A Review of Tissue Substitutes for Ultrasound Imaging

The characterization and calibration of ultrasound imaging systems requires tissue-mimicking phantoms with known acoustic properties, dimensions and internal features. Tissue phantoms are available commercially for a range of medical applications. However, commercial phantoms may not be suitable in ultrasound system design or for evaluation of novel imaging techniques. It is often desirable to have the ability to tailor acoustic properties and phantom configurations for specific applications. A multitude of tissue-mimicking materials and phantoms are described in the literature that have been created using a variety of materials and preparation techniques and that have modeled a range of biological systems. This paper reviews ultrasound tissue-mimicking materials and phantom fabrication techniques that have been developed over the past four decades, and describes the benefits and disadvantages of the processes. Both soft tissue and hard tissue substitutes are explored. (E-mail: mculjat@mednet.ucla.edu)     

Simultaneous backscatter and attenuation estimation using a least squares method with constraints

Backscatter and attenuation variations are essential contrast mechanisms in ultrasound B-mode imaging. Emerging quantitative ultrasound methods extract and display absolute values of these tissue properties. However, in clinical applications, backscatter and attenuation parameters sometimes are not easily measured because of tissues inhomogeneities above the region-of-interest (ROI). We describe a least squares method (LSM) that fits the echo signal power spectra from a ROI to a three-parameter tissue model that simultaneously yields estimates of attenuation losses and backscatter coefficients. To test the method, tissue-mimicking phantoms with backscatter and attenuation contrast as well as uniform phantoms were scanned with linear array transducers on a Siemens S2000. Attenuation and backscatter coefficients estimated by the LSM were compared with those derived using a reference phantom method (Yao et al. 1990). Results show that the LSM yields effective attenuation coefficients for uniform phantoms comparable to values derived using the reference phantom method. For layered phantoms exhibiting nonuniform backscatter, the LSM resulted in smaller attenuation estimation errors than the reference phantom method. Backscatter coefficients derived using the LSM were in excellent agreement with values obtained from laboratory measurements on test samples and with theory. The LSM is more immune to depth-dependent backscatter changes than commonly used reference phantom methods.     

Construction of 3‐Dimensional Printed Ultrasound Phantoms With Wall‐less Vessels

Ultrasound phantoms are invaluable as training tools for vascular access procedures. We developed ultrasound phantoms with wall‐less vessels using 3‐dimensional printed chambers. Agar was used as a soft tissue–mimicking material, and the wall‐less vessels were created with rods that were retracted after the agar was set. The chambers had integrated luer connectors to allow for fluid injections with clinical syringes. Several variations on this design are presented, which include branched and stenotic vessels. The results show that 3‐dimensional printing can be well suited to the construction of wall‐less ultrasound phantoms, with designs that can be readily customized and shared electronically.     

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)     

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)     

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)     

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 novel and inexpensive ballistic gel phantom for ultrasound training

BACKGROUND: Ultrasonography use is increasing in emergency departments, and ultrasound education is now recommended in resident training. Ultrasound phantoms are used in many institutions for training purposes. The purpose of this study is to describe an inexpensive and simple method to create ultrasound-imaging models for the purpose of education and practice using clear ballistic gel.METHODS: Clear ballistic gel is used to simulate tissue for firing practice and other military evaluations.RESULTS: The transparent and durable ultrasound phantom we produced was clear and contained four vessel lumens. The images obtained using the phantom were of high quality and compared well to normal sonographic anatomy.CONCLUSIONS: The clear ballistic brand gel is unique because it is inexpensive, does not dry out, does not decay, is odorless, and is reusable. The ultrasound images obtained using the phantom are realistic and useful for ultrasound education.     

Photo-acoustic Excitation and Optical Detection of Fundamental Flexural Guided Wave in Coated Bone Phantoms

Photo-acoustic (PA) imaging was combined with skeletal quantitative ultrasound (QUS) for assessment of human long bones. This approach permitted low-frequency excitation and detection of ultrasound so as to efficiently receive the thickness-sensitive fundamental flexural guided wave (FFGW) through a coating of soft tissue. The method was tested on seven axisymmetric bone phantoms, whose 1- to 5-mm wall thickness and 16-mm diameter mimicked those of the human radius. Phantoms were made of a composite material and coated with a 2.5- to 7.5-mm layer of soft material that mimicked soft tissue. Ultrasound was excited with a pulsed Nd:YAG laser at 1064-nm wavelength and received on the same side of the coated phantom with a heterodyne interferometer. The FFGW was detected at 30-kHz frequency. Fitting the FFGW phase velocity by the F_LC(1,1) tube mode provided an accurate (9.5 ± 4.0%) wall thickness estimate. Ultrasonic in vivo characterization of cortical bone thickness may thus become possible.     

