Contrast protocol assists 3D clarity in MR angiography

Twitter icon
Facebook icon
LinkedIn icon
e-mail icon
Google icon

In MR angiography (MRA) of peripheral arteries, injecting contrast medium at a slow, balanced rate can alleviate contamination of veins in the calves while maintaining image quality, according to at multinational team of researchers from China and the United States.

Their study, published this month in the American Journal of Roentgenology, suggests that it is possible to increase arterial visibility in the calf while signal-to-noise and contrast-to-noise ratios are maintained at high levels. Researchers also discussed with Cardiovascular Business News the importance of 3D advanced visualization in these exams.

 
  A 46-year-old man with Fontaine stage III peripheral arterial occlusive disease on right side and Fontaine stage II disease on left. Injection protocol was 2 mL of contrast agent and 1 mL of saline solution at 1 mL/s. MR angiogram shows veins enhanced in calves. Arterial visibility score is 2 on right side and 3 on left. Venous contamination score is 3 on both sides. Calf contrast-to-noise ratio is 26.6 on right side and 27.8 on left. Decrease in signal intensity in femoral artery may be due to dielectric effect. Image and caption courtesy of the American Roentgen Ray Society.

“While the postprocessing of the MRA images adds considerable time to the diagnostic process, it is important to do 3D reconstructions because the images may be shared by other physicians from other departments or hospitals,” lead author Kai Lin, MD, a radiologist at Johns Hopkins University School of Medicine in Baltimore, told Cardiovascular Business News.

“We need to optimize images for diagnosis. Although postprocessing does not affect diagnosis greatly, it is my pleasure to diagnose with perfect images. As a radiologist, I always believe MRA is not only a diagnosis method but also an art,” Lin said.

Lin and colleagues from the Beijing Anzhen Hospital in China examined 80 consecutively registered patients with symptomatic peripheral arterial occlusive disease, who underwent a bolus-chase three-level MRA of the abdominal and lower-limb arteries. The patients were randomly and evenly split into two cohorts, Group A and Group B.

Both groups underwent their MRA on a 3T system (Signa Excite, GE Healthcare) with similar parameters:

• field of view (cm) -- 48 x 48
• reconstruction matrix size -- 512 x 512
• anteroposterior slice thickness (mm) -- 2.2-2.4 (abdomen and pelvis), 1.2-1.6 (thigh) and 0.6-0.8 (calf)
• pulse sequence -- 3D enhanced fast gradient-echo
• scan duration (s) -- 19-24 (abdomen and pelvis), 17-22 (thigh) and 32-36 (calf).

Researchers processed all images on a dedicated workstation (Advantage Workstation, software version 4.2, GE Healthcare). They measured the signal-to-noise ratio (SNR) at the origins of the renal artery, superficial femoral artery and posterior tibial artery. If the posterior tibial artery was occluded, they chose the anterior tibial artery (three limbs) and the peroneal artery (one limb) for analysis.

To postprocess the images, researchers first generate 3D MIPs (maximum intensity projections) at the workstation. “We then rotate the 3D-MIPs and cut of some of the unwanted part of the images, including venous contamination, to show the arteries clearly,” Lin said.

Contrast was administered at different levels and intervals for each group. “In group A, the test bolus was 2 mL of contrast agent (Magnevist, Bayer Schering) administered at 2 mL/s followed by 20 mL of saline solution administered at 1 mL/s. In group B, the test bolus was 2 mL of contrast agent administered at 1.2 mL/s followed by 20 mL of saline solution administered at 0.7 mL/s,” the authors wrote.

From the beginning of contrast injection, complete arterial phase image acquisition lasted approximately 100 seconds, according to the research team. Group A’s injection duration was 38 seconds, while Group B’s injection duration was 57 seconds. The scientists reported no statistical difference between the two groups in SNR or in contrast-to-noise (CNR) ratio.

The researchers found that the venous contamination score was higher in Group A with the faster injection rate than in Group B and that the arterial visibility score was higher in Group B than in Group A.

“With a 3T system, use of a slow injection rate can alleviate venous contamination and increase arterial CNR in the calf without sacrificing SNR and CNR at the two upper levels,” they wrote. “As such, high magnetic field strength unconventionally contributes to the image