A real vessel phantom for flow imaging: 3-D Doppler ultrasound of steady flow

Vascular phantoms are used to assess the capabilities of various imaging techniques, such as x-ray CT and angiography, and B-mode, power Doppler, and colour Doppler ultrasound (US). They should, therefore, accurately mimic the vasculature, blood, and surrounding tissue, in regard to both imaging properties and vessel geometry. In the past, a variety of walled and wall-less vessel models have been used. However, these models only approximate the true vessel geometry, and generally lack pathologic features such as plaques or calcifications. To amend these deficiencies, we have developed a real vessel phantom for US and x-ray studies, which comprises a fixed human vessel specimen, cannulated onto two acrylic tubes, and embedded in agar in an acrylic box. Earlier, we demonstrated a good overall correlation between x-ray angiography, CT, and 3-D B-mode US images of this phantom. Here, we extend its use to flow imaging with 3-D power and 3-D colour Doppler US.     

Ultrasound Phantom Using Sodium Alginate as a Gelling Agent

For medical workers, ultrasound phantoms for human soft tissue are used not only for accuracy management of ultrasound diagnosis but also to aid ultrasound‐guided needle and blind catheter insertion training without risk to real patients. For the phantoms, ultrasound characteristics and a texture are required to mimic the human soft tissue. The proposed phantom was composed of sodium alginate, calcium sulfate dihydrate, trisodium phosphate 12‐hydrate, glycerol, and water. The propagation speed, attenuation coefficient, acoustic impedance, and texture of the proposed phantom were almost the same as those of human soft tissue. Expensive chemicals and special equipment are not required.     

Acoustic characterization of tissue-mimicking materials for ultrasound perfusion imaging research

Materials with well-characterized acoustic properties are of great interest for the development of tissue-mimicking phantoms with designed (micro)vasculature networks. These represent a useful means for controlled in-vitro experiments to validate perfusion imaging methods such as Doppler and contrast-enhanced ultrasound (CEUS) imaging. In this work, acoustic properties of seven tissue-mimicking phantom materials at different concentrations of their compounds and five phantom case materials are characterized and compared at room temperature. The goal of this research is to determine the most suitable phantom and case material for ultrasound perfusion imaging experiments. The measurements show a wide range in speed of sound varying from 1057 to 1616 m/s, acoustic impedance varying from 1.09 to 1.71 × 10⁶ kg/m²s, and attenuation coefficients varying from 0.1 to 22.18 dB/cm at frequencies varying from 1 MHz to 6 MHz for different phantom materials. The nonlinearity parameter B/A varies from 6.1 to 12.3 for most phantom materials. This work also reports the speed of sound, acoustic impedance and attenuation coefficient for case materials. According to our results, polyacrylamide (PAA) and polymethylpentene (TPX) are the optimal materials for phantoms and their cases, respectively. To demonstrate the performance of the optimal materials, we performed power Doppler ultrasound imaging of a perfusable phantom, and CEUS imaging of that phantom and a perfusion system. The obtained results can assist researchers in the selection of the most suited materials for in-vitro studies with ultrasound imaging.     

Doppler ultrasound compatible plastic material for use in rigid flow models

A technique for the rapid but accurate fabrication of multiple flow phantoms with variations in vascular geometry would be desirable in the investigation of carotid atherosclerosis. This study demonstrates the feasibility and efficacy of implementing numerically controlled direct-machining of vascular geometries into Doppler ultrasound (DUS)-compatible plastic for the easy fabrication of DUS flow phantoms. Candidate plastics were tested for longitudinal speed of sound (SoS) and acoustic attenuation at the diagnostic frequency of 5 MHz. Teflon was found to have the most appropriate SoS (1376 +/- 40 m s(-1) compared with 1540 m s(-1) in soft tissue) and thus was selected to construct a carotid bifurcation flow model with moderate eccentric stenosis. The vessel geometry was machined directly into Teflon using a numerically controlled milling technique. Geometric accuracy of the phantom lumen was verified using nondestructive micro-computed tomography. Although Teflon displayed a higher attenuation coefficient than other tested materials, Doppler data acquired in the Teflon flow model indicated that sufficient signal power was delivered throughout the depth of the vessel and provided comparable velocity profiles to that obtained in the tissue-mimicking phantom. Our results indicate that Teflon provides the best combination of machinability and DUS compatibility, making it an appropriate choice for the fabrication of rigid DUS flow models using a direct-machining method.     

