|Factors affecting the appearance|
COMPOSITION OF ADJACENT TISSUES
The greater the difference in acoustic impedance between two adjacent structures, the greater the reflected sound waves and the more echogenic (white) the border between the tissues appears on the image.
The difference in acoustic impedance between bone or air and adjacent tissues is very high, and thus at such an interface most of the sound waves are reflected. Very few sound waves are left to penetrate and reach deeper structures. This results in failure to image the area behind bone or air and the lack of reflected echos from this area causes it to look black on the US image.
The image on the right is a superficial ultrasound of the left chest wall. The ribs, seen in cross section, are marked with stars. Note the anterior edges of the ribs are white, or echogenic. Note the black "shadow" posterior to the ribs, where no ultrasound beam can penetrate. Also note the lung edge, running just posterior to the ribs, is also echogenic, and the lung parenchyma is not imaged and appears black.