Basic physics and imaging characteristics of ultrasound

The imaging characteristics of diagnostic ultrasound (US) are determined by the ultrasonic properties of tissue. The velocity of propagation of US and the attenuation are the 2 most important parameters. These determine the f requency with which the tissues may be imaged, which in turn sets a fundame ntal limit on the axial and the lateral resolution. Ultrasonic imaging equi pment is designed on the premise that the ultrasonic energy propagates thro ugh tissue in a straight line and that the ultrasonic beam is very narrow, In fact, the ultrasonic energy propagates through tissue as a beam of finit e dimensions set by the physical dimensions of the transducer, the way it i s constructed, and the way it is energized. Also, the velocity of propagati on in different tissues varies and this can lead to deviation of the ultras onic beam from the assumed direction of propagation. This breakdown in assu mptions leads to the creation of artifacts that must be appreciated in the interpretation of ultrasonic images. For this reason skilled interpreters o f ultrasonic images follow 3 golden rules: never make an interpretation on a single image; just because a feature is displayed do not consider that it is necessarily real; and just because a feature is not displayed do not co nsider that it is necessarily not there.