Comparison of vortical structures induced by arteriovenous grafts using vector Doppler ultrasound

A Polyvinyl Alcohol-Based Thermochromic Material for Ultrasound Therapy Phantoms

Temperature estimation is a fundamental step in assessment of the efficacy of thermal therapy. A thermochromic material sensitive within the temperature range 52.5°C-75°C has been developed. The material is based on polyvinyl alcohol cryogel with the addition of a commercial thermochromic ink. It is simple to manufacture, low cost, non-toxic and versatile. The thermal response of the material was evaluated using multiple methods, including immersion in a temperature-controlled water bath, a temperature-controlled heated needle and high-intensity focused ultrasound (HIFU) sonication. Changes in colour were evaluated using both RGB (red, green, blue) maps and pixel intensities. Acoustic and thermal properties of the material were measured. Thermo-acoustic simulations were run with an open-source software, and results were compared with the HIFU experiments, showing good agreement. The material has good potential for the development of ultrasound therapy phantoms.

Review of Experimental Modelling in Vascular Access for Hemodialysis

This paper reviews applications of experimental modelling in vascular access for hemodialysis. Different techniques that are used in in-vitro experiments are bulk pressure and flow rate measurements, Laser Doppler Velocimetry and Vector Doppler Ultrasound point velocity measurements, and whole-field measurements such as Particle Image Velocimetry, Ultrasound Imaging Velocimetry, Colour Doppler Ultrasound, and Planar Laser Induced Fluorescence. Of these methods, the ultrasound techniques can also be used in-vivo, to provide realistic boundary conditions to in-vitro experiments or numerical simulations. In the reviewed work, experimental modelling is mainly used to support computational models, but also in some cases as a tool on its own. It is concluded that, to further advance the utility of computational modelling in vascular access research, a rigorous verification and validation procedure should be adopted. Experimental modelling can play an important role in both in-vitro validation, and the quantification of the accuracy, uncertainty, and reproducibility of in-vivo measurement methods.

Investigation of Crossbeam Multi-receiver Configurations for Accurate 3-D Vector Doppler Velocity Estimation

An accurate estimation of low blood velocities whose Doppler shifts span the wall filter cutoff, such as near the wall in recirculation or disturbed flow regions, is important for accurate mapping of velocities to achieve improved estimations of wall shear stress and turbulence, which are known risk factors for atherosclerosis and stroke. This paper presents the comparative benefit of increasing the number of receiver beams above three for an improved estimation of low 3-D velocities. The 3-D crossbeam vector Doppler ultrasound configurations were studied in terms of the number of receiver beams, interbeam angle, and beam selection method (criterion for discriminating between tissue and blood Doppler signals) for a range of velocity orientations, which may prove useful in the design of a future 2-D array for vascular imaging. For maximum velocity resolution, a shallow gradient of low flow velocities up to 5 cm/s was generated across a large-diameter (2.46 cm) straight vessel. Data were acquired using a linear array rotated around the central transmit beam axis to generate three- to eight-receiver (3R-8R) configurations;the rotation of each configuration relative to the flow axis was used to mimic a broad range of velocity vector orientations. Accuracy and precision for ≥5 receivers were consistently better over all velocity orientations and for all selection methods. For a velocity magnitude of 2 cm/s, the best accuracy and precision in both magnitude and direction (~21% ± 13%, <1° ± 9°, respectively) were seen with a 5R configuration using a weighted least-squares selection method. Asymmetry in the 5R configuration led to an improved accuracy and precision compared with that in symmetrical 6R and 8R configurations. The results demonstrated relatively little to no benefit from more than five receiver beams.

Graft Modification Strategies to Improve Patency of Prosthetic Arteriovenous Grafts for Hemodialysis

Prosthetic arteriovenous grafts (AVGs) are indicated for vascular access for long-term hemodialysis in patients in whom creation or maintenance of an arteriovenous fistula (AVF) has failed or is contraindicated. AVGs have an inferior long-term patency as compared to AVFs. To ameliorate patency rates of prosthetic AVGs, different strategies have emerged to improve graft materials. This review aims to describe current strategies and future perspectives on graft modification, by graft geometry, drug coatings and graft surface technology, to improve AVG patency.

Haemodynamics and Flow Modification Stents for Peripheral Arterial Disease: A Review

Endovascular stents are widely used for the treatment of peripheral arterial disease (PAD). However, the development of in-stent restenosis and downstream PAD progression remain a challenge. Stent revascularisation of PAD causes arterial trauma and introduces abnormal haemodynamics, which initiate complicated biological processes detrimental to the arterial wall. The interaction between stent struts and arterial cells in contact, and the blood flow field created in a stented region, are highly affected by stent design. Spiral flow is known as a normal physiologic characteristic of arterial circulation and is believed to prevent the development of flow disturbances. This secondary flow motion is lost in atheromatous disease and its re-introduction after endovascular treatment of PAD has been suggested as a method to induce stabilised and coherent haemodynamics. Stent designs able to generate spiral flow may support endothelial function and therefore increase patency rates. This review is focused on secondary flow phenomena in arteries and the development of flow modification stent technologies for the treatment of PAD.