This prospective observational study aims to validate three-dimensional electrical impedance tomography (3D-EIT) as a non-invasive bedside tool for the diagnosis and longitudinal monitoring of pulmonary embolism (PE). The study will include a PE cohort and a control cohort without PE or venous thromboembolism (VTE). Using algorithms that separate ventilation and perfusion signals, 3D-EIT-derived indices, including the Matching Index (MI), Dead Space Index (DI), and Shunt Index (SI), will be quantified. The diagnostic performance of MI and DI will be evaluated against computed tomography pulmonary angiography (CTPA), and their associations with PE severity, risk stratification, and treatment response will be explored. This study is expected to support the clinical translation of 3D-EIT as a radiation-free bedside functional imaging tool for PE management.

Pulmonary embolism (PE) is a major cause of morbidity and mortality. Computed tomography pulmonary angiography (CTPA) is the current reference standard for PE diagnosis, but it is limited by radiation exposure, contrast administration, and poor suitability for bedside or repeated dynamic assessment. Three-dimensional electrical impedance tomography (3D-EIT) is a non-invasive, radiation-free imaging technique that enables bedside evaluation of regional lung ventilation and perfusion.

The objective of this study is to validate the clinical utility of 3D-EIT for the diagnosis, risk stratification, and longitudinal monitoring of PE. The study will focus on EIT-derived ventilation-perfusion parameters, including the Matching Index (MI), Dead Space Index (DI), and Shunt Index (SI).

This is a prospective, single-center, observational case-control study conducted at Beijing Tsinghua Changgung Hospital. Two cohorts will be enrolled:

1. A control cohort without PE or VTE, recruited from patients scheduled for saphenous vein radiofrequency ablation for varicose veins who meet predefined screening criteria.
2. A PE cohort including patients with a first episode of PE confirmed by CTPA. Participants in the control cohort will undergo one 3D-EIT examination to establish reference ranges for MI, DI, and SI. Participants in the PE cohort will undergo 3D-EIT within 24 hours after diagnostic CTPA. Repeated EIT examinations will be performed after thrombolytic therapy when applicable, at hospital discharge, and at 1-month follow-up to evaluate dynamic changes over time.

The primary objective is to assess the diagnostic performance of MI and DI for identifying PE using CTPA as the reference standard. Secondary objectives include evaluating the association of EIT-derived indices with PE severity, CTPA-based obstruction burden, risk stratification category, and temporal changes following treatment.

Statistical analyses will include estimation of reference ranges in the control cohort, receiver operating characteristic (ROC) analysis for diagnostic performance, correlation analyses between EIT and CTPA-derived parameters, and longitudinal analyses of repeated EIT measurements in the PE cohort.

Inclusion Criteria:

• Age 18 years or older
• Body mass index (BMI) less than 50 kg/m²
• Able to provide written informed consent
• first episode of pulmonary embolism confirmed by CT pulmonary angiography (CTPA), with or without CT venography (CTV)

Exclusion Criteria:

• Previous diagnosis of pulmonary embolism
• Pregnancy
• Thoracic deformity or chest wall abnormality preventing proper placement of the EIT electrode belt
• Contraindications to EIT use, including implantable cardioverter-defibrillator, pacemaker, implanted pumps, or chest wounds limiting electrode placement
• Concurrent participation in another interventional clinical study Hemodynamic instability requiring emergency intervention that precludes completion of the EIT examination (PE cohort only)
• Any other condition judged by the investigator to make the participant unsuitable for study participation

Participants will be recruited from Beijing Tsinghua Changgung Hospital. The study population will include: (1) adult patients with a first episode of pulmonary embolism confirmed by CT pulmonary angiography, and (2) control participants without pulmonary embolism or deep vein thrombosis who are scheduled for saphenous vein radiofrequency ablation for varicose veins. Control participants will undergo screening to exclude venous thromboembolism before enrollment.