Dedicated coronary CT scan cannot match triple rule-out for PE detection

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In patients with suspected acute coronary syndrome presenting to the emergency department for the primary purpose of evaluation of the coronary arteries, a dedicated coronary CT protocol provides excellent quality images of the coronary arteries and proximal ascending aorta, but images of the pulmonary vasculature are insufficient for the exclusion of pulmonary embolism (PE).

The so-called triple rule-out—a CT scan of the coronary, aortic and pulmonary circulation—has advantages, such as the one-stop-shop approach—and disadvantages, such as high radiation exposure and a longer breathold. Consequently, its usefulness continues to be debated.

  
   
  
A 58-year-old man presented to the emergency department with acute chest pain. Righted-sided pulmonary arterial circulation (A) and left-sided pulmonary arterial circulation (B) coronary CT scans show mean contrast enhancement (HU) of pulmonary arterial segments. Mean vessel opacification decreases in craniocaudal direction as contrast agent passes from pulmonary to systemic circulation during dedicated cardiac CT acquisition. Legend: apic = apical; seg = segment; ant = anterior; post = posterior; rul = right upper lobe; rpa = right pulmonary artery; rll = right lower lobe; rml = right middle lobe; sup = superior; med = medial; lat = lateral; lul = left upper lobe; lpa = left pulmonary artery; lll = left lower lobe; ling = lingula; inf = inferior. Source: American Roentgen Ray Society
 

In that regard, researchers from the Massachusetts General Hospital sought to determine if a dedicated coronary CTA protocol could provide enough detail to rule in or rule out aortic dissection and PE. They found that the CT protocol falls short of opacifying the total pulmonary tree.

The study was published in the September issue of the American Journal of Roentgenology.

Jonathan D. Dodd, MD, and colleagues retrospectively examined the CT scans of 50 patients consecutively admitted from the emergency department from May to June 2004 with acute chest pain but normal or nondiagnostic ECG findings.

Patients were scanned using a single contrast infusion followed by a bolus chaser rather than multiphasic contrast infusion.

Qualitatively, image quality of the coronary arteries, proximal ascending aorta and the main pulmonary artery was deemed excellent. Pulmonary artery quality suffered in the right upper and lower lobes and in the segmental branches. Poor quality was often associated with a dense collection of contrast material in the superior vena cava (SVC). ECG lead placement was another cause of streak artifact.

“Dense collection of contrast material in the SVC particularly affected the right pulmonary artery and anterior segmental branch of the right upper lobe. Such artifacts highlight the importance of the use of a saline bolus chaser after contrast administration and careful ECG lead placement off the anterior part of the chest,” the authors wrote.

Quantitatively, researchers found statistically significant differences in aortic contrast opacification between various levels in the ascending and descending aorta, but none of them amounted to absolute differences in mean attenuation.

The poorest opacification in the pulmonary circulation was detected in the right lower lobe lateral segmental artery (44 percent < 200 Hounsfield units). Vessel opacification decreased both with progressive pulmonary arterial branching and from the upper to the lower lobes, according to the study.

“For exclusion of all pulmonary embolisms, formal CT pulmonary angiography appears necessary,” the authors wrote.

Researchers also commented that the variability in opacification is related to the contrast bolus, most of which has passed into the aorta at the start of acquisition. “Such scanning-time characteristics have implications for PE detection because most PEs are known to occur in the lower lobes.”

They suggested that caudocranial acquisition for coronary CT would improve detection of PE.