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Background
Chronic thromboembolic pulmonary hypertension (CTEPH) is characterized by persistent fibrotic thromboemboli in the pulmonary arteries and resultant remodeling of the pulmonary vasculature leading to obstruction and increased vascular resistance. Although this process is only observed in a small percentage of patients following acute pulmonary thromboembolism, CTEPH remains a significant cause of pulmonary hypertension and is widely under-recognized and under-diagnosed. Although the clinical presentation is often ambiguous, the imaging manifestations are characteristic, and definitive diagnosis often rests in the hands of the radiologist.

Educational Goals / Teaching Points
This exhibit will help the learner recognize the clinical presentation, risk factors, and pathophysiology of CTEPH, and identify the imaging patterns of CTEPH across all imaging modalities including radiography, CT/high-resolution (HRCT), CT pulmonary angiography, conventional angiography, magnetic resonance angiography (MRA), and nuclear scintigraphy.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Knowledge of the multimodality imaging features of CTEPH allows the radiologist to provide a confident and sometimes previously unsuspected accurate diagnosis of this entity. This exhibit will present and illustrate the imaging features of CTEPH on radiography, CT/HRCT, CT pulmonary angiography, conventional angiography, and nuclear scintigraphy. Although radiography is typically insensitive for the diagnosis of CTEPH, as imaging abnormalities are often nonspecific, it may prompt additional imaging to establish the definitive diagnosis. Radiographic features of CTEPH include cardiomegaly, dilatation of the pulmonary trunk and central pulmonary arteries, and pruning of peripheral pulmonary arteries. Nonenhanced chest CT may demonstrate mosaic attenuation/perfusion (often more conspicuous than that seen in patients with small airways disease), calcification of the pulmonary arteries (in long-standing CTEPH), enlargement of the pulmonary trunk, and dilatation of the right ventricle. CT pulmonary angiography demonstrates web-like filling defects, peripherally located eccentric thrombi within vascular lumina, enlargement of the pulmonary trunk and central pulmonary arteries, pulmonary artery calcifications (in prolonged CTEPH), right ventricular dilatation, flattening of the interventricular septum, and hypertrophied bronchial arteries. Ventilation-perfusion imaging, often utilized to help establish the diagnosis, demonstrates markedly heterogenous mismatch perfusion defects. Catheter-based pulmonary angiography may demonstrate web-like (linear) or eccentric filling defects.

Conclusion
Making the often unsuspected diagnosis of CTEPH has significant clinical implications as medical management and pulmonary endarterectomy have been demonstrated to improve clinical outcomes in selected patients. Thus, the radiologist plays a critical role in the identification and diagnosis of CTEPH and may help facilitate timely diagnosis and guide appropriate management.