Dual-source CT boosts patient flow, diagnostic confidence

March 1, 2007

In the past, a person with chest pain entering the emergency department at the Hospital of the University of Pennsylvania could spend up to six hours there before being discharged.

In the past, a person with chest pain entering the emergency department at the Hospital of the University of Pennsylvania could spend up to six hours there before being discharged. The acquisition of a Siemens Definition dual x-ray source CT scanner helps put patients back on the street in less than two hours, HUP officials say. The time that outpatients with atypical chest pain spend in the imaging environment to rule out coronary artery disease has also been greatly reduced.

The time savings revolve around the scanner's temporal resolution, which has improved from 165 msec to 83 msec. Such speed gives radiologists flexibility regarding the administration of beta blockers. Patients at HUP receive beta blockers if their heart rate is above 90 bpm. About half an hour after the beta blockers are administered, patients get scanned. Checking the heart rate again or making decisions about whether to give intravenous beta blockers is no longer necessary.

"Having a lower heart rate is certainly preferable whenever possible, even when you have the improved temporal resolution. But you can live without the lower heart rate much more readily with the dual-source scanner," said Dr. Harold Litt, chief of cardiovascular imaging at HUP.

Previously, with the single x-ray source 64-slice CT scanner, residents or fellows would talk to patients, check their heart rate, and either send them to the scanner if it was under 65 bpm or administer oral beta blockers if it was elevated-which was the case with almost everyone, according to Litt. In 30 to 45 minutes, the heart rate would be checked again and decisions often had to be made whether to give more oral beta blockers or take the patient to the scanner and give IV beta blockers. In about one-third of cases, other problems arose that required more decisions and more time spent talking with patients.

"We don't have any of those conversations any more, and it has helped our throughput remarkably, particularly in the emergency department," he said.

About 75% of low-risk patients with chest pain now admitted to HUP's emergency department can be discharged after a negative CT angiogram, said Dr. Judd Hollander, a professor of emergency medicine and director of the clinical emergency program. Previously, about 65% of them would have been admitted. Hollander and colleagues are following these patients to determine how they do over time. Preliminary data on 200 patients 30 days postexam revealed no cardiac events.

"The real issue is, Are the patients safe to go home? In fact, they are," Hollander said.

HUP's dual-source scanner, acquired last October, does not live in the emergency department. Litt decided to locate the scanner in the main radiology department to have easier access for research.

The situation is similar at the Cleveland Clinic. The addition of a dual-source CT scanner has eliminated the need to have a target heart rate for moderate risk patients with atypical chest pain. Those with heart rates above 70 bpm are given IV beta blockers and scanned within minutes. The beta blockers serve less to reduce the heart rate and more to smooth out heart rate variability, said Dr. Scott D. Flamm, section head of cardiovascular imaging at the Clinic.

"Higher heart rates are less problematic with the dual-source scanner than with single-source scanners. But if we can reduce the variation-no matter what the heart rate is-we get better images and can make more confident diagnoses," Flamm said.

The scanner is used for a broad variety of studies, including symptomatic, moderate risk chest pain patients; pre- and post-thoracic aortic endovascular stent placement; and pre- and post pulmonary vein ablation. One big advantage of the dual-source scanner, according to Flamm, is the ability to scan patients who previously were either difficult or impossible to scan, particularly those with arrhythmias such as atrial fibrillation.

For now, the clinic does not use the dual-source scanner for emergency department patients. Typical protocol for patients in the ED is to place them in the chest pain unit, draw enzymes, and do a nuclear stress test. There's been some in-house lobbying to skip the stress test and go right to coronary CTA, Flamm said.

"The earth is shaking in that direction," he said.

Those pushing for the change noted that CTA is less expensive than a nuclear stress test or a trip to the cardiac cath lab.

DUAL-ENERGY RESEARCH

An exciting aspect of the dual-source scanner is the ability to perform dual-energy studies. In other words, each tube scans at a different level of power during a single scan. One project in the works at HUP involves scanning patients with aortic stent-grafts to check for endoleaks. The current follow-up protocol involves a three-phase study: precontrast, arterial, and delayed. Using dual-energy technology, one tube could be set at 120 kVp, while the other is set at 80 kVp. At those two energy levels, calcium and contrast will not have the same Hounsfield unit attenuation. Reconstructed images can subtract either the calcium or the contrast medium, essentially creating virtual noncontrast images.

"If the bright area in the aortic sac is calcium, we don't have to worry about it. If it's contrast, then it's an endoleak and the patient may need further follow-up or treatment," Litt said.

A project planned at HUP (and the Cleveland Clinic) is using dual energy to remove structures that interfere with visualization, such as bone at the skull base in a CTA of the head. Current software can take out the bone but it doesn't get rid of the associated streak artifacts. Dual-energy scanning can strip away bone as if it weren't there, leaving no artifacts. Researchers intend to compare the two techniques.

Investigators at the Cleveland Clinic also plan to use dual energy to see how well it can subtract calcification from the thoracic aorta. A number of surgeons and interventional cardiologists are interested in percutaneous placement of aortic valves.

"Getting a good look at the aortic valve with and without calcium could be critical in surgical planning," Flamm said.

Another potential use of dual-energy technology is tissue characterization, particularly in the liver. Many different types of lesions occur in the liver, and sometimes it's difficult to differentiate them. MRI can do so now, but using it means giving the patient an additional test, and MRI is not always right, Litt said.

Dual-energy scanning might also be beneficial to look at perfusion of organs, such as the brain or heart, and tumors. Images can be reconstructed to show only where the iodine has traveled, and pixel intensities can mark the degree of perfusion. Current CT perfusion imaging requires heavy doses of radiation. The dual-energy technique may be a way to reduce that output, he said.

Other potential areas of research include quantifying the amount of iron in the liver, quantifying calcium in osteoporotic bone, and quantifying gray and white matter in the brain.

Mr. Kaiser is news editor for Diagnostic Imaging.