Flow Magnetic Resonance Imaging Measurements Research Articles

On the identification of hypoxic regions in subject-specific cerebral vasculature by combined CFD/MRI.

A long-time exposure to lack of oxygen (hypoxia) in some regions of the cerebrovascular system is believed to be one of the causes of cerebral neurological diseases. In the present study, we show how a combination of magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) can provide a non-invasive alternative for studying blood flow and transport of oxygen within the cerebral vasculature. We perform computer simulations of oxygen mass transfer in the subject-specific geometry of the circle of Willis. The computational domain and boundary conditions are based on four-dimensional (4D)-flow MRI measurements. Two different oxygen mass transfer models are considered: passive (where oxygen is treated as a dilute chemical species in plasma) and active (where oxygen is bonded to haemoglobin) models. We show that neglecting haemoglobin transport results in a significant underestimation of the arterial wall mass transfer of oxygen. We identified the hypoxic regions along the arterial walls by introducing the critical thresholds that are obtained by comparison of the estimated range of Damköhler number (Da ⊂ 〈9; 57〉) with the local Sherwood number. Finally, we recommend additional validations of the combined MRI/CFD approach proposed here for larger groups of subject- or patient-specific brain vasculature systems.

Modeling Bias Error in 4D Flow MRI Velocity Measurements.

We present a model to estimate the bias error of 4D flow magnetic resonance imaging (MRI) velocity measurements. The local instantaneous bias error is defined as the difference between the expectation of the voxel's measured velocity and actual velocity at the voxel center. The model accounts for bias error introduced by the intra-voxel velocity distribution and partial volume (PV) effects. We assess the intra-voxel velocity distribution using a 3D Taylor Series expansion. PV effects and numerical errors are considered using a Richardson extrapolation. The model is applied to synthetic Womersley flow and in vitro and in vivo 4D flow MRI measurements in a cerebral aneurysm. The bias error model is valid for measurements with at least 3.75 voxels across the vessel diameter and signal-to-noise ratio greater than 5. All test cases exceeded this diameter to voxel size ratio with diameters, isotropic voxel sizes, and velocity ranging from 3-15mm, 0.5-1mm, and 0-60cm/s, respectively. The model accurately estimates the bias error in voxels not affected by PV effects. In PV voxels, the bias error is an order of magnitude higher, and the accuracy of the bias error estimation in PV voxels ranges from 67.3% to 108% relative to the actual bias error. The bias error estimated for in vivo measurements increased two-fold at systole compared to diastole in partial volume and non-partial volume voxels, suggesting the bias error varies over the cardiac cycle. This bias error model quantifies 4D flow MRI measurement accuracy and can help plan 4D flow MRI scans.

Assessment of 4D flow MRI's quality by verifying its Navier-Stokes compatibility.

4D Flow Magnetic Resonance Imaging (MRI) is the state‐of‐the‐art technique to comprehensively measure the complex spatio‐temporal and multidirectional patterns of blood flow. However, it is subject to artifacts such as noise and aliasing, which due to the 3D and dynamic structure is difficult to detect in clinical practice. In this work, a new mathematical and computational model to determine the quality of 4D Flow MRI is presented. The model is derived by assuming the true velocity satisfies the incompressible Navier–Stokes equations and that can be decomposed by the measurements u→meas plus an extra field w→. Therefore, a non‐linear problem with w→ as unknown arises, which serves as a measure of data quality. A stabilized finite element formulation tailored to this problem is proposed and analyzed. Then, extensive numerical examples—using synthetic 4D Flow MRI data as well as real measurements on experimental phantom and subjects—illustrate the ability to use w→ for assessing the quality of 4D Flow MRI measurements over space and time.

