In this study, we analyzed turbulent flows through a phantom (a 180Formula: see text bend with narrowing) at peak systole and a patient-specific coarctation of the aorta (CoA), with a pulsating flow, ...using magnetic resonance imaging (MRI) and computational fluid dynamics (CFD). For MRI, a 4D-flow MRI is performed using a 3T scanner. For CFD, the standard Formula: see text, shear stress transport Formula: see text, and Reynolds stress (RSM) models are applied. A good agreement between measured and simulated velocity is obtained for the phantom, especially for CFD with RSM. The wall shear stress (WSS) shows significant differences between CFD and MRI in absolute values, due to the limited near-wall resolution of MRI. However, normalized WSS shows qualitatively very similar distributions of the local values between MRI and CFD. Finally, a direct comparison between in vivo 4D-flow MRI and CFD with the RSM turbulence model is performed in the CoA. MRI can properly identify regions with locally elevated or suppressed WSS. If the exact values of the WSS are necessary, CFD is the preferred method. For future applications, we recommend the use of the combined MRI/CFD method for analysis and evaluation of the local flow patterns and WSS in the aorta.
Doppler echocardiographic aortic valve peak velocity and peak pressure gradient assessment across the aortic valve (AV) is the mainstay for diagnosing aortic stenosis. Four-dimensional flow ...cardiovascular magnetic resonance (4D flow CMR) is emerging as a valuable diagnostic tool for estimating the peak pressure drop across the aortic valve, but assessment remains cumbersome. We aimed to validate a novel semi-automated pipeline 4D flow CMR method of assessing peak aortic value pressure gradient (AVPG) using the commercially available software solution, CAAS MR Solutions, against invasive angiographic methods.
We enrolled 11 patients with severe AS on echocardiography from the EurValve programme. All patients had pre-intervention doppler echocardiography, invasive cardiac catheterisation with peak pressure drop assessment across the AV and 4D flow CMR. The peak AVPG was 51.9 ± 35.2 mmHg using the invasive pressure drop method and 52.2 ± 29.2 mmHg for the 4D flow CMR method (semi-automated pipeline), with good correlation between the two methods (r = 0.70, p = 0.017). Assessment of AVPG by 4D flow CMR using the novel semi-automated pipeline method shows excellent agreement to invasive assessment when compared to doppler-based methods and advocate for its use as complementary to echocardiography.
Noninvasive evaluation of vessel wall properties in humans is hampered by the absence of methods to assess directly local distensibility, compliance, and Young’s modulus. Contemporary ultrasound ...methods are capable of assessing end-diastolic artery diameter, the local change in artery diameter as a function of time, and local wall thickness. However, to assess vessel wall properties of the carotid artery, for example, the pulse pressure in the brachial artery still must be used as a substitute for local pulse pressure. The assessment of local pulse wave velocity as described in the present article provides a direct estimate of local vessel wall properties (distensibility, compliance, and Young’s modulus) and, in combination with the relative change in artery cross-sectional area, an estimate of the local pulse pressure. The local pulse wave velocity is obtained by processing radio frequency ultrasound signals acquired simultaneously along two M-lines spaced at a known distance along the artery. A full derivation and mathematical description of the method to assess local pulse wave velocity, using the temporal and longitudinal gradients of the change in diameter, are presented. A performance evaluation of the method was carried out by means of experiments in an elastic tube under pulsatile pressure conditions. It is concluded that, in a phantom set-up, the assessed local pulse wave velocity provides reliable estimates for local distensibility.
We aim to validate four-dimensional flow cardiovascular magnetic resonance (4D flow CMR) peak velocity tracking methods for measuring the peak velocity of mitral inflow against Doppler ...echocardiography.
Fifty patients were recruited who had 4D flow CMR and Doppler Echocardiography. After transvalvular flow segmentation using established valve tracking methods, peak velocity was automatically derived using three-dimensional streamlines of transvalvular flow. In addition, a static-planar method was used at the tip of mitral valve to mimic Doppler technique.
Peak E-wave mitral inflow velocity was comparable between TTE and the novel 4D flow automated dynamic method (0.9 ± 0.5 vs 0.94 ± 0.6 m/s; p = 0.29) however there was a statistically significant difference when compared with the static planar method (0.85 ± 0.5 m/s; p = 0.01). Median A-wave peak velocity was also comparable across TTE and the automated dynamic streamline (0.77 ± 0.4 vs 0.76 ± 0.4 m/s; p = 0.77). A significant difference was seen with the static planar method (0.68 ± 0.5 m/s; p = 0.04). E/A ratio was comparable between TTE and both the automated dynamic and static planar method (1.1 ± 0.7 vs 1.15 ± 0.5 m/s; p = 0.74 and 1.15 ± 0.5 m/s; p = 0.5 respectively). Both novel 4D flow methods showed good correlation with TTE for E-wave (dynamic method; r = 0.70; P < 0.001 and static-planar method; r = 0.67; P < 0.001) and A-wave velocity measurements (dynamic method; r = 0.83; P < 0.001 and static method; r = 0.71; P < 0.001). The automated dynamic method demonstrated excellent intra/inter-observer reproducibility for all parameters.
Automated dynamic peak velocity tracing method using 4D flow CMR is comparable to Doppler echocardiography for mitral inflow assessment and has excellent reproducibility for clinical use.
•4D flow CMR shows good agreement with doppler echocardiography for mitral inflow peak velocity measurement.•This study suggests that 4D flow CMR is highly reproducible in mitral inflow peak velocity measurement.•4D flow CMR is an accurate and reliable non-invasive imaging method for left ventricular diastolic assessment.
