Review
A Review of Tissue Substitutes for Ultrasound Imaging

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Abstract

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: [email protected])

Introduction

Tissue phantoms have been used for characterization and calibration of ultrasound imaging systems since the 1960s. Phantoms are also used to compare the performance of ultrasound systems for training of ultrasound technicians, for comparison to computer models and to assist in the development of new ultrasound transducers, systems or diagnostic techniques. The advantage of phantoms is that idealized tissue models can be constructed with well-defined acoustic properties, dimensions and internal features, thereby simplifying and standardizing the imaging environment.

Phantoms are composed of tissue-mimicking materials, with the majority of phantoms having a simple homogeneous internal structure. Simple or complex targets are sometimes embedded within phantoms to mimic internal structures or to serve as characterization targets. Phantoms that accurately mimic heterogeneous organs or organ systems are often referred to as anthropomorphic phantoms. The term tissue substitute encompasses both phantoms and tissue-mimicking materials.

Phantoms and anthropomorphic phantoms are available commercially, mimicking many tissues organs and organ systems. Commercial phantoms range in price from hundreds to thousands of dollars and are often preferred for training and calibration of ultrasound systems. However, commercial phantoms are typically designed for broad markets and specific applications, and are not customizable. For this reason, customized design and fabrication of tissue phantoms is required for more specialized applications requiring tailored properties or dimensions, or when seeking to reduce cost.

This paper reviews many of the materials and techniques used to prepare both soft and hard tissue-mimicking materials and phantoms, focusing primarily on those developed for traditional ultrasound imaging rather than those developed specifically for elasticity imaging (elastography), Doppler (string phantoms) or alternate ultrasound techniques such as high-intensity focused ultrasound (HIFU). Many of the relevant acoustic properties and measurements are first discussed, followed by common materials and preparation techniques used to develop general soft tissue phantoms. The subsequent sections focus on the development of specific soft tissue phantoms and on the materials and techniques used to develop hard tissues phantoms. This paper is intended to allow the ultrasound researcher to better understand the advantages and disadvantages of various techniques and to select the appropriate approach for their own work.

Section snippets

Phantom and Tissue Properties

Tissue substitutes used in ultrasonography must possess acoustic properties near those of the tissues of interest, with the most critical acoustic properties of soft tissue substitutes being the compressional speed of sound, characteristic acoustic impedance, attenuation, backscattering coefficient and nonlinearity parameter (ICRU 1998). The most relevant acoustic properties for hard tissues include the compressional and shear wave speeds of sound, characteristic acoustic impedance and

Soft Tissue-Mimicking Materials

Soft tissues are composed of muscles, tendons, ligaments, fascia, fat, fibrous tissue, synovial membranes, nerves and blood vessels. Although some soft tissue phantoms have been developed to include many of the components of soft tissues, the majority of tissue substitutes have modeled each tissue as isotropic, homogeneous materials. It is also often desirable to prepare homogeneous tissue substitutes that mimic the broader soft tissue environment rather than individual tissues or groups of

Soft Tissue Phantoms

Many of the soft tissue substitute preparation techniques described before have been modified and tailored to mimic specific tissues or organs. Some have combined multiple techniques to develop phantoms or more realistic anthropomorphic phantoms. A sampling of the techniques used to mimic specific soft tissues and organs are provided.

Hard Tissue-Mimicking Materials and Phantoms

Hard tissues are mineralized tissues with a firm intercellular substance and include cortical bone, trabecular bone, dental enamel and dentin. Bone substitutes and phantoms have been developed primarily to evaluate and calibrate ultrasound systems designed specifically for detecting bone pathologies (Young et al., 1993, Clarke et al., 1994). Ultrasound imaging of teeth has not yet become clinically accepted, but has been the subject of various studies because of its ability to penetrate hard

Discussion and Conclusion

Many soft tissue-mimicking materials have been described that have a compressional speed of sound, density, attenuation and acoustic impedance within the measured range of soft tissues (Table 1, Table 2). The backscattering coefficient, nonlinearity parameter and shear wave speed of sound (in the case of hard tissue substitutes) have rarely been reported and therefore are not included in the tables. Agarose-based materials have been the most widely used and are very well characterized in the

Acknowledgment

Partial funding for this work provided by the Telemedicine and Advanced Technology Research Center (TATRC)/Department of Defense under award numbers W81XWH-07-1-0672 and W81XWH-07-1-0668.

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