Christian Binter

Bayesian multipoint velocity encoding for concurrent flow and turbulence mapping.

An approach to efficiently measure three‐dimensional velocity vector fields and turbulent kinetic energy of blood flow is presented. Multipoint phase‐contrast imaging is used in combination with Bayesian analysis to map both mean and fluctuating velocities over a large dynamic range and for practically relevant signal‐to‐noise ratios. It is demonstrated that the approach permits significant spatiotemporal undersampling to allow for clinically acceptable scan times. Using numerical simulations and in vitro measurements in aortic valve phantoms, it is shown that for given scan time, Bayesian multipoint velocity encoding provides consistently lower errors of velocity and turbulent kinetic energy over a larger dynamic range relative to previous methods. In vitro, significant differences in both peak velocity and turbulent kinetic energy between the aortic CoreValve (150 cm/s, 293 J/m3) and the St. Jude Medical mechanical valve (120 cm/s, 149 J/m3) were found. Comparison of peak turbulent kinetic energy measured in a patient with aortic stenosis (950 J/m3) and in a patient with an implanted aortic CoreValve (540 J/m3) revealed considerable differences relative to the values detected in healthy subjects (149 ± 12 J/m3) indicating the potential of the method to provide a comprehensive hemodynamic assessment of valve performance in vivo. Magn Reson Med, 2013.

Mapping mean and fluctuating velocities by Bayesian multipoint MR velocity encoding-validation against 3D particle tracking velocimetry

To validate Bayesian multipoint MR velocity encoding against particle tracking velocimetry for measuring velocity vector fields and fluctuating velocities in a realistic aortic model.

On the accuracy of viscous and turbulent loss quantification in stenotic aortic flow using phase-contrast MRI.

To investigate the limits of phase contrast MRI (PC‐MRI)–based measurements of viscous losses and turbulent kinetic energy (TKE) pertaining to spatial resolution, signal‐to‐noise ratio (SNR), and non‐Gaussian intravoxel velocity distributions.

Arterial, Venous, and Cerebrospinal Fluid Flow: Simultaneous Assessment with Bayesian Multipoint Velocity-encoded MR Imaging

PURPOSE To measure arterial, venous, and cerebrospinal fluid (CSF) velocities simultaneously by using Bayesian multipoint velocity-encoded magnetic resonance (MR) imaging and to compare interacquisition reproducibility relative to that of standard phase-contrast MR imaging for sequential measurements of arterial, venous, and CSF velocities. MATERIALS AND METHODS This study was approved by the local ethics committee, and informed consent was obtained from all subjects. Simultaneous measurement of blood and CSF flow was performed at the C1-C2 level in 10 healthy subjects (mean age, 24.4 years ± 2.7; five men, five women) by using accelerated Bayesian multipoint velocity-encoded MR imaging. Data were compared with those obtained from two separate conventional phase-contrast MR imaging acquisitions, one optimized for arterial and venous blood flow (velocity encoding range, ±50 cm/sec) and the other optimized for CSF flow (velocity encoding range, ±10 cm/sec), with an imaging time of approximately 2 minutes each. Data acquisition was repeated six times. Intraclass correlation coefficient (ICC) and linear regression were used to quantify interacquisition reproducibility. RESULTS There was no significant difference in arterial blood flow measured with Bayesian multipoint velocity-encoded MR imaging and that measured with phase-contrast MR imaging (mean ICC, 0.96 ± 0.03 vs 0.97 ± 0.02, respectively). Likewise, there was no significant difference between CSF flow measured with Bayesian multipoint velocity-encoded MR imaging and that measured with phase-contrast MR imaging (mean ICC, 0.97 ± 0.02 vs 0.96 ± 0.05, respectively). For venous blood flow, the ICC with Bayesian multipoint MR imaging was significantly larger than that with conventional phase-contrast MR imaging (mean, 0.75 ± 0.23 vs 0.65 ± 0.26, respectively; P = .016). CONCLUSION Bayesian multipoint velocity-encoded MR imaging allows for simultaneous assessment of fast and slow flows in arterial, venous, and CSF lumina in a single acquisition. It eliminates the need for vessel-dependent adjustment of the velocity-encoding range, as required for conventional sequential phase-contrast MR imaging measurements.

