Assessing the performance of ultrafast vector flow imaging in the neonatal heart via multiphysics modeling and In vitro experiments

Ultrafast vector flow imaging would benefit newborn patients with congenital heart disorders, but still requires thorough validation before translation to clinical practice. This paper investigates 2-D speckle tracking (ST) of intraventricular blood flow in neonates when transmitting diverging waves at ultrafast frame rate. Computational and in vitro studies enabled us to quantify the performance and identify artifacts related to the flow and the imaging sequence. First, synthetic ultrasound images of a neonate's left ventricular flow pattern were obtained with the ultrasound simulator Field II by propagating point scatterers according to 3-D intraventricular flow fields obtained with computational fluid dynamics (CFD). Noncompounded diverging waves (opening angle of 60 degrees) were transmitted at a pulse repetition frequency of 9 kHz. ST of the B-mode data provided 2-D flow estimates at 180 Hz, which were compared with the CFD flow field. We demonstrated that the diastolic inflow jet showed a strong bias in the lateral velocity estimates at the edges of the jet, as confirmed by additional in vitro tests on a jet flow phantom. Furthermore, ST performance was highly dependent on the cardiac phase with low flows (< 5 cm/s), high spatial flow gradients, and out-of-plane flow as deteriorating factors. Despite the observed artifacts, a good overall performance of 2-D ST was obtained with a median magnitude underestimation and angular deviation of, respectively, 28% and 13.5 degrees during systole and 16% and 10.5 degrees during diastole

The multigate Doppler approach for assessing hemodynamics in a forearm vascular access for hemodialysis purposes

We compared the multigate and single gate pulsed wave Doppler (PWD) approach to assess the hemodynamics in an arteriovenous fistula for hemodialysis purposes. In particular, we evaluated volume flow and wall shear rate measurements based on PWD because: (i) volume flow measurements are typically performed to assess the maturation of the arteriovenous fistula (AVF), with low postoperative fistula flow being a prognostic marker for failure of the AVF; (ii) wall shear stress may play an important role in vascular remodeling processes of the fistula. As in-vivo validation of fistula flow measurements is cumbersome, we performed simulations integrating computational fluid dynamics with an ultrasound (US) simulator. Flow in the arm was calculated, based on a patient-specific model of the arm vasculature pre and post AVF creation. Next, raw ultrasound signals were simulated and processed using a multigate and single gate Doppler approach in the radial artery, both for the preop and postop setting. Results showed that the extreme postop flow conditions (high velocity magnitudes and complex flow profiles) hampered accurate assessment of both wall shear rate and volume flow, while reasonable estimates were obtained preop (lower velocity magnitudes and laminar flow)

Vector flow mapping using plane wave ultrasound imaging

Conventional pulse wave Doppler techniques can only provide one-dimensional blood velocity components parallel to the direction of the beam and conventional focusing provides limited frame rates of about 30-40 frames per second. As a solution to these well known limitations we perform a two-dimensional vector mapping using compounded coplanar oriented plane waves, analogous to vector-Doppler. Our method was tested using Field II simulations of both stationary parabolic pipe flow and computational fluid dynamics determined flow through a patient specific carotid artery. Our results show the ability for this method to provide more discernible representation of the flow dynamics compared with conventional color-Doppler imaging, while maintaining a frame rate of roughly 500 frames per second. Quantitative comparison with known velocity fields provides robust validation and demonstrates error comparable to that found in literature using conventional Doppler measurements. Moreover, this method provides a promising means to quantify quick transitory events and complex flow structures unattainable with clinical color-Doppler

Impact of competitive flow on wall shear stress in coronary surgery: computational fluid dynamics of a LIMA-LAD model

Competitive flow from native coronary vessels is considered a major factor in the failure of coronary bypass grafts. However, the pathophysiological effects are not fully understood. Low and oscillatory wall shear stress (WSS) is known to induce endothelial dysfunction and vascular disease, like atherosclerosis and intimal hyperplasia. The aim was to investigate the impact of competitive flow on WSS in mammary artery bypass grafts. Using computational fluid dynamics, WSS was calculated in a left internal mammary artery (LIMA) graft to the left anterior descending artery in a three-dimensional in vivo porcine coronary artery bypass graft model. The following conditions were investigated: high competitive flow (non-significant coronary lesion), partial competitive flow (significant coronary lesion), and no competitive flow (totally occluded coronary vessel). Time-averaged WSS of LIMA at high, partial, and no competitive flow were 0.3-0.6, 0.6-3.0, and 0.9-3.0 Pa, respectively. Further, oscillatory WSS quantified as the oscillatory shear index (OSI) ranged from (maximum OSI = 0.5 equals zero net WSS) 0.15 to 0.35, < 0.05, and < 0.05, respectively. Thus, high competitive flow resulted in substantial oscillatory and low WSS. Moderate competitive flow resulted in WSS and OSI similar to the no competitive flow condition. Graft flow is highly dependent on the degree of competitive flow. High competitive flow was found to produce unfavourable WSS consistent with endothelial dysfunction and subsequent graft narrowing and failure. Partial competitive flow, however, may be better tolerated as it was found to be similar to the ideal condition of no competitive flow