Simultaneous Evaluation of Thermal and Non-Thermal Effects of High-Intensity Focused Ultrasound on a Tissue-Mimicking Phantom

Physiologically relevant phantoms with high reliability are essential for extending the therapeutic applications of high-intensity therapeutic ultrasound. Here we describe a tissue-mimicking phantom capable of quantifying temperature changes and observing non-thermal phenomena by high-intensity therapeutic ultrasound. Using polydiacetylene liposomes, we fabricated agar-based polydiacetylene hydrogel phantoms (PHPs) that not only respond to temperature, but also have acoustic properties similar to those of human liver tissue. The color of PHPs changed from blue to red depending on the temperature in the range 40°C–70°C, where the red/blue ratio of PHP had a good linearity of 99.06% for the temperature changes. Furthermore, repeated high-intensity focused ultrasound led to histotripsy on the PHP with liquefied and damaged areas measuring 0.7 and 4.0 cm², respectively, at the signal generator amplitude setting voltage of 80 mV. Our results indicate not only the usability of the thermochromic phantom, but also its potential for evaluating non-thermal phenomena in various high-intensity focused ultrasound therapies.     

A novel noncontact ultrasound indentation system for measurement of tissue material properties using water jet compression

This study is aimed to develop a novel noncontact ultrasonic indentation system for measuring quantitative mechanical properties of soft tissues, which are increasingly important for tissue assessment and characterization. The key idea of this method is to use a water jet as an indenter to compress the soft tissue while at the same time as a medium for an ultrasound beam to propagate through. The use of water jet indentation does not require a rigid compressor in front of the focused high frequency ultrasound transducer to compress the tissue, so that the additional attenuation caused by the rigid compressor and the strong echoes reflected from its surfaces can be avoided. The indentation deformation was estimated from the ultrasound echoes using a cross-correlation algorithm and the indentation force was calculated from the water pressure measured inside the water pipe. Experiments were performed on uniform tissue-mimicking phantoms with different stiffness. The Young's moduli and Poisson's ratios of these phantoms were measured using a uniaxial ultrasound compression system. The ratio of the indentation pressure to the tissue relative deformation was obtained from the water indentation. This ratio was well correlated with the Young's modulus (r = 0.87). The results also demonstrated that the water indentation approach could differentiate materials with different stiffness in a combined phantom (288 kPa and 433 kPa). This novel noncontact water indentation approach could be potentially used for the measurement of the elasticity of small samples and with a fast scanning speed.     

Fabrication of SEBS Block Copolymer-Based Ultrasound Phantom Containing Mimic Tumors for Ultrasound-Guided Needle Biopsy Training

The creation of various phantoms is important in medical education, especially for intern physicians who need to practice their skills. Ultrasound phantoms are particularly useful for training in ultrasound-guided needle biopsy. SEBS, or poly(styrene–ethylene–butylene–styrene), is a thermoplastic elastomer that can be used with mineral oil to make ultrasound phantoms and a tumor-like structure. SEBS block copolymer-based phantoms are inexpensive, non-toxic and shelf-stable, and are easy to modulate. Most importantly, such ultrasound phantoms have acoustic and mechanical properties similar to those of human soft tissues. The quality of ultrasound images of phantoms and mimic tumors is excellent and can be maintained even after several biopsy needle punctures, making them excellent ultrasound phantoms for physician practice as needed.     

A novel realistic three-layer phantom for intravascular ultrasound imaging

Intravascular ultrasound (IVUS) is an imaging modality that experienced a tremendous development over the last 20 years. Phantoms for IVUS are rare and poorly documented. The aim of this paper is to propose an original IVUS phantom that has geometries and specular textures closer to those of coronary arteries than conventional tube-like phantoms. The proposed phantom has a three-layer aspect, reproducing the intima, media and adventitia that compose the arterial wall. It is made of an agar-based compound, with water, glycerol and cellulose particles. Fourteen phantoms were quantified using IVUS. Six phantoms were evaluated by both photomacroscopy and IVUS. There was an excellent correlation between phantom dimensions evaluated by photomacroscopy and the nominal values (mold dimensions). The IVUS quantification of the phantom was closely correlated to the measurements obtained by photomacroscopy. These results demonstrate that a multilayer phantom, with known and reproducible dimensions and with realistic geometric and echographic properties has been developed.     

Integration of a Low‐Cost Introductory Ultrasound Curriculum Into Existing Procedural Skills Education for Preclinical Medical Students

We evaluated integration of an introductory ultrasound curriculum into our existing mandatory procedural skills program for preclinical medical students. Phantoms consisting of olives, pimento olives, and grapes embedded in opaque gelatin were developed. Four classes encouraged progressive refinement of phantom‐scanning and object identification skills. Students improved their ability to identify hidden objects, although each object type achieved a statistically significant improvement in correct identification at different time points. The total phantom cost per student was $0.76. Our results suggest that short repeated experiences scanning simple, low‐cost ultrasound phantoms confer basic ultrasound skills.     