Anatomical flow phantoms of the nonplanar carotid bifurcation, part II: experimental validation with Doppler ultrasound

A nonplanar wall-less anatomical flow phantom of a healthy human carotid artery is described, the construction of which is based on a lost-core technique described in the companion paper (Part I) by . The core was made by rapid prototyping of an idealized three-dimensional computer model of the carotid artery. Flow phantoms were built using these idealized non planar carotid artery bifurcations. Physiologically realistic flow waveforms were produced with resistance index values of 0.75, 0.72 and 0.63 in the common, external and internal carotid artery branches, respectively. Distension of the common carotid using M-mode imaging was found to be at 10% of diameter. Although differences in vessel diameter between the phantom and that of the original computer model were statistically significant (p < 0.05), there was no difference (p > 0.05) in measurements made on the lost-cores and those obtained by B-mode ultrasound on the resulting flow phantoms. In conclusion, it was possible to reliably reproduce geometrically similar anatomical flow phantoms that are capable of producing realistic physiological flow patterns and distensions.     

Ultrasound Doppler Monitoring of Soft Tissues In Vitro and Tissue Phantoms Heating and Thermal Destruction Induced by Acoustic Remote Palpation

The rise of shear strain value under temperature increase in biological tissue samples in vitro and tissue phantoms was studied and the range of shear modulus and viscosity calculated. It has been shown that the acoustic radiation force-based methods with the usage of ultrasound Doppler probing provides the potential ability of noninvasive real-time monitoring of tissues' ultrasound thermal destruction process. At that, the thermal destruction is possible under action of wave beam that creates the radiation force and local tissue displacements so that tissue ablation and acoustic remote palpation could be realized by means of the same ultrasound transducer. The experiments were performed using gelatin-based tissue-mimicking phantoms and freshly excised samples of bovine muscle tissue. It was determined also that fluctuating pattern of detected displacement amplitude variation is the indicator of the phase transitions beginning in the heated field of soft tissue or tissue phantom. (Email: Evgenij.A.Barannik@univer.kharkov.ua; barannik@pht.univer.kharkov.ua)     

Simulation and validation of arterial ultrasound imaging and blood flow

We reviewed the simulation and validation of arterial ultrasound imaging and blood flow assessment. The physical process of ultrasound imaging and measurement is complex, especially in disease. Simulation of physiological flow in a phantom with tissue equivalence of soft tissue, vessel wall and blood is now achievable. Outstanding issues are concerned with production of anatomical models, simulation of arterial disease, refinement of blood mimics to account for non-Newtonian behavior and validation of velocity measurements against an independent technique such as particle image velocimetry. String and belt phantoms offer simplicity of design, especially for evaluation of velocity estimators, and have a role as portable test objects. Electronic injection and vibrating test objects produce nonphysiologic Doppler signals, and their role is limited. Computational models of the ultrasound imaging and measurement process offer considerable flexibility in their ability to alter multiple parameters of both the propagation medium and ultrasound instrument. For these models, outstanding issues are concerned with the inclusion of different tissue types, multilayer arteries, inhomogeneous tissues and diseased tissues. (E-mail: P.Hoskins@ed.ac.uk)     

Copolymer-in-oil Phantom Materials for Elastography

Phantoms that mimic mechanical and acoustic properties of soft biological tissues are essential to elasticity imaging investigation and to elastography device characterization. Several materials including agar/gelatin, polyvinyl alcohol and polyacrylamide gels have been used successfully in the past to produce tissue phantoms, as reported in the literature. However, it is difficult to find a phantom material with a wide range of stiffness, good stability over time and high resistance to rupture. We aim at developing and testing a new copolymer-in-oil phantom material for elastography. The phantom is composed of a mixture of copolymer, mineral oil and additives for acoustic scattering. The mechanical properties of phantoms were evaluated with a mechanical test instrument and an ultrasound-based elastography technique. The acoustic properties were investigated using a through-transmission water-substituting method. We showed that copolymer-in-oil phantoms are stable over time. Their mechanical and acoustic properties mimic those of most soft tissues: the Young's modulus ranges from 2.2–150 kPa, the attenuation coefficient from 0.4–4.0 dB.cm^–1 and the ultrasound speed from 1420–1464 m/s. Their density is equal to 0.90 ± 0.04 g/cm³. The results suggest that copolymer-in-oil phantoms are attractive materials for elastography. (E-mail: jennifer.oudry@echosens.com)     

