flow Dynamics Result in Low Wall Shear Stress and High Oscillatory Shear Index in Abdominal Aortic Dilatation: Initial in vivo Assessment with 4D-flow MRI

PURPOSE

To characterize the non-laminar flow dynamics and resultant decreased wall shear stress (WSS) and high oscillatory shear index (OSI) of the infrarenal abdominal aortic dilatation, cardiac phase-resolved 3D phase-contrast MRI (4D-flow MRI) was performed.

METHODS

The prospective single-arm study was approved by the Institutional Review Board and included 18 subjects (median 67.5 years) with the dilated infrarenal aorta (median diameter 35 mm). 4D-flow MRI was conducted on a 1.5T MRI system. On 3D streamline images, laminar and non-laminar (i.e., vortex or helical) flow patterns were visually assessed both for the dilated aorta and for the undilated upstream aorta. Cardiac phase-resolved flow velocities, WSS and OSI, were also measured for the dilated aorta and the upstream undilated aorta.

RESULTS

Non-laminar flow represented by vortex or helical flow was more frequent and overt in the dilated aorta than in the undilated upstream aorta (P < 0.0156) with a very good interobserver agreement (weighted kappa: 0.82-1.0). The WSS was lower, and the OSI was higher on the dilated aortic wall compared with the proximal undilated segments. In mid-systole, mean spatially-averaged WSS was 0.20 ± 0.016 Pa for the dilated aorta vs. 0.68 ± 0.071 Pa for undilated upstream aorta (P < 0.0001), and OSI on the dilated aortic wall was 0.093 ± 0.010 vs. 0.041 ± 0.0089 (P = 0.013). The maximum values and the amplitudes of the WSS at the dilated aorta were inversely proportional to the ratio of dilated/undilated aortic diameter (r = -0.694, P = 0.0014).

CONCLUSION

4D-flow can characterize abnormal non-laminar flow dynamics within the dilated aorta in vivo. The wall of the infrarenal aortic dilatation is continuously and increasingly affected by atherogenic stimuli due to the flow disturbances represented by vortex or helical flow, which is reflected by lower WSS and higher OSI.

4D Flow with MRI

Magnetic resonance imaging (MRI) has become an important tool for the clinical evaluation of patients with cardiac and vascular diseases. Since its introduction in the late 1980s, quantitative flow imaging with MRI has become a routine part of standard-of-care cardiothoracic and vascular MRI for the assessment of pathological changes in blood flow in patients with cardiovascular disease. More recently, time-resolved flow imaging with velocity encoding along all three flow directions and three-dimensional (3D) anatomic coverage (4D flow MRI) has been developed and applied to enable comprehensive 3D visualization and quantification of hemodynamics throughout the human circulatory system. This article provides an overview of the use of 4D flow applications in different cardiac and vascular regions in the human circulatory system, with a focus on using 4D flow MRI in cardiothoracic and cerebrovascular diseases.