2D Phase Contrast Research Articles

Optimization of 3D phase contrast venography for the assessment of the cranio-cervical venous system at 1.5T.

The aim of this work was to optimize a three-dimensional (3D) phase-contrast venography (PCV) product MR pulse sequence in order to obtain clinically reliable images with less artifacts for an improved depiction of the cranio-cervical venous vessels. Starting from the product sequence, the 3D PCV protocol was optimized in eight steps with respect to the velocity encoding (Venc) direction and value, slice thickness, reduction of susceptibility artifacts and arterial contamination, gradient mode and radio-frequency (RF)-spoiling, B0-Shimming, asymmetric echo technique and RF-pulse type, and flip angle. The product and optimized protocol was used to perform 3D PCV in 12 healthy male volunteers with a median age of 50years using a state-of-the-art 1.5-T MR system. For evaluation, the cranio-cervical venous system was divided into 15 segments. These segments were evaluated by three radiologists with experience in neuroradiology. An ordinal scoring system was used to access the overall diagnostic quality, arterial contamination, and the quality of visualization. Image quality in the optimized 3D PCV was graded as "excellent" by all readers in 65.3% of the cases (p < 0.0001). The visualization of venous segments was strongly improved: it was considered diagnostic in 81.8% of all cases using the optimized sequence and in 47.6% for the product 3D PCV (p < 0.0001), respectively. The optimized protocol improved the imaging of all venous segments (p < 0.0001). The optimized 3D PCV pulse sequence showed superior results compared to the product 3D PCV for the visualization and evaluation of the venous system in all healthy volunteers.

Fetal hemodynamics and cardiac streaming assessed by 4D flow cardiovascular magnetic resonance in fetal sheep

BackgroundTo date it has not been possible to obtain a comprehensive 3D assessment of fetal hemodynamics because of the technical challenges inherent in imaging small cardiac structures, movement of the fetus during data acquisition, and the difficulty of fusing data from multiple cardiac cycles when a cardiac gating signal is absent. Here we propose the combination of volumetric velocity-sensitive cardiovascular magnetic resonance imaging (“4D flow” CMR) and a specialized animal preparation (catheters to monitor fetal heart rate, anesthesia to immobilize mother and fetus) to examine fetal sheep cardiac hemodynamics in utero.MethodsTen pregnant Merino sheep underwent surgery to implant arterial catheters in the target fetuses. Anesthetized ewes underwent 4D flow CMR with acquisition at 3 T for fetal whole-heart coverage with 1.2–1.5 mm spatial resolution and 45–62 ms temporal resolution. Flow was measured in the heart and major vessels, and particle traces were used to visualize circulatory patterns in fetal cardiovascular shunts. Conservation of mass was used to test internal 4D flow consistency, and comparison to standard 2D phase contrast (PC) CMR was performed for validation.ResultsStreaming of blood from the ductus venosus through the foramen ovale was visualized. Flow waveforms in the major thoracic vessels and shunts displayed normal arterial and venous patterns. Combined ventricular output (CVO) was 546 mL/min per kg, and the distribution of flows (%CVO) were comparable to values obtained using other methods. Internal 4D flow consistency across 23 measurement locations was established with differences of 14.2 ± 12.1%. Compared with 2D PC CMR, 4D flow showed a strong correlation (R2 = 0.85) but underestimated flow (bias = − 21.88 mL/min per kg, p < 0.05).ConclusionsThe combination of fetal surgical preparation and 4D flow CMR enables characterization and quantification of complex flow patterns in utero. Visualized streaming of blood through normal physiological shunts confirms the complex mechanism of substrate delivery to the fetal heart and brain. Besides offering insight into normal physiology, this technology has the potential to qualitatively characterize complex flow patterns in congenital heart disease phenotypes in a large animal model, which can support the development of new interventions to improve outcomes in this population.

