Ventilation-perfusion scans and CT remain the standards for diagnosing pulmonary embolism, but the position of MRI improves.
"Does MRI have a role in the evaluation of pulmonary embolism? Yes, but it's a qualified yes," said Dr. H. Dirk Sostman, a professor of radiology at Cornell University in New York.
For a number of reasons, V-Q scintigraphy remains the gold standard for diagnosis of pulmonary embolism. The technique is safe, and a normal scan virtually rules out the possibility of pulmonary embolism, Sostman said. The nuclear imaging modality is widely available, has an extensive clinical experience, and can be highly specific under certain circumstances.
A considerable number of V-Q scans turn out to be inconclusive, however. In such cases, CT offers a highly robust imaging modality to clarify results, Sostman said. The technology is more widely available than MRI, and CT also does an adequate job of providing both pulmonary and extremity imaging together, a major consideration given the association between pulmonary embolism and deep vein thrombosis.
MR imaging with gadolinium enhancement has helped solve one of the major limitations of noncontrast MR imaging for pulmonary embolism: slow flow leading to spin saturation. The development has led to substantially improved images.
"With good equipment and experienced readers, you can get results similar to those obtained with contrast CT," Sostman said.
MR offers a number of advantages that warrant continued evaluation of the modality as an alternative to CT and ventilation-perfusion (V-Q) scans. MR does not require iodinated contrast. MR is capable of imaging capillaries and may offer the potential for perfusion imaging. Unlike V-Q scans, MR can image the pulmonary arteries and leg veins in a single sitting.
"By imaging only the lungs or legs, you might miss patients who have thromboembolic conditions," Sostman said. "You have to be concerned about both vascular beds."
A limitation of CT and MR is their inability to image subsegmental emboli.
"The radionuclide scan is still needed because it provides 100% sensitivity," Sostman said. "CT is still the preferred cross-sectional modality. MR is useful when it is performed correctly at experienced, motivated centers."