乳腺肿块超声血流显示的影响因素及其良恶性诊断价值

目的探讨乳腺肿块超声血流显示的影响因素及其对良、恶性肿块的鉴别诊断价值。方法对408个乳腺良、恶性肿块进行灰阶及彩色多普勒超声血流检查，分别记录其血流数、灰阶声像图特征进行回顾性分析。结果超声显示的血流数随肿块大小不同而具有显著差异，肿瘤后方衰减征象、内部坏死液性区征象在未检出血流的乳腺癌组中均显著高于有血流组。以超声检出的血流数为指标诊断乳腺良、恶性肿块的敏感性、特异性、阳性预测值、阴性预测值均较低。结论肿块大小、肿瘤后方衰减、内部坏死液性区征象均显著影响彩色多普勒超声对乳腺癌内血流的检测，超声检出的血流数不能作为乳腺良、恶性肿块鉴别诊断的直接有效依据。     

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.     

Design of Anthropomorphic Flow Phantoms Based on Rapid Prototyping of Compliant Vessel Geometries

Anatomically realistic flow phantoms are essential experimental tools for vascular ultrasound. Here we describe how these flow phantoms can be efficiently developed via a rapid prototyping (RP) framework that involves direct fabrication of compliant vessel geometries. In this framework, anthropomorphic vessel models were drafted in computer-aided design software, and they were fabricated using stereolithography (one type of RP). To produce elastic vessels, a compliant photopolymer was used for stereolithography. We fabricated a series of compliant, diseased carotid bifurcation models with eccentric stenosis (50%) and plaque ulceration (types I and III), and they were used to form thin-walled flow phantoms by coupling the vessels to an agar-based tissue-mimicking material. These phantoms were found to yield Doppler spectrograms with significant spectral broadening and color flow images with mosaic patterns, as typical of disturbed flow under stenosed and ulcerated disease conditions. Also, their wall distension behavior was found to be similar to that observed in vivo, and this corresponded with the vessel wall’s average elastic modulus (391 kPa), which was within the nominal range for human arteries. The vessel material’s acoustic properties were found to be sub-optimal: the estimated average acoustic speed was 1801 m/s, and the attenuation coefficient was 1.58 dB/(mm·MHzⁿ) with a power-law coefficient of 0.97. Such an acoustic mismatch nevertheless did not notably affect our Doppler spectrograms and color flow image results. These findings suggest that phantoms produced from our design framework have the potential to serve as ultrasound-compatible test beds that can simulate complex flow dynamics similar to those observed in real vasculature.     

Tissue-Mimicking Materials for Ultrasound-Guided Needle Intervention Phantoms: A Comprehensive Review

Ultrasound-guided needle interventions are common procedures in medicine, and tissue-mimicking phantoms are widely used for simulation training to bridge the gap between theory and clinical practice in a controlled environment. This review assesses tissue-mimicking materials from 24 studies as candidates for a high-fidelity ultrasound phantom, including methods for evaluating relevant acoustic and mechanical properties and to what extent the reported materials mimic the superficial layers of biological tissue. Speed of sound, acoustic attenuation, Young's modulus, hardness, needle interaction forces, training efficiency and material limitations were systematically evaluated. Although gelatin and agar have the closest acoustic values to tissue, mechanical properties are limited, and strict storage protocols must be employed to counteract dehydration and microbial growth. Polyvinyl chloride (PVC) has superior mechanical properties and is a suitable alternative if durability is desired and some ultrasound realism to human tissue may be sacrificed. Polyvinyl alcohol (PVA), while also requiring hydration, performs well across all categories. Furthermore, we propose a framework for the evaluation of future ultrasound-guided needle intervention tissue phantoms to increase the fidelity of training programs and thereby improve clinical performance.     

Echoscintigraphy: a new imaging modality for the reduction of color blooming and acoustic shadowing in contrast sonography

The purpose of this study was to develop and evaluate a new imaging modality (echoscintigraphy) to reduce color blooming and acoustic shadowing in contrast sonography. After injection of various amounts (700 to 40,000 bubbles/mL) of the echo contrast agent SH-U 563A into a flow phantom, artificial vessels were insonated in the intermittent harmonic-power Doppler imaging (H-PDI) mode. The receive gain was varied from 50% to 75%. The cross-sectional area (CSA) of the tube was assessed using a new summation algorithm (echoscintigraphy) and a conventional single-frame analysis (S-FA) of the H-PDI-signals. Echoscintigraphy is based on the recording and summation of low-intensity signals that are emitted during the ultrasound (US)-induced destruction of microbubbles. Application of the summation algorithm at low-contrast concentration allowed a gain-independent automatic calculation of the CSA at medium and high gain settings. Using the S-FA method, the assessment of the vessel diameter and the CSA was gain-dependent and allowed correct measurements only from 60% to 65% gain. At a high receive-gain and high contrast concentration, S-FA resulted in an overestimation of the CSA up to 35.5%. Echoscintigraphy allows correct display of contrast-filled vessels over a wide range of gain settings at low contrast concentrations, where S-FA does not adequately display echo contrast. Thus, echoscintigraphy minimizes artefacts resulting from color blooming and acoustic shadowing.     

