Imaging raises more questions in acute pulmonary embolism

Advances in CT technology have made it easier to detect blood clots located in segmental and more proximal pulmonary vessels. Physicians equipped with CT should have no problem making a positive diagnosis of acute pulmonary embolism or asserting that patients are clot-free, In reality, clinicians and radiologists are still left wondering how best to use CT in routine practice.

Presenters at a symposium on PE showed how and why the development of multislice technology promises to make CT the ultimate reference tool for acute PE assessment. But access to state-of-the-art CT scans will not necessarily simplify diagnostic and clinical decision-making, said Prof. Arnaud Perrier, head of general internal medicine at Geneva University Hospital.

Identification of an isolated clot in a subsegmental vessel might be a great demonstration of the power of CT, but if the patient is at low risk of PE and shows no clinical symptoms, how should this radiological finding be treated?

"As the performance of CT allows for more distal vascular imaging, we should be more concerned about false positives," he said.

Studies assessing the accuracy of CT for acute PE evaluation have shown how the value of CT can depend on clinical information. The PIOPED II study published in 2006 reported a high specificity for MSCT (96%) but a disappointingly low sensitivity (83%). Most false negatives were in patients with a high probability of PE. For patients with a low clinical probability of PE, 42% of positive calls turned out to be false.

Perrier recommends performing plasma D-dimer testing on all patients considered to be at low or intermediate risk of acute PE. Those with a negative D-dimer result can be considered clear, while those with a positive test result should be referred for MSCT.

Patients with a high clinical probability of PE should go straight to MSCT, without D-dimer testing. Any clots picked up on the scan should be treated. If no emboli are found on CT, another diagnostic test will be needed. Which test that should be remains a matter for debate.

The quality of data generated from latest-generation CT scanners poses another dilemma, said Prof. Martine Rémy-Jardin, head of the thoracic imaging department at Hôpital Calmette, University Center of Lille, France. Exactly what should radiologists do with all of this information?

Some patients with acute PE who also have right ventricular dysfunction may benefit from more aggressive treatments, such as fibrinolysis or embolectomy. It may therefore be helpful to evaluate cardiac function as well as the morphology of pulmonary arteries. Both should be possible now from the same high-resolution transverse CT scans.

Extended CT-based examinations involve possible higher radiation exposure to patients. Dual-source technology should make it possible to assess right ventricular dysfunction, estimate clot burden, and detect PE with a dose of just 150 mGy/cm. ECG-gated examinations may be used in the future for coronary CT angiography, though for now, the radiation burden is probably unjustified, Rémy-Jardin said.