Diagnostic Value of Photon-Counting CT in Subsegmental Pulmonary Embolism

Pulmonary embolism (PE) is a leading cause of morbidity and mortality worldwide. Subsegmental pulmonary embolism (SSPE), defined as thrombi confined to subsegmental pulmonary arteries, may account for up to 20% of acute PE cases detected by multidetector CT. While distal in location, SSPE carries a risk of recurrent venous thromboembolism and mortality that is comparable to more proximal PE, suggesting that SSPE is not a benign entity. Accurate diagnosis is therefore clinically important, but remains challenging due to motion artifacts, poor contrast opacification, and the large number of small subsegmental vessels that must be evaluated.

Photon-counting computed tomography (PCCT) is a recently developed imaging technology that offers significant improvements over conventional energy-integrating detector CT (EID-CT). PCCT provides higher spatial resolution, direct spectral information, and the ability to generate virtual monoenergetic images and iodine maps in a single acquisition, while reducing radiation dose. Preliminary studies indicate that PCCT can minimize motion artifacts, improve image quality, and enhance visualization of small pulmonary arteries, potentially increasing the detection of SSPE.

This prospective observational cohort study will compare the diagnostic performance of PCCT with dual-source CT in patients with suspected acute PE. The primary outcome is the detection rate of SSPE. Secondary outcomes include image quality scores, radiation dose, and the added diagnostic value of PCCT in patients with negative findings on dual-source CT. By systematically evaluating image quality, detection yield, and radiation exposure, this study seeks to establish the potential role of PCCT as a superior diagnostic tool for SSPE in real-world clinical practice.

Inclusion criteria

• Age ≥ 18 years
• Clinical manifestations supporting the diagnosis of acute pulmonary embolism, including:

Acute onset or progressive dyspnea Unexplained hypoxemia Chest pain Hemoptysis Syncope or presyncope Combined with abnormal findings on electrocardiogram, echocardiography, or pulse oximetry

• Revised simplified Geneva score ≥ 3
• Written informed consent obtained from the patient or the patient's legally authorized representative

Exclusion criteria

• Patients requiring prophylactic or therapeutic doses of anticoagulant medication for reasons other than venous thromboembolism (VTE).
• Life expectancy less than three months.
• Patients unable to undergo CT scanning due to severe condition or hemodynamic instability.
• History of allergy to contrast media, renal insufficiency (creatinine clearance < 30 mL/min), or ongoing long-term dialysis.
• Pregnancy.
• Refusal to provide informed consent or inability to complete follow-up for any reason.