Dynamic MR imaging of cerebral perfusion during bicycling exercise

Habitual physical activity is beneficial for cerebrovascular health and cognitive function. Physical exercise therefore constitutes a clinically relevant cerebrovascular stimulus. This study demonstrates the feasibility of quantitative cerebral blood flow (CBF) measurements during supine bicycling exercise with pseudo-continuous arterial spin labeling (pCASL) magnetic resonance imaging (MRI) at 3 Tesla.Twelve healthy volunteers performed a steady-state exercise-recovery protocol on an MR-compatible bicycle ergometer, while dynamic pCASL data were acquired at rest, during moderate (60% of the age-predicted supine maximal heart rate (HRmax)) and vigorous (80% of supine HRmax) exercise, and subsequent recovery. These CBF measurements were compared with 2D phase-contrast MRI measurements of blood flow through the carotid arteries. Procedures were repeated on a separate day for an assessment of measurement repeatability.Whole-brain (WB) CBF was 41.2 ± 6.9 mL/100 g/min at rest (heart rate 63 [57-71] beats/min), remained similar at moderate exercise (102 [97-107] beats/min), decreased by 10% to 37.1 ± 5.7 mL/100 g/min (p = 0.001) during vigorous exercise (139 [136-142] beats/min) and decreased further to 34.2 ± 6.0 mL/100 g/min (p < 0.001) during recovery. Hippocampus CBF decreased by 12% (p = 0.001) during moderate exercise, decreased further during vigorous exercise (-21%; p < 0.001) and was even lower during recovery (-31%; p < 0.001). In contrast, motor cortex CBF increased by 12% (p = 0.027) during moderate exercise, returned to resting-state values during vigorous exercise, and decreased by 17% (p = 0.006) during recovery. The inter-session repeatability coefficients for WB CBF were approximately 20% for all stages of the exercise-recovery protocol. Phase-contrast blood flow measurements through the common carotid arteries overestimated the WB CBF because of flow directed to the face and scalp. This bias increased with exercise.We have demonstrated the feasibility of dynamic pCASL-MRI of the human brain for a quantitative evaluation of cerebral perfusion during bicycling exercise. Our spatially resolved measurements revealed a differential response of CBF in the motor cortex as well as the hippocampus compared with the brain as a whole. Caution is warranted when using flow through the common carotid arteries as a surrogate measure for cerebral perfusion.

Four-Dimensional Flow Magnetic Resonance Imaging for Assessment of Velocity Magnitudes and Flow Patterns in The Human Carotid Artery Bifurcation: Comparison with Computational Fluid Dynamics.

Purpose: Knowledge of the hemodynamics in the vascular system is important to understand and treat vascular pathology. The present study aimed to evaluate the hemodynamics in the human carotid artery bifurcation measured by four-dimensional (4D) flow magnetic resonance imaging (MRI) as compared to computational fluid dynamics (CFD). Methods: This protocol used MRI data of 12 healthy volunteers for the 3D vascular models and 4D flow MRI measurements for the boundary conditions in CFD simulation. We compared the velocities measured at the carotid bifurcation and the 3D velocity streamlines of the carotid arteries obtained by these two methods. Results: There was a good agreement for both maximum and minimum velocity values between the 2 methods for velocity magnitude at the bifurcation plane. However, on the 3D blood flow visualization, secondary flows, and recirculation regions are of poorer quality when visualized through the 4D flow MRI. Conclusion: 4D flow MRI and CFD show reasonable agreement in demonstrated velocity magnitudes at the carotid artery bifurcation. However, the visualization of blood flow at the recirculation regions and the assessment of secondary flow characteristics should be enhanced for the use of 4D flow MRI in clinical situations.

Noise reduction of flow MRI measurements using a lattice Boltzmann based topology optimisation approach

In a previous work, the feasibility of coupling magnetic resonance imaging (MRI) measurements and computational fluid dynamics (CFD) was presented, called CFD-MRI. Using a lattice Boltzmann based topology optimisation approach, the method can be described as a Navier–Stokes filter for flow MRI measurements. The main objective of this article is the analysis and quantification of CFD-MRI for its ability to reduce statistical measurement noise. For this, MRI data was analysed and used as basis for synthetic data, where noise was added to a simulation result. Thus, the noise-free data is known and a thorough analysis can be performed. The results show a very high agreement with the original data, even with high statistical noise in the input data and limited information available.

