Technical and positional causes

The initial step in evaluating unilateral hemithoracic transradiancy is to identify any radiographic artifacts that may mimic this appearance. Common artifacts include patient rotation and grid cutoff.

When a patient is rotated, the rotated side appears more radiolucent because soft tissues overlap one side of the chest while being displaced from the other. In order to assess if the patient is well aligned, the medial ends of the clavicles should like equidistant and symmetrical to the spinous process of the thoracic vertebrae.

Grid cutoff occurs when the primary x-ray beam geometry is not matched to the angle of the grid strips. This causes an unequal exposure across the peripheries of the film. A reliable way to identify grid cutoff is by comparing the bony details on both sides of the image. In cases of grid cutoff, such as in an AP erect chest radiograph, there will be a noticeable difference in the clarity of the shoulder girdle structures.

Asymmetry of chest wall soft tissues

Variations of the chest wall soft tissues are often very obvious to the clinician but overlooked by the radiologist.

Pectoralis muscle absence

The absence of pectoralis musculature can either be congenital (Poland syndrome) or acquired (Surgical removal for flap surgery).

Poland syndrome is a congenital condition characterized by the absence or underdevelopment of the pectoralis muscles on one side of the body. On radiographs, due to the absence of overlying soft tissue, the affected side appears more transradiant. (Fig 1,2)

Mastectomy

Radical mastectomy will likely be the most common cause of chest wall asymmetry. The lung field ipsilateral to the side of the breast removal will appear more transradiant compared to the normal side, due to the lack of overlying soft tissue to attenuate the primary X-ray beam. (Fig 3)

Assessment of the pleura

Contralateral pleural opacity

When assessing transradiancy, the importance in determining if the trans radiancy or radio-opacity is abnormal is often underappreciated.

One major cause of this is pleural effusions that are imaged when the patient is supine. Due to the dependent nature of fluid, it often collects posteriorly and presents as a veil like opacity over the affected lung. (Fig 4)

A similar situation can also arise from pleural thickening which has both benign and malignant causes. This may cause the affected side to appear more dense, giving a false appearance of transradiancy of the contralateral side.

Ipsilateral pleural transradiancy

Air does not significantly attenuate the primary x-ray beam, and so structures containing air appear hyperlucent on x-rays. In the case of a pneumothorax the affected side will appear transradiant compared to the unaffected side. It is usually well appreciated on erect radiographs, with a visible thin pleural line seen with the absence of lung markings. (Fig 5)

Assessment of the parenchyma

Unilateral parenchymal transradiancy

An increase in the amount of air within the lung parenchyma can occur due to either air trapping or compensatory hyperinflation. Unilateral transradiancy of the lung associated with air trapping, is observed in various conditions, including foreign body aspiration, giant bullae, emphysema, and Swyer-James syndrome.

It is possible for the diseased lung volume to be smaller, normal, or enlarged. The mediastinum frequently shifts contralaterally in conditions when the lung capacity is enlarged, such as or

bullous emphysema. Where the lung capacity is reduced with a unilateral small lung with hyperlucency, conditions such as Swyer-James syndrome should come to mind.

Contralateral parenchymal opacity

Similar to evaluation of the pleura, one should be certain that it is not the contralateral side being more opacified giving the appearance of a unilateral transradiancy.

Unilateral lung transplant is becoming increasingly prevalent as an intervention for end stage lung disease, where the transradiant side may represent either the native lung as seen in emphysema, or the transplanted lung such as in interstitial lung disease. The side of the mediastinal shift often gives clues to which side is the abnormal side.(Fig 6)

Other causes of contralateral parenchymal opacity may include underlying mass lesions or consolidation which may cause the unaffected side to appear more hyperlucent. (Fig 7, 8)

Assessment of the diaphragm

Herniation of the intra-abdominal organs through the hemidiaphragm can be due to congenital or acquired defects through the hemidiaphragm.

Due to the propulsion of abdominal organs through the diaphragmatic defect, the affect side often appears more opacified than the unaffected side, causing the appearance of contralateral trans radiancy. (fig 9)