ABSTRACT
Imaging with ultrasound involves transmission of high-frequency (3-15MHz), inaudible sound waves from a probe, through a coupling device gel (which is applied topically to the area of interest). These sound waves are directed into the body, and travel through the different layers, i.e. skin, subcutaneous fat, muscle and bone. The velocity and wavelength with which the waves travel through each layer is dependent on the density of the substance of that layer. At a junction between one layer and another layer, at the interfaces, a proportion of the waves are reflected back to the probe. The probe then acts as a receiver, detecting these reflected waves and creating an ultrasound image from them. The greater the difference between the density of the two structures, the greater proportion of the waves that are reflected at interface of the two structures. This is why it is important to use gel between the probe and the skin, as without gel the large difference between the densities of the probe and the air pocket would imply that almost all the waves are reflected back and none will go through the body.
