Sonographic contrast effect of combined steroid and anesthetic injections: in vitro analysis

The purpose of this study was to investigate the extent and duration of the sonographic contrast effect during guided steroid-anesthetic injections and to propose a potential mechanism based on mixture immiscibility and density differences.

A steroid-anesthetic mixture was injected into cyst phantoms under sonographic visualization, and time-dependent data were acquired. Regions of interest were drawn within each cyst, and measured mean pixel intensities were compared with a gel phantom background that served as a control. Two test tubes were also prepared similarly with the steroid-anesthetic mixture and saline in one test tube and synovial fluid in the other; these were observed to estimate separation time to form a fluid-fluid level.

There was a substantial contrast effect after injection of the steroid-anesthetic suspension into a cyst phantom containing saline. The background levels remained constant during the period of observation. The contrast effect decreased as a function of time to 4 (94% decrease from the time of injection) in the upper half and to 34 (64% decrease from the time of injection) in the lower half throughout the course of the experiment. The test tube containing the injected material in saline achieved separation in approximately 15 minutes, whereas in the synovial fluid, separation was achieved in 48 hours.

We have verified, in vitro, an apparent clinically observed contrast effect noted during sonographically guided therapeutic injections of cortisone-anesthetic mixtures. We have shown that this effect relates to differences in acoustic impedance and immiscibility, differences in density of the mixture, or both.