To maximize spatial resolution and anatomic detail, cardiac CT (CCT) studies use a coned-down or limited field of view (FOV) that visualizes the heart and approximately one-third of the chest volume. However, if only limited-FOV images are viewed, more than 66.7 percent of pulmonary nodules larger than 1 cm in diameter and more than 80 percent of nodules smaller than 1 cm would be missed, according to a study published this month in the American Journal of Roentgenology.
Incidental findings are common in the practice of radiology; research demonstrates that extra-cardiac findings are made with a frequency of 10 percent to as high as 60 percent at CCT, with pulmonary neoplasms being found at rates of 1.2 percent – similar to results reported in lung cancer screening trials.
In a retrospective study conducted in the department of radiology at the Medical University of South Carolina in Charleston, researchers reviewed the records of 1,764 patients who underwent CCT studies over a three-year period.
“Cardiac CT was performed for calcium scoring in 463 cases, coronary CT angiography (CTA) in 737 cases, evaluation for pulmonary venous stenosis after radiofrequency ablation in 341 cases, and evaluation of a coronary artery bypass graft (CABG) in 223 cases,” the authors wrote.
All studies were performed using a 64-slice CT scanner, either a Siemens Medical Solutions’ Somatom Definition or Somatom Sensation 64 cardiac system. In all cases, limited FOV images, encompassing the heart and 1 cm from the farthest anterior, posterior and lateral extents of the cardiac chamber, were reconstructed from the full FOV data set.
The scientists then reviewed both FOV data sets to determine the percentage of pulmonary nodules that would be missed with viewing only limited FOV images.
They found that pulmonary nodules, including 15 larger than 1 cm, were found in 11.5 percent (202) of the 1,764 reviewed cases. Of the 123 men and 64 women between 36 and 88 years of age with nodules less than 1 cm, 15.5 percent were seen in limited FOV, 2.7 percent were partially visible and 81.8 percent were excluded from view. In the subgroup with nodules larger than 1 cm, 26.7 percent were visible in limited FOV, 6.7 percent were partially visible and 66.7 percent were excluded from view.
“Because FOV can be adjusted with relative ease and without the consequence of increased radiation exposure, it seems reasonable to require full FOV for cardiac CT interpretation,” the authors suggested.
The dilemma in CCT study evaluation, for patients, is what specialist interprets the results of their exam—a radiologist or cardiologist.
The American College of Radiology guidelines for CCT requires that interpreters also meet its guidelines for interpreting diagnostic CT scans and must assess and document important extra-cardiac findings in a diagnostic report. The American College of Cardiology makes no implicit or explicit competence requirement for evaluation of extra-cardiac structures.
In an accompanying commentary, Patrick M. Colletti, MD, of the department of imaging science at the University of Southern California in Los Angeles, noted that the moral implications of reviewing the full FOV CCT data set are clear.
“Once an examination is performed, the noblest approach is to view and evaluate all
available data, to apply appropriate judgment and to proceed in the best interest of the patient and society,” he stated.