Two- and three-dimensional ultrasound in the development of a needle-free injection system

Ultrasound was used to assess a needle-free injection device for both intradermal and subcutaneous injections. The aim of this study was, first, to differentiate intradermal from subcutaneous injections, both in vivo and in vitro using 2D ultrasound, and second, to quantify the amount of injectate that actually arrives within the dermis or subcutaneous tissues using volume measurements derived from high-resolution 3D ultrasound data sets, using a freehand system (Stradx), developed by the Cambridge University Departments of Engineering and Radiology. For the in vitro study the devices were filled with dye and injected into a pig preparation. The injection site was examined with high-resolution ultrasound and subsequently dissected to locate the injected dye with respect to the dermis. For the in vivo study, 8 volunteers received needle-free injections of normal saline. High-resolution 2D images and 3D data sets were obtained of the injected sites. Proprioceptive information for the 3D data sets was produced using an optically tracked freehand system. Segmentation of the 3D data sets gave an estimation of the volume of injected material (injectate) within the dermis. The results demonstrated that 2D ultrasound could identify the location of the injectate in the in vitro experiments and successfully distinguished an intradermal from a subcutaneous injection. In the in vivo study, 2D ultrasound clearly demonstrated the injectate location within the volunteers' dermis but was less able to demonstrate the dispersion of injectate within the subcutaneous tissues.