Pleural Effusion: This is often the most common cause of a peripheral hemithorax opacity, especially if it's located inferiorly. The fluid accumulation in the pleural space can be due to various reasons such as infection, malignancy, or heart failure.

Pneumonia: An infection in the lung tissue can cause an opacity that might appear in the peripheral hemithorax, especially if it's a lobar pneumonia or an atypical pneumonia.
Pulmonary Contusion or Laceration: Trauma to the chest can cause bleeding into the lung tissue, leading to an opacity on imaging.
Diaphragmatic Elevation: Conditions that cause the diaphragm to elevate, such as diaphragmatic paralysis or eventration, can mimic a peripheral opacity.

Pulmonary Embolism: Although it might not directly cause a peripheral hemithorax opacity, pulmonary embolism can lead to infarction, which might appear as an opacity. It's crucial to consider this diagnosis due to its high mortality rate if untreated.
Malignancy: Both primary lung cancer and metastatic disease can present as peripheral opacities. Missing these diagnoses can have significant implications for patient outcomes.
Empyema: A collection of pus in the pleural space, which can appear similar to a pleural effusion but requires urgent drainage.

Pulmonary Sequestration: A rare congenital anomaly where a portion of lung tissue receives its blood supply from an anomalous systemic artery, which can appear as a peripheral opacity.
Bronchogenic Cyst: A congenital anomaly that can appear as a well-defined opacity in the lung periphery, often filled with fluid or air.
Hyalinizing Granuloma: A rare, benign condition that can cause pulmonary masses or opacities, often in the periphery of the lungs.