Cardiac CT triple rule-out passes several tests
Three studies have found that the “triple rule-out” exam is advantageous in certain patient populations and the senior author told Cardiovascular Business News that the postprocessing involved is simple and straightforward.

The triple rule-out exam is a single CT angiography scan to evaluate the coronary and pulmonary arteries, as well as the aorta in low- to moderate-risk patients who present to the emergency department (ED) with symptoms suggestive of acute coronary syndrome (ACS).

Researchers from Thomas Jefferson University found that the exam can quickly identify patients with coronary disease and triage those with non-cardiac origins of chest pain; can reduce hospital time compared to nuclear stress testing; and delivers optimal image quality with minimal additional contrast media. They reported their studies at the 2008 Radiological Society of North American meeting in Chicago.

Triple rule-out study (top) showing a pulmonary embolus (arrow) and another (bottom) showing an aortic dissection (arrowheads), with a normal right coronary artery (arrow). Source: Halpern, Ethan J. Clinical Cardiac CT: Anatomy and Function. Thieme Medical Publishers, 2008. Reprinted with permission by the author.  
In the first study, Ethan Halpern, MD, an assistant professor in the department of radiology, and Kevin Takakuwa, MD, an assistant professor of emergency medicine, found that the chest pain of 76 percent of 197 consecutive low- to intermediate-risk patients who underwent the triple rule-out protocol did not originate in the coronary arteries, aorta or pulmonary arteries. These patients received no further diagnostic testing.

Eleven percent of the patients had a non-cardiac cause of their symptoms, such as pulmonary embolism or aortic dissection, and 11 percent had either severe or moderate coronary artery disease (CAD).

At 30-day follow-up, the negative predictive value of a CCTA with no more than mild disease was 99.4 percent. There were no adverse outcomes at 30-days. CT exams were performed on a Brilliance-64 scanner (Philips Healthcare).

The postprocessing of CT data adds no significant time to the diagnostic process, Halpern said.

“For the majority of patients, diagnosis is made by a combination of reading the original axial images and slab MIP [maximum intensity projection] reconstructions that are created and interpreted on a workstation in real time. The same images are used to evaluate for pulmonary embolism and aortic dissection,” he said.

He added that each segment of each coronary artery should be evaluated in at least two perpendicular projections and can be done in real time by rotating the slab MIP images on a workstation. In more complicated cases, they use vessel tracking with curved MIP reconstructions.
In the second study, led by Takakuwa, researchers found that the use of a triple rule-out CTA strategy compared to a stress testing strategy translated into overall decreased total hospital and observation time in the ED, but higher imaging costs.

The standard observation protocol included a minimum of two sets of cardiac enzymes at least six hours apart, followed by a nuclear stress test. Once a week, observation patients were offered a triple rule-out exam with the option of further stress testing for those patients found to have any evidence of coronary disease.

Mean imaging costs were $1299 for triple rule-out patients versus $870 for traditional stress testing. Total length of stay was 16 hours for triple rule-out patients versus 22 hours for traditional stress testing. Mean observation time was 4 hours for triple rule-out patients versus 6 hours for stress testing patients. All comparisons were statistically significant.

“The level of increased costs was driven in part by the frequency of stress testing for patients with minimal and mild coronary disease on triple-rule-out,” Takakuwa said, adding that the analysis did not consider cost savings based upon decreased length of stay in triple rule-out patients.
In the third study, led by Halpern, researchers found that a dedicated triple rule-out CT scan using 95 cc of contrast medium provided good image quality of the three areas of concern compared with a dedicated coronary CTA protocol using 70 cc of contrast (Omnipaque-350, GE Healthcare).    
Since the triple rule-out scan is approximately 30 percent longer than a dedicated CCTA exam, the radiation dose is increased by a factor of 30 percent, Halpern said.  

“In order to reduce the radiation dose, we use tube current modulation whenever possible, which achieves a 50 percent dose reduction. In selected patients with regular sinus bradycardia, we switch to prospective ECG gating with a dose reduction of 75 percent.”

The triple rule-out exam is not for everyone. The selection of the test is related to patient presentation, Halpern said. The triple rule-out study is most appropriate for a low- to moderate-risk ACS population when there is also a clinical consideration for other non-cardiac causes of chest pain. “It is most appropriate when there is a clinical suspicion for pulmonary embolism as well as either ACS or aortic dissection,” he said.