能量多普勒显像与彩色多普勒显像显示血管内径准确性的实验比较

目的：比较能量多普勒显像（ＰＤＩ）和彩色多普勒血流显像（ＣＤＦＩ）显示离体模拟血流血管内边界的准确性。方法：在０．４ｍｍ、３ｍｍ和８ｍｍ内径的硅胶管中模拟低速血流,分别用二维超声（２ＤＵＳ）、ＣＤＦＩ及ＰＤＩ进行显像,准确测量各显像模式中各血管的血管内径或彩色血流直径。结果：①、２ＤＵＳ测量的各种血管内径与实际内径均无明显差别（Ｐ＞０．０５）;②、ＰＤＩ在０．４ｍｍ内径血管的血流显示中,ＰＤＩ血流直径比２ＤＵＳ的血管内径大２．４倍（Ｐ＜０．０１）;在３ｍｍ和８ｍｍ内径血管,ＰＤＩ血流直径与２ＤＵＳ的血管内径无明显差异（Ｐ＞０．０５）;③、在三种内径血管显示中,ＣＤＦＩ血流直径均明显大小ＰＤＩ血流直径（Ｐ＜０．０１）,且血流前后径明显大于血流横径（Ｐ＜０．０１）。结论：ＰＤＩ用血流直径代表血管内径的准确性明显高于ＣＤＦＩ,尤其在较粗的血管中,ＰＤＩ血流直径能准确反映血管内径。     

Properties of phantom tissuelike polymethylpentene in the frequency range 20-70 MHZ

Quantitative ultrasound (QUS) has been used to characterize soft tissues at ordinary abdominal ultrasound frequencies (2 to 15 MHz) and is beginning application at high frequencies (20 to 70 MHz). For example, backscatter and attenuation coefficients can be estimated in vivo using a reference phantom. At high frequencies, it is crucial that reverberations do not compromise the measurements. Such reverberations can occur between the phantom’s scanning window and transducer components as well as within the scanning window between its surfaces. Transducers are designed to minimize reverberations between the transducer and soft tissue. Thus, the acoustic impedance of a phantom scanning window should be tissuelike; polymethylpentene (TPX) is commonly used because of its tissuelike acoustic impedance. For QUS, it is also crucial to correct for the transmission coefficient of the scanning window. Computation of the latter requires knowledge of the ultrasonic properties, viz, density, speed and attenuation coefficients. This work reports values for the ultrasonic properties of two versions of TPX over the high-frequency range. One form (TPX film) is used as a scanning window on high-frequency phantoms, and at 40 MHz and 22°C was found to have an attenuation coefficient of 120 dB/cm and a propagation speed of 2093 m/s.     

A microcirculation phantom for performance testing of blood perfusion measurement equipment

Objective: This paper describes a new flow phantom, designed to simulate slow, small volume flow, such as that found in the capillary beds. Existing flow phantoms imitate flow in blood vessels with a narrow flow area and the flow velocities are normally not extendible down to capillary blood flow velocities. Recent progress in ultrasonic blood perfusion measurements has resulted in devices that operate at blood flow velocities of the order 0.1−5 m/s and flow that is spatially and directionally distributed over the sample volume. Method: The phantom uses a ball of spatially random-oriented thin plastic tubes to carry scatterers through the sample volume. To avoid stationary echoes, and thereby ensure that the ultrasound reflections emanate mainly from the circulating blood mimicking particle suspension inside the tubing, the surrounding liquid has been acoustically matched to the tubing material. Results: The phantom has been tested on a novel blood perfusion measurement equipment and shown to set up a reasonably reproducible approximation of blood perfusion in tissue. Conclusion: It is believed that the design of the phantom is satisfactory for mimicking blood perfusion.     