Fast self-navigated wall shear stress measurements in the murine aortic arch using radial 4D-phase contrast cardiovascular magnetic resonance at 17.6\u2009T

Purpose4D flow cardiovascular magnetic resonance (CMR) and the assessment of wall shear stress (WSS) are non-invasive tools to study cardiovascular risks in vivo. Major limitations of conventional triggered methods are the long measurement times needed for high-resolution data sets and the necessity of stable electrocardiographic (ECG) triggering. In this work an ECG-free retrospectively synchronized method is presented that enables accelerated high-resolution measurements of 4D flow and WSS in the aortic arch of mice.Methods4D flow and WSS were measured in the aortic arch of 12-week-old wildtype C57BL/6 J mice (n = 7) with a radial 4D-phase-contrast (PC)-CMR sequence, which was validated in a flow phantom. Cardiac and respiratory motion signals were extracted from the radial CMR signal and were used for the reconstruction of 4D-flow data. Rigid motion correction and a first order B0 correction was used to improve the robustness of magnitude and velocity data.The aortic lumen was segmented semi-automatically. Temporally averaged and time-resolved WSS and oscillatory shear index (OSI) were calculated from the spatial velocity gradients at the lumen surface at 14 locations along the aortic arch. Reproducibility was tested in 3 animals and the influence of subsampling was investigated.ResultsVolume flow, cross-sectional areas, WSS and the OSI were determined in a measurement time of only 32 min. Longitudinal and circumferential WSS and radial stress were assessed at 14 analysis planes along the aortic arch. The average longitudinal, circumferential and radial stress values were 1.52 ± 0.29 N/m2, 0.28 ± 0.24 N/m2 and − 0.21 ± 0.19 N/m2, respectively. Good reproducibility of WSS values was observed.ConclusionThis work presents a robust measurement of 4D flow and WSS in mice without the need of ECG trigger signals. The retrospective approach provides fast flow quantification within 35 min and a flexible reconstruction framework.

Bulk Flow and Near Wall Hemodynamics of the Rabbit Aortic Arch and Descending Thoracic Aorta: A 4D PC-MRI Derived Computational Fluid Dynamics Study.

Animal models offer a flexible experimental environment for studying atherosclerosis. The mouse is the most commonly used animal, however, the underlying hemodynamics in larger animals such as the rabbit are far closer to that of humans. The aortic arch is a vessel with complex helical flow and highly heterogeneous shear stress patterns which may influence where atherosclerotic lesions form. A better understanding of intraspecies flow variation and the impact of geometry on flow may improve our understanding of where disease forms. In this work, we use magnetic resonance angiography (MRA) and 4D phase contrast magnetic resonance imaging (PC-MRI) to image and measure blood velocity in the rabbit aortic arch. Measured flow rates from the PC-MRI were used as boundary conditions in computational fluid dynamics (CFD) models of the arches. Helical flow, cross flow index (CFI), and time-averaged wall shear stress (TAWSS) were determined from the simulated flow field. Both traditional geometric metrics and shape modes derived from statistical shape analysis were analyzed with respect to flow helicity. High CFI and low TAWSS were found to colocalize in the ascending aorta and to a lesser extent on the inner curvature of the aortic arch. The Reynolds number was linearly associated with an increase in helical flow intensity (R = 0.85, p < 0.05). Both traditional and statistical shape analyses correlated with increased helical flow symmetry. However, a stronger correlation was obtained from the statistical shape analysis demonstrating its potential for discerning the role of shape in hemodynamic studies.

MR Venous Flow in Sigmoid Sinus Diverticulum.

