Ultrasound Backscatter of Liver with Particulate Contrast Agents

Abstract

A solid-particle contrast agent in liver has provided some encouraging initial results [7, 8]. Iodipamide ethyl ester (IDE) can be formulated with a narrow diameter distribution about a mean which can be selected within the range of 0.1–2.0 µm.The particles can be prepared under sterile conditions, are stable over time, and do not clump in the blood after intravenous injection. The circulating particles are then captured by the Kupffer’s cells which line the sinusoids of normal liver, and the IDE is eliminated within 2 days. The high density (2.4g/cm3) of these particles compared to surrounding tissue (1.0–1.1 g/cm3) produces an impedance mismatch which is responsible for backscatter enhancement. Since tumors and other lesions lack Kupffer’s cells, they do not concentrate the particles and therefore lack contrast effects compared to surrounding normal tissue. In previous work we have reported some of the ultrasonic properties ex vivo of liver with IDE [7], and imaging results in vivo of rabbit liver implanted with VX2 carcinomas [8]. This paper describes the measurement of ultrasonic backscatter in rabbit liver with standard IDE particles and with a newly formulated bubble-IDE particle. The changes in backscatter due to IDE uptake are related to those predicted by simple theory. The influence of biodistribution is examined. The results elucidate some of the underlying mechanisms of ultrasound propagation through normal liver and also underscore the importance of biodistribution in determining the ultrasonic effects of contrast agents.