Our objectives are to determine the intrinsic ultrasonic scattering properties of human soft tissues and to produce phantoms which have scattering identical to organs imaged with diagnostic ultrasound. Since phantom materials already exist which have speeds of sound, density and ultrasonic attenuation matched to soft tissue parenchyma, the matching scatter will result in tissue equivalent phantoms. These would be extremely useful for performance evaluation of clinical instruments, for developing ultrasonic tissue characterization techniques and for studying mechanisms underlying the generation of scattered echo signals and production of image details on clinical scans. Backscatter from large volume samples will be studied, analogous to similar work carried out by other investigators. In addition, apparatus has been developed for measuring the differential scattering cross section per unit volume. The latter technique allows the scatter as a function of angle to be quantitatively determined for frequencies throughout the diagnostic range. Measurements from volumetric scattering standards will be used to verify that the measurement process provides instrument independent (intrinsic) scatter values. The differential scatter cross section will be measured for normal human liver, fat, muscle and brain tissue. Comparisons will be made between in vivo measurements and in vitro measurements of ultrasonic backscattering for organs in large laboratory animals. Tissue equivalent phantoms will be developed using newly developed materials, combining mixtures of small particle and intermediate size scatterers. Water based gels used in this project will allow an iterative approach to the problem of matching scatter. The role of scatter on clinical ultrasound images will be investigated by studying the generation of echo signals and gray scale texture in phantoms. Texture analysis studies will account for the intrinsic scatter properties of the phantom as well as the temporal and spatial transducer characteristics and other components of the imaging chain.