Case reports demonstrate that coiling of a sigmoid sinus diverticulum can treat pulsatile tinnitus. We hypothesized that MR imaging 4D flow and computational fluid dynamics would reveal distinct blood flow patterns in the venous outflow tract in these patients. Patients with pulsatile tinnitus of suspected venous etiology underwent MR imaging at 3T, using venous phase contrast-enhanced MR angiography, 4D flow, and 2D phase contrast. The contrast-enhanced MRA contours were evaluated to determine the presence and extent of a sigmoid sinus diverticulum. Computational fluid dynamics analysis was performed using the 4D flow inlet flow and the luminal contours from contrast-enhanced MRA as boundary conditions. In addition, computational fluid dynamics was performed for the expected post treatment conditions by smoothing the venous geometry to exclude the sigmoid sinus diverticulum from the anatomic boundary conditions. Streamlines were generated from the 4D flow and computational fluid dynamics velocity maps, and flow patterns were examined for the presence of rotational components. Twenty-five patients with pulsatile tinnitus of suspected venous etiology and 10 control subjects were enrolled. Five (20%) of the symptomatic subjects had sigmoid sinus diverticula, all associated with an upstream stenosis. In each of these patients, but none of the controls, a stenosis-related flow jet was directed toward the opening of the sigmoid sinus diverticulum with rotational flow patterns in the sigmoid sinus diverticulum and parent sigmoid sinus on both 4D flow and computational fluid dynamics. Consistent patterns of blood flow can be visualized in a sigmoid sinus diverticulum and the parent sinus using 4D flow and computational fluid dynamics. Strong components of rotational blood flow were seen in subjects with sigmoid sinus diverticula that were absent in controls.

Errors in power-law estimations of inflow rates for intracranial aneurysm CFD

Patient-specific inflow rates are rarely available for computational fluid dynamics (CFD) studies of intracranial aneurysms. Instead, inflow rates are often estimated from parent artery diameters via power laws, i.e. Q ∝ Dn, reflecting adaptation of conduit arteries to demanded flow. The present study aimed to validate the accuracy of these power laws. Internal carotid artery (ICA) flow rates were measured from 25 ICA aneurysm patients via 2D phase contrast MRI. ICA diameters, derived from 3D segmentation of rotational angiograms, were used to estimate inflow rates via power laws from the aneurysm CFD literature assuming the same inlet wall shear stress (WSS) (n = 3), velocity (n = 2) or flow rate (n = 0) for all cases. To illustrate the potential impact of errors in flow rate estimates, pulsatile CFD was carried out for four cases having large errors for at least one power law. Flow rates estimated by n = 3 and n = 0 power laws had significant (p < 0.01) mean biases of −22% to +32%, respectively, but with individual errors ranging from −78% to +120%. The n = 2 power law had no significant bias, but had non-negligible individual errors of −58% to +71%. CFD showed similarly large errors for time-averaged sac WSS; however, these were reduced after normalizing by parent artery WSS. High frequency WSS fluctuations, evident in 2/4 aneurysms, were also sensitive to inflow rate errors. Care should therefore be exercised in the interpretation of aneurysm CFD studies that rely on power law estimates of inflow rates, especially if absolute (vs. normalized) WSS, or WSS instabilities, are of interest.

Accelerated 4D phase contrast MRI in skeletal muscle contraction

Magnetic Resonance in MedicineVolume 80, Issue 5 p. C1-C1 Cover ImageFree Access Accelerated 4D phase contrast MRI in skeletal muscle contraction First published: 23 August 2018 https://doi.org/10.1002/mrm.27452AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume80, Issue5November 2018Pages C1-C1 RelatedInformation

Non-contrast MRI methods as a tool for the pre-operative assessment and surveillance of the arterio-venous fistula for haemodialysis

ObjectiveTo compare non-contrast enhanced MRI with ultrasound (US) for measurement of arm blood vessel geometries and flow velocities in volunteers and patients with end-stage renal disease.Materials and methodsSubjects were scanned using US (reference standard), and MRI 2D time-of-flight (ToF), 2D phase contrast (PC), and 3D multi-echo data image combination (MEDIC). Patients were also scanned after arteriovenous fistula (AVF) surgery.ResultsFor mean vessel diameters (radial and brachial arteries; cephalic vein) MEDIC measurements were similar to US (p > 0.05). However, ToF underestimated the mean diameter of the cephalic vein relative to US (p < 0.05). For arterial velocity measurements, the mean values derived by PC-MR and US were similar (p > 0.05). Post-operatively, the intra-luminal signal intensity was hypo-intense at the anastomosis site using ToF and MEDIC. At the same site the outer boundary of the vessel was consistently lost on ToF, but remained clearly delineated on the MEDIC images.DiscussionWith the exception of ToF, the MRI data demonstrated excellent agreement with US for measurements of vessel geometry and flow velocity. Further, the ability to clearly delineate the post-surgery vessel edges with MEDIC MRI suggests that the technique may be useful for surveillance after AVF creation or for patient-specific modelling studies.

