Companion shadow is a term used in describing radiographs that denotes the appearance of a smooth, homogenous, radiodensity with a well-defined margin that runs parallel with a bony landmark. Companion shadows represent soft tissue

Soft tissue

In anatomy, the term soft tissue refers to tissues that connect, support, or surround other structures and organs of the body, not being bone. Soft tissue includes tendons, ligaments, fascia, skin, fibrous tissues, fat, and synovial membranes , and muscles, nerves and blood vessels .It is sometimes...

 that overlies the respective bony landmark in profile. They are not seen in every radiograph and can be misinterpreted as pathology

Pathology

Pathology is the precise study and diagnosis of disease. The word pathology is from Ancient Greek , pathos, "feeling, suffering"; and , -logia, "the study of". Pathologization, to pathologize, refers to the process of defining a condition or behavior as pathological, e.g. pathological gambling....

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Types of companion shadow

• Clavicular companion shadow is a thin soft-tissue stripe along the upper edge of the clavicle
  Clavicle
  In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
  
  .

• Rib companion shadows parallel the ribs and measure 1–5 mm in diameter project adjacent to the inferior and inferolateral margins of the first and second ribs and the axillary portions of the lower ribs. These companion shadows of the first and second ribs occur in 35% and 31% of the population, respectively. Rib companion shadows represent the fat
  Fat
  Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
  
   and muscles in the intercostal space
  Intercostal space
  The intercostal space is the space between two ribs . Since there are 12 ribs on each side, there are 11 intercostal spaces, each numbered for the rib superior to it.-Structures in intercostal space:* several kinds of intercostal muscle...
  
  . The shadows that accompany the ribs may mimic pleural
  Pleural disease
  Pleural diseases include:* Pneumothorax: a collection of air within the pleural cavity, arising either from the outside or from the lung. Pneumothoraces may be traumatic, iatrogenic, or spontaneous. A tension pneumothorax is a particular type of pneumothorax where the air may enter on...
  
   and lung disease.

• Scapular companion shadow overlie the scapula
  Scapula
  In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
  
  , with a smooth, well-defined margin parallel to the medial border of the scapula. The companion shadow results from unusual radiographic position of the scapula
  Scapula
  In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
  
  , which causes a soft-tissue fold to occur along its medial border. Winging of the scapula
  Winged scapula
  A winged scapula is a condition in which the shoulder blade, or shoulder bone, protrudes from a person’s back in an abnormal position. It is a rare condition with the potential to lead to limited functional activity in the upper extremity with which it is adjacent. It can affect a person’s ability...
  
   may also be responsible for the shadow. Scapular companion shadows may be mistaken for a soft-tissue or pleural lesion
  Pleural disease
  Pleural diseases include:* Pneumothorax: a collection of air within the pleural cavity, arising either from the outside or from the lung. Pneumothoraces may be traumatic, iatrogenic, or spontaneous. A tension pneumothorax is a particular type of pneumothorax where the air may enter on...
  
  .