radiology Radiology ( ) is the medical specialty that uses medical imaging to diagnose diseases and guide treatment within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiation), but tod ...

, the silhouette sign refers to the loss of normal borders between thoracic structures. It is usually caused by an

intrathoracic The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum. There ...

radiopaque mass that touches the border of the

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

aorta The aorta ( ; : aortas or aortae) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart, branching upwards immediately after, and extending down to the abdomen, where it splits at ...

.The silhouette sign and possible mechanisms
Ian Maddison Nov 1994, revised Nov. 2007. Retrieved 9 February 2010. In other words, it is difficult to make out the borders of a particular structure - normal or otherwise - because it is next to another dense structure, both of which will appear white on a standard

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

. It may occur, for example, in right middle lobe syndrome, where the right heart margin is obscured, and in right lower lobe

pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...

, where the border of the

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

on the right side is obscured, while the right heart margin remains distinct. Silhouette sign is very useful in localizing lung lesions as all structures forming cardiac silhouette are in contact with a specific portion of the lung.

References

Further reading