CO₂-enhanced CT imaging for the detection of pulmonary emboli: Feasibility study in a porcine model

Rationale and Objectives. The authors investigated the feasibility of using computed tomography (CT) with CO₂ gas as a negative contrast agent for detecting pulmonary emboli in a porcine model. Materials and Methods. Seven pigs with or without pulmonary emboli underwent thoracic imaging with multi-detector row spiral CT. To identify optimal injection and scanning protocols, the first four pigs were scanned repeatedly in the supine and prone positions with different scan delays (10, 15, and 20 seconds) and different volumes of CO₂ (60, 120, 180, and 240 mL), which were hand infused (each infusion took 10-15 seconds). The last five pigs with emboli were scanned with iodinated contrast medium and then rescanned with 120 or 180 mL of CO₂. The CO₂ volumes and scan delays were qualitatively assessed. The supine and prone CT scans and the number and location of thrombi depicted in the CO₂- and contrast material-enhanced CT scans were compared. Results. Because the pulmonary artery in pigs is in the posterior anatomy, the prone position was more effective than the supine position with CO₂ enhancement. An infusion of 120 mL of CO₂ was sufficient to enhance the entire pulmonary artery, and scanning timed to coincide with the completion of infusion was the most effective. Both the CO₂- and contrast-enhanced CT scans demonstrated all thrombi. Thrombi were more apparent on the CO₂-enhanced CT scans than on the contrast-enhanced scans because of the high contrast interface between soft tissue and gas. However, two of the seven pigs with thrombi experienced abrupt cardiac arrest after CO₂-enhanced scanning and could not be resuscitated. The cause of these events was not determined in the current study. Conclusion. The CT depiction of pulmonary emboli is feasible with CO₂ gas as a negative contrast agent and may even be superior to that with iodinated contrast media. Further studies are required to evaluate the safety of this method and to develop an improved delivery of CO₂ gas for this application.