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In this study, newly developed two-equation turbulence models and transitional variants are employed for the prediction of blood flow patterns in a diseased carotid artery where the growth, progression, and structure of the plaque at rupture are closely linked to low and oscillating wall shear stresses. Moreover, the laminar-turbulent transition in the poststenotic zone can alter the separation zone length, wall shear stress, and pressure distribution over the plaque, with potential implications for stresses within the plaque. Following the validation with well established experimental measurements and numerical studies, a magnetic-resonance (MR) image-based model of the carotid bifurcation with 70% stenosis was reconstructed and simulated using realistic patient-specific conditions. Laminar flow, a correlation-based transitional version of Menter's hybrid k-epsilon/k-omega shear stress transport (SST) model and its "scale adaptive simulation" (SAS) variant were implemented in pulsatile simulations from which analyses of velocity profiles, wall shear stress, and turbulence intensity were conducted. In general, the transitional version of SST and its SAS variant are shown to give a better overall agreement than their standard counterparts with experimental data for pulsatile flow in an axisymmetric stenosed tube. For the patient-specific case reported, the wall shear stress analysis showed discernable differences between the laminar flow and SST transitional models but virtually no difference between the SST transitional model and its SAS variant.

The aim of this study is to investigate the blood flow pattern in carotid bifurcation with a high degree of luminal stenosis, combining in vivo magnetic resonance imaging (MRI) and computational fluid dynamics (CFD). A newly developed two-equation transitional model was employed to evaluate wall shear stress (WSS) distribution and pressure drop across the stenosis, which are closely related to plaque vulnerability. A patient with an 80% left carotid stenosis was imaged using high resolution MRI, from which a patient-specific geometry was reconstructed and flow boundary conditions were acquired for CFD simulation. A transitional model was implemented to investigate the flow velocity and WSS distribution in the patient-specific model. The peak time-averaged WSS value of approximately 73 Pa was predicted by the transitional flow model, and the regions of high WSS occurred at the throat of the stenosis. High oscillatory shear index values up to 0.50 were present in a helical flow pattern from the outer wall of the internal carotid artery immediately after the throat. This study shows the potential suitability of a transitional turbulent flow model in capturing the flow phenomena in severely stenosed carotid arteries using patient-specific MRI data and provides the basis for further investigation of the links between haemodynamic variables and plaque vulnerability. It may be useful in the future for risk assessment of patients with carotid disease.

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Halloysite nanotubes (HNTs) are nanomaterials composed of double layered aluminosilicate minerals characterized by a wide range of medical applications. Nonetheless, systematic investigations of their imaging potential are still poorly documented. This paper shows a parametric assessment of the effectiveness of HNTs as scatterers for safe ultrasound (US)-based molecular imaging. Quantitative evaluation of average signal enhancement produced by HNTs with varying set up configuration was performed. The influence of different levels of power (20%, 50%, and 80%) of the signal emitted by clinical equipment was determined, to assess the efficacy of different HNT concentrations (1.5, 3, and 5 mg/mL) at conventional ultrasonic frequencies (5.7-7 MHz), even in case of specific limitation regarding US mechanical interaction with target tissues. Different samples of HNT containing agarose gel were imaged through a commercially available echographic system and acquired data were processed through a dedicated prototypal platform to extract the average ultrasonic signal amplitude. The rate of signal enhancement achieved by different concentration values was quantified and the contribution of frequency increment was separately evaluated. Despite influencing the level of mechanical excitation on HNTs and tissues, our results demonstrated how increasing the power of the emitted signal negatively affected the measured backscatter. Conversely, noticeable improvements in signal backscatter could be achieved incrementing HNT concentration and the echographic frequency employed; specifically the signal enhancement over the used concentration range could be improved by averagely 20%, corresponding to 4.86 ± 0.80 (a.u.), when employing the higher value of echographic frequency.

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Halloysite Nanotubes (HNTs) are nanomaterials composed of double layered aluminosilicate minerals with a predominantly hollow tubular structure in submicron range. HNTs are characterized by a wide range of applications in anticancer therapy, sustained agent delivery, being particularly interesting because of their tunable release rates and fast adsorption rates. However systematic investigations of their acoustic properties are still poorly documented. This paper shows a quantitative assessment of the effectiveness of HNTs as scatterers at conventional ultrasonic frequencies (5.7 -7 MHz) in low range of concentrations (1.5-5 mg/mL). Different samples of HNT (diameter: 40-50 nm; length: 0.5 to 2 microns, empty lumen diameter: 15-20 nm) containing agarose gel were imaged through a commercially available echographic system and acquired data were processed through a dedicated prototypal platform in order to extract the average ultrasonic signal amplitude associated to the considered sample. Relationships have been established among backscatter, HNT concentration and the employed echographic frequency. Our results demonstrated that improvement in image backscatter could be achieved incrementing HNT concentration, determining a non-linear signal enhancement due to the fact that they are poly-disperse in length. On the other hand the effect of different echographic frequencies used was almost constant at all concentrations, specifically using higher values of echographic frequency allows yielding a signal enhanced of a factor 1.75±0.26.