1. MEDIASTINAL MASSES
Whenever you see a mass on a chest x-ray that is possibly located within the mediastinum, your goal is to determine the following:
Is it a mediastinal mass?
Is it in the anterior, middle or posterior mediastinum?
Are you able to characterize the lesion by determining whether it has any fatty, fluid or vascular components?

4. Statistically, it is important to remember the following
Most masses (> 60%) are:
Thymomas
Neurogenic Tumors
Benign Cysts
Lymphadenopathy (LAD)
In children the most common (> 80%) are:
Neurogenic tumors
Germ cell tumors
Foregut cysts
In adults the most common are:
Lymphomas
LAD
Thyroid masses

5. Localize to the mediastinum
Unlike lung lesions, a mediastinal mass will not contain air bronchograms.
A lung mass abuts the mediastinal surface and creates acute angles with the lung, while a mediastinal mass will sit under the surface creating obtuse angles with the lung.
There can be associated spinal, costal or sternal abnormalities.

8. The four T's make up the mnemonic for anterior mediastinal masses::
Thymus
Teratoma (germ cell)
Thyroid
Terrible Lymphoma

9. Hilum Overlay Sign
When there is a mediastinal mass and you still can see the hilar vessels through this mass, then you know the mass does not arise from the hilum.  This is known as the hilum overlay sign.

12. Cervicothoracic sign
The anterior mediastinum stops at the level of the superior clavicle.
Therefore, when a mass extends above the superior clavicle, it is located either in the neck or in the posterior mediastinum.
When lung tissue comes between the mass and the neck, the mass is probably in the posterior mediastinum. 
This is known as the Cervicothoracic Sign.

16. Silhouette Sign
When two radio opaque densities comes in anatomical contact with each other the line of demarcation between them is lost.
When examining the lung fields of a normal CXR, the silhouettes of the heart borders, the ascending and descending aorta, the aortic knob and the hemidiaphragms should be clear.

17.  Obliteration of any of these silhouettes by a water density can be caused by infection in the lung, blood, pus, etc.
 All of these silhouettes, or structures, are in contact with a specific portion of the lung. 
Therefore, by determining exactly which structure is obliterated, you can determine where the lung pathology is located.

20. Silhouette/Structure
Contact with Lung
Upper right heart border/ascending aorta
Anterior segment of RUL
Right heart border
RML (medial)
Upper left heart border
Anterior segment of LUL
Left heart border
Lingula (anterior)
Aortic knob
Apical portion of LUL (posterior)
Anterior hemidiaphragms
Lower lobes (anterior)

22. D.Y.P.H Pulmonary Medicine Department. @ Face Book