How images are obtained  
 

Sound waves are reflected at the interfaces of tissues with different acoustic impedance. Acoustic impedance is a product of the density of the tissue and the velocity of sound in the medium. The intensity of the reflected US beam depends upon the difference in acoutsic impedance between adjacent tissues and tissue interfaces. As the difference increases, the intensity of the reflected echo increases. High intensity echos appear white on the US image, "echogenic or hyperechoic". Low intensity echos appear dark, or "hypoechoic".

An US scanner is shown on the right. As the transducer is moved over the patient, the screen displays the image in real-time. Since the transducer can be held in any position, images in the axial, sagittal, coronal planes, as well as any obliquity can be generated.