Cardiac CT with Somatom Definition Image source: Siemens Healthcare

Dueling articles in the November issue of Radiology tackled the touchy topic of extracardiac findings on cardiac CT exams, debating whether to reconstruct and review studies at the maximum field of view (FOV) or to limit interpretation to a restricted FOV.

Cardiac CT exams produce diagnostic images of multiple extracardiac structures, including the lungs, mediastinum, upper abdomen and bones. Consequently, incidental findings occur fairly frequently, with previous studies suggesting that the prevalence of incidental findings on cardiac CT ranges from 10 percent to 60 percent. Pulmonary nodules account for the lion’s share of the findings and occur in five to 20 percent of exams.

The question of how to reconstruct and review cardiac CT data took on new urgency in 2010 with the release of National Lung Screening Trial (NLST) data indicating a 20 percent mortality reduction among a high-risk population with screening CT exams.

A survey of members of the Society of Cardiovascular Computed Tomography and North American Society of Cardiovascular Imaging revealed that 68 percent of physicians used a full FOV and 32 percent used a limited FOV.

James P. Earls, MD, of Fairfax Radiological Consultants in Clifton, Va., argued that studies should be reconstructed and reviewed in the maximum FOV. Charles S. White, MD, of the department of diagnostic radiology and nuclear medicine at University of Maryland School of Medicine in Baltimore, took the opposing stance and posited that it is acceptable to restrict the FOV to the heart.

“Most debate on proper coronary CT angiographic reconstruction centers on the contention that patients stand to benefit from the detection of the greater number of pulmonary nodules that would be found with full FOV reconstruction of the thorax,” White wrote.

Earls acknowledged that 99 percent of pulmonary nodules detected at cardiac CT are benign. However, studies show that the majority do not meet established criteria for evaluation or follow-up and present in only 0.4 to 16.5 percent of patients.

While White questioned the economic and morbidity impacts of evaluating incidental findings, suggesting that costs may outweigh benefits, Earls countered that this argument is based on an overestimation of the costs and risks of follow-up of indeterminate nodules.

Although evidence demonstrates that a full FOV shows a greater number of nodules, White questioned the utility of detection and follow-up. He pointed to key differences between cardiac CT patients and NLST participants. Patients imaged with cardiac CT are more likely to be nonsmokers younger than age 55 who present with symptoms such as chest pain or dyspnea.

“Nevertheless, while cardiac CT and lung cancer CT screening populations are each unique, they may not be as dissimilar as one might think,” wrote Earls. More than half of cardiac CT patients are current and former smokers, and their mean age falls within NLST criteria. Thus, Earls suggested that cardiac CT patients may accrue some of the mortality benefit associated with lung CT screening.

But White suggested that among a symptomatic population a wider FOV might detect advanced, incurable disease.

In addition, wide FOV images exclude portions of the lungs and do not allow a complete evaluation of the lung, noted White. However, Earls cited a study by Northam et al which showed that a small FOV excluded 80 percent of all lung nodules and 67 percent of all lung cancers.

White pointed out that differences in protocols used in lung CT screening and cardiac CT could play a role. For example, cardiac CT angiography studies are reconstructed with very thin sections and higher dose, which could translate into increased identification of incidental findings.

Given the lack of guidelines to inform practice, White summarized, “While there is no obvious harm in reconstructing a wide FOV at cardiac CT, on the basis of logistical considerations and current practice patterns, at present, it appears equally justifiable to use a FOV restricted to the heart.”

Earls, in contrast, wrote, “Until the data are further clarified, all cardiac CT studies should be reconstructed in the maximum FOV available, and the images should be adequately reviewed by a qualified thoracic reader to detect pathologic findings.”