Augmented Reality with Virtual Cerebral Aneurysms: A Feasibility Study

Advanced magnetic resonance imaging (MRI) acquisition methods such as 4D phase contrast magnetic resonance imaging (4D pcMRI) have found widespread applications in assessing hemodynamics in cerebral aneurysms. One of the goals is the identification of a hemodynamic scenario predictive of aneurysm rupture. Because of the technical complexity the 4D pcMRI technology, challenges exist in extracting and communicating the wealth of information contained in the MRI image data, in particular, to a multidisciplinary team of health professionals and basic scientists. Here we introduce a new resource in the form of a commercially available optical see-through head-mounted display, which allows augmenting the real world by computer-generated virtual objects. An image processing algorithm was presented for integrating 3D surface models derived from 3D rotational angiography data, 3D time-of-flight MRI data, and 4D pcMRI data into the augmented reality environment. This new approach was successfully evaluated on blood flow data acquired before and after flow diverter treatment in 6 patients harboring a cerebral aneurysm. Augmented reality may become an additional means for visualizing complex medical data, in particular complex flows as illustrated here.

Analysis of Peak Velocity and Mean Velocity According to Shimming Technique in 2D Phase Contrast : Comparison of 1.5 Tesla and 3.0 Tesla

The purpose of this study was to compare the difference in peak velocity (PV) and average flow (AF) values between the shimming of the aortic arch (AA) and the left ventricle (LV) using the 2D phase contrast (PC) technique at 1.5 Tesla and 3.0 Tesla. The sino-tubular junction (S-T junction), the proximal AA, and the LV, an aortic valve, were examined using a 2D PC technique at 1.5 Tesla and 3.0 Tesla. At 1.5 Tesla and 3.0 Tesla, 2D PC technique was used to examine the proximal AA Sino-Tubular (S-T) junction and LV was examined at the aortic valve area. shim was not used (no_Shim) in 1.5 Tesla and 3.0 Tesla to compare differences in heart blood flow due to magnetic field strength. To assess the difference due to shimming, an auto_shim, target_shim was used and an all_shimthat used auto and target shim simultaneously was used. The average value between 1.5 Tesla and 3.0 Tesla as a result of inspection was different when AA_AF was no_shim and all_shim. In LV_AF, the difference in mean values was found in 1.5 Tesla and 3.0 Tesla when it was no_shim and all_shim. Other tests did not show any significant differences. Also, according to the shimming method in 1.5 Tesla, the mean value difference occurred in AA_AF and the mean value difference in LV_AF in 3.0 Tesla. In conclusion, it is necessary to improve uniformity of magnetic field through shimming for accurate blood flow evaluation in 1.5 Tesla and 3.0 Tesla. Therefore, this study is expected to be used as a basis for improving the uniformity of magnetic field.

Neurovascular stent artifacts in 3D‐TOF and 3D‐PCMRI: Influence of stent design on flow measurement

