CT angiography in pulmonary embolism raises radiation dose

August 1, 2005

Can a test be too effective for its own good? Current research indicates that CT pulmonary angiography is the test of choice for confirming or ruling out a diagnosis of pulmonary embolism. It achieves higher interobserver agreement than other embolic imaging tests such as ventilation/perfusion scintigraphy and conventional pulmonary angiography, and its easy accessibility encourages many physicians to use it.

Can a test be too effective for its own good? Current research indicates that CT pulmonary angiography is the test of choice for confirming or ruling out a diagnosis of pulmonary embolism. It achieves higher interobserver agreement than other embolic imaging tests such as ventilation/perfusion scintigraphy and conventional pulmonary angiography, and its easy accessibility encourages many physicians to use it.

But studies reported at the American Roentgen Ray Society meeting in May indicate that while pulmonary embolism diagnoses are actually decreasing, CT pulmonary angiography use is rising, potentially placing vulnerable populations at risk for radiation-induced cancers.

In a study analyzing the outcomes of 3500 patients, researchers at Brigham and Women's Hospital found that a negative spiral CT scan is sufficient for excluding clinically significant pulmonary embolism, with a negative predictive value similar to that of invasive pulmonary angiography (JAMA 2005 27;293[16]:2012-2017). And researchers from Rhode Island Hospital in Providence reported at the ARRS meeting that multislice CT pulmonary angiography is replacing the ventilation/perfusion lung scan for diagnosis of suspected pulmonary embolism in hospitalized patients.

The Rhode Island group found that the average number of ventilation/perfusion scans dropped from 5.7 to 3.6 per month over 30 months, while the average number of MSCT pulmonary angiograms increased from 21.9 to 42.5 per month.

Although the degree of change represents only the experience of Rhode Island Hospital, it mirrors a national trend toward diagnosis of pulmonary embolism using CT, said lead author Dr. Jay H. Donohoo.

The modality's accessibility, high negative predictive value, and ability to make alternative diagnoses have contributed to its increasing use, but the spike in demand also raises clinical questions about the possibility of overutilization.

In a study conducted at University Hospitals of Cleveland, researchers identified both hospitalized and emergency room patients who had undergone CT pulmonary angiography for suspected pulmonary embolism during one of two periods: 1997 to 1998 and 2002 to 2003. The patients were examined with single-slice CT with 3-mm collimation and a pitch of 1.7 and multislice CT with 2-mm collimation and a pitch of 1.

The number of CT pulmonary angiographies jumped from 81 patients between 1997 and 1998 to 349 between 2002 and 2003 (AJR 2004; 183:1093-1096).

"These data demonstrate that clinicians have accepted the evidence regarding CT for pulmonary embolism and are incorporating it into their practices," said lead author Dr. John David Prologo, chief radiology resident at University Hospitals.

The burgeoning use of the test does not necessarily translate into increased diagnoses of pulmonary embolism, however.

Researchers from the Mallinckrodt Institute of Radiology and Washington University School of Medicine examined the frequency of pulmonary embolism and alternative diagnoses in 650 patients. They found that only 13% of the pulmonary embolism protocols were positive for pulmonary embolism. A study at Albert Einstein College of Medicine found that the percentage of positive diagnoses dropped from 14% in 2002 to 9% in 2004.

The sustained growth in utilization indicates that referring clinicians are using the study for patients with shortness of breath and chest pain, and they are using it in younger patients, said coauthor, Dr. Sanjeev Bhalla, codirector of body CT and emergency radiology at Washington University.

"We have seen a true indication creep for the pulmonary embolism protocol in that the positive detection rate has declined," he said. "What will this added radiation exposure over the long term do to our population?"

TOO MUCH RADIATION

In addition to the dangers inherent in contrast imaging, such as adverse reactions to the dye and the need for intravenous injection, another specter looms over the greater use of the technique: radiation dose and the potential risks for radiation-induced cancers such as breast cancer.

"This is a huge question and one filled with uncertainty. In our busy emergency department, it is not unusual for a patient to get more than one PE protocol in a year," Bhalla said. "Another potential problem is the added use of retrospective cardiac-gating, which is being used in some centers for the 'triple rule-out' study, or CT to exclude PE, coronary ischemia, and dissection."

PROPER PROTOCOL

Exposure to radiation could be reduced with the right diagnostic protocol. Many physicians suggest the use of the D-dimer screening test after a clinical evaluation to test for likelihood of the disease.

"D-dimer testing prior to CT pulmonary angiography should be part of the diagnostic algorithm in outpatients," said Dr. U. Joseph Schoepf, director of CT research and development at the Medical University of South Carolina.

Outpatients with a negative D-dimer are unlikely to have a venous thromboembolism, Schoepf said. Young patients with normal chest x-rays and low pretest likelihood could potentially be managed with a low-dose perfusion scintigram, without the ventilation component.

All inpatients, as well as patients with a high pretest likelihood, should undergo CT, he said. They should be scanned with MSCT with 500-msec rotation or less, so that the entire chest can be imaged using 1-mm or thinner sections. Thin-slice acquisition has been shown to significantly improve the diagnosis of pulmonary embolism.

Dr. Lisa K. Moores, an associate professor of medicine at Walter Reed Army Medical Center, recommends determining a pretest probability from either expert anecdotal experience or the Wells score. A D-dimer should then be done in patients with a low pretest probability, as patients with low probabilities and negative D-dimer scores do not require further testing.

Moores performs CT pulmonary angiography with indirect venography in patients who do not have low pretest probabilities or do have a positive D-dimer.

"If this is a good study, we are done, and patients with clot get treated, while those with a negative study do not," she said.

For patients with indeterminate studies, either a repeat CT pulmonary angiography, MRI, or conventional angiography should be considered, based on whether an alternative diagnosis has been made, Moores said.

"Although less studied, it might be argued to start with carotid ultrasound since this does not require the dye load or radiation. If this is negative, then a CTPA could be performed," she said.

Ideally, physicians should start off with lower extremity ultrasound, according to Bhalla. However, the protocol may not be practical in centers that do not have sonography technologists or where services other than radiology perform these studies.

"At our center, we try and determine if the patient has had any prior embolic imaging. If the answer is yes, we try to duplicate that test so that we can take advantage of the comparison. If the answer is no, we try and use the chest radiograph and the patient's age to guide us," he said.

FUTURE STUDIES

Given the risks surrounding exposure to radiation at an early age, proper patient selection is an important area for future research.

"Because the signs and symptoms are so nonspecific, the diagnosis of PE has been elusive," Bhalla said. "Nonimaging laboratory tests and PE criteria need to be further honed. Also, radiation reduction techniques such as the use of breast shields and tube-modulation techniques need to be investigated."

MR imaging could potentially be a valuable diagnostic tool as well, he said, but studies are needed to prove the modality's performance in this application.

Additionally, development of improved workflow concepts with dedicated visualization tools and computer-aided detection for pulmonary angiography could help sharpen the diagnostic technique, Schoepf said.

"The test is so good that there is a risk of overutilization in unsuitable patient populations. If the test is ordered as a result of a rationalized decision tree, radiation is no real issue. The very imminent risk of missing a potentially fatal diagnosis far outweighs the theoretical radiation risk," he said.

Ms. Trevino is an assistant editor of Diagnostic Imaging.