What anatomical structures are well seen with US  

US is widely used in obstetrics (OB), as the fluid in the amniotic sac provides a great "acoustic window" through which to image the fetus. The two images above are from an OB US of a 19 week old fetus. The amniotic fluid is black or anechoic. The superior portion of the image on the left shows the medium gray of the uterine wall. Just deep to this is the black amniotic fluid, and then the fetus. This is a sagittal image of the fetus with the head on the left and the body on the right.

The image on the right shows the skin, uterine wall, amniotic fluid, and the fetus's hand in the center of the image.

Fluid or blood filled structures, for example the gallbladder, urinary bladder, and the heart, can be evaluated well with US. Depending on their location and depending on the composition of adjacent tissues, some blood vessels can also be seen well. For example, the descending thoracic aorta cannot be seen with ultrasound, due to its location deep to bone and air.