Comparison of simultaneous arterial spin labeling MRI and 15O-H2O PET measurements of regional cerebral blood flow in rest and altered perfusion states

Arterial spin labelling (ASL) is a non-invasive magnetic resonance imaging (MRI) technique that may provide fully quantitative regional cerebral blood flow (rCBF) images. However, before its application in clinical routine, ASL needs to be validated against the clinical gold standard, 15O-H2O positron emission tomography (PET). We aimed to compare the two techniques by performing simultaneous quantitative ASL-MRI and 15O-H2O-PET examinations in a hybrid PET/MRI scanner. Duplicate rCBF measurements were performed in healthy young subjects (n = 14) in rest, during hyperventilation, and after acetazolamide (post-ACZ), yielding 63 combined PET/MRI datasets in total. Average global CBF by ASL-MRI and 15O-H2O-PET was not significantly different in any state (40.0 ± 6.5 and 40.6 ± 4.1 mL/100 g/min, respectively in rest, 24.5 ± 5.1 and 23.4 ± 4.8 mL/100 g/min, respectively, during hyperventilation, and 59.1 ± 10.4 and 64.7 ± 10.0 mL/100 g/min, respectively, post-ACZ). Overall, strong correlation between the two methods was found across all states (slope = 1.01, R2 = 0.82), while the correlations within individual states and of reactivity measures were weaker, in particular in rest (R2 = 0.05, p = 0.03). Regional distribution was similar, although ASL yielded higher perfusion and absolute reactivity in highly vascularized areas. In conclusion, ASL-MRI and 15O-H2O-PET measurements of rCBF are highly correlated across different perfusion states, but with variable correlation within and between hemodynamic states, and systematic differences in regional distribution.

Phase contrast mapping MRI measurements of global cerebral blood flow across different perfusion states – A direct comparison with 15O-H2O positron emission tomography using a hybrid PET/MR system

Phase-contrast mapping (PCM) magnetic resonance imaging (MRI) provides easy-access non-invasive quantification of global cerebral blood flow (gCBF) but its accuracy in altered perfusion states is not established. We aimed to compare paired PCM MRI and 15O-H2O positron emission tomography (PET) measurements of gCBF in different perfusion states in a single scanning session. Duplicate combined gCBF PCM-MRI and 15O-H2O PET measurements were performed in the resting condition, during hyperventilation and after acetazolamide administration (post-ACZ) using a 3T hybrid PET/MR system. A total of 62 paired gCBF measurements were acquired in 14 healthy young male volunteers. Average gCBF in resting state measured by PCM-MRI and 15O-H2O PET were 58.5 ± 10.7 and 38.6 ± 5.7 mL/100 g/min, respectively, during hyperventilation 33 ± 8.6 and 24.7 ± 5.8 mL/100 g/min, respectively, and post-ACZ 89.6 ± 27.1 and 57.3 ± 9.6 mL/100 g/min, respectively. On average, gCBF measured by PCM-MRI was 49% higher compared to 15O-H2O PET. A strong correlation between the two methods across all states was observed (R2 = 0.72, p < 0.001). Bland–Altman analysis suggested a perfusion dependent relative bias resulting in higher relative difference at higher CBF values. In conclusion, measurements of gCBF by PCM-MRI in healthy volunteers show a strong correlation with 15O-H2O PET, but are associated with a large and non-linear perfusion-dependent difference.

3D-printed rotating spinnerets create membranes with a twist

Round hollow fiber membranes have been well established in applications such as gas separation, ultrafiltration and blood dialysis. Yet, it is well known that geometrical topologies can introduce secondary flow patterns counteracting mass transport limitations, stemming from diffusion resistances and fouling. We present a new systematic methodology to fabricate novel membrane architectures. We use the freedom of design by 3D-printing spinnerets, having multiple bore channels of any geometry. First, such spinnerets are stationary to fabricate straight bore channels inside a monolithic membrane. Second, in an even more complex design, a new mechanical system enables rotating the spinneret. Such rotating multibore spinnerets enable (A) the preparation of twisted channels inside a porous monolithic membrane as well as (B) a helical twist of the outside geometry. The spun material systems comprise classical polymer solutions as well as metal-polymer slurries resulting in solid porous metallic monolithic membrane after thermal post-processing. It is known that twisted spiral-type bore channel geometries are potentially superior to straight channels with respect to mass and heat polarization phenomena, however their fabrication was cumbersome in the past. Now, the described methodology enables membrane fabrication to tailor the membrane geometry to the needs of the membrane process. To showcase the delicate interplay between the geometry and radial and axial flow conditions, we report fluid mechanical simulations and flow magnetic resonance imaging measurements for a twisted tri-bore membrane during permeation.