Ultrasound contrast agents, i.e., small gas filled microbubbles, enhance the echogenicity of blood and have the potential to be used for tissue perfusion assessment. The contrast agents scatter ...ultrasound in a nonlinear manner and thereby introduce harmonics in the ultrasound signal. This property is exploited in new ultrasound techniques like harmonic imaging, which aims to display only the contrast agent presence. Much attention has already been given to the physical properties of the contrast agent. The present study focuses on practical aspects of the measurement of the intrinsic harmonic response of ultrasound contrast agents with single transducer pulse waved ultrasound systems. Furthermore, the consequences of two other sources of harmonics are discussed. These sources are the nonlinear distortion of ultrasound in a medium generating native harmonics, and the emitted signal itself which might contain contaminating harmonics. It is demonstrated conceptually and by experiments that optimization of the contrast agent harmonic response measured with a single transducer is governed by the transducer spectral sensitivity distribution rather than the resonance properties of the contrast agent. Both native and contaminating harmonics may be of considerable strength and can be misinterpreted as intrinsic harmonics of the contrast agent. Practical difficulties to filter out the harmonic component selectively, without deteriorating the image, may cause misinterpretation of the fundamental as a harmonic.
Conventional pulsed ultrasound systems are only able to detect motion along the ultrasound beam (
i.e., axial motion). If the angle between the actual motion direction and the ultrasound beam is ...known, then the magnitude of the actual motion can be derived. This technique can be applied for laminar blood-flow measurements in straight vessels, but for tissue motion it is inadequate because the local tissue motion direction is unknown and may be position-dependent. Assessment of both the axial motion and the lateral motion (
i.e., in the direction perpendicular to the ultrasound beam) makes angle-independent assessment of the magnitude of the actual motion feasible. Information about the axial and lateral motion is available in a set of radiofrequency (RF) signals obtained along the same line of observation (M-mode). The experiments described in the present paper show that axial and lateral motion can be estimated from the shape of the envelope of the 2-D (spatial and temporal) correlation function of analytic M-mode RF signals. Furthermore, it is demonstrated that the shape is also affected by the Band width of the received RF signals, signal-to–noise ratio, and local amplitude and phase characteristics of the ultrasound beam.
All Doppler systems, whether conventional Doppler domain or radio frequency (RF) processing is employed, relate the temporal frequency characteristics of the signal at a certain point in depth as ...function of time to the spatial frequency characteristics of the received signal as function of depth. The mean frequency of the latter may change as a result of depth-dependent attenuation, nonlinear scattering mechanisms, as in harmonic imaging of ultrasound contrast agents, or RF signal demodulation. For all these cases, the relationship between spatial and temporal mean frequency and target velocity is still governed by the familiar Doppler expression if the signal modifications have been properly accounted for. A major drawback of RF signal processing to extract the target velocity is the large number of data points to consider. The computational complexity increases further for harmonic imaging. It is shown conceptually, and demonstrated by signal simulations, that prior to velocity estimation RF demodulation followed by decimation (1) does not affect the Doppler equation, (2) enhances the information content of the samples, (3) reduces the computational load by a factor of four and for harmonic signals by a higher factor, and (4) while demodulation does not have to be actually performed, but can be accounted for by a scaling factor in the cross-correlation function. It is concluded that decimation hardly affects the precision of the velocity estimate if possible frequency aliasing is maintained within bounds, suggesting that the decimation factor is not critical.
The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, ...harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.
In pulsed Doppler ultrasound systems, the ultrasound radiofrequency (RF) signals received can be employed to estimate noninvasively the time-dependent blood velocity distribution within and artery. ...The RF signals are composed of signals originating from clutter (e.g., vessel walls) and scatterers (e.g., red blood cells). The clutter, which is induced by stationary or slowly-moving structure interfaces, must be suppressed to get reliable estimates of the mean blood flow velocities. In conventional pulsed Doppler systems, this is achieved with a static temporal high-pass filter. The static cut-off frequency and the roll-off of these filters cause the culture not always to be optimally suppressed. This paper introduces a clutter removal filter that is based on Singular Value Decomposition (SVD). Unlike conventional high-pass filters, which take into account only the information of the temporal direction, the SVD filter makes use of the information of the temporal and spatial directions. The advantage of this approach is that it does not matter where the clutter is located in the RF signal. The performance of the SVD filter is examined with computer-generated Doppler RF signals. The results are compared with those of standard linear regression (SLR) filter. The performance of the SVD filter is good, especially if a large temporal window (i.e., approximately 100 RF signals) is applied, which improves the performance for low blood flow velocities, A major disadvantage of the SVD filter is its computational complexity, which increases considerably for larger temporal windows.
This article introduces a mean frequency estimator based on a radio frequency (RF) domain complex cross-correlation model (C3M). The C3M estimator differs from the real cross-correlation model (CCM) ...estimator in two respects; it is an unbiased estimator of blood flow velocity and/or tissue motion independent of the bandwidth of the RF ultrasound signals, and it provides an estimate of the spatial bandwidth of the RF-signal. The estimators derived from the complex cross-correlation model (mean spatial frequency, mean temporal frequency, spatial bandwidth and signal-to-noise ratio) are based on three complex cross-correlation coefficients. A full derivation and mathematical description of both estimators (C3M and CCM), starting from a Gaussian model of the complex power spectral density distribution of sampled RF signals, are presented. In addition, a thorough performance evaluation of the C3M estimator in comparison with the CCM estimator is carried out by means of simulations to document the effect of signal-to-noise ratio, bandwidth and sample frequency. In the context of the specific simulation conditions considered, the quality of the C3M estimator is shown to offer the best performance (no bias, low standard deviation of the estimate). Taking into account the computational load and the robustness of the C3M estimator, it may be concluded that the C3M estimator combines high quality and modest complexity.
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