The morphological and hemodynamic evaluations of neurovascular diseases treated with stents would benefit from noninvasive imaging techniques such as 3D time-of-flight MRI (3D-TOF) and 3D phase contrast MRI (3D-PCMRI). For this purpose, a comprehensive evaluation of the stent artifacts and their impact on the flow measurement is critical. The artifacts of a representative sample of neurovascular stents were evaluated in vitro with 3D-TOF and 3D-PCMRI sequences. The dependency of the artifacts with respect to the orientation was analyzed for each stent design as well as the impact on the flow measurement accuracy. Furthermore, the 3D-PCMRI data of four patients carrying intracranial aneurysms treated with flow diverter stents were analyzed as illustrative examples. The stent artifacts were mainly confined to the stent lumen therefore indicating the leading role of shielding effect. The influence of the stent design and its orientation with respect to the transmitting MR coils were highlighted. The artifacts impacted the 3D-PCMRI velocities mainly in the low magnitude domains, which were discarded from the analysis ensuring reliable near-stent velocities. The feasibility of in-stent flow measurements was confirmed in vivo on two patients who showed strong correlation between flow and geometric features. In two other patients, the consistency of out-of-stent velocities was verified qualitatively through intra-aneurysmal streamlines except when susceptibility artifacts occurred. The present results motivate the conception of low inductance or nonconductive stent design. Furthermore, the feasibility of near-stent 3D-PCMRI measurements opens the door to clinical applications like the post-treatment follow-up of stenoses or intracranial aneurysms.

Whole-heart 4D flow cardiac magnetic resonance in healthy dogs.

In cardiac magnetic resonance imaging (CMR), accurate flow measurements rely on perpendicular plane-alignment with flow direction. For 2D phase contrast (PC) cardiac magnetic resonance measurements, planes have to be defined during the examination of the heart, which is time consuming and error-prone. Collection of flow information of the entire volume of the heart by a 4D flow CMR postpones plane alignment to post-processing. Sampling of such a large amount of data requires acceleration of data acquisition with techniques such as SENSitivity Encoding (k-t SENSE) or Broad-use Linear Acquisition Speed-up Technique (k-t BLAST). Objectives of the study were to compare 4D flow CMR, accelerated with two different acceleration methods with the established 2D PC CMR based on assessment of stroke volume at all four cardiac valves. The values of stroke volume acquired with the 4D flow CMR SENSE did not differ significantly when compared to the 2D PC CMR SENSE at the left side of the heart (aortic and mitral valve). Significant differences between the techniques were seen at the pulmonic and tricuspid valves. Acceleration with k-t BLAST revealed significantly lower values of stroke volume at all cardiac valves, except at the mitral valve.

4D flow MRI quantification of mitral and tricuspid regurgitation: Reproducibility and consistency relative to conventional MRI

In patients with mitral or tricuspid valve regurgitation, evaluation of regurgitant severity is essential for determining the need for surgery. While transthoracic echocardiography is widely accessible, it has limited reproducibility for grading inlet valve regurgitation. Multiplanar cardiac MRI is the quantitative standard but requires specialized local expertise, and is thus not widely available. Volumetric 4D flow MRI has potential for quantitatively grading the severity of inlet valve regurgitation in adult patients. To evaluate the accuracy and reproducibility of volumetric 4D flow MRI for quantification of inlet valvular regurgitation compared to conventional multiplanar MRI, which may simplify and improve accessibility of cardiac MRI. This retrospective, HIPAA-compliant imaging-based comparison study was conducted at a single institution. Twenty-one patients who underwent concurrent multiplanar and 4D flow cardiac MRI between April 2015 and January 2017. 3T; steady-state free-precession (SSFP), 2D phase contrast (2D-PC), and postcontrast 4D flow. We evaluated the intertechnique (4D flow vs. 2D-PC), intermethod (direct vs. indirect measurement), interobserver and intraobserver reproducibility of measurements of regurgitant flow volume (RFV), fraction (RF), and volume (RVol). Statistical analysis included Pearson correlation, Bland-Altman statistics, and intraclass correlation coefficients. There was high concordance between 4D flow and multiplanar MRI, whether using direct or indirect methods of quantifying regurgitation (r = 0.813-0.985). Direct interrogation of the regurgitant jet with 4D flow showed high intraobserver consistency (r = 0.976-0.999) and interobserver consistency (r = 0.861-0.992), and correlated well with traditional indirect measurements obtained as the difference between stroke volume and forward outlet valve flow. 4D flow MRI provides highly reproducible measurements of mitral and tricuspid regurgitant volume, and may be used in place of conventional multiplanar MRI. 4 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;48:1147-1158.

