The diaphragm. The position of the diaphragm can change depending upon whether you are lying down prone or supine. Scanning from both directions is sometimes used to minimize interference from the diaphragm when imaging the heart.
Complete imaging. The heart is a 3 dimensional structure and the images are 2 dimensional. To completely visualize the entire heart, you need more than one view. Hard to fully describe the moon without flying around to the back side.