Turbulent Kinetic Energy Assessed by Multipoint 4-Dimensional Flow Magnetic Resonance Imaging Provides Additional Information Relative to Echocardiography for the Determination of Aortic Stenosis Severity

Background— Turbulent kinetic energy (TKE), assessed by 4-dimensional (4D) flow magnetic resonance imaging, is a measure of energy loss in disturbed flow as it occurs, for instance, in aortic stenosis (AS). This work investigates the additional information provided by quantifying TKE for the assessment of AS severity in comparison to clinical echocardiographic measures. Methods and Results— Fifty-one patients with AS (67±15 years, 20 female) and 10 healthy age-matched controls (69±5 years, 5 female) were prospectively enrolled to undergo multipoint 4D flow magnetic resonance imaging. Patients were split into 2 groups (severe and mild/moderate AS) according to their echocardiographic mean pressure gradient. TKE values were integrated over the aortic arch to obtain peak TKE. Integrating over systole yielded total TKEsys and by normalizing for stroke volume, normalized TKEsys was obtained. Mean pressure gradient and TKE correlated only weakly (R2=0.26 for peak TKE and R2=0.32 for normalized TKEsys) in the entire study population including control subjects, while no significant correlation was observed in the AS patient group. In the patient population with dilated ascending aorta, both peak TKE and total TKEsys were significantly elevated (P<0.01), whereas mean pressure gradient was significantly lower (P<0.05). Patients with bicuspid aortic valves also showed significantly increased TKE metrics (P<0.01), although no significant difference was found for mean pressure gradient. Conclusions— Elevated TKE levels imply higher energy losses associated with bicuspid aortic valves and dilated ascending aortic geometries that are not assessable by current echocardiographic measures. These findings indicate that TKE may provide complementary information to echocardiography, helping to distinguish within the heterogeneous population of patients with moderate to severe AS.

A g‐factor metric for k‐t SENSE and k‐t PCA based parallel imaging

To propose and validate a g‐factor formalism for k‐t SENSE, k‐t PCA and related k‐t methods for assessing SNR and temporal fidelity.

A g-factor metric for k-t-GRAPPA- and PEAK-GRAPPA-based parallel imaging

The aim of this work is to derive a theoretical framework for quantitative noise and temporal fidelity analysis of time‐resolved k‐space‐based parallel imaging methods.

Accelerating 4D flow MRI by exploiting low-rank matrix structure and hadamard sparsity

To develop accelerated 4D flow MRI by exploiting low‐rank matrix structure and Hadamard sparsity.

Assessment of energy loss in aortic stenosis using Bayesian multipoint phase-contrast MRI

(TKE) and energy loss indices (ELI) are listed in Table 1. Compared to our previous data obtained in healthy subjects [3], peak TKE in the patients presented here was found to be significantly increased (149±12 J/m 3 vs. 1350 and 1630 J/m 3 , respectively). It is noteworthy that the patient with higher TKE values had a lower energy loss index. Values from Doppler echocardiography are given for comparison. Figure 1 shows maps of TKE in the aortic arch in both patients along with flow patterns derived from the velocity data. Conclusions Bayesian multipoint PC-MRI permits concurrent mapping of both mean kinetic and turbulent kinetic energy in patients and allows the assessment of relative energy loss and pressure gradients associated with aortic valve stenosis. The energy loss index was found to be approximately 8-fold higher as compared to healthy subjects [2] and may hold promise to serve as a novel marker for grading valve disease.

Assessment of 3D velocity vector fields and turbulent kinetic energy in a realistic aortic phantom using multi-point variable-density velocity encoding

A multi-point velocity encoding approach for the assessment of velocity vector fields and TKE is shown in this work. The method is applied in an aortic arch phantom under different flow conditions.