The velocity fields created by ultrasound in chambers that model cysts are presented. The objective was to determine the effects of cyst size and location on the acoustic streaming velocity field, using Particle-Image Velocimetry (PIV), a laser-based technique independent of the applied ultrasound.
Some clinicians have been using the observation of acoustic streaming, which can be generated by standard examination modes, to confirm if a lesion is a fluid-filled cavity and to assess its consistency. This is of particular relevance in gynaecological US, where acoustic streaming has been proposed as a diagnostic. There is uncertaintly as to the relation between the streaming velocity measured by US and cyst size, location, shape and rheology. A complicating factor is that Doppler US itself creates the streaming velocity field it is measuring.
Continuous-wave 2.25 MHz US drove a streaming field in chambers modelling cysts of various sizes. PIV datasets were acquired using a twin-pulse laser system. Velocity fields were found to form a jet with its largest magnitude at the distal end of the model cyst. The velocity profile was found to change systematically with distance from the source, developing a marked asymmetry the closer the cyst was to the transducer. Meanwhile, the intercystal velocity magnitude at a given distance from the transducer appeared independent of cyst size. Preliminary conclusions were that measurement of the intercystal velocity profile along the beam centreline, rather than absolute velocity magnitude, may provide the most useful discriminating diagnostic.