The most appropriate diagnostic test to perform next is high-resolution chest CT (HRCT). This patient has symptoms (progressive dyspnea on exertion and nonproductive cough), physical examination findings (impaired oxygenation, dry crackles on lung examination, and clubbing), and plain chest radiographic imaging findings (interstitial thickening at the lung bases) consistent with diffuse parenchymal lung disease. HRCT is the most effective CT study for evaluating parenchymal lung disease and is the preferred modality for evaluating patients with possible diffuse parenchymal lung disease. Because of its high resolution, it can help narrow the differential diagnosis based on the distribution of the parenchymal disease and the presence or absence of associated findings such as fibrosis. The HRCT protocol employs very thin image sections that are reconstructed at high resolution to obtain very detailed information about the lung parenchyma. However, these image sections are obtained at relatively wide intervals (typically 1 cm between images); they represent a sampling of the lung parenchyma and do not provide a complete picture of the lungs. Because of this, HRCT should not be performed for suspected localized lung disease (such as pulmonary nodules) as it may miss smaller lesions.

Unenhanced chest CT scanning obtains images at closer intervals and is indicated for evaluation of focal lung lesions, including pulmonary nodules. Contrast may be added to the study when better definition of the mediastinal structures is needed (for example, to assess for lymphadenopathy).

CT pulmonary angiography opacifies the pulmonary arteries and is not designed for detailed definition of the pulmonary parenchyma. It is mainly used in the diagnosis of pulmonary embolism or aortic dissection and would not be an appropriate next diagnostic study in this patient.