The study's authors believe the noninvasive test could be a nearly foolproof, and ultimately cost-effective, way of identifying people with deep venous thromboses (DVT). These blockages are a leading complication of surgery and hospitalization, and they can be fatal if they become lodged in the lungs. Once there, the clots can starve the heart of blood and cause heart failure.
DVTs have also been linked to the controversial "economy class syndrome," sudden clot-related deaths associated with long-haul air travel and other prolonged immobility.Dr. Alan Moody, a University of Nottingham radiologist and a co-author of the study, says he'd like to see MRI become the routine screening tool for leg clots. However, that's not likely in the immediate future, because the machines aren't yet in every hospital and are pricey to operate.
"It's going to be tricky going from zero to hero in one go," says Moody, whose study appears this week in the Annals of Internal Medicine. What's more plausible, he says, is that doctors could turn to a MRI when the current screening tools, ultrasound and venography, deliver murky results.
Moody says MRI is especially useful in patients who've suffered a previous DVT. The episode can misshape vessels and lead to erroneous diagnoses of a second clot -- which generally warrants a lifetime of blood-thinning drugs to prevent any more. The MRI is much better than ultrasound or venography at distinguishing vessel warping from a genuine clot, and so may save patients unnecessary blood treatments, Moody says.
Moody and his colleagues tested MRI in 101 patients suspected of having DVTs after undergoing venography. In the latter procedure, which has been all but abandoned in this country, doctors pump contrast dye into a patient's leg and use X-rays to locate blockages.
Two reviewers examined the MRI films, which proved better than 90 percent accurate, on average, at identifying clots in the calf. The technique was also effective at identifying clots above the knee and in the groin area.
Moody says the results with MRI were at least as good as venography or ultrasound at finding clots. Yet, because both of the other methods are vulnerable to different interpretations among observers, MRI may even be more accurate than the conventional methods.
"Compared to the other standards, this technique is very reproducible and very accurate," he says.
At under 15 minutes a scan, MRI is about twice as fast as either ultrasound or venography, the latter of which has the added disadvantage of being quite painful.
The study did not include pregnant women. However, the researchers say MRI could also be a good way of ferreting out clots in this group of patients, among whom DVTs are a potentially grave problem.
Frederick Anderson Jr., a University of Massachusetts researcher who studies DVT, says he's skeptical MRIs will ever be cheaper than ultrasound.
What's more, he says, ultrasound is already a dramatic improvement over a physical exam even by the most expert physicians, who misdiagnose leg clots roughly half the time. A modest addition in accuracy may not justify the extra cost, he adds.
On the other hand, Anderson adds, if MRI can step in when ultrasound falters, it could be an important advance in diagnosing clots. The large doses of anticoagulants used to fight blockages are hazardous, he says.
"Certainly, you don't want to subject patients to that risk unless you're very confident they have a blood clot," he says.
What To Do
For more on deep vein thrombosis, check out Boston University.