Effect of graphite concentration on shear-wave speed in gelatin-based tissue-mimicking phantoms

Elasticity-based imaging modalities are becoming popular diagnostic tools in clinical practice. Gelatin-based, tissue mimicking phantoms that contain graphite as the acoustic scattering material are commonly used in testing and validating elasticity-imaging methods to quantify tissue stiffness. The gelatin bloom strength and concentration are used to control phantom stiffness. While it is known that graphite concentration can be modulated to control acoustic attenuation, the impact of graphite concentration on phantom elasticity has not been characterized in these gelatin phantoms. This work investigates the impact of graphite concentration on phantom shear stiffness as characterized by shear-wave speed measurements using impulsive acoustic-radiation-force excitations. Phantom shear-wave speed increased by 0.83 (m/s)/(dB/(cm MHz)) when increasing the attenuation coefficient slope of the phantom material through increasing graphite concentration. Therefore, gelatin-phantom stiffness can be affected by the conventional ways that attenuation is modulated through graphite concentration in these phantoms.     

Development of a vessel-mimicking material for use in anatomically realistic Doppler flow phantoms

Polyvinyl alcohol cryogel (PVA-C) is presented as a vessel-mimicking material for use in anatomically realistic Doppler flow phantoms. Three different batches of 10% wt PVA-C containing (i) PVA-C alone, (ii) PVA-C with antibacterial agent and (iii) PVA-C with silicon carbide particles were produced, each with 1-6 freeze-thaw cycles. The resulting PVA-C samples were characterized acoustically (over a range 2.65 to 10.5 MHz) and mechanically to determine the optimum mixture and preparation for mimicking the properties of healthy and diseased arteries found in vivo. This optimum mix was reached with the PVA-C with antibacterial agent sample, prepared after two freeze/thaw cycles, which achieved a speed of sound of 1538 ± 5 m s⁻¹ and a Young’s elastic modulus of 79 ± 11 kPa. This material was used to make a range of anatomically realistic flow phantoms with varying degrees of stenoses, and subsequent flow experiments revealed that higher degrees of stenoses and higher velocities could be achieved without phantom rupturing compared with a phantom containing conventional wall-less vessels. (E-mail:jacinta.browne@dit.ie)     

Pulsatile flow phantom for ultrasound image-guided HIFU treatment of vascular injuries

A pulsatile flow phantom was developed for studies of ultrasound image-guided high intensity focused ultrasound (HIFU) application in transcutaneous hemostasis of injured blood vessels. The flow phantom consisted of a pulsatile pump system with instrumented excised porcine carotid artery, which was imbedded in a transparent agarose gel to model structural configuration of in vivo tissues. Heparinized porcine blood was circulated through the phantom. The artery was injured using an 18-gauge needle to model a penetrating injury in human peripheral vasculature. A HIFU transducer with the diameter of 7 cm, focal length of 6.3 cm and frequency of 3.4 MHz was used to seal the puncture. Ultrasound imaging was used to localize and target the puncture site and to monitor the HIFU treatment. Triphasic blood flows present in the human arteries were reproduced, with flow rates of 50 to 500 mL/min, pulse rates of 62 to 138 beats/min and peak pressures of 100 to 250 mm Hg. The penetrating injury of an artery was mimicked successfully in the flow phantom setting and was easily visualized both optically through the transparent gel and with power Doppler ultrasound imaging. Hemostasis was achieved in 55 +/- 31 s (n = 9) of HIFU application. Histologic observations showed that a HIFU-sealed puncture was filled with clotted blood and covered with a fibrin cap. The pulsatile flow phantom provides a controlled and repeatable environment for studies of transcutaneous image-guided HIFU application in hemostasis of a variety of blood vessel injuries.     

彩色多普勒超声检测颈部动脉227例的结果分析

目的:通过彩色多普勒超声检测颈部动脉结果分析探讨其临床诊断价值。材料与方法:利用彩色多普勒超声诊断仪对227例患者进行颈总动脉、颈内动脉、椎动脉检查,观察血管壁结构、有无斑块、斑块的大小、类型、管腔狭窄率及血管走行情况。结果:227例患者颈部血管B超检测异常率为84.58%,且与患者年龄增长呈正相关,与性别无明显性差异。颈动脉系统血管病变主要为颈动脉粥样硬化及颈动脉狭窄,而椎动脉则主要为走行迂曲或内径变细,二者的动脉损害是有一定差异的。结论:在临床对脑血管病进行病理生理诊断时,应常规做颈部血管彩色B超,除进行中风筛查外,并根据检查结果及早采取防治措施,尤其重视颈椎病的防治。