Measurement issues in quantitative ultrasonic imaging

Ultrasonic imaging of " flesh and blood " is vulnerable to the natural variability of these media: the speed of sound is not known, not constant, and not amenable to calibration using simply shaped manufactured samples. When images of high dimensional accuracy are needed, as for image-guided surgery, the " average " or " typical " values used in diagnostic ultrasound may not be good enough. In this paper we identify the main sources of uncertainty, and we suggest and model experimental approaches to in situ calibration. I. INTRODUCTION Applications recently proposed for ultrasonic imaging, for example, in " image guided surgery " [1][2] and in the custom design of wheelchair cushions [3], demand much higher dimensional measurement accuracy than is demanded by any of the traditional diagnostic visual-ization applications of ultrasound [4]. To render dimensionally accurate ultrasonic images it is necessary either to have a priori, or alternatively to acquire as an integral part of the measurement process, accurate knowledge of the speed of sound in the skin, fat, muscle, bone, and other living media whose interfaces spawn echoes. To correctly interpret the intensities of the echoes it is also necessary to have accurate knowledge of the acoustic impedances of these media.