CTA showing pulmonary embolus in 48-year-old man. Source: Radiology 2012;263:271-278

Adding two MRI sequences to a common MR pulmonary angiogram (MRPA) significantly improves detection of pulmonary embolism and could provide an alternative to CT angiography (CTA) for diagnosis, according to a study published in the April issue of Radiology.

MRPA has been used for pulmonary embolism detection in certain patient populations, such as pregnant women and patients whose kidneys may be harmed by contrast agents, but previous research has demonstrated the difficulty of obtaining adequate quality MRPA for suspected pulmonary embolism, according to study authors Diego R. Martin, MD, PhD, head of the department of radiology at the University of Arizona College of Medicine in Tucson, and colleagues. CTA is the standard for diagnosis, but it exposes patients to ionizing radiation and iodinated contrast agent.

Martin and colleagues wanted to assess the impact of two additional MRI sequences on MRPA’s accuracy—contrast-enhanced volumetric interpolated breath-hold examination (VIBE) and non-contrast true fast imaging with steady-state precession (true FISP). The additional sequences complement MRPA, according to the authors. The true FISP test does not require contrast agent or a breath hold, and VIBE provides a gray scale that enables readers to distinguish between the clot and the lung, which both appear dark on MRPA.

“Also, VIBE is not time sensitive,” Martin said in a statement. “If the patient coughs, you can do it again. You don't have that option with MRPA.”

The researchers studied the three techniques on 22 patients with CTA diagnosis of pulmonary embolism. Each sequence was analyzed separately by two independent reviewers.

Results showed a sensitivity of 55 percent, 67 percent and 73 percent for MRPA, true FISP and VIBE, respectively, but when combining the sequences, the overall detection rate improved to 84 percent. Specificity was 100 percent for all detection methods except for MRPA, which demonstrated one false positive. A total of 67 pulmonary emboli were detected at CT, 11 of which were not identified with any of the MRI techniques. Eight of the undetected emboli were in the segmental pulmonary arterial branch and the remaining three were present in a subsegmental branch.

The authors noted that location of the pulmonary embolus may affect the sensitivity for detection. Emboli within the lingual in all MR techniques may be harder to detect relative to other vascular distributions.

Martin explained that the MRI sequences used in the study are offered on all major MR systems, and the entire three-sequence protocol takes only 15 minutes to set up and perform.

“MRI is developing much faster than CT,” said Martin.“The images we're getting are already significantly better than they were a year ago. There is no doubt that in the future we will be able to offer a non-radiation-based alternative to CT for the diagnosis of pulmonary embolism.”