The sound beam reflects within tissues at acoustic interfaces. An acoustic interface is where two different tissues are adjacent to each other, and may mean the capsules surrounding tissues, adjacent structures within organs or different cells within tissues. When the sound beam is reflected and returns to the transducer, the strength of the returning beam (amplitude) is displayed as a grey scale pixel where the whiter or brighter the pixel, the greater the amplitude of the returning sound. The time the sound beam took to return determines the depth that the sound beam reached and determines where on the image that pixel is placed. 



The higher the frequency of sound used, the better the resolution of the end image and the more likely small changes will be identifiable. However higher frequency = less penetration. So for deeper structures a lower frequency is needed. As an example a 12MHz sound beam can usually penetrate to a maximum depth of 8-10cm while a 2MHz beam can penetrate to around 25cm. 



Another consideration for ultrasound is the region of interest. The strongest reflections are at gas-fluid/soft tissue and bone-fluid/soft tissue interfaces. This means that when the sound beam meets air or bone, almost the entire beam is reflected back and tissue deep to the interface cannot be evaluated. Gastrointestinal gas can prevent full evaluation of the abdomen and lung changes cannot be seen at all unless they extend to the visceral pleura.



Recently the use of ultrasound to identify the presence of abnormal gas in marine mammals was described (Dennison et al., Bubbles in marine mammals. 2011, Proc B Royal Soc).



In general, ultrasound is most useful for evaluation of superficial soft tissue damage, ocular disease, peripheral pulmonary or pleural disease and abdominal disease. The size of the patient can become a limiting factor depending on the organ system that needs to be evaluated.