Four-dimensional flow magnetic resonance imaging in cirrhosis.

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.

760 Management of Hydrocephalus Using a Noninvasive Magnetic Resonance Imaging Method for Measurement of Intracranial Compliance and Pressure

INTRODUCTION: Measurements of intracranial compliance (ICC) and intracranial pressure (ICP) are clinically important in the management of patients with hydrocephalus, especially when treating patients with possible shunt malfunction. In this work, we present a noninvasive magnetic resonance imaging (MRI) technique for determination of ICP and ICC. Using standardized measurements obtained from normal patients and patients with severe head trauma (with ICP monitors) we now applied this method to patients with hydrocephalus. METHODS: The compliance index is derived from the measured changes in the ICP and volume of blood and cerebrospinal fluid (CSF) in the intracranial space that occur naturally during each cardiac cycle, obtained with a motion-sensitive MRI scan. Intracranial volume and ICP changes are calculated from MRI measurements of CSF flow and blood volumetric flow rates. The change in pressure is derived from the pressure gradient calculated from velocity. Twenty patients presenting with symptoms suggestive of symptomatic hydrocephalus were studied to determine the need for surgery. RESULTS: The clinical cases included congenital hydrocephalus (some of which had no shunt surgery for >10 yr), tumor, toxoplasmosis, slit ventricles, aqueductal stenosis, and Chiari malformation. Patients presented with a variety of symptoms, including headaches, visual blurring, seizures, neck pain, nausea/vomiting, gait disturbances, and lethargy. This technique correctly identified three patients requiring surgery, and ICP measured at surgery correlated with MRI-ICP. Two patients with third ventriculostomies and enlarged ventricles avoided surgery because extraventricular drain (EVD) measurements correlated with normal MRI-ICP. In three patients with EVDs, pressures correlated with low MRI-ICP measurements. Nonsurgical cases were closely observed and have remained stable. CONCLUSION: These results suggest MRI-ICP sensitivity sufficient to differentiate between normal and elevated ICP in patients with a variety of subtypes of hydrocephalus. Noninvasive MRI-ICP may be a useful tool in the determination of the need for a shunt or shunt revision and may enable patients to avoid unnecessary surgery or invasive monitoring procedures.

760 Management of Hydrocephalus Using a Noninvasive Magnetic Resonance Imaging Method for Measurement of Intracranial Compliance and Pressure

INTRODUCTION: Measurements of intracranial compliance (ICC) and intracranial pressure (ICP) are clinically important in the management of patients with hydrocephalus, especially when treating patients with possible shunt malfunction. In this work, we present a noninvasive magnetic resonance imaging (MRI) technique for determination of ICP and ICC. Using standardized measurements obtained from normal patients and patients with severe head trauma (with ICP monitors) we now applied this method to patients with hydrocephalus. METHODS: The compliance index is derived from the measured changes in the ICP and volume of blood and cerebrospinal fluid (CSF) in the intracranial space that occur naturally during each cardiac cycle, obtained with a motion-sensitive MRI scan. Intracranial volume and ICP changes are calculated from MRI measurements of CSF flow and blood volumetric flow rates. The change in pressure is derived from the pressure gradient calculated from velocity. Twenty patients presenting with symptoms suggestive of symptomatic hydrocephalus were studied to determine the need for surgery. RESULTS: The clinical cases included congenital hydrocephalus (some of which had no shunt surgery for >10 yr), tumor, toxoplasmosis, slit ventricles, aqueductal stenosis, and Chiari malformation. Patients presented with a variety of symptoms, including headaches, visual blurring, seizures, neck pain, nausea/vomiting, gait disturbances, and lethargy. This technique correctly identified three patients requiring surgery, and ICP measured at surgery correlated with MRI-ICP. Two patients with third ventriculostomies and enlarged ventricles avoided surgery because extraventricular drain (EVD) measurements correlated with normal MRI-ICP. In three patients with EVDs, pressures correlated with low MRI-ICP measurements. Nonsurgical cases were closely observed and have remained stable. CONCLUSION: These results suggest MRI-ICP sensitivity sufficient to differentiate between normal and elevated ICP in patients with a variety of subtypes of hydrocephalus. Noninvasive MRI-ICP may be a useful tool in the determination of the need for a shunt or shunt revision and may enable patients to avoid unnecessary surgery or invasive monitoring procedures.