3D phase contrast MRI in models of human airways: Validation of computational fluid dynamics simulations of steady inspiratory flow.

Knowledge of airflow patterns in the large airways is of interest in obstructive airways disease and in the development of inhaled therapies. Computational fluid dynamics (CFD) simulations are used to study airflow in realistic airway models but usually need experimental validation. To develop MRI-based methods to study airway flow in realistic 3D-printed models. Case control. Two 3D-printed lung models. 1.5-3T, flow MRI. Two human airway models, respectively including and excluding the oral cavity and upper airways derived from MR and CT imaging, were 3D-printed. 3D flow MRI was performed at different flow conditions corresponding to slow and steady airflow inhalation rates. Water was used as the working fluid to mimic airflow. Dynamic acquisition of 1D velocity profiles was also performed at different locations in the trachea to observe variability during nonsteady conditions. Linear regression analysis to compare both flow velocity fields and local flow rates from CFD simulations and experimental measurement with flow MRI. A good agreement was obtained between 3D velocity maps measured with flow MRI and predicted by CFD simulations, with linear regression R-squared values ranging from 0.39 to 0.94 when performing a pixel-by-pixel comparison of each velocity component. The flow distribution inside the lung models was also similar, with average slope and R-squared values of 0.96 and 0.99, respectively, when comparing local flow rates assessed at different branching locations. In the model including the upper airways, a turbulent laryngeal jet flow was observed with both methods and affected remarkably the velocity profiles in the trachea. We propose flow MRI using water as a surrogate fluid to air, as a validation tool for CFD simulations of airflow in geometrically realistic models of the human airways. 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:1400-1409.

Accelerated 4D phase contrast MRI in skeletal muscle contraction.

3D time-resolved (4D) phase contrast MRI can be used to study muscle contraction. However, 3D coverage with sufficient spatiotemporal resolution can only be achieved by interleaved acquisitions during many repetitions of the motion task, resulting in long scan times. The aim of this study was to develop a compressed sensing accelerated 4D phase contrast MRI technique for quantification of velocities and strain rate of the muscles in the lower leg during active plantarflexion/dorsiflexion. Nine healthy volunteers were scanned during active dorsiflexion/plantarflexion task. For each volunteer, we acquired a reference scan, as well as 4 different accelerated scans (k-space undersampling factors: 3.14X, 4.09X, 4.89X, and 6.41X) obtained using Cartesian Poisson disk undersampling schemes. The data was reconstructed using a compressed sensing pipeline. For each scan, velocity and strain rate values were quantified in the gastrocnemius lateralis, gastrocnemius medialis, tibialis anterior, and soleus. No significant differences in velocity values were observed as a function acceleration factor in the investigated muscles. The strain rate calculation resulted in one positive (s+ ) and one negative (s- ) eigenvalue, whereas the third eigenvalue (s3 ) was consistently 0 for all the acquisitions. No significant differences were observed for the strain rate eigenvalues as a function of acceleration factor. Data undersampling combined with compressed sensing reconstruction allowed obtainment of time-resolved phase contrast acquisitions with 3D coverage and quantitative information comparable to the reference scan. The 3D sensitivity of the method can help in understanding the connection between muscle architecture and muscle function in future studies.

Role of Helium–Hydrogen ratio on energetic interchange mode behaviour and its effect on ion temperature and micro-turbulence in LHD