A wall-less vessel phantom for Doppler ultrasound studies

Doppler ultrasound flow measurement techniques are often validated using phantoms that simulate the vasculature, surrounding tissue and blood. Many researchers use rubber tubing to mimic blood vessels because of the realistic acoustic impedance, robust physical properties and wide range of available sizes. However, rubber tubing has a very high acoustic attenuation, which may introduce artefacts into the Doppler measurements. We describe the construction of a wall-less vessel phantom that eliminates the highly attenuating wall and reduces impedance mismatches between the vessel lumen and tissue mimic. An agar-based tissue mimic and a blood mimic are described and their acoustic attenuation coefficients and velocities are characterised. The high attenuation of the latex rubber tubing resulted in pronounced shadowing in B-mode images; however, an image of a wall-less vessel phantom did not show any shadowing. We show that the effects of the highly attenuating latex rubber vessels on Doppler amplitude spectra depend on the vessel diameter and ultrasound beam width. In this study, only small differences were observed in spectra obtained from 0.6 cm inside diameter thin-wall latex, thick-wall latex and wall-less vessel phantoms. However, a computer model predicted that the spectrum obtained from a 0.3-cm inside diameter latex-wall vessel would be significantly different than the spectrum obtained from a wall-less vessel phantom, thus resulting in an overestimation of the average fluid velocity. These results suggest that care must be taken to ensure that the Doppler measurements are not distorted by the highly attenuating wall material. In addition, the results show that a wall-less vessel phantom is preferable when measuring flow in small vessels.     

3-D Freehand Ultrasound Calibration Using a Tissue-Mimicking Phantom With Parallel Wires

This article describes a method used to calibrate 3-D freehand ultrasound systems based on phantoms with parallel wires forming two perpendicular planes, such as the usual general-purpose commercial phantoms. In our algorithm, the phantom pose is co-optimized with the calibration to avoid the need to precisely track the phantom. We provide a geometrical analysis to explain the proposed acquisition protocol. Finally, we give an estimate of the system accuracy and precision based on measurements acquired on an independent test phantom. We obtained error norms of 1.6 mm up to 6 cm of depth and 3.5 mm between 6 and 14 cm of depth, in total average. In conclusion, it is possible to calibrate ultrasound tracked-probe systems with a reasonable accuracy based on a general-purpose phantom. Contrarily to most calibration methods that imply the construction of the phantom, the present algorithm is based on a standard phantom geometry that is commercially available.     

Tissue-mimicking oil-in-gelatin dispersions for use in heterogeneous elastography phantoms

A ten-month study is presented of materials for use in heterogeneous elastography phantoms. The materials consist of gelatin with or without a suspension of microscopic safflower oil droplets. The highest volume percent of oil in the materials is 50%. Thimerosal acts as a preservative. The greater the safflower oil concentration, the lower the Young's modulus. Elastographic data for heterogeneous phantoms, in which the only variable is safflower oil concentration, demonstrate stability of inclusion geometry and elastic strain contrast. Young's modulus ratios (elastic contrasts) producible in a heterogeneous phantom are as high as 2.7. The phantoms are particularly useful for ultrasound elastography. They can also be employed in MR elastography, although the highest achievable ratio of longitudinal to transverse relaxation times is considerably less than is the case for soft tissues.     

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.     

A Method for Characterization of Tissue Elastic Properties Combining Ultrasonic Computed Tomography With Elastography

Objective. The correlation between various diseases and the change in the local mechanical properties of soft tissues has been long known. Over the past 20 years, there have been increasing research efforts to characterize mechanical properties of biological tissues using ultrasonic elastography. However, most of these works were based on characterization of only 1 type of waves (longitudinal or shear). The goal of this work was to devise a comprehensive ultrasound‐based imaging method capable of measuring elastic parameters by combining both backscattered elastography and through‐transmitted ultrasonic computed tomography. Methods. Our suggested technique provides measurements of both longitudinal and shear wave velocities. This enables the noninvasive computation of several tissue elasticity parameters such as Young's and shear moduli, Poisson's ratio, and, more importantly, the bulk modulus, the determination of which requires both wave velocities. Four different phantom types were examined: agar‐gelatin–based phantoms and porcine fat tissue, turkey breast tissue, and bovine liver tissue in vitro specimens. The values of Young's modulus, the shear modulus, and Poisson's ratio were estimated and were consistent with values published in the literature. Results. The average bulk modulus values of the phantoms ± SD were 2.83 ± 0.001, 2.25 ± 0.01, 2.48 ± 0.01, and 2.53 ± 0.02 GPa, respectively. A statistically significant difference (P < .001) in the values of the bulk modulus of the different phantoms was found. Conclusions. The bulk modulus is suitable for differentiation between different tissue types. The obtained results show the feasibility of using a comprehensive ultrasonic imaging technique for noninvasive quantitative tissue characterization.     

Creation of a High‐fidelity,Low‐cost Pediatric Skull Fracture Ultrasound Phantom

Over the past decade, point‐of‐care ultrasound has become a common tool used for both procedures and diagnosis. Developing high‐fidelity phantoms is critical for training in new and novel point‐of‐care ultrasound applications. Detecting skull fractures on ultrasound imaging in the younger‐than‐2‐year‐old patient is an emerging area of point‐of‐care ultrasound research. Identifying a skull fracture on ultrasound imaging in this age group requires knowledge of the appearance and location of sutures to distinguish them from fractures. There are currently no commercially available pediatric skull fracture models. We outline a novel approach to building a cost‐effective, simple, high‐fidelity pediatric skull fracture phantom to meet a unique training requirement.