Prompt detection of deep vein thrombosis (DVT) can play a vital role in preventing the condition from evolving into a potentially fatal pulmonary embolism (PE). Although a range of imaging tools can help spot the problematic venous blood clots, practitioners remain divided over their competing merits. As the quest to improve DVT diagnosis continues, MRI may emerge as a viable contender, according to a comprehensive study presented at December's RSNA meeting.

The clinical symptoms associated with DVT are well known and include pain and swelling in the affected limb, which upon further examination is likely to show redness, warmth, and tenderness. But clinical evaluation is notoriously unreliable, and to make matters worse, most people with DVT exhibit no symptoms, said Dr. Oliver Tann and colleagues from the department of radiology at King's College Hospital in London.

The group's educational exhibit described the options for DVT detection in the lower limbs of at-risk and symptomatic patients, highlighting the pros and cons of competing imaging techniques. Ascending contrast-enhanced venography is regarded as the gold standard for finding small thrombi deep within the venous system. The technique is invasive, however, leading many radiologists to seek a viable alternative.

Ultrasound stands out as a prime contender in the search for a less invasive diagnostic method. Both B-mode and Doppler are suitable for assessing lower limb veins. Color Doppler boosts examination speed and reliability, and an absence of color flow indicates a blood clot in the vessel under investigation. Ultrasound is relatively quick in skilled hands and has a high degree of accuracy in detecting femoral and popliteal DVT, Tann said.

CT venography offers an alternative range of benefits. The technique is noninvasive and very accurate, providing good visualization of the proximal venous system and deep calf veins. As an added advantage, CT venography can be combined with CT pulmonary angiography for a simultaneous investigation of both DVT and PE.

But neither ultrasound nor CT is without its problems, Tann said. Ultrasound's accuracy in DVT detection may depend on the operator's experience. The modality also has shortcomings in evaluating asymptomatic postoperative patients, searching for thrombi below the knee, and examining patients with a past history of DVT. Complex leg anatomy such as venous duplication presents further interpretation difficulties.

Patients who undergo CT venography, on the other hand, will receive a higher radiation dose than with standard venography. Sluggish blood flow in enlarged veins that causes contrast media layering and beam hardening artifacts generated from bone, orthopedic material, and calcifications affect image quality and hinder accurate diagnosis of vessel pathology.

The use of MRI for DVT diagnosis avoids many of these pitfalls. MR venography, performed with time-of-flight and phase contrast imaging, is highly accurate at showing blood flow in the proximal venous system, Tann said. MR direct thrombus imaging uses high signal generated from the thrombi to calculate the volume of intravenous blood clots and assess the risk of subsequent PE. Evaluation of pregnant women and patients with plaster casts is possible, as is differentiation between acute or chronic DVT and mimicking pathology.

On the downside, the costs of MRI are relatively high, the modality is not sufficiently available to be considered as a screening tool, and not all patients are willing or able to enter MR scanners. Like ultrasound, MR venography provides poor visualization of below-knee clots, but the significance of this failing is debatable.

"It is known that 20% of below-knee DVT will propagate proximally, but thrombosis that remains contained within the calf may not embolize," Tann said. "A wait-and-see policy is often adopted, with serial imaging of the affected limb."