In the Large helical device, a change of energetic particle mode is observed as He concentration is varied in ion-ITB type experiments, having constant electron density and input heating power but with a clear increase of central ion temperature in He rich discharges. This activity consists of bursty, but damped energetic interchange modes (EICs, Du et al 2015 Phys. Rev. Lett. 114 155003), whose occurrence rate is dramatically lower in the He-rich discharges. Mechanisms are discussed for the changes in drive and damping of the modes with He concentration. These EIC bursts consist of marked changes in the radial electric field, which is derived from the phase velocity of turbulence measured with the 2D phase contrast imaging (PCI) system. Similar bursts are detected in edge fast ion diagnostics. Ion thermal transport by gyro-Bohm scaling is recognised as a contribution to the change in ion temperature, though fast ion losses by these EIC modes may also contribute to the ion temperature dependence on He concentration, most particularly controlling the height of an ‘edge-pedestal’ in the profile. The steady-state level of fast ions is shown to be larger in helium rich discharges on the basis of a compact neutral particle analyser (CNPA), and the fast-ion component of the diamagnetic stored energy. These events also have an influence on turbulence and transport. The large velocity shear induced produced during these events transiently improves confinement and suppresses turbulence, and has a larger net effect when bursts are more frequent in hydrogen discharges. This exactly offsets the increased gyro-Bohm related turbulence drive in hydrogen which results in the same time-averaged turbulence level in hydrogen as in helium.

Aortic valve stenosis and aortic diameters determine the extent of increased wall shear stress in bicuspid aortic valve disease.

BackgroundUse of 4‐dimensional flow magnetic resonance imaging (4D‐flow MRI) derived wall shear stress (WSS) heat maps enables identification of regions in the ascending aorta with increased WSS. These regions are subject to dysregulation of the extracellular matrix and elastic fiber degeneration, which is associated with aortic dilatation and dissection.PurposeTo evaluate the effect of the presence of aortic valve stenosis and the aortic diameter on the peak WSS and surface area of increased WSS in the ascending aorta.Study TypeProspective.SubjectsIn all, 48 bicuspid aortic valve (BAV) patients (38.1 ± 12.4 years) and 25 age‐ and gender‐matched healthy individuals.Field Strength/SequenceTime‐resolved 3D phase contrast MRI with three‐directional velocity encoding at 3.0T.AssessmentPeak systolic velocity, WSS, and aortic diameters were assessed in the ascending aorta and 3D heat maps were used to identify regions with elevated WSS.Statistical TestsComparisons between groups were performed by t‐tests. Correlations were investigated by univariate and multivariate regression analysis.ResultsElevated WSS was present in 15 ± 11% (range; 1–35%) of the surface area of the ascending aorta of BAV patients with aortic valve stenosis (AS) (n = 10) and in 6 ± 8% (range; 0–31%) of the ascending aorta of BAV patients without AS (P = 0.005). The mid‐ascending aortic diameter negatively correlated with the peak ascending aortic WSS (R = –0.413, P = 0.004) and the surface area of elevated WSS (R = –0.419, P = 0.003). Multivariate linear regression analysis yielded that the height of peak WSS and the amount of elevated WSS depended individually on the presence of aortic valve stenosis and the diameter of the ascending aorta.Data ConclusionThe extent of increased WSS in the ascending aorta of BAV patients depends on the presence of aortic valve stenosis and aortic dilatation and is most pronounced in the presence of AS and a nondilated ascending aorta. Level of Evidence: 2 Technical Efficacy: Stage 2J. MAGN. RESON. IMAGING 2018;48:522–530.

Vascular reactivity in small cerebral perforating arteries with 7 T phase contrast MRI – A proof of concept study

Existing cerebrovascular reactivity (CVR) techniques assess flow reactivity in either the largest cerebral vessels or at the level of the parenchyma. We examined the ability of 2D phase contrast MRI at 7 T to measure CVR in small cerebral perforating arteries.Blood flow velocity in perforators was measured in 10 healthy volunteers (mean age 26 years) using a 7 T MR scanner, using phase contrast acquisitions in the semioval center (CSO), the basal ganglia (BG) and the middle cerebral artery (MCA). Changes in flow velocity in response to a hypercapnic breathing challenge were assessed, and expressed as the percentual increase of flow velocity as a function of the increase in end tidal partial pressure of CO2.The hypercapnic challenge increased (fit ± standard error) flow velocity by 0.7 ± 0.3%/mmHg in the CSO (P < 0.01). Moreover, the number of detected perforators (mean [range]) increased from 63 [27–88] to 108 [61–178] (P < 0.001). In the BG, the hypercapnic challenge increased flow velocity by 1.6 ± 0.5%/mmHg (P < 0.001), and the number of detected perforators increased from 48 [24–66] to 63 [32–91] (P < 0.01). The flow in the MCA increased by 5.2 ± 1.4%/mmHg (P < 0.01).Small vessel specific reactivity can now be measured in perforators of the CSO and BG, using 2D phase contrast at 7 T.

