Young women may be receiving too much radiation when physicians rely on CT to make a diagnosis of pulmonary embolism, according to researchers at the Medical College of Virginia in the U.S. CT pulmonary angiograms, an increasingly popular strategy for diagnosing PE, can deliver more than three times the amount of radiation to the female breast as standard mammography exams. For younger women with suspected PE, CT tube current modulation, use of different imaging modalities, and radiation shielding alternatives may provide ways to reduce radiation dose.
The MCV group is not calling for an end to the use of CT in this application, but they ask that radiologists urge referring clinicians to use caution in ordering CT exams for PE.
"We don't want to use scare tactics. The benefit of one given exam is going to outweigh the risk," said Dr. Mark S. Parker, an associate professor of thoracic radiology at MCV. "We want to see what we can do to modify the amount of radiation people are exposed to."
Parker and colleagues retrospectively reviewed 1325 CT exams performed for PE at their institution between May 2000 and December 2002. They presented the results at the American Roentgen Ray Society meeting in May. Sixty percent of the patients were female, and 25% of them were younger than 40, the youngest being 15. The researchers calculated radiation dose using the ImPACT CT dosimetry calculator and the CTDI (dose index) for the Siemens Plus 4 scanner, on which all the scans were performed.
Noncontrast 5-mm images were taken from the thoracic inlet to the diaphragm using 140 kVp, 120 mAs, and pitch of 1.5. Additionally, 1.5-mm contrast-enhanced caudiocranial images were acquired using 140 kVp, 150 mAs, and pitch of 2. Diagnostic outcomes included 51% negative for PE, 30% positive, 15% inconclusive, and 4% nondiagnostic.
For the average 130-pound woman, radiation dose averaged 2 rads per breast, much higher than the 0.6 rad of standard two-view mammography, Parker said.
The best way to reduce dose is to use 100 or 80 kVp, according to Dr. Denis Tack, chief of medical imaging at Clinique Louis Caty in Baudour, Belgium. The signal-to-noise ratio is higher using lower kVp values. Another approach would be to adapt the mAs to the body mass index.
For example, a young woman 5 foot 2 inches tall and weighing about 130 pounds can undergo CT pulmonary angiography with 90 kVp and 75 mAs with high-quality images. Compared with 140 kVp and 150 mAs, this acquisition can reduce the radiation dose by a factor of 6, he said.
"In my department, the use of 140 kVp for chest CT is strictly prohibited," he said.
Parker suggests using alternatives such as MR, ventilation/perfusion scans, and blood screening D-dimer tests to triage patients for further diagnosis. New 16-slice CT scanners, which automatically modulate tube current according to body geometry, could also reduce exposure.
MCV has plans to perform phantom studies testing shields made from bismuth, a soft metal that reduces radiation without altering diagnostic information.