Radiologist study warns about extra-cardiac CT findings; cardiologist responds

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Researchers from the department of radiology at the Medical University of South Carolina in Charleston found that using a limited field of view with cardiac CT misses a large percentage of pulmonary nodules. UCLA cardiologist Matthew Budoff, MD, however, says the current evidence suggests it is “better to keep the field of view small, rather than find a lot of nodules, especially in a population at low risk for lung cancer.”

In a retrospective study appearing in the September issue of the American Journal of Roentgenology, Meredith Northam, MD, and colleagues at MUSC 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.

  
   
  
   
A 58-year-old man with indeterminate nodule considered suspicious for lung cancer partially visualized at edge of limited field of view. A, Full-field-of-view axial CT scan (3-mm slice thickness, lung window) obtained in evaluation of pulmonary venous stenosis shows 1.8-cm mixed-density nodule ( arrow) in left lower lobe. B, Limited-field-of-view axial CT scan (1-mm slice thickness, lung window) at same level as A shows partially visible nodule ( arrow). Image and caption by permission of the American Roentgen Ray Society.  
   

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 field-of-view (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 of the reviewed cases. Of those 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 wrote.

Budoff said that expanding the FOV by using a larger bow-tie filter will afford more radiation to the patient. “On a current Lightspeed VCT (GE Healthcare), a medium bowtie filter for a CCTA affords 13.4 mSv of radiation, while a small bowtie filter—which would still allow reconstructions of the heart to 25 cm, but not a full field of view—reduces this to 8 mSv in the same patient, same protocol,” he said.

The dilemma in CCT study evaluation is what specialist interprets the results of their exam—a radiologist or cardiologist.

The American College of Radiology guidelines for CCT require 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.

Buddoff, director of the Harbor-UCLA LA BioMed CT Reading Center, told Cardiovascular Business News that the issue related to overreading cardiac CT scans raised by Colletti in the recent commentary is misdirected.

“The question is not whether we can find millions of lung nodules, but rather should we. There is no proven benefit, and may be proven harm, to identifying these incidental nodules on cardiac CT.” (1)

He stated that