Early insight of 4D flow imaging in patients with congenital heart disease

Cardiac magnetic resonance has become common in the management of congenital heart disease because of its ability to visualize structures that remain obscure by echocardiography without ionizing radiation. The 4D flow is an MRI method that simultaneously processes the quantification of blood flow, the quantification of ventricular volumes and function, the visualization of intracardiac and extracardiac structures and blood flow. The objectives of this study are to assess the feasibility of 4D flow routinely in children and young adults, to compare 4D flow with 2D phase-contrast MRI for quantification of aortic and pulmonary flow and to evaluate the advantage of 4D flow-based volumetric flow analysis compared to 2D phase contrast MRI. Two-dimensional phase-contrast MRI of the aortic root, main pulmonary artery, right and left ventricles and atrias and 4D flow with volumetric coverage of the aorta and pulmonary arteries were performed in 14 patients, 2 (12 and 14 years old) and 12 adults (mean age: 24.1 ± 7.3 years). This CMR concerned various diseases: repaired tetralogy of Fallot ( n = 10), aortic coarctation ( n = 1), single ventricle heart defect ( n = 1) and ventricular communicating shunt (2). Four-dimensional volumetric assessments of ventricles, pulmonary flow and aortic flow were performed and compared to 2D phase-contrast. Four dimensional flow analyses included calculation of net flow and regurgitant fraction with 4D flow analysis. Excellent correlations were found between 2D phase-contrast MRI and 4D flow for stroke volumes, aortic and pulmonary flows. Four-dimensional flow provided both anatomic visualization and comprehensive functional analysis showing stenosis, aneurysm and complex turbulent flows. In each of these circumstances, CMR has been achieved and has been found to be contributory for complete visualization and adequate evaluation.

Quantitative Flow Imaging in Human Umbilical Vessels In Utero Using Nongated 2D Phase Contrast MRI.

Volumetric assessment of afferent blood flow rate provides a measure of global organ perfusion. Phase-contrast magnetic resonance imaging (PCMRI) is a reliable tool for volumetric flow quantification, but given the challenges with motion and lack of physiologic gating signal, such studies, in vivo on the human placenta, are scant. To evaluate and apply a nongated (ng) PCMRI technique for quantifying blood flow rates in utero in umbilical vessels. Prospective study design. Twenty-four pregnant women with median gestational age (GA) 30 4/7 weeks and interquartile range (IQR) 8 1/7 weeks. All scans were performed on a 3.0T Siemens Verio system using the ng-PCMRI technique. The GA-dependent increase in umbilical vein (UV) and arterial (UA) flow was compared to previously published values. Systematic error to be expected from ng-PCMRI, in the context of pulsatile UA flow and partial voluming, was studied through Monte-Carlo simulations, as a function of resolution and number of averages. Correlation between the UA and UV was evaluated using a generalized linear model. Simulations showed that ng-PCMRI measurement variance reduced by increasing the number of averages. For vessels on the order of 2 voxels in radius, partial voluming led to 10% underestimation in the flow. In fetuses, the average flow rates in UAs and UV were measured to be 203 ± 80 ml/min and 232 ± 92 ml/min and the normalized average flow rates were 140 ± 59 ml/min/kg and 155 ± 57 ml/min/kg, respectively. Excellent correlation was found between the total arterial flow vs. corresponding venous flow, with a slope of 1.08 (P = 0.036). Ng-PCMRI can provide accurate volumetric flow measurements in utero in the human umbilical vessels. Care needs to be taken to ensure sufficiently high-resolution data are acquired to minimize partial voluming-related errors. 2 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2017.