Ultrasonic tissue-equivalent materials using inorganic gel mixtures

Magnesium silicate gels and gel mixtures offer alternatives from which to fabricate mixtures whose ultrasonic properties mimic those of living tissue. This note describes the preparation of such mixtures, and gives details of some of their properties. Many substances have already been investigated as ultrasonic tissue-equivalent materials. The objectives of these past investigations have been: to provide phantoms for the quantitative assessment of diagnostic ultrasound imaging equipment; to construct realistic anthropomorphic phantoms for training purposes; and to provide models for the study of the interaction of sound with tissue. The materials investigated include various water-based organic gels, for instance gelatin (Edmonds et al, 1977; Madsen et al, 1978), agar (Burlew et al, 1980), or sodium alginate (Eggleton & Whitcombe, 1977). Silicone rubbers (Edmonds et al, 1977) and urethane polymers (Carson et al, 1977) have also been studied. Of these, the gelatin and agar-based materials have been most widely investigated and are now used as the basis for commercially available tissue phantoms of a range of types. This is so primarily because suitable mixtures of these gels can be manufactured whose acoustic properties (velocity, frequencydependent attenuation, back-scatter cross-section) are very close to those of tissue. The possible disadvantages are concerned with long-term stability of the gels; they may melt at elevated temperatures (the melting point of pure gelatin is about 33°C), are prone to bacteriological growth which must be inhibited, and will dry rapidly unless enclosed.