Faster scans, contrast add sensitivity to difficult diagnostic challenge

CT vies for top spot in embolus detection

By: Harold Abella

Although almost a decade has passed since researchers first proposed the use of CT in diagnosing pulmonary embolism, debate continues about its advantages and disadvantages compared with other diagnostic techniques. As its accuracy and cost-effectiveness are being assessed, CT appears to be emerging as a solid challenger to traditional techniques.

"Multidetector CT allows for direct visualization of the embolus in the pulmonary artery, as opposed to ultrasound for deep vein thrombosis or D-dimer," said Dr. Max Paul Rosen, an assistant professor of radiology at Harvard Medical School. "MDCT is usually our first-line test in the workup of suspected PE."

The imaging method is extremely useful in patients with underlying lung disease, in whom a ventilation/perfusion (V/Q) scan would likely be indeterminate, he said.

Researchers in Europe and the U.S. have compared MDCT technology with standard tests; many of them suggest using MDCT as an adjunct to V/Q scintigraphy. Others recommend primary screening with duplex ultrasound for patients with leg symptoms, since more than 90% of PE cases originate with deep vein thrombosis of the lower extremities or pelvis. V/Q scans, however, have shown lower sensitivity and specificity than MDCT when performed alone, and ultrasound examination of the legs shows positive results in only about 50% of patients with the condition.

Four- and eight-row multislice scanners offer a key addition to their predecessors' capabilities: the ability to provide larger data sets to be viewed and analyzed as volumes.

With the advent of 16-slice scanners-and the potential for up to 32 or even 64 slices-some researchers, including Dr. Martine Remy-Jardin of the University of Lille in France, predict that thinner section collimation and spatial resolution will provide improved views of the lungs, including airways, minuscule tissues, and subsegmental blood vessels.

To understand such complexity, the respiratory system must be examined in the intact, dynamic state, and only MDCT can provide the necessary spatial and temporal resolution to evaluate both anatomy and function, said Eric A. Hoffman, Ph.D., chief of physiologic imaging at the University of Iowa. Research in this area should help improve the ability to detect the onset, progression, extent, and location of ventilation or perfusion disorders like pulmonary emboli.

"MDCT is very cost-effective, especially if you compare it to pulmonary angiography," Hoffman said.

One of MDCT's main advantages is the ability to do faster scans in a single breath-hold, reducing the odds of missing certain regions or producing motion artifact. It also provides complete imaging of the chest, Rosen said. In approximately 25% of cases, there is no PE, but the MDCT provides an alternative diagnosis that explains the patient's symptoms.

More CT rows should also increase the odds of obtaining a conclusive diagnosis. The evolution of multislice CT from four to 16 slices was important from an anatomic imaging standpoint, according to Hoffman. Four-slice scanners allowed physicians to image the full lung volume in 30 to 40-second breath-holds, whereas 16-slice scanners allow for breath-holds as short as 10 to 15 seconds, a reasonable time for sick patients to hold their breath, he said. As MDCT evolves from 16 to 32 and up to 64 slices, clinicians should be able to perform functional studies such as V/Q without moving the table.

The contrast media and blood-pool agents that can identify the potential source of a clot as well as its location in the lung may help increase acceptance of MDCT as a replacement for pulmonary angiography as well, Hoffman said.

Contrast enhancement tends to vary among patients, however, according to Rosen. And radiologists don't always report details about the quality of the scan and how that influences their ability to make a concrete diagnosis.

"A better approach is to include in the radiology report a statement on the quality of the study as well as the radiologist's confidence in the diagnosis," Rosen said.

PHYSIOLOGY AND 3D IMAGING

A number of research groups are investigating potential x-ray blood-pool contrast agents. Hoffman and colleagues are working on a technique that consists of injecting a bolus of contrast agent and then following the brightness changes in the lungs as the contrast passes from the right side to the left side of the heart. The regions that don't exhibit changes in brightness are not receiving blood flow. There may be reasons for this other than PE, he said, but the technique is a highly sensitive indicator of pathology.

"The injection of blood-pool agents would allow you to identify the potential source of a clot as well as detect the presence of clots in the lungs," he said.

Physiological approaches to imaging will become increasingly popular in the coming years, Hoffman said. While four-slice CT allowed imagers to create dynamic scans of sections of lung several centimeters thick, 32- or 64-row scanners should greatly improve the ability to merge physiological and structural studies.

In addition to these new approaches, volumetric visualization and analysis is taking MDCT one step further. Besides allowing physicians to look at the data more rapidly, 16-slice multidetector scanners should make it easier to image the whole lung and to automate the process of locating the pulmonary arteries without having to examine the data slice by slice, Hoffman said.