Method to correct for the effects of limited spatial resolution in phase‐contrast flow MRI measurements

Phase-contrast (PC) magnetic resonance imaging (MRI) flow measurements suffer from the effect of the point spread function (PSF) due to the limited sampling of k-space. The PSF, which in this case is a sinc function, deforms the flow profile and forms a ringing pattern around the vessel. In this work, an empirical method is presented that corrects for errors due to the deformation of the flow profile. The ringing pattern is used to obtain a well-defined vessel segmentation, which after correction provides more accurate vessel radius and volume flow rate (VFR). The correction method was developed from phantom measurements at constant flow and applied on phantom measurements at moderately pulsatile flow. After correction, the error of the estimated tube radius and the VFR was less than 10% and 5%, respectively. Corresponding errors without correction overestimated the radius by 60% and the VFR by 35%. Preliminary results indicate that the method is also valid in vivo. The variation in the estimated radius and VFR for different spatial resolution decreased when the method was applied. The presented method gives a more accurate estimation of the radius and VFR in vessels of the size of a few pixels without prior knowledge about the true vessel radius.

Assessment of Left-to-Right Intracardiac Shunting by Velocity-Encoded, Phase-Difference Magnetic Resonance Imaging

Velocity-encoded, phase-difference magnetic resonance imaging (MRI) has been shown to provide an accurate assessment of shunt magnitude in patients with large atrial septal defects, but its ability to determine shunt magnitude in patients with intracardiac left-to-right shunts of various locations and sizes has not been evaluated in a prospective and blinded manner. The objective of the present study was to determine whether velocity-encoded, phase-difference MRI can assess the magnitude of intracardiac left-to-right shunting in humans. Twenty-one subjects (15 women and 6 men; age range, 15 to 72 years) underwent velocity-encoded, phase-difference MRI measurements of flow in the proximal aorta and pulmonary artery, followed immediately by cardiac catheterization. The presence of left-to-right intracardiac shunting was assessed with hydrogen inhalation, after which shunt magnitude was measured by the oximetric and indocyanine green techniques. Of the 21 patients, 12 had left-to-right intracardiac shunting detected by hydrogen inhalation. There was a good correlation (r = .94) between the invasive and MRI assessments of shunt magnitude. In comparison to oximetry and indocyanine green, MRI correctly identified the 12 patients with a ratio of pulmonary to systemic flow (Qp/Qs) of < 1.5 (9 without intracardiac shunting and 3 with small shunts) and the 9 patients with a Qp/Qs of > or = 1.5 (6 with atrial septal defect, 1 with ventricular septal defect, 1 with patent ductus arteriosus, and 1 with both atrial septal defect and patent ductus arteriosus). Compared with measurements obtained during cardiac catheterization, velocity-encoded, phase-difference MRI measurements of flow in the proximal great vessels can reliably assess the magnitude of intracardiac left-to-right shunting.

Normal renal blood flow measurement using phase-contrast cine magnetic resonance imaging.

This study assesses the ability of a cardiac-gated phase-contrast magnetic resonance imaging (MRI) technique to measure renal blood flow (RBF) noninvasively in humans. In nine normal volunteers, total RBF in the renal arteries and in the left renal vein was estimated by MRI and correlated with RBF determined by the clearance of para-aminohippuric acid (CPAH) and the hematocrit level. Correlation of RBF estimated from left renal vein flow, with RBF by CPAH-hematocrit, yielded r = .86 (P less than .003). Repeated measurement of RBF by MRI demonstrated a high degree of reproducibility, with coefficients of variation ranging from 4.8% to 8.9%. However, the MRI measurements of arterial flow did not significantly correlate with the standard measurements. Reproducible noninvasive measurement of normal RBF is possible with the phase-contrast MRI technique